High-affinity Variants Research Articles

Affinity maturation and fine functional mapping of an antibody fragment against a novel neutralizing epitope on human vascular endothelial growth factor

We have previously reported the isolation of a novel single-chain variable fragment (scFv) against vascular endothelial growth factor (VEGF), from a phage-displayed human antibody repertoire. This scFv, denominated 2H1, was shown to block the binding of VEGF to its receptor but exhibited a moderate binding affinity. Here, we describe the affinity maturation of the 2H1 scFv. Two phage-displayed libraries were constructed by diversification of the third complementarity-determining regions (CDRs) of the light (VL) and heavy (VH) chain variable domains of 2H1 using parsimonious mutagenesis. A competitive phage-selection strategy in the presence of the parental scFv as a competitor was used to eliminate low affinity binders. High affinity variants were retrieved from both libraries. An optimized VL variant was designed and constructed by combining recurrent replacements found among selected variants in a single molecule, resulting in an additional affinity increase. Further affinity improvements were achieved by combining this optimized VL with the best VH variants. The final variant obtained here, L3H6, showed an overall affinity improvement of 18-fold over the parental scFv and exhibited an enhanced potency to block the binding of VEGF to its receptor. Using phage display and extensive mutagenesis of VEGF, we determined the fine specificity of L3H6. This functional mapping revealed a novel neutralizing epitope on human VEGF defined by the residues Y25, T71, E72, N100, K101, E103 and R105. The conformational epitope recognized by L3H6 was recapitulated by grafting human VEGF residues into the mouse molecule, providing further confirmation of the nature of the identified epitope.

Haemoglobinopathia Ypsilanti – A rare, but important differential diagnosis to polycythaemia vera

We present a case of a mother and daughter who were initially diagnosed with polycythaemia vera and treated with venesectio. As JAK2 V6217F/exon 12 mutation analyses became available, these were performed and turned out negative. Haemoglobin electrophoresis was performed and the patients were found to have high oxygen affinity haemoglobin Ypsilanti. It is important and relevant to look for high oxygen affinity variants of haemoglobin when there is a family history of erythrocytosis, in young persons and when there is no apparent reason or clonal marker present.

FcγRIIa and FcγRIIIa polymorphisms in childhood primary immune thrombocytopenia

Primary immune thrombocytopenia (ITP) is the commonest acquired cause of bleeding in childhood. The aim of the present study was to evaluate the role of FcγRIIa and FcγRIIIa polymorphisms in the pathogenesis and therapeutic result of childhood ITP. The genotypic frequencies for two Fcγ receptor single-nucleotide polymorphisms, FcγRIIa-131 arginine (R) versus histidine (H) and FcγRIIIa-158 valine (V) versus phenylalanine (F) were examined in 53 children diagnosed with ITP. The genotype frequencies were compared with those of 45 healthy controls. The association between the above frequencies and disease natural course as well as therapeutic result following intravenous immunoglobulin (IVIG) administration was investigated. FcγRIIIa-158V was significantly overrepresented in children with ITP versus controls (P = 0.029), whereas no statistically significant difference was noted in FcγRIIa polymorphism distribution. No statistically significant difference was noted in the above genotype frequencies' distribution between children with newly diagnosed and chronic ITP, as well as with regards to the therapeutic result following IVIG administration. High-affinity FcγRIIIa variant (158 V) is possibly implicated in disease susceptibility, but neither of the two Fcγ receptor single-nucleotide polymorphisms seem to have any impact on chronicity or therapeutic effect of IVIG.

Engineering a soluble high-affinity receptor domain that neutralizes staphylococcal enterotoxin C in rabbit models of disease.

