CD3 mAb Research Articles

HMBD004, a Novel Anti-CD47xCD33 Bispecific Antibody Displays Potent Anti-Tumor Effects in Pre-Clinical Models of AML

CD47 is the “don't-eat-me” signal and is ubiquitously expressed on normal cells where binding to SIRPalpha on macrophages inhibits phagocytosis. CD47 is commonly over-expressed in tumors where it correlates with immune evasion and poor prognosis. Blocking CD47-SIRPalpha interaction restores macrophage phagocytosis of tumor cells and anti-CD47 mAbs have shown anti-tumor efficacy in mouse models of solid tumors and hematological malignancies. Nevertheless, the widespread expression of CD47 decreases the bioavailability of anti-CD47 mAbs at the tumor due to antigen sink effects, and also risks off-target toxicities, most significantly from cross-linking of erythrocytes that show high expression of CD47. Therefore, increasing the specificity for binding CD47, and inhibiting the interaction with SIRPalpha, to tumor cells alone should increase the efficacy and safety of anti-CD47 therapy.CD33 is expressed on myeloid cells, but is highly over-expressed in 90% of AML patients, where it marks leukemic blast cells. The safety of CD33 as an antibody target for directing effector molecules to tumor cells has been convincingly demonstrated by the efficacy of gemtuzumab ozogamicin (Mylotarg), an antibody-drug conjugate (ADC) that binds CD33 with high affinity, in the treatment of relapsed AML. However, ADCs are notable for their significant risk of toxicities, and gemtuzumab ozogmicin was withdrawn for this reason in 2010. Still, other companies continue development on next generation anti-CD33 ADCs, e.g. vadastuximab talirine.Hummingbird Bioscience has used its proprietary Rational Antibody Development Platform for the design and development of novel therapeutic antibodies against CD47. Computational sequence and structural analyses predicted a highly specific surface epitope, with strong antigenicity and good safety profile, where antibody binding would inhibit binding to SIRPalpha. Subsequently, an in-house mouse immunization and screening strategy efficiently yielded a monoclonal antibody (mAb, 11a1) that bound with low nM (KD 5 nM) affinity and high specificity to the desired epitope on human CD47. In vitro, 11a1 demonstrated 100% inhibition of SIRPalpha binding at 1 nM, and potently induced phagocytosis of Raji cells by isolated macrophages.11a1 was subsequently humanized by CDR grafting and back-mutation, and the resulting humanized mAb maintained high affinity binding to CD47. Hummingbird Bioscience then constructed a bispecific anti-CD47xCD33 antibody (bsAb), based on a standard 1+1 IgG format, with a humanized anti-CD47 variable domain as an effector arm and the gemtuzumab variable domain as an anti-CD33 specificity arm. This bsAb (HMBD004) maintained the potent inhibition of SIRPalpha binding and induction of phagocytosis but, unlike many CD47 mAbs, caused negligible hemagglutination (i.e. cross-linking) of erythrocytes in vitro . Furthermore, HMBD004 showed preferential sorting of CD47+ CD33+ cells during FACS of mixed populations of CD47+ cells. Finally, in vivo CDX mouse models of AML were established by engrafting NCr nude mice with the CD47+ CD33+ AML cell line, HL-60, via subcutaneous implantation. Treatment of these mice with HMBD004 showed a significant decrease in tumor burden and increased PFS.Together, these data highlight the benefit of using CD33 binding to enhance the specificity, efficacy and safety of anti-CD47 antibody therapy in AML. Hummingbird Bioscience is currently advancing HMBD004 towards the clinic in 2018. DisclosuresNo relevant conflicts of interest to declare.

Ibrutinib, Fludarabine, Cyclophosphamide, and Obinutuzumab (GA101) (iFCG) for First-Line Treatment of Patients with CLL with Mutated IGHV and without TP53 Aberrations

Ibrutinib, Fludarabine, Cyclophosphamide, and Obinutuzumab (GA101) (iFCG) for First-Line Treatment of Patients with CLL with Mutated IGHV and without TP53 Aberrations

