Primary Human Antigen-presenting Cells Research Articles

Abstract 576: ACTM-838, a novel immunotherapy that enriches in solid tumors after IV dosing and comprehensively reverses the immunosuppressive TME to promote durable anti-tumor immunity

Abstract ACTM-838 is an avirulent bacterial immunotherapy that encodes an engineered IL-15 (IL-15plex) and a constitutively active STING variant (eSTING). The ACTM-838 strain is a highly modified, attenuated S. Typhimurium that lacks several major inflammatory and immunogenic components on the surface of the microbe and is designed to naturally and specifically enrich in the tumor microenvironment (TME) via auxotrophic dependency on metabolites of the adenosine pathway and purines, to achieve tumor-specific payload delivery. ACTM-838 was selectively internalized by phagocytic, primary human antigen presenting cells (APCs) but not endothelial cells. ACTM-838 was stable in human whole blood and is not inactivated by complement. Human macrophage uptake of labeled ACTM-838 led to acidic lysosomes and nuclear delivery with a concomitant increase in expression of encoded IL-15plex and eSTING payloads such as IL-15, IRF3 reporter activity and IFN ⍺/β secretion, respectively. Primary human and murine macrophages polarized towards a dual co-stimulatory and phagocytic M1/M2 dual phenotype in response to ACTM-838. ACTM-838 delivered eSTING payload in STING knockout macrophages and induced significantly higher Type I IFN production over a negative control strain. IV delivered ACTM-838 showed dose and payload dependent tumor enrichment and potent single-agent efficacy in two independent syngeneic tumor models, EMT6 (orthotopic) and MC38. Sustained memory anti-tumor immunity upon tumor rechallenge in cured animals was demonstrated. ACTM-838 treated tumors exhibited a significant increase in human IL-15plex protein payload. ACTM-838 as a single agent induced profound immune reprogramming and remodeling of the TME over time through increased infiltration and activation of myeloid, NK, B and cytolytic CD8 T cells, MHC class I and II, antigen presentation as well as decreased Treg, PD1+Lag3+ exhausted T cells, and a remodeled stromal signature, as assessed by bulk RNAseq, and confirmed by flow cytometry. Synergistic anti-tumor activity was observed in multiple tumor models when ACTM-838 was dosed in combination with anti-PD1, safely inducing complete responses. No significant body weight loss or death was observed in any treatment regimens of ACTM-838 alone (single or multiple doses) or in combination with anti-PD1. Analyses of public RNAseq datasets from patients with primary or metastatic disease revealed cancer types with high levels of myeloid infiltration and adenosine enrichment, suggesting that ACTM-838 efficacy could provide benefit in these indications. ACTM-838 is a novel immunotherapy delivering IL-15plex + eSTING payloads to phagocytic APCs, inducing a durable anti-tumor immune response, after IV dosing. It possesses a compelling safety profile, and is currently in IND-enabling, preclinical development. Citation Format: Ping Fang, Kyle R. Cron, Oanh Pham, Julie R. Janes, John Brandenburg, Fauna Yarza, Bret Peterson, Christopher Thanos, Chan Whiting, Akshata R. Udyavar. ACTM-838, a novel immunotherapy that enriches in solid tumors after IV dosing and comprehensively reverses the immunosuppressive TME to promote durable anti-tumor immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 576.

Preferential uptake of chitosan-coated PLGA nanoparticles by primary human antigen presenting cells

Biodegradable polymeric nanoparticles (NP) made from poly (lactid-co-glycolide) acid (PLGA) and chitosan (CS) hold promise as innovative formulations for targeted delivery. Since interactions of such NP with primary human immune cells have not been characterized, yet, here we assessed the effect of PLGA or CS-PLGA NP treatment on human peripheral blood mononuclear cells (PBMC), as well as on monocyte-derived DC (moDC). Amongst PBMC, antigen presenting cells (APC) showed higher uptake of both NP preparations than lymphocytes. Furthermore, moDC internalized CS-PLGA NP more efficiently than PLGA NP, presumably because of receptor-mediated endocytosis. Consequently, CS-PLGA NP were delivered mostly to endosomal compartments, whereas PLGA NP primarily ended up in lysosomes. Thus, CS-PLGA NP confer enhanced delivery to endosomal compartments of APC, offering new therapeutic options to either induce or modulate APC function and to inhibit pathogens that preferentially infect APC.