Superantigens (SAgs) are a class of immunostimulatory exotoxins that activate large numbers of T cells, leading to overproduction of cytokines and subsequent inflammatory reactions and systemic toxicity. Staphylococcal enterotoxin C (SEC), a SAg secreted by Staphylococcus aureus, has been implicated in various illnesses including non-menstrual toxic shock syndrome (TSS) and necrotizing pneumonia. SEC has been shown to cause TSS illness in rabbits and the toxin contributes to lethality associated with methicillin-resistant S.aureus (MRSA) in a rabbit model of pneumonia. With the goal of reducing morbidity and mortality associated with SEC, a high-affinity variant of the extracellular variable domain of the T-cell receptor beta-chain for SEC (~14 kDa) was generated by directed evolution using yeast display. This protein was characterized biochemically and shown to cross-react with the homologous (65% identical) SAg staphylococcal enterotoxin B (SEB). The soluble, high-affinity T-cell receptor protein neutralized SEC and SEB in vitro and also significantly reduced the bacterial burden of an SEC-positive strain of MRSA (USA400 MW2) in an infective endocarditis model. The neutralizing agent also prevented lethality due to MW2 in a necrotizing pneumonia rabbit model. These studies characterize a soluble high-affinity neutralizing agent against SEC, which is cross-reactive with SEB, and that has potential to be used intravenously with antibiotics to manage staphylococcal diseases that involve these SAgs.

Affinity and Cross-Reactivity Engineering of CTLA4-Ig To Modulate T Cell Costimulation

CTLA4-Ig is an Fc fusion protein containing the extracellular domain of CTLA-4, a receptor known to deliver a negative signal to T cells. CTLA4-Ig modulates T cell costimulatory signals by blocking the CD80 and CD86 ligands from binding to CD28, which delivers a positive T cell costimulatory signal. To engineer CTLA4-Ig variants with altered binding affinity to CD80 and CD86, we employed a high-throughput protein engineering method to map the ligand binding surface of CTLA-4. The resulting mutagenesis map identified positions critical for the recognition of each ligand on the three CDR-like loops of CTLA-4, consistent with the published site-directed mutagenesis and x-ray crystal structures of the CTLA-4/CD80 and CTLA-4/CD86 complexes. A number of single amino acid substitutions were identified that equally affected the binding affinity of CTLA4-Ig for both ligands as well as those that differentially affected binding. All of the high-affinity variants showed improved off-rates, with the best one being a 17.5-fold improved off-rate over parental CTLA4-Ig binding to CD86. Allostimulation of human CD4(+) T cells showed that improvement of CD80 and CD86 binding activity augmented inhibition of naive and primed T cell activation. In general, increased affinity for CD86 resulted in more potent inhibition of T cell response than did increased affinity for CD80. Optimization of the affinity balance to CD80 and CD86 to particular disease settings may lead to development of a CTLA4-Ig molecule with improved efficacy and safety profiles.

Robust in vitro affinity maturation strategy based on interface-focused high-throughput mutational scanning

Development of protein therapeutics or biosensors often requires in vitro affinity maturation. Here we report a robust affinity engineering strategy using a custom designed library. The strategy consists of two steps beginning with identification of beneficial single amino acid substitutions then combination. A high quality combinatorial library specifically customized to a given binding-interface can be rapidly designed by high-throughput mutational scanning of single substitution libraries. When applied to the optimization of a model antibody Fab fragment, the strategy created a diverse panel of high affinity variants. The most potent variant achieved 2110-fold affinity improvement to an equilibrium dissociation constant (Kd) of 3.45pM with only 7 amino acid substitutions. The method should facilitate affinity engineering of a wide variety of protein–protein interactions due to its context-dependent library design strategy.

Design of T-cell receptor libraries with diverse binding properties to examine adoptive T-cell responses