Targeting CD47 As a Novel Therapeutic Strategy in Multiple Myeloma

Introduction: CD47 is an immune checkpoint coordinating the innate and adaptive immune systems which allows cancer cells to escape the innate immune response by sending inhibitory signals to macrophages and regulates the adaptive immune anti-tumor response. Hence, CD47 has been identified as a novel therapeutic target in cancers, including multiple myeloma (MM) which is localized mainly in the bone marrow (BM). MM cells rely heavily on the interactions with tumor microenvironment (TME) and immune system. Macrophages are enriched in the BM of MM patients compared to healthy controls, and were demonstrated to support MM growth, in opposition to their intrinsic phagocytic properties. We study the new class of immune checkpoint inhibitor such as anti-CD47 monoclonal antibody (mAb) which enhance MM killing. We tested two anti-CD47 mAbs provided; Vx1000R (blocking only) - blocking CD47/SIRPα binding and Vx1004R (dual function) - blocking CD47/SIRPα binding and inducing cell death activity.The objectives of this study were: (1) to test the expression of CD47 (Vx1000R and Vx1004R binding) to MM cells, stromal cells (MSP-1 and HS5) and peripheral blood mononuclear cell (PBMC) sub-populations; (2) to study anti-CD47 mAbs binding in hypoxic conditions; (3) to investigate the effect of anti-CD47 mAbs on MM cell apoptosis in 2D and the 3D tissue engineered BM (3DTEBM; a scaffold incorporating BM plasma from MM patients, MM cells, and accessory cells) to reproduce the TME; (4) to test the cytotoxicity of Vx1004R on PBMC sub-populations in 3D culture; and (5) to examine the effect of Vx1004R on MM cell phagocytosis ex vivo by macrophages.Methods: We performed meta-analysis of CD47 mRNA expression in dataset available online in healthy, MGUS, smoldering MM and MM subjects. We tested the binding of Vx1000R and Vx1004R mAbs on MM.1S, H929, RPMI8226 and U266 cells and stromal cells (MSP-1 and HS5) in normoxia (21% O2) and hypoxia (1% O2), and in the 3DTEBM at 24h and 72h. We also tested Vx1000R binding to PBMC sub-populations (from 12 MM patients) including CD3 (T cells), CD14 (monocytes/macrophages), CD16 (NK cells, eosinophils, and neutrophils), CD19 (B cells), CD123 (pDC and basophils) and CD138 (MM cells).Next, we examined the cytotoxic activity of CD47 mAbs on MM cells alone and in co-culture with the BM negative fraction (CD138-depleted) and stromal cells (MSP-1) in 2D cultures by performing MTT survival assay, and in 3DTEBM cultures by flow cytometry. The cytotoxicity of Vx1004R was studied on PBMCs incorporated into a similar 3D model and treated with Vx1004R; proliferation and apoptosis of different sub-populations of PBMC were assessed via flow cytometry. Lastly, the effect of Vx1004R on MM cell phagocytosis by macrophages (CD14+; green) was tested after 2h of co-culture with MM cells (pre-labeled with DiD+; red) in the presence or absence of Vx1004R. Phagocytosis was determined as double positive (green+ red+) cells by flow cytometry.Results: The meta-analysis results demonstrated that CD47 mRNA expression increases with tumor progression (compared to early stage disease and healthy subjects). We demonstrated that MM.1S, H929, U266 and RPMI8226 cells and stromal cells (MSP-1 and HS5) express high levels of CD47, which does not alter in hypoxia, and that MM cells increase CD47 expression in 3DTEBM at 72h. CD47 (Vx1000R) is expressed in different sub-populations of PBMCs with the highest expression in MM cells. In addition, we have shown that anti-CD47 dual function mAb inhibits proliferation of MM cells, especially when co-cultured with negative fraction and MSP-1 stroma in 3DTEBM cultures. Moreover, we showed that anti-CD47 mAb induces phagocytosis of MM cells ex vivo by macrophages.Conclusions: CD47 is a promising target candidate for MM treatment, since CD47 mRNA increases with MM progression, and CD47 protein increases in MM cells in the presence of TME in the 3DTEBM, suggesting involvement in pathophysiology of MM. Moreover, anti-CD47 mAb enhances killing of MM cells when co-cultured with TME; however, it does not affect apoptosis of PBMC sub-populations in the 3DTEBM. Finally, anti-CD47 mAb induces phagocytosis of MM cells ex vivo by macrophages. These results suggest that anti-CD47 mAbs are applicable for MM treatment. Further studies are warranted to examine the effect of anti-CD47 mAbs as a novel strategy to inhibit the progression and dissemination of MM in vivo, especially when combined with chemotherapy. DisclosuresVij:Takeda: Honoraria, Research Funding; Jazz: Honoraria; Abbvie: Honoraria; Celgene: Honoraria; Amgen: Honoraria, Research Funding; Janssen: Honoraria; Bristol-Meyers-Squibb: Honoraria; Konypharma: Honoraria. Capoccia:Tioma: Employment. Manning:Tioma: Employment. Azab:Abbvie: Research Funding; Verastem: Research Funding; Targeted Therapeutics LLC: Other: Founder and owner; Selexys: Research Funding; Karyopharm: Research Funding; Cell Works: Research Funding; Cleave Bioscience: Research Funding; Cellatrix LLC: Other: Founder and owner; Glycomimetics: Research Funding; Tioma: Research Funding.

Co-Stimulatory versus Cell Death Aspects of Agonistic CD40 Monoclonal Antibody Selicrelumab in Chronic Lymphocytic Leukemia.

Simple SummaryPrevious observations have shown that CD40 activation of CLL cells via coculture with CD40L-expressing fibroblasts increases sensitivity to cell death by CD20 mAbs rituximab and obinutuzumab. We studied the activity of the fully human-agonistic CD40 mAb selicrelumab in primary CLL cells in relation to cell activation, induced pro-survival profile and sensitization for cell death by aCD20 mAbs. We found that the pro-survival effect of selicrelumab is minimal, while cell death by combined selicrelumab plus anti-CD20 antibody treatment is maintained. Thus, further investigation of applying selicrelumab combined with anti-CD20 mAbs in a therapeutic setting might be considered.Objectives: Chronic lymphocytic leukemia (CLL) is a common form of leukemia with a heterogeneous clinical course that remains incurable due to the development of therapy resistance. In lymph node proliferation centers, signals from the microenvironment such as CD40 ligation through interaction with follicular T helper cells shield CLL cells from apoptosis. Previous observations have shown that, despite CD40-induced changes in apoptotic mediators resulting in cell survival, CD40 activation also increases sensitivity to cell death by CD20 mAbs rituximab and obinutuzumab. To further investigate these observations, we here studied the activity of the fully human agonistic CD40 mAb selicrelumab in primary CLL cells in relation to cell activation, induced pro-survival profile, and sensitization for cell death by aCD20 mAbs, in vitro. Methods: CLL cells from peripheral blood were isolated by the Ficoll density method. The expression of activation markers and cytokine production following CD40 stimulation was quantified by flow cytometry and ELISA. The anti-apoptotic profile of CLL induced by stimulation was evaluated by the expression of BCL-2 proteins with Western blot, and resistance to venetoclax with flow cytometry. Cell death induced by the combination of selicrelumab and aCD20 mAbs was quantified by flow cytometry. Results: CLL cells treated with selicrelumab upregulated co-stimulatory molecules such as CD86, TNF-α and death receptor CD95/Fas. In contrast to the CD40 ligand-transfected NIH3T3 cells, induction of resistance to venetoclax by selicrelumab was very moderate. Importantly, selicrelumab stimulation positively sensitized CLL cells to CD20-induced cell death, comparable to CD40 ligand-transfected NIH3T3 cells. Conclusions: Taken together, these novel insights into selicrelumab-stimulatory effects in CLL may be considered for developing new therapeutic strategies, particularly in combination with obinutuzumab.

Idecabtagene vicleucel (ide-cel, bb2121), a BCMA-directed CAR T cell therapy, in relapsed and refractory multiple myeloma: Updated KarMMa results.