Vaccinia virus preferentially infects primary human memory B cells

Abstract Vaccinia virus (VACV) has shown an extreme preference for binding to and infection of primary human antigen-presenting cells including monocytes, macrophages, and dendritic cells in the peripheral blood. Although VACV infection in monocytes, macrophages, and dendritic cells has been examined, few studies have evaluated VACV infection in B cells. In this study, we characterized VACV binding to and infection of primary human B cells. Flow cytometric analysis revealed that B cells isolated from human peripheral blood mononuclear cells exhibited two distinct populations of B cells, referred to as Bsmall and Blarge. Phenotypic analysis demonstrated that the Bsmall compartment was more quiescent compared to the Blarge population and enriched with CD138-positive plasma cells. VACV preferentially entered into and infected memory B cells. Especially, VACV preferentially infected ex vivo CXCR5+ B cells, particularly CXCR5+HLA-DRhigh B cells. Assessment of VACV gene expression by RT-PCR in total B cells to evaluate permissiveness of VACV infection in B cells revealed early and intermediate viral gene expression, but little to no late viral gene expression, suggesting that VACV infection was aborted at the late viral gene expression stage in unstimulated B cells. Additionally, preliminary data demonstrated enhanced VACV binding to and infection of B cells stimulated by B cell receptor crosslinking and TLR-9 agonist treatment. With the risk of smallpox or monkeypox being used as bioterrorism agents and the multitude of adverse reactions to the current vaccine, understanding VACV infection in B cells is important for developing a better smallpox vaccine.

ID: 240: HCMV infected human monocyte-derived macrophages and DC produce type I IFN in a cGAS-dependent manner

The majority of the human population is latently infected with human cytomegalovirus (HCMV). In the immunocompromised host HCMV reactivation can lead to serious symptoms and vertical infection can cause severe disabilities in the infant. We addressed the susceptibility of primary human antigen presenting cells to HCMV infection and their capacity to mount type I interferon (IFN-I) responses. HCMV treated plasmacytoid dendritic cells (pDC) did not efficiently support viral gene expression, while abundant to intermediate viral gene expression was detected in M-CSF macrophages (M-CSF M Φ ) , monocyte-derived DC (moDC), and GM-CSF macrophages (GM-CSF M Φ ) in descending order. Even though HCMV encodes multiple evasion mechanisms, several of which exploit the interferon system, the magnitude of IFN-I responses mounted by the different monocyte-derived subsets correlated with the degree of infection. In contrast, pDC that were largely resistant to HCMV infection showed the highest IFN-I expression. These data implied that pDC sensed HCMV in a different manner than monocyte-derived cells. Since microbial dsDNA can activate cyclic GMP/AMP synthase (cGAS) to produce cyclic GMP-AMP dinucleotides (cGAMP), which trigger the stimulator of interferon genes (STING) that in turn induces IFN-I, we analyzed cGAS-dependent IFN-I responses in pDC, moDC, GM-CSF M Φ , and M-CSF M Φ . We found that upon HCMV infection monocyte-derived macrophages and DC, but not pDC, showed enhanced cGAMP production as a measure of cGAS activity. Moreover, monocyte-derived cells devoid of cGAS showed impaired IFN-I responses. Thus, HCMV stimulated primary monocyte-derived macrophages and DC, but not pDC, showed cGAS dependent cGAMP formation that triggered IFN-I.

Intratumoral Inhibition of C5a Using Anti-C5a Monoclonal Antibody Inhibits Lymphoma Progression