Adoptive T cell therapies have shown significant promise in the treatment of cancer and viral diseases. One approach, that introduces antigen-specific T cell receptors (TCRs) into ex vivo activated T cells, is designed to overcome central tolerance mechanisms that prevent responses by endogenous T cell repertoires. Studies have suggested that use of higher affinity TCRs against class I MHC antigens could drive the activity of both CD4+ and CD8+ T cells, but the rules that govern the TCR binding optimal for in vivo activity are unknown. Here we describe a high-throughput platform of “reverse biochemistry” whereby a library of TCRs with a wide range of binding properties to the same antigen is introduced into T cells and adoptively transferred into mice with antigen-positive tumors. Extraction of RNA from tumor-infiltrating lymphocytes or lymphoid organs allowed high-throughput sequencing to determine which TCRs were selected in vivo. The results showed that CD8+ T cells expressing the highest affinity TCR variants were deleted in both the tumor infiltrating lymphocyte population and in peripheral lymphoid tissues. In contrast, these same high-affinity TCR variants were preferentially expressed within CD4+ T cells in the tumor, suggesting they played a role in antigen-specific tumor control. The findings thus revealed that the affinity of the transduced TCRs controlled the survival and tumor infiltration of the transferred T cells. Accordingly, the TCR library strategy enables rapid assessment of TCR binding properties that promote peripheral T cell survival and tumor elimination.

Complex structure of engineered modular domains defining molecular interaction between ICAM-1 and integrin LFA-1.

Intermolecular contacts between integrin LFA-1 (αLβ2) and ICAM-1 derive solely from the integrin αL I domain and the first domain (D1) of ICAM-1. This study presents a crystal structure of the engineered complex of the αL I domain and ICAM-1 D1. Previously, we engineered the I domain for high affinity by point mutations that were identified by a directed evolution approach. In order to examine αL I domain allostery between the C-terminal α7-helix (allosteric site) and the metal-ion dependent adhesion site (active site), we have chosen a high affinity variant without mutations directly influencing either the position of the α7-helix or the active sites. In our crystal, the αL I domain was found to have a high affinity conformation to D1 with its α7-helix displaced downward away from the binding interface, recapitulating a current understanding of the allostery in the I domain and its linkage to neighboring domains of integrins in signaling. To enable soluble D1 of ICAM-1 to fold on its own, we also engineered D1 to be functional by mutations, which were found to be those that would convert hydrogen bond networks in the solvent-excluded core into vdW contacts. The backbone structure of the β-sandwich fold and the epitope for I domain binding of the engineered D1 were essentially identical to those of wild-type D1. Most deviations in engineered D1 were found in the loops at the N-terminal region that interacts with human rhinovirus (HRV). Structural deviation found in engineered D1 was overall in agreement with the function of engineered D1 observed previously, i.e., full capacity binding to αL I domain but reduced interaction with HRV.

Increased Antibody Affinity Confers Broad In Vitro Protection against Escape Mutants of Severe Acute Respiratory Syndrome Coronavirus

Even though the effect of antibody affinity on neutralization potency is well documented, surprisingly, its impact on neutralization breadth and escape has not been systematically determined. Here, random mutagenesis and DNA shuffling of the single-chain variable fragment of the neutralizing antibody 80R followed by bacterial display screening using anchored periplasmic expression (APEx) were used to generate a number of higher-affinity variants of the severe acute respiratory syndrome coronavirus (SARS-CoV)-neutralizing antibody 80R with equilibrium dissociation constants (K(D)) as low as 37 pM, a >270-fold improvement relative to that of the parental 80R single-chain variable fragment (scFv). As expected, antigen affinity was shown to correlate directly with neutralization potency toward the icUrbani strain of SARS-CoV. Additionally, the highest-affinity antibody fragment displayed 10-fold-increased broad neutralization in vitro and completely protected against several SARS-CoV strains containing substitutions associated with antibody escape. Importantly, higher affinity also led to the suppression of viral escape mutants in vitro. Escape from the highest-affinity variant required reduced selective pressure and multiple substitutions in the binding epitope. Collectively, these results support the hypothesis that engineered antibodies with picomolar dissociation constants for a neutralizing epitope can confer escape-resistant protection.