8016 Background: Patients (pts) with RRMM previously exposed to immunomodulatory agents, proteasome inhibitors (PIs), and CD38 antibodies (mAbs) have poor outcomes with subsequent treatments. Ide-cel, a BCMA-directed CAR T cell therapy, showed frequent, deep, and durable responses in heavily pretreated pts with RRMM in the pivotal KarMMa trial (Munshi NC, et al. J Clin Oncol 2020;38[suppl 15]. Abstract 8503). Here, we present updated data. Methods: Pts with ≥ 3 prior regimens (including immunomodulatory agent, PI, and CD38 mAb) and refractory to their last regimen per IMWG criteria were eligible (NCT03361748). Pts received 150─450 × 106 CAR+ T cells (target dose range) after 3 days of lymphodepletion (cyclophosphamide 300 mg/m2 + fludarabine 30 mg/m2). Endpoints included overall response rate (ORR; primary) and complete response (CR) rate (key secondary). Additional secondary endpoints included progression-free survival (PFS), overall survival (OS), and safety. Results: KarMMa enrolled 140 pts, and 128 received ide-cel. Pts had a median age of 61 years and a median of 6 (range, 3-16) prior regimens; 84% were triple-class refractory, and 26% were penta-class refractory (lenalidomide, pomalidomide, bortezomib, carfilzomib, and daratumumab). Most pts (88%) had bridging therapy. Median follow-up was 15.4 mo (data cutoff, 7 Apr 2020). ORR was 73% and median PFS was 8.8 mo in all treated pts; both increased with higher dose (Table). At the highest target dose (450 × 106 CAR+ T cells), the ORR was 81%, the CR rate was 39%, and the median PFS increased to 12.2 months with longer follow-up. Responses were observed in all subgroups including difficult-to-treat subsets (eg, extramedullary disease [ORR, 70%], high tumor burden [71%], and R-ISS stage III disease [48%]). OS continues to mature and the median has not been reached; the 15-month event-free rate for OS was 71%. Cytopenias (97%) and cytokine release syndrome (CRS; 84%) were the most common any-grade toxicities. CRS was mostly grade 1/2; 5 pts (4%) had grade 3, 1 had grade 4 (at 300 × 106), and 1 had grade 5 (at 300 × 106). Investigator-identified neurotoxicity was reported in 23 pts (18%); 4 pts (3%) had grade 3 and 0 had grade ≥ 4. Tocilizumab was used in 67 and 3 pts with CRS and neurotoxicity, respectively. Conclusions: Updated results from the KarMMa trial continue to demonstrate deep, durable responses with ide-cel in heavily pretreated pts with RRMM. Efficacy and safety reflect prior reports and support a favorable clinical benefit-risk profile for ide-cel across the target dose range. Clinical trial information: NCT03361748. [Table: see text]

Phase 1 trial of anti-CD22 recombinant immunotoxin moxetumomab pasudotox combined with rituximab for relapsed/refractory hairy cell leukemia.

7036 Background: Anti-CD22 recombinant immunotoxin moxetumomab pasudotox (Moxe) is FDA-approved for hairy cell leukemia (HCL) patients who have received at least two prior systemic therapies including a purine nucleoside analog. In phase 3 testing the complete remission (CR) rate was 41%, and response was higher in patients with lower tumor burden and lower titers of antidrug antibodies (ADA). Phase 1 testing indicated that most CRs were without minimal residual disease (MRD) and eradication of MRD was associated with prolonged CR duration. Monoclonal antibody (Mab) rituximab binds to CD20 on HCL cells and induces apoptosis or immune-mediated killing, but as a single-agent achieved only 13% CRs in relapsed HCL requiring therapy. In a phase 1 trial to determine safety, rituximab was combined with Moxe, with the goal to help reduce tumor burden and to prevent or delay ADA by killing normal B-cells. Methods: To allow rituximab sufficient time to accomplish both goals, it was infused 3 days before day 1 of cycle 1 at 375 mg/m2, and Moxe was given by 30-minute infusion on days 1, 3 and 5. On repeat cycles of Moxe days 1, 3 and 5, rituximab was given on day 1. Cycles were generally spaced 4 weeks apart. Moxe was begun at a lower dose, 30 rather than the 40 mcg/kg dose used in phase 3 in case the rituximab would increase its toxicity. Bone marrow aspirate flow cytometry, which can detect 0.002% HCL cells, was the most sensitive test used for MRD detection, much more sensitive than BRAF V600E digital droplet PCR (ddPCR) or bone marrow biopsy immunohistochemistry (IHC). Patients could receive 4 cycles past MRD-free CR, but not more than 8 cycles. Results: Three patients received Moxe at 30 mcg/Kg/dose and 6 received 40 mcg/Kg/dose, all without dose limiting toxicity (DLT). There was no evidence of hemolytic uremic syndrome or capillary leak syndrome. To prevent intravascular hypovolemia due to expected third spacing, patients were encouraged to drink one cup per hour of water or other fluid from days 1 to 8 and take dexamethasone 4 mg orally if headache or nausea prevented good oral hydration. Of the 9 patients, 7 (78%) achieved CR after 2 (n = 6) or 3 (n = 1) cycles, and achieved MRD-free CR after 2 (n = 3), 4 (n = 3) or 6 (n = 1) cycles. No patients became infected with COVID-19. Conclusions: This phase 1 trial met its primary endpoint of determining whether rituximab could be safely combined with Moxe and will enroll 4 additional patients to further access clinical activity. Further testing will determine whether addition of a CD20 Mab to Moxe significantly improves clinical outcome compared to Moxe alone, particularly long-term MRD-free CR rate. Clinical trial information: NCT03805932.

An Fc-Optimized CD133 Antibody for Induction of NK Cell Reactivity against B Cell Acute Lymphoblastic Leukemia.

Simple SummaryB cell acute lymphoblastic leukemia (B-ALL) is a common blood cancer characterized by proliferating and accumulating malignant, immature B cells within the body. Despite recent successes in B-ALL therapy, there is still a need for new therapeutic options. In the present study, we report on the characterization of 293C3-SDIE for the treatment of B-ALL. 293C3-SDIE is an improved anti-tumor antibody targeting CD133, a common protein on the surface of B-ALL cells. We demonstrated that 293C3-SDIE specifically induces activation of natural killer cells, which leads to lysis of B-ALL cells. Based on this study, we conclude that CD133 serves as a target for immune therapy, and treatment with 293C3-SDIE represents a promising therapeutic option in B-ALL therapy and warrants further preclinical and clinical evaluation.In recent decades, antibody-dependent cellular cytotoxicity (ADCC)-inducing monoclonal antibodies (mAbs) have revolutionized cancer immunotherapy, and Fc engineering strategies have been utilized to further improve efficacy. A promising option is to enhance the affinity of an antibody’s Fc-part to the Fc-receptor CD16 by altering the amino acid sequence. Herein, we characterized an S239D/I332E-modified CD133 mAb termed 293C3-SDIE for treatment of B cell acute lymphoblastic leukemia (B-ALL). Flow cytometric analysis revealed CD133 expression on B-ALL cell lines and leukemic cells of 50% (14 of 28) B-ALL patients. 293C3-SDIE potently induced NK cell reactivity against the B-ALL cell lines SEM and RS4;11, as well as leukemic cells of B-ALL patients in a target antigen-dependent manner, as revealed by analysis of NK cell activation, degranulation, and cytotoxicity. Of note, CD133 expression did not correlate with BCR-ABL, CD19, CD20, or CD22, which are presently used as therapeutic targets in B-ALL, which revealed CD133 as an independent target for B-ALL treatment. Increased CD133 expression was also observed in MLL-AF4-rearranged B-ALL, indicating that 293C3-SDIE may constitute a particularly suitable treatment option in this hard-to-treat subpopulation. Taken together, our results identify 293C3-SDIE as a promising therapeutic agent for the treatment of B-ALL.