Background: The complement system can have both pro-inflammatory and immunosuppressive effects. Our preliminary studies demonstrated that C5a can limit antigen-specific responses, in part, by enhancing the induction of immunosuppressive T regulatory (Treg) cells by primary human antigen-presenting cells and malignant B cells. The current study explored the effect of intratumoral C5a on B lymphoma progression and intratumoral T cells using the A20 murine lymphoma mouse model.Methods:(A) Co-injection approach: Female Balb/C mice were subcutaneously injected with A20 B lymphoma cells mixed with either 20 μg of rat anti-mouse C5a neutralizing monoclonal antibody (mAb) or matching isotype control mAb. Tumor growth was followed and the phenotype of tumor-infiltrating lymphocytes analyzed by flow cytometry.(B) Therapeutic approach: Established subcutaneous A20 tumors (7-9 mm diameter) were treated intratumorally with 20 µg of anti-mouse C5a neutralizing mAb (considered as day 1). A second dose (20 µg) of mAb was given intratumorally on day 5. Control mice were treated similarly with matching isotype control mAb. Tumor growth was followed.Data was analyzed using standard t-test (for tumor growth and immune responses) and Log-rank (Mantel-Cox) test (for comparing survival).Results:(A) Co-injection approach: Co-injection of anti-C5a mAb at the time of tumor cell inoculation significantly delayed A20 tumor growth (Fig. 1; p<0.05) and led to prolonged survival of anti-C5a-treated tumor-bearing mice when compared to the control mice (p=0.001).Tumors from mice that were co-injected with anti-C5a mAb showed significantly higher infiltration of activated CD8+ T cells producing IFNg than the control mice (Fig. 2; p=0.034). Anti-C5a-treated mice also had a lower percentage of circulating Tregs (p=0.023) and splenic Tregs (p=0.0001) compared to control mice.(B) Therapeutic approach: Treatment of pre-established subcutaneous A20 tumors with anti-mouse C5a mAb slowed tumor growth significantly when compared to the control mice (Fig. 3; **p<0.01; *p<0.05). Tumor-bearing mice treated intratumorally with anti-C5a also showed significant improvement in the survival when compared to tumor-bearing mice treated with isotype control Ab (p=0.008).Conclusions:Intratumoral anti-C5a mAb therapy inhibits progression of B lymphoma and improves survival, likely by reducing immunosuppression. We are currently exploring the mechanisms behind C5a-promoted tumor immunosuppression. [Display omitted] [Display omitted] [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

A conserved polylysine motif in CD86 cytoplasmic tail is necessary for cytoskeletal association and effective co-stimulation

T cell activation requires both antigen specific and co-stimulatory signals that include the interaction of CD28 with its ligands CD80 and CD86. These signals are delivered by antigen presenting cells (APC) in the context of the immunological synapse (IS). Reorganization of the cytoskeleton is required for the formation and maintenance of the IS. Our results show that a highly conserved polylysine motif in CD86 cytoplasmic tail, herein referred to as the K4 motif, is responsible for the constitutive association of CD86 to the cytoskeleton in primary human APC as well as in a murine APC model. This motif is not involved in initial APC:T cell conjugate formation but mutation of the K4 motif affects CD86 reorientation at the IS. Importantly, APCs expressing CD86 with mutated K4 motif are severely compromised in their capacity to trigger complete T cell activation upon peptide presentation as measured by IL-2 secretion. Altogether, our results reveal the critical importance of the cytoskeleton-dependent CD86 polarization to the IS and more specifically the K4 motif for effective co-signaling.

Application of a novel highly sensitive activity-based probe for detection of cathepsin G

Cathepsins are crucial in antigen processing in the major histocompatibility complex class II (MHC II) pathway. Within the proteolytic machinery, three classes of proteases (i.e., cysteine, aspartic, and serine proteases) are present in the endocytic compartments. The combined action of these proteases generates antigenic peptides from antigens, which are loaded to MHC II molecules for CD4+ T cell presentation. Detection of active serine proteases in primary human antigen-presenting cells (APCs) is restricted because of the small numbers of cells isolated from the peripheral blood. For this purpose, we developed a novel highly sensitive α-aminoalkylphosphonate diphenyl ester (DAP) activity-based probe to detect the serine protease cathepsin G (CatG) in primary APCs and after Epstein–Barr virus (EBV) exposure. Although CatG activity was not altered after short-term exposure of EBV in primary myeloid dendritic cells 1 (mDC1s), the aspartic protease cathepsin D (CatD) was reduced, suggesting that EBV is responsible for mitigating the presentation of a model antigen tetanus toxoid C-fragment (TTCF) by reduction of CatD. In addition, CatG activity was reduced to background levels in B cells during cell culture; however, these findings were independent of EBV transformation. In conclusion, our activity-based probe can be used for both Western blot and 96-well-based high-throughput CatG detection when cell numbers are limited.