Xenografted Islet Cell Clusters From INSLEA29Y Transgenic Pigs Rescue Diabetes and Prevent Immune Rejection in Humanized Mice

Islet transplantation is a potential treatment for type 1 diabetes, but the shortage of donor organs limits its routine application. As potential donor animals, we generated transgenic pigs expressing LEA29Y, a high-affinity variant of the T-cell costimulation inhibitor CTLA-4Ig, under the control of the porcine insulin gene promoter. Neonatal islet cell clusters (ICCs) from INSLEA29Y transgenic (LEA-tg) pigs and wild-type controls were transplanted into streptozotocin-induced hyperglycemic NOD-scid IL2Rγnull mice. Cloned LEA-tg pigs are healthy and exhibit a strong β-cell–specific transgene expression. LEA-tg ICCs displayed the same potential to normalize glucose homeostasis as wild-type ICCs after transplantation. After adoptive transfer of human peripheral blood mononuclear cells, transplanted LEA-tg ICCs were completely protected from rejection, whereas reoccurrence of hyperglycemia was observed in 80% of mice transplanted with wild-type ICCs. In the current study, we provide the first proof-of-principle report on transgenic pigs with β-cell–specific expression of LEA29Y and their successful application as donors in a xenotransplantation model. This approach may represent a major step toward the development of a novel strategy for pig-to-human islet transplantation without side effects of systemic immunosuppression.

Symptomatic Erythrocytosis Associated with a Compound Heterozygosity for Hb Lepore-Boston-Washington (δ87-β116) and Hb Johnstown [β109(G11)Val→Leu, GTG>TTG

Hb Johnstown [β109(G11)Val→Leu, GTG>TTG] has previously been described as a high oxygen affinity variant in a heterozygous state and in combination with β0-thalassemia (β0-thal). Because the variant does not separate from Hb A by routine methods it may be easily missed unless clinical suspicion is high. Hb Lepore-Boston-Washington (Hb LBW; δ87-β116) is a δβ hybrid variant that clinically manifests similarly to a β+-thal. Hb LBW is not detected by routine polymerase chain reaction (PCR) sequencing but is easily detected by electrophoretic methods. We describe a 19-year-old African American male with a compound heterozygosity for Hb Johnstown and Hb LBW. The patient presented with progressively worsening chest pains, headaches and erythrocytosis. He was repeatedly phlebotomized with symptomatic improvement and subsequently was confirmed to have the high oxygen affinity hemoglobin (Hb) variant. The lowest Hb and hematocrit (packed cell volume, PCV) achieved by phlebotomy was 16.1 g/dL and 0.51 L/L, respectively. Currently, he is no longer being phlebotomized, and is feeling relatively well except for minor chest pain. It is unclear to what degree the phlebotomies contributed to his subjective improvement. The combination of Hbs Johnstown and LBW has not been heretofore described, and in this case, was associated with marked symptomatic erythrocytosis. This unique combination results in a more pronounced phenotype, similar to or slightly more severe than, compound Hb Johnstown/β0-thal. This compound hemoglobinopathy will likely not be correctly classified using a single method of Hb detection and underscores the need for multiple characterization methods when indicated by the clinical picture.

Examining the role of T cell co-receptors CD4 and CD8 in T cell activation by using high-affinity T cell receptors (121.28)

Abstract The recognition of foreign antigens by T cells is accomplished through the binding of the T cell receptor (TCR) in collaboration with co-receptor (CD4 or CD8) recognition of MHC. It is clear that CD4 and CD8 function by binding to their cognate MHCs and by delivering the kinase Lck to the TCR complex for initiation of signaling, however, the mechanistic details of this process remain unclear. For example, it remains unknown if co-receptors must engage the same MHC as the TCR, or if engagement of a nearby non-cognate MHC molecule is sufficient to initiate signaling. To examine the details of co-receptor function, we used a class II MHC specific TCR system consisting of a wild-type (KD 20 μM) and high-affinity variant (KD 25 nM). A panel of T cell hybridomas was generated expressing either the wild-type or high-affinity TCR paired with either the CD4 or CD8 co-receptor. The low-affinity, but not the high-affinity, TCR required cognate CD4 for activity. Unexpectedly, non-cognate CD8 was able to enhance the activity of the low-affinity TCR, and CD8:class I MHC interactions were required for activity mediated by the high-affinity TCR. Unexpectedly, CD8 also operated by preventing apoptosis of the antigen presenting cells (APCs). These data show that CD8 can act by binding to non-cognate MHC, and that its functions extend to the prevention of apoptosis of APCs. The findings thus provide a framework for a new model of co-receptor function.