Antihuman CD44 antibody BJ18 inhibits platelet phagocytosis by correcting aberrant FcɣR expression and M1 polarization in immune thrombocytopenia

BackgroundImmune thrombocytopenia (ITP) is an autoimmune hemorrhagic disease with a low platelet count. CD44 is a pivotal component involved in phagocytosis and inflammation, and monoclonal antibodies (mAbs) against CD44 have been shown to be beneficial in several autoimmune diseases. In the present study, we investigated the correlation between CD44 levels and disease severity in patients with ITP and explored the immunomodulatory mechanisms of the antihuman CD44 mAb BJ18 on platelet phagocytosis mediated by monocytes/macrophages. MethodsPlasma was collected from 45 participants to measure the circulating concentration of CD44 using ELISA. Peripheral blood mononuclear cells from patients and controls were isolated and induced to differentiate into monocytes/macrophages utilizing cytokines and drugs. CD44 expression on circulating cells and the effects of BJ18 on platelet phagocytosis, Fcɣ receptor (FcɣR) expression and M1/M2 polarization of macrophages were evaluated using flow cytometry and qPCR. ResultsCD44 levels of both the soluble form found in plasma and the form expressed on the surface of circulating monocytes/macrophages were significantly elevated in ITP patients. Linear correlations were verified between the CD44 levels and major clinical characteristics. In an in vitro study, BJ18 successfully inhibited platelet phagocytosis by monocytes/macrophages obtained from ITP patients. Further studies indicated that BJ18 corrected abnormal FcγR expression on monocytes/macrophages. Moreover, the polarization of proinflammatory M1 macrophages could also be regulated by BJ18. ConclusionsOur data indicated that the CD44 level has potential predictive value for disease severity and that the antihuman CD44 mAb BJ18 may be a promising therapy for ITP patients.

Epcoritamab induces potent anti-tumor activity against malignant B-cells from patients with DLBCL, FL and MCL, irrespective of prior CD20 monoclonal antibody treatment

Epcoritamab (DuoBody-CD3xCD20, GEN3013) is a novel bispecific IgG1 antibody redirecting T-cells toward CD20+ tumor cells. Here, we assessed the preclinical efficacy of epcoritamab against primary tumor cells present in the lymph node biopsies from newly diagnosed (ND) and relapsed/refractory (RR) B-NHL patients. In the presence of T-cells from a healthy donor, epcoritamab demonstrated potent activity against primary tumor cells, irrespective of prior treatments, including CD20 mAbs. Median lysis of 65, 74, and 84% were achieved in diffuse large B-cell lymphoma (n = 16), follicular lymphoma (n = 15), and mantle cell lymphoma (n = 8), respectively. Furthermore, in this allogeneic setting, we discovered that the capacity of B-cell tumors to activate T-cells was heterogeneous and showed an inverse association with their surface expression levels of the immune checkpoint molecule Herpesvirus Entry Mediator (HVEM). In the autologous setting, when lymph node (LN)-residing T-cells were the only source of effector cells, the epcoritamab-dependent cytotoxicity strongly correlated with local effector cell-to-target cell ratios. Further analyses revealed that LN-residing-derived or peripheral blood-derived T-cells of B-NHL patients, as well as heathy donor T-cells equally mediated epcoritamab-dependent cytotoxicity. These results show the promise of epcoritamab for treatment of newly-diagnosed or relapsed/refractory B-NHL patients, including those who became refractory to previous CD20-directed therapies.

Inhibition of B cell activation following in vivo co-engagement of B cell antigen receptor and Fcγ receptor IIb in non-autoimmune-prone and SLE-prone mice

Engagement of Fcγ receptor IIb (FcγRIIb) suppresses B cell activation and represents a promising target for therapy in autoimmunity. Obexelimab is a non-depleting anti-human CD19 mAb with an Fc region engineered to have high affinity for human FcγRIIb, thereby co-engaging BCR and FcγRIIb. To assess its ability to suppress B cell activation in vivo, we generated non-autoimmune-prone C57BL/6 (B6) and SLE-prone NZM 2328 (NZM) mice in which the human FcγRIIb extracellular domain was knocked into the mouse Fcgr2b locus (B6.hRIIb and NZM.hRIIb mice, respectively, the latter retaining features of SLE). XENP8206, a mAb which bears the same FcγRIIb-enhanced human Fc domain as does obexelimab but which recognizes murine CD19 rather than human CD19, inhibited in vitro BCR-triggered activation of B cells from both B6.hRIIb and NZM.hRIIb mice. Following administration of XENP8206 to B6.hRIIb or NZM.hRIIb mice, B cell numbers in the spleen and lymph nodes remained stable but became hyporesponsive to BCR-triggered activation for at least 14 days. These findings demonstrate proof-of-principle that pharmacologic co-engagement of BCR and human FcγRIIb inhibits B cell activation in non-autoimmune and SLE-prone hosts while preserving B cell numbers. These observations lay a strong foundation for clinical trials in human SLE with agents that co-engage BCR and FcγRIIb. Moreover, B6.hRIIb and NZM.hRIIb should serve as powerful in vivo models in the elucidation of the cellular and molecular underpinnings of the changes induced by BCR/FcγRIIb co-engagement.