Cathepsin A is expressed in primary human antigen-presenting cells

Cathepsins are expressed in antigen-presenting cells (APC). These cathepsins are known to regulate antigen processing and degradation of the invariant chain (Ii) into the class II-associated Ii peptide (CLIP), which occupies the peptide-binding groove of the major histocompatibility complex (MHC) class II molecule. Previous studies have identified the serine carboxypeptidase cathepsin A (CatA) in various tissues and cells; however, it is not clear whether CatA is also expressed in primary human APC. We demonstrate the expression of CatA in B lymphoblastoid cells (BLC), primary human B cells, both subsets of myeloid dendritic cells (mDC1 and mDC2), as well as in plasmacytoid DC. PMSF or lactacystin-mediated inhibition of serine proteases in BLC-derived lysosomal proteases resulted in the inhibition of amino acid release from the C-terminal end of two model peptides. This inhibition did not occur by using a proline rich peptide. Our data suggest that CatA is involved in the C-terminal fine-tuning of antigenic T cell epitopes in human APC.

Specific cathepsin B inhibitor is cell-permeable and activates presentation of TTC in primary human dendritic cells

Cathepsins of the cysteine, aspartyl, and serine classes are involved in antigen processing in the class II major histocompatibility complex (MHC) loading compartment. Investigation of these proteases in living cells is difficult to perform due to the lack of highly specific cell-permeable inhibitors. Recently, a highly selective cathepsin B (CatB) inhibitor, Z-Arg-Leu-Arg-α-aza-glycyl-Ile-Val-OMe (ZRLR), was described. We found that ZRLR is cell-permeable and specifically inhibits CatB, in contrast to the CatB inhibitor, CA074-OMe, which blocks cysteine cathepsins in addition to CatB in primary human antigen-presenting cells (APC). Furthermore, we compared both CA074-OMe and ZRLR in the ability to alter tetanus toxin C-fragment (TTC) presentation to T cells by different APC. As a result, we found enhanced presentation of TTC in the presence of ZRLR, as determined by detection of pro-inflammatory cytokines. We conclude that ZRLR is a specific, cell-permeable CatB inhibitor which can be used for antigen presenting studies in situ.

Cathepsin G is differentially expressed in primary human antigen-presenting cells

Cathepsins are required for the processing of antigens in order to make them suitable for loading on major histocompatibility complex (MHC) class II molecules, for subsequent presentation to CD4 + T cells. It was shown that antigen processing in monocyte-derived dendritic cells (DC), a commonly used DC model, is different from that of primary human DC. Here, we report that the two subsets of human myeloid DC (mDC) and plasmacytoid DC (pDC) differ in their cathepsin distribution. The serine protease cathepsin G (CatG) was detected in mDC1, mDC2, pDC, cortical thymic epithelial cells (cTEC) and high levels of CatG were determined in pDC. To address the role of CatG in the processing and presentation of a Multiple Sclerosis-associated autoantigen myelin basic protein (MBP), we used a non-CatG expressing fibroblast cell line and fibroblasts, which were preloaded with purified CatG. We find that preloading fibroblasts with CatG results in a decrease of MBP84-98-specific T cell proliferation, when compared to control cells. Our data suggest a different processing signature in primary human antigen-presenting cells and CatG may be of functional importance.

Activity and subcellular distribution of cathepsins in primary human monocytes.

Cathepsins (Cat) in antigen presenting cells (APC) control antigen processing as well as major histocompatibility complex class II transport and function. The set of active Cat and the subcellular architecture of the class II antigen presentation compartment are largely unknown in primary human APC, including peripheral blood monocytes. We used novel chemical tools to visualize Cat in an activity-dependent manner. Primary human monocytes contained active CatS, -B, and -H, while CatL was absent. Expression and activity patterns of Cat in human myelo-monocytoid cell lines were distinct from those found in primary cells. On a subcellular scale, the bulk of active Cat was concentrated in lysosomes in primary monocytes. In late endosomes, only active CatS was found in sizable amounts, colocalizing with C-terminal processing of the class II invariant chain and with cystatin C, the major endogenous Cat inhibitor. Late endosomes of human peripheral blood monocytes contain a well-controlled proteolytic machinery distinct from lysosomes, which is likely to play a key role in class II function.