Induction of Donor-Specific Tolerance: Is This Achievable?

To the Editor, The final goal of transplant physicians is the induction of donor-specific tolerance (DST), in which the host permanently accepts the graft, but immunity against other antigens is maintained. There have been reports of successful induction of DST in rodent models. However, the majority of studies have focused primarily on the inhibition of co-stimulatory signals that regulate the activation of T cells, and belatacept, a high-affinity variant of CTLA4-IgG, is the only biological drug that has been applied to human organ transplantation [1]. Antigen-presenting cells (APCs) are activated by the innate immune response, and mature APCs are the key regulators of rejection through activation of the acquired immune response. Therefore, the control of APC maturation is the key step in induction of DST in the field of organ transplantation. Previously, we reported induction of DST through control of APC maturation [2]. However, we were not able to delineate the underlying mechanisms, nor apply the methods to other models of transplantation. Recently, Jung et al. [3] reported a significant contribution where they showed successful induction of DST through the control of APC maturation. They developed a way to maintain APCs in a semi-arrested state by administration of an antibody ligating a particular epitope on intercellular adhesion molecule 1 (ICAM-1). Signaling through ICAM-1 is important for the activation of APCs. They evaluated the effect of semi-arrested APCs on xenogeneic grafts in humanized mice and non-human primates. Thus, they provided a feasible method that may assist the realization of the DST concept. Further work will likely be connected to clinical trials. However, care should be taken when interpreting these results and applying them directly to humans. The study was performed not on 'organs' but, rather, on a 'group of cells', because 'group of cells' induces a milder immune response, as compared to that against an organ. Moreover, a thorough method for screening various microorganisms in xenogeneic species has not yet been developed. Therefore, the safety issue remains to be resolved. Despite these limitations, this study provides new insight and will help overcome the problem of non-specific immunosuppression. Furthermore, this study may facilitate control of immune responses more meticulously and safely by providing a method of controlling the immune response at the site of interest.

In Vitro Affinity Maturation of an Anti-PSA Antibody for Prostate Cancer Diagnostic Assay

Prostate-specific antigen (PSA) is a serum marker that is widely used for the diagnosis of prostatic diseases. Various subforms of free PSA, which are associated with prostate cancer differently, have been identified in sera. Thus, specific detection of certain subforms could permit discrimination between benign and malignant cases. Although the monoclonal antibody 5D3D11 displays the desired selectivity, its relative weak binding affinity prevents its development into an effective diagnostic tool. The directed-evolution strategy presented here succeeds in enhancing affinity and immunoassay sensitivity while maintaining selectivity. Starting without structural data, we constructed four independent phage-display single-chain variable fragment (scFv) libraries targeting hot spots from CDR-L1, H1, H2, and H3. Mutations derived from each library were combined, yielding further affinity gains. This constitutes the first demonstration of additivity for independently selected complementarity-determining region (CDR) hot-spot mutations. The X-ray structure of the Fab′ 5D3D11–PSA complex (after it became available) inspired the design of two new libraries targeting CDR-L3 that resulted in other higher-affinity variants. Attempts at combining the new variants with previous ones did not result in further gains, suggesting that mutations from the two strategies provide alternative but noncomplementary solutions for affinity enhancement of 5D3D11. The results can be interpreted to provide a plausible explanation for the observed lack of additivity. Finally, with respect to the wild-type scFv, the best binders show an enhancement of sensitivity in sandwich immunoassay. Its ability to discriminate between prostate cancer sera and benign prostatic hyperplasia sera has now been confirmed through the dosage of 63 patients.