Characterization of a variant CD4 molecule in Japanese Black cattle

Monoclonal antibodies (mAbs) that recognize cluster of differentiation (CD) molecules on lymphocytes are useful tools for the study of different lymphocyte subsets in flow cytometry (FCM) analysis. CD4 is a glycoprotein found on the surfaces of helper T cells, monocytes, macrophages, and dendritic cells. In this study, we describe Japanese Black (JB) calves in a farm whose peripheral blood mononuclear cells (PBMCs) did not react with a CD4-specific mAb. To identify calves with PBMCs with low mAb reactivity, PBMCs from 21 JB calves (1–12 months of age) bred at the same farm were examined using two different bovine CD4 mAbs (clones #CC8 and #CACT138A). FCM analysis indicated that the calves fell into two groups based on reactivity against the two mAbs, i.e., double-positive (DP) calves, whose PBMCs were recognized by both mAbs clones, and single-positive (SP) calves, whose PBMCs were only recognized by #CACT138A. PBMCs from seven calves were not recognized by #CC8, although they had normal reactivity with another mAb, #CACT138A. Sequencing analysis of the CD4 gene in these calves revealed four nucleotide substitutions (G918 T, A930C, G970A, and G1074A) in the coding region in the SP group when compared to the DP group. Three of the four mutations were associated with amino acid substitution (Q306H, K310 N, and A324 T). The substitution at A324 T was located in the D4 domain of CD4 gene. Homology modeling based on the amino acid sequences revealed that the surface structure of this part of the molecule was significantly different between the SP and the DP groups. Therefore, the epitope recognized by the #CC8 CD4 mAb was altered in calves with this genetic mutation, and this led to the low reactivity of the PBMCs from calves in the SP group aginst the #CC8 mAb. In conclusion, this is the first study to identify CD4 variants in JB cattle. We confirmed that the variants did not affect lymphocyte functions, such as mitogen stimulation or lipopolysaccharide-induced cytokine gene expression.

Labeling efficiency of Tb3+ conjugated CD133 monoclonal antibody nanocomplex targeting in vitro metastatic cancer cells

Efficient cancer treatment remains a huge challenge worldwide. As reported by the World HealthOrganization, the number of cancer patients is estimated to be 24 million in 2035, which is a 70% increasecompared to 14.1 million in 2012. The severity of cancer is due to the presence of cancer stem cells (CSCs),which are directly related to drug resistance, metastasis, and tumor relapse. Because of the unknown location ofthe primary tumor and/or the residency of CSCs, standard therapies deliver a high dose of drugs to the whole body,which can have negative effects and deadly consequences for patients undergoing treatment. Therefore, efficientluminescent materials for labeling and tracking CSCs are urgently needed to determine their distribution andtarget treatment. Herein, a fluorescent Tb3+ nano-ion and CD133 monoclonal antibody (mAb) were conjugatedinto a nano probe-complex (ET2). Tb3+ nano-ion is a rare-earth element and the CD133 mAb targets CD133,which is a CSC surface marker. The Tb3+ nanorods were surface treated with silica and activated with -NH2 forfunctioning before being coupled with CD133 mAb. Strong fluorescent Tb3+ nanorods were used to decrease thetoxicity of high-dose medicines, and the purpose of the CD133 mAb was to increase the specific binding capacityof CSCs to the ET2 nanocomplex. The luminescent properties of this coupled ET2 complex were determinedand its ability to target and label CSCs was determined using the pluripotent human embryonic carcinoma cellline, NTERA-2. Fluorescence microscopy showed strong luminescent signals from ET2-exposed NTERA-2cells. It was also demonstrated that the ET2 nanocomplex effectively labeled up to 97.74% of the tested NTERA-2 cells, but only 2.35% of CCD-18Co human colon normal cells. Therefore, these results show that the ET2luminescent nanocomplex specifically targeted and labeled CSCs, and may be used for further applications infundamental and clinical research.

Effects of CD20 antibodies and kinase inhibitors on B-cell receptor signalling and survival of chronic lymphocytic leukaemia cells.

Recently, clinical trial results have established inhibitors of B-cell receptor (BCR)-associated kinase (BAKi), with or without CD20 moniclonal antibodies (mAbs), as the preferred first-line treatment for most chronic lymphocytic leukaemia (CLL) patients. Using phosphospecific flow cytometry, we showed that in leukaemic cells from CLL patients the CD20 therapeutic antibodies - rituximab, ofatumumab, and obinutuzumab - inhibited BCR signalling pathways targeting preferentially pBTKY551 - but not BTKY223 - and pAKT. On the contrary, ibrutinib and idelalisib reduced pBTKY223 to a higher extent than pBTKY551 . The strong reduction of pAKT induced by idelalisib was enhanced by its combination with rituximab or ofatumumab. Moreover, CD20 mAbs and BAKi induced the death of leukaemia cells that was significantly potentiated by their combination. Analysis of the enhancement of cell death in these combinations revealed an approximately additive enhancement induced by rituximab or obinutuzumab combined with ibrutinib or idelalisib. Taken together, our data identified negative regulatory effects of CD20 mAbs and their combinations with BAKi on BCR signalling and cell survival in CLL. In conclusion, this study advances our understanding of mechanisms of action of CD20 mAbs as single agents or in combination with BAKi and could inform on the potential of combined therapies in ongoing and future clinical trials in patients with CLL.

A soluble activator that favors the ex vivo expansion of CD8+CD27+ T cells.

Adoptive cell therapy involves the infusion of tumor-reactive T cells into patients with cancer to provide antitumor immunity. The ex vivo expansion and differentiation of such T cells are key parameters that affect their therapeutic potential. Human T cells are presently expanded in culture through the use of anti-CD3 and anti-CD28 mAbs immobilized on beads, expressed on cells, or assembled in the context of soluble antibody complexes. Here we report the design of a small, bispecific single-chain variable fragment construct agonizing both CD3 and CD28 pathways. This soluble T cell expansion protein, termed T-CEP, activates, expands, and differentiates human T cells ex vivo at concentrations in the femtomolar range. Importantly, T-CEP promotes the preferential growth of human CD8+ T cells over the course of 12 days in comparison with methods involving immobilized anti-CD3 mAb/soluble anti-CD28 mAb or soluble anti-CD3/CD28 mAb complexes. The differentiation profile of the resulting human T cell population is also singularly affected by T-CEP, favoring the expansion of a preferred CD8+CD27+ T cell phenotype. The activity profile of T-CEP on human T cells ex vivo suggests its use in generating human T cell populations that are more suited for adoptive cell therapy.

Abstract 16582: Costimulation Blockade With Anti Cd80 and Cd86 Monoclonal Antibodies Prolonged Graft Survival in Allogeneic Ips Cell-Derived Cardiomyocyte Patch Transplantation

Introduction: How to avoid immune rejection after transplantation should be elucidated for confirmed efficacy in clinical application of allogenic induced pluripotent stem cell derived cardiomyocyte patch. The blockade of CD28-CD80/86 costimulation is known to induce T cell anergy and immune tolerance by the recruitment of regulatory T cells (T reg) and prolong graft survival in organ transplantation. Hypothesis: We hypothesized that the blockade of CD28-CD80/CD86 costimulatory pathway by anti-CD80/CD86 monoclonal antibodies (mAbs) induces T cell anergy in vivo and suppresses immune rejection in allogeneic iPSC-CMs subcutaneous transplantation mice model. Methods: T cell anergy was induced in vitro by co-culture of Balb/c and 25Gy irradiated C57BL/6 splenocytes with anti-CD80/86 mAbs for 5 days. The inhibitory effect of anergic cells was evaluated by mixed lymphocyte reaction (MLR) in C57BL/6 and Balb/c splenocytes with anergic cells. IFN-g in MLR supernatants was measured by ELISAs to assess immune response. C57BL/6 iPSC-CMs expressing luciferase were subcutaneously transplanted into Balb/c. Anti-CD80/CD86 mAbs were injected intraperitoneally 250μg/dose on day 0, 1, and 2 (treated mice, n=6). In control mice (n=6), equivalent volume of saline was injected. To evaluate iPSC-CMs graft survival, photon counts of iPSC-CMs were measured by bioluminescence imaging system (BLI). Cells of harvested grafts were analyzed by immunofluorescence staining (IF). Results: On ELISAs, IFN-g in MLR supernatants co-cultured with anergic cells was significantly lower as compared with those without anergic cells (12.3±14.4 vs. 251±80.2 pg/ml, p=0.007). The ability to suppress the alloresponses was dose-dependent. BLI showed that photon counts on day 14 in treated mice were significantly higher than in control mice (7397±2651 vs. 2703±1271, p=0.003). IF showed a siginificantly higher ratio of CD4 and Foxp3 double positive cells to CD4-postive cells in the grafts on day 7 in treated mice as compared with in control mice (23±3% vs. 9±3%, p=0.009) Conclusions: The blockade of CD28-CD80/CD86 costimulatory pathway might suppress immune rejection in allogeneic iPSC-CMs transplantation mice model and T reg might involve this immune tolerance.

Descriptive Analysis of Isatuximab Use Following Prior Daratumumab in Patients with Relapsed/Refractory Multiple Myeloma

Background: Anti-CD38 monoclonal antibodies (mAb) like daratumumab (dara) have become integral in managing relapsed/refractory (RR) and newly diagnosed (ND) multiple myeloma (MM). Isatuximab (isa), a newer CD38 mAb, induces direct rather than indirect apoptosis of MM cells. However, little is known about whether the use of one prior CD38 mAb will alter the efficacy of another in subsequent lines of therapy. Methods: All patients (pts) with MM treated at MD Anderson with isa after receiving dara in prior lines of therapy were identified. We conducted a retrospective analysis with data points including patient and disease characteristics, responses to dara, response to isa, the presence of high risk features, and the presence of t(11,14). Results: 9 pts were identified, ages 56-72. 5 pts (55%) were male. 5 pts (55%) were alive at the time of data cutoff. 5 pts were Hispanic, 3 White, and 1 Black. 8 pts (89%) had high risk features as represented by the presence of del17p, t(4,14), t(14,16), t(14,20), p53 mutations, gain 1q, extramedullary disease (EMD), CNS disease, early relapse (within 1 year) after autologous transplant, or an increased (>5%) peripheral blood plasma cells (PBPC). 2 (22%) had t(11,14). 4 (44%) had IgG MM. 2 (22%) with light chain disease, 2 (22%) with IgA MM, and 1 (11%) with IgD MM. Dara was initially used in lines 2-7. Dara combinations with pomalidomide (pom), bortezomib (bor), thalidomide (thal), lenalidomide (len), or carfilzomib (car); and pom combinations that also included elotuzumab (elo) or Cytoxan (cytox) are noted in table 1. Dara was discontinued (dc'd) in 8 pts due to progressive disease (PD) and in 1 pt due to toxicity. 8 pts (89%) experienced a best overall response (ORR) of partial response (PR) to dara; 1 pt had stable disease (SD). All pts received prior len and 8 pts received prior pom at some time during the treatment of MM. All pts received isa in combination with pom/dexamethasone (dex). Best ORR to isa/pom/dex: 5 pt (55%) had PR, 2 pt with minimal response (MR), 1 SD, 1 PD. Median treatment duration of isa/pom/dex was 5 weeks (2-14 weeks) at data cutoff. 3 pts dc'd isa/pom/dex due to infections, and 2 due to later progression. 2 pts remain on therapy. 1 pt chose to dc all MM therapy for quality of life purposes despite PR with isa/pom/dex. 1 pt died from cardiac disease unrelated to MM or treatment. Conclusions: Our current study of heavily pretreated pts with RRMM demonstrates that despite prior anti-CD38 therapy with dara, most patients (77%) experienced a response of MR or better with treatment with another anti-CD38 therapy isa. To our knowledge, this is the first report of outcomes to isa in patients with prior dara therapy. Further long term follow up will be needed to determine the length of response. Additional studies are planned to further evaluate this patient population. Table 1 Disclosures Thomas: Pharmacyclics: Other: Advisory Boards; BMS: Research Funding; Ascentage: Membership on an entity's Board of Directors or advisory committees, Research Funding; X4 Pharma: Research Funding; Xencor: Research Funding; Genentech: Research Funding. Iyer:Rhizen: Research Funding; CRISPR: Research Funding; Spectrum: Research Funding; Merck: Research Funding; Curio Biosciences: Honoraria; Target Oncology: Honoraria; Afffimed: Research Funding; Daiichi Sankyo: Consultancy; Legend Biotech: Consultancy; Trillium: Research Funding; Seattle Genetics, Inc.: Research Funding. Patel:Celgene: Consultancy, Research Funding; Cellectis: Research Funding; Nektar: Consultancy, Research Funding; Oncopeptides: Consultancy; Poseida: Research Funding; Precision Biosciences: Research Funding; Takeda: Consultancy, Research Funding; Janssen: Consultancy, Research Funding; Bristol Myers Squibb: Consultancy, Research Funding. Manasanch:Adaptive Biotechnologies: Honoraria; GSK: Honoraria; Sanofi: Honoraria; BMS: Honoraria; Takeda: Honoraria; Quest Diagnostics: Research Funding; Merck: Research Funding; JW Pharma: Research Funding; Novartis: Research Funding; Sanofi: Research Funding. Kaufman:Janssen: Research Funding; Bristol Myers Squibb: Research Funding; Karyopharm: Honoraria. Lee:Genentech: Consultancy; GlaxoSmithKline: Consultancy, Research Funding; Takeda: Consultancy, Research Funding; Amgen: Consultancy, Research Funding; Celgene: Consultancy, Research Funding; Janssen: Consultancy, Research Funding; Sanofi: Consultancy; Daiichi Sankyo: Research Funding; Regeneron: Research Funding; Genentech: Consultancy. Orlowski:Sanofi-Aventis, Servier, Takeda Pharmaceuticals North America, Inc.: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen, Inc., AstraZeneca, BMS, Celgene, EcoR1 Capital LLC, Forma Therapeutics, Genzyme, GSK Biologicals, Ionis Pharmaceuticals, Inc., Janssen Biotech, Juno Therapeutics, Kite Pharma, Legend Biotech USA, Molecular Partners, Regeneron Pharmaceuticals, Inc.,: Honoraria, Membership on an entity's Board of Directors or advisory committees; STATinMED Research: Consultancy; Founder of Asylia Therapeutics, Inc., with associated patents and an equity interest, though this technology does not bear on the current submission.: Current equity holder in private company, Patents & Royalties; Laboratory research funding from BioTheryX, and clinical research funding from CARsgen Therapeutics, Celgene, Exelixis, Janssen Biotech, Sanofi-Aventis, Takeda Pharmaceuticals North America, Inc.: Research Funding.