Systematic mutational analysis of an ubiquitin ligase (MDM2)-binding peptide: computational studies

To understand the importance of amino acids that comprise the peptide PMI (p53-MDM2/MDMX inhibitor), a p53-mimicking peptide with high affinity for the ubiquitin ligase MDM2, computational alanine scanning has been carried out using various protocols. This approach is very useful for identifying regions of a peptide that can be mutated to yield peptides that bind to their targets with higher affinities. Computational alanine scanning is a very useful technique that involves mutating each amino acid of the peptide in its complex with its target (MDM2 in the current study) to alanine, running short simulations on the mutated complex and computing the difference in interaction energies between the mutant peptides and the target protein (MDM2 in the current study) relative to the interaction energy of the original (wild-type) peptide and the target protein (MDM2 in the current study). We find that running multiple short simulations yield values of computed binding affinities (enthalpies) that are similar to those obtained from a long simulation and are well correlated with the trends in the data available from experiments that used Surface Plasmon Resonance to obtain dissociation constants. The p53-mimicking peptides contain three amino acids (F19, W23 and L26) that are major determinants of the interactions between the peptides and MDM2 and form an essential motif. We find in the current study that the trends amongst the contributions to experimental binding affinities of the hydrophobic residues F19, W23 and L26 are the best reproduced in all the computational protocols examined here. This study suggests that running such short simulations may provide a rapid method to redesign peptides to obtain high-affinity variants against a target protein. We further observe that modelling an extended conformation at the C-terminus of the helical PMI peptides, in accord with the conformation of the p53-peptide complexed to MDM2, reproduces the trends seen amongst the experimental affinities of the peptides that carry the alanine mutations at their C-termini. This suggests that some of the mutant peptides possibly interconvert between helical and extended states and can bind to MDM2 in either conformation. This novel feature, not obvious from the crystallographic data, if factored into modelling protocols, may yield novel high-affinity peptides. Our findings suggest that such protocols may enable rapid investigations of at least certain types of amino acid mutations, notably from large to small amino acids.

Production and characterization of LEA29Y, a variant of cytotoxic T-lymphocyte antigen 4-immunoglobulin, in Pichia pastoris

Blocking the CD28/B7 costimulatory pathway is a promising strategy in the treatment of graft rejection, graft-versus-host disease and autoimmune diseases. LEA29Y, a high-affinity variant of cytotoxic T-lymphocyte antigen 4-immunoglobulin (CTLA4Ig), is a more potent inhibitor of the interaction between CD28 and B7 than is CTLA4Ig. In a previous study, LEA29Y was produced in a mammalian cell system, which is time-consuming and expensive. To obtain LEA29Y more efficiently and cost effectively, we attempted to produce LEA29Y using a Pichia pastoris expression system. The gene encoding LEA29Y, with an additional 6-His tag at the N-terminus, was cloned into the yeast vector pPIC9K and expressed in the P. pastoris strain GS115. Under the optimized induction conditions for protein expression (inoculum density, OD(600) = 80; methanol concentration added daily, 1.0-3.0%; induction time point, 72-96 h; culture medium pH = 6.0), the yield of purified LEA29Y was approximately 30 mg l(-1) by one-step Ni-agarose affinity chromatography. PNGase F treatment showed the purified LEA29Y to be post-translational modified by N-linked glycosylation. In biological function assays, LEA29Y expressed in P. pastoris demonstrated specific binding to B7-1/B7-2-positive Raji cells and also suppressed lymphocyte proliferation in a dose-dependent manner. These results suggest that LEA29Y produced in P. pastoris is biologically active and will be useful for experimental therapy on immunotherapy for transplant rejection and autoimmune diseases.

LFA-1–specific therapy prolongs allograft survival in rhesus macaques

Outcomes in transplantation have been limited by suboptimal long-term graft survival and toxicities associated with current immunosuppressive approaches. T cell costimulation blockade has shown promise as an alternative strategy to avoid the side effects of conventional immunosuppressive therapies, but targeting CD28-mediated costimulation alone has proven insufficient to prevent graft rejection in primates. Donor-specific memory T (TM) cells have been implicated in costimulation blockade-resistant transplant rejection, due to their enhanced effector function and decreased reliance on costimulatory signaling. Thus, we have tested a potential strategy to overcome TM cell-driven rejection by targeting molecules preferentially expressed on these cells, such as the adhesion molecule lymphocyte function-associated antigen 1 (LFA-1). Here, we show that short-term treatment (i.e., induction therapy) with the LFA-1-specific antibody TS-1/22 in combination with either basiliximab (an IL-2Rα-specific mAb) and sirolimus (a mammalian target of rapamycin inhibitor) or belatacept (a high-affinity variant of the CD28 costimulation-blocker CTLA4Ig) prolonged islet allograft survival in nonhuman primates relative to control treatments. Moreover, TS-1/22 masked LFA-1 on TM cells in vivo and inhibited the generation of alloproliferative and cytokine-producing effector T cells that expressed high levels of LFA-1 in vitro. These results support the use of LFA-1-specific induction therapy to neutralize costimulation blockade-resistant populations of T cells and further evaluation of LFA-1-specific therapeutics for use in transplantation.

GlycoVariant Anti-CD37 Small Modular Immuno-Pharmaceutical Exhibits Superior Natural Killer Cell Mediated Cytotoxicity Against Chronic Lymphocytic Leukemia Cells at Low Concentrations and Low Antigen Density

AbstractAbstract 1847CD37 is a tetraspanin transmembrane family protein that is strongly expressed on the surface of mature human B cells and transformed mature B cell lymphoma and leukemia cells, including CLL cells. It is absent or minimally expressed on normal T cells, NK cells, monocytes, and granulocytes. Predominant expression of CD37 on CLL cells makes it an ideal candidate to target with potential agents for treatment of CLL. TRU-016, a small modular immunopharmaceutical protein (SMIP) that specifically binds to an extracellular region of CD37, is presently in clinical trials in CLL patients. TRU-016 includes humanized immunoglobulin variable regions (scFv) fused to a human IgG1 Fc region. We have previously reported that SMIP-016, the chimeric version of the humanized TRU-016, induced apoptosis in CLL B cells in the presence of goat anti-human Fc antibody cross-linker through a novel, caspase-independent pathway. Furthermore, SMIP-016 showed potent in vivo activity in a SCID xenograft mouse model. Aside from direct cytotoxicity, SMIP-016 mediates antibody-dependent cellular cytotoxicity (ADCC) by NK cells both in vitro and in vivo. In an attempt to enhance its ADCC function, a new variant of SMIP-016, SMIP-016GV, was designed with a modification of the glycosylation of the Fc portion of the molecule. SMIP-016GV exhibits enhanced binding to both low- and high-affinity molecular variants of human CD16 (FcγRIII) and augments ADCC potency when compared to SMIP-016. In this study, we compared SMIP-016GV and SMIP-016 in direct cytotoxicity and ADCC against CLL B cells. While SMIP-016 and SMIP-016GV mediated comparable direct cytotoxicity at 48 hrs in the presence of goat anti-human Fc crosslinker, the SMIP-016GV resulted in 2 to 4 fold increase in NK cell mediated ADCC function at all effector to target ratios tested. This increased ADCC with SMIP-016GV was observed using NK cell effectors derived from both normal as well as CLL-affected individuals. In addition, this enhanced cytotoxicity was sustained at concentrations of SMIP-016GV as low at 5E-6 μg/ml. These low concentrations of SMIP-016GV were also able to mediate superior ADCC in 697 cells expressing as few as 10,000 molecules of surface CD37 antigen. Furthermore, NK cells stimulated with the glycovariant were potently activated and released 3 to 4 fold more IFNγ compared to SMIP-016. Ongoing studies are aimed at defining other effector cells which may interact with SMIP-016GV via different Fcγ Receptors. Collectively, these results suggest potential use of the SMIP-016GV with enhanced ADCC function as an alternate for TRU-016 in B cell malignancies including CLL therapy. Disclosures:Siadak:Trubion Pharmaceuticals: Employment.