Systematic Literature Review of the Clinical Efficacy, Safety, and Patient-Reported Outcomes of Treatments in Patients with Relapsed or Refractory Follicular Lymphoma after Two Prior Therapies

Introduction For patients (pts) with relapsed or refractory follicular lymphoma (r/r FL) beyond front-line therapy, there is no well-defined standard of care (SOC) treatment, especially in the third-line or later (3L+) setting. Treatment decisions for symptomatic patients depend on comorbidities, extent of disease, lines of prior therapy and duration of response to initial anti-CD20 containing treatment. Treatment options for 3L+ may include similar options to the ones in earlier lines, with a preference for non-cross-resistant schemes. Recently, a plethora of new compounds are being studied in clinical trials. A systematic literature review (SLR) was conducted to identify relevant evidence on clinical outcomes in pts with r/r FL, including conventional treatments and emerging compounds, within the 3L+ setting. Methods We performed a SLR on March 17, 2020. Clinical trials and observational studies were searched through Embase, PubMed, and Cochrane Central Register of Controlled Clinical Trials from 1998 to 2020, followed by relevant conference proceedings and regulatory documents. Evidence assessing any intervention as 3L+ FL and published in English language was included. If a study included broader indolent non-Hodgkin's lymphoma (iNHL) pts, only FL data was reported; if a study included pts with fewer than 3L+, 3L+ data was included, with mixed line results excluded. Conventional treatments, defined as approved or clinical guideline recommended treatments, included anti-CD20 monoclonal antibody (mAb)-containing regimen, mAb alone, chemotherapy alone, phosphatidylinositol 3-kinase (PI3K) inhibitors (idelalisib, copanlisib, duvelisib), lenalidomide + rituximab (R2), radio-immunotherapy (RIT) with yttrium-90 (90Y) ibritumomab tiuxetan, tazemetostat, autologous and allogeneic stem cell transplantation (auto- and allo-SCT). Emerging compounds included new treatments that are not approved but were tested in the context of clinical trials (bruton tyrosine kinase inhibitors, bortezomib, polatuzumab vedotin, daratumumab, inotuzumab, bi-specific T-cell engaging CD19 mAb, anti-CD19 chimeric antigen receptor T-cell [CAR-T] therapy). Results Of the 3747 publications identified, 74 studies assessing 26 treatment regimens, including conventional ones like rituximab (R)-containing immunochemotherapy to emerging compounds such as CAR-T therapies, were selected. Across the conventional regimens, 7 studies reported clinical trial data with relatively large 3L+ FL populations (Table 1). With R monotherapy, PI3K inhibitors and tazemetostat, the reported complete response (CR) rates and overall response rates (ORR) ranged from 1-20% and 34-77%, while median duration of response (mDOR) ranged between 7.9-16.3 months. Three clinical trials with PI3K inhibitors reported proportions of pts achieving a response of at least 6 months in duration ranging from 18-30%; median progression-free survival (mPFS) ranging from 8.3-11.2 months. For tazemetostat, mPFS was 11.0 months in EZH2 mutant vs 5.7 months in wild-type FL. With allo-SCT, 2-yr PFS rate was 88% and 57% for pts with CR and PR, respectively. Overall survival (OS) data varied: median overall survival (mOS) was 28-38 months for PI3K inhibitors while mOS was up to 85 months after allo-SCT, despite high non-relapse mortality (NRM) rates with most common causes of death being graft-versus-host disease (GvHD) and infections. Safety profiles were also different across treatments, with most common side effects being hepatotoxicity, diarrhea and infections (PI3K inhibitors) to GvHD (allo-SCT). Three trials presented patient-reported outcomes (PRO) data, all using the Functional Assessment of Cancer Therapy (FACT) questionnaire. Two studies with PI3K inhibitors demonstrated favorable or clinically significant improvement on pts quality of life (QoL), but 1 allo-SCT trial did not show a significant difference between baseline and 2 years post-transplant scores. Conclusion To our knowledge, this is the first SLR focusing on 3L+ treatments of FL. Heterogeneity in study design, patient population, safety profile and reported outcomes make it challenging to identify an optimal treatment regimen for FL in the 3L+ setting. More PRO data are needed considering the important role pt QoL plays in treatment selection. Disclosures Ma: Novartis:Current Employment.Kaur:Novartis:Current Employment.Zhao:Novartis:Current Employment.Zhang:Novartis:Current Employment.Ramos:Novartis:Current Employment.Kumar:Novartis:Current Employment.John:Novartis:Current Employment.Bubuteishvili Pacaud:Novartis:Current Employment.Forcina:Novartis:Current Employment.

Abstract IA03: Optimizing immunostimulatory antibodies for cancer immunotherapy

Abstract Agonistic antibodies directed to immunostimulatory receptors are a currently untapped source for immunotherapy. Whereas checkpoint blockers have translated into the clinic, the rules for agonistic antibodies have been more difficult to discern and these reagents await further optimization. Here we discuss the salient properties of monoclonal antibodies (mAb) required to strongly agonize these receptors and discuss potential strategies for the future. We show that immunostimulatory mAb (ISA) can agonize key stimulatory receptors on the cell surface—many of which are from the TNFR family—and highlight the importance of isotype. Using CD40 mAb as a paradigm, we show that receptor clustering is key for ISA responses—probably mimicking ligand. This clustering can be achieved via Fc gamma receptor (FcgR) engagement or independently with the hIgG2(B) isotype. For CD40 this is highly epitope dependent, showing that epitope, isotype, and domain location all interact to drive mAb agonism. Finally, we show that isotype engineering can overcome “weak” epitopes. This can be completely independent of FcgR interaction in the case of hIgG2B, which may be of use in humans where the FcgR profile within the tumor microenvironment may vary. Citation Format: Xiaojie Yu, Anne White, Martin Glennie, Ivo Tews, Hayden Fisher, Chris Orr, Ruth French, Stephen Beers, Aymen Al'Shamkhani, Osman Dadas, Ali Roghanian, Jane Willoughby, Mark S. Cragg. Optimizing immunostimulatory antibodies for cancer immunotherapy [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr IA03.

Abstract 2770: Imaging cancer immunology: Monitoring CD47 mAb treatment in vivo by magnetic particle imaging

Abstract Introduction: The rapid growth of research into immuno-oncology research has fueled a need to be able to evaluate the efficacy of immunotherapies in a timely and comprehensive fashion. However, established non-invasive imaging approaches have been insufficient to meet all requirements. Magnetic Particle Imaging (MPI) is a novel tomographic molecular imaging technique that can be used to track iron-oxide in 3D in vivo, with sensitivity and specificity similar to nuclear medicine but without the complex workflow, safety, and half-life limitations. Methods: A murine breast tumor model was established by injecting 3 × 105 4T1 cells into the 4th mammary fat pad of 8-10 week female Blab/c mice. A CD47 monoclonal antibody (mAb) treatment was initiated by intraperitoneal injection of CD47 mAb (Bio-X Cell) at day 6 after tumor implantation. The treated and the control groups were then injected with an iron-oxide MPI tracer (6mg/kg) and then 3D MPI images were acquired 8, 11, 14 and 17 days after implantation. µCT and MRI were also acquired with a T2-weighted multi-slice, multi-echo (MSME) sequences at above time points. Both MPI and MRI images were co-registered and quantified. Tumors, liver, spleen and draining lymph nodes were then harvested, imaged, fixed, and stained with Perls Prussian blue. Results: Superparamagnetic iron oxide nanoparticles accumulation has been shown qualitatively to increase following CD47 mAb treatment [1]. To determine iron accumulation within the tumor, MPI was co-registered to both MRI µCT for anatomical context. All mice showed an accumulation of nanoparticles in the tumor and liver following injection in both MRI and MPI. Quantitation of iron at the tumor was performed and compared between the CD47-treated and untreated groups at every imaging time point with MPI. CD47 mAb treated breast tumors demonstrated enhanced positive contrast on 3D MPI images while a shortened T2 relaxation on MRI images. Iron content within the tumor was confirmed by ex vivo scanning followed by staining. Conclusions: By combining the sensitivity, specificity and quantitation potentials of MPI, information can be obtained on a preclinical model that monitors the efficacy of CD47 mAb cancer immunotherapy. With the high spatial resolution provided by MRI, coregistered MRI-MPI can be utilized to evaluate the response of tumor-associated macrophages to CD47 mAb and other cancer immunotherapies.

Abstract 2288: Investigational CD25 x CTLA-4 bispecific DART® molecule for depletion of tumor infiltrating Tregs via an enhanced Fc-dependent effector mechanism

Abstract Background: Tregs are suppressive cells whose presence in the tumor microenvironment (TME) has been correlated with tumor progression and may be a major impediment in the effectiveness of tumor immunotherapy. Removal of Tregs has been shown to induce cancer regression in animal models; however, selective depletion of tumor-infiltrating Tregs (TI-Tregs) is complicated as they share phenotypic markers with Tregs in healthy tissues as well as with tumor-infiltrating effector cells. To overcome cross-reactivity and achieve enhanced targeting specificity, we employed a bispecific strategy to simultaneously target two cell surface antigens (CD25 and CTLA-4) with overlapping, high level expression on TI-Tregs. Furthermore, we incorporated Fc engineering to enable target-cell depletion via enhanced Fc-mediated effector function. Methods: CD25 x CTLA-4 DART proteins were constructed based on CD25 and CTLA-4 mAbs and incorporating an Fc domain with enhanced binding to the activating Fc-gamma RIIIa receptor (CD16A) and reduced binding to the inhibitory Fc-gamma RIIb receptor (CD32B). Depletion of FoxP3-positive cells ex vivo was monitored by flow cytometry. Immunomodulatory effects of the DART proteins were evaluated in cultured PBMC or dissociated tumor cells in various in vitro immune assays, including mixed lymphocytes reaction (MLR). Results: Consistent with their design, we observed increased binding of CD25 x CTLA-4 DART molecules to cells when both target antigens were engaged compared to single antigen engagement. The bispecific DART molecule depleted in vitro induced FoxP3+ CD4+ TI-like Tregs cells by an Fc-dependent mechanism, with minimal effect on the viability of FoxP3-negative T cells. A molecule bearing a null Fc was ineffective. The extent of FoxP3+ T-cell depletion by the Fc-enhanced CD25 x CTLA-4 DART molecule was greater than that exhibited by the combination of its monovalent components, with the depleting activity requiring both arms of the bispecific molecule. The DART molecule had only minor effect on the capacity of effector T cells to secrete IL-2 and interferon-gamma upon restimulation or to exert cytotoxic function. In contrast the combination of CD25 and CTLA-4 mAbs reduced the ability of effector T cells to produce cytokines in a restimulation assay. An increased response in the presence of the Fc-enhanced CD25 x CTLA-4 DART molecule was also observed in various immune assays, consistent with Treg suppression and preservation of T-cell effector function. Conclusions: A CD25 x CTLA-4 DART molecule was effective in depleting activated FoxP3+ CD4+ Tregs in vitro, with reduced effects on resting Tregs or activated FoxP3- effector cells. Targeting CD25 and CTLA-4 in a bispecific molecular format with enhanced Fc-dependent effector function may be suitable for specifically depleting TI-Tregs. Citation Format: Alexey Berezhnoy, Kalpana Shah, Daorong Liu, Rillema Jill, Claudia Fieger, Douglas Smith, Francine Chen, James Tamura, Ezio Bonvini, Gundo Diedrich, Paul A. Moore. Investigational CD25 x CTLA-4 bispecific DART® molecule for depletion of tumor infiltrating Tregs via an enhanced Fc-dependent effector mechanism [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2288.