A Recombinant Antibody to EphA3 with Pro-Apoptotic and Enhanced ADCC Activity Against Various Hematologic Malignancies Shows Selective Inhibition of Colony Formation From Long-Term Culture-Initiating Cells (LTC-ICs) In Primary Leukemia Samples

AbstractAbstract 2897EphA3 is a receptor tyrosine kinase expressed at high level on the surface of some solid tumors and certain leukemias. KB004 is a high-affinity non-fucosylated, recombinant antibody developed by KaloBios derived from a mouse monoclonal anti-EphA3 antibody using Antibody Humaneering™ Technology. The antibody binds EphA3 from human and cynomolgus macaque with equivalent affinity (610 pM for human and 570 pM for cynomolgus as determined by surface plasmon resonance (spr) analysis) making the latter a relevant toxicology species. KB004 has a direct pro-apoptotic effect on EphA3-positive cells and a induces potent antibody-dependent cellular cytotoxicity (ADCC) activity mediated by CD16-expressing effector cells. The non-fucosylated antibody has enhanced affinity for both human and cynomolgus CD16a. The affinity of KB004 for cynomolgus CD16a (7.2 nM, determined by spr analysis) was comparable to the affinity for the high-affinity variant of CD16a from humans (6.6 nM), indicating that KB004 has similar potential for ADCC in cynomolgus macaques.We first demonstrated cell-surface expression of EphA3 by flow cytometry in approximately 50% of primary human leukemia cells. Here we extend the analysis to multiple myeloma (MM) in which 3/5 patient samples show EphA3 expression. EphA3 expression was confirmed in a panel of chronic myeloid leukemia (CML) samples [n=14] and in acute myeloid leukemia (AML) [n=40] in which EphA3+ cells were identified in each French American British (FAB) classification sub-type except M0, for which only two samples were studied. CD123-positive leukemia stem cells (CD34+ CD38− CD123+) also displayed surface expression of EphA3. KB004 induced apoptosis in EphA3+ primary cells from each of the diseases analyzed, including imatinib-sensitive and resistant CML, with no activity in EphA3-negative specimens, including normal CD34+ bone marrow cells. KB004 inhibited myeloid leukemia colony formation (CFU-L) from primary AML samples in methylcellulose colony assays without affecting normal hematopoietic colony recovery and also showed dose-dependent inhibition of long-term culture-initiating cells (LTC-IC) from AML stem cells. EphA3 expression was not detected on primary human CD34+ bone marrow cells from healthy individuals.These data support the development of an anti-EphA3 antibody for the treatment of hematologic malignancies. The selective activity against leukemic stem cells supports a novel strategy for therapeutic targeting of leukemia-initiating cells. Disclosures:No relevant conflicts of interest to declare.

Results of a phase I/II study of ocrelizumab, a fully humanized anti-CD20 mAb, in patients with relapsed/refractory follicular lymphoma

BackgroundOcrelizumab is a humanized anti-CD20 antibody with increased antibody-dependent cellular cytotoxicity compared with rituximab. This phase I/II study evaluated its safety and efficacy in patients with relapsed/refractory follicular lymphoma (FL) after prior rituximab therapy. Design and methodsForty-seven patients were treated in three dose cohorts and received eight infusions every 3 weeks: cohort A, 200 mg/m2 (n = 15); cohort B, 375 mg/m2 (n = 16); cohort C, first dose 375 mg/m2, seven subsequent doses of 750 mg/m2 (n = 16). Patients were assessed for safety, efficacy, pharmacodynamics and pharmacokinetics. ResultsThe median patient age was 58 years, the majority had Ann Arbor stage III/IV disease and had received a median of 2 (range 1–6) prior regimens. Ocrelizumab was well tolerated with grade 3/4 toxicity occurring in 9% of patients. The most common toxicity was infusion-related reactions (74% patients), all grade 1/2 except one grade 3 event. The objective response rate was 38% and was similar in patients with low-affinity and high-affinity variants of the Fcγ receptor IIIa (FcγRIIIa). With follow-up of ∼28 months, the median progression-free survival was 11.4 months. ConclusionOcrelizumab demonstrated activity in patients with relapsed/refractory FL following prior rituximab treatment, with safety similar to rituximab although adverse events appeared milder.