The repeated emergence of pathogenic coronaviruses highlights the continual threat of zoonotic spillover events and the need to understand conserved regions of the viral spike protein that influence immune recognition. Here, we employ high-resolution proteomic analysis of circulating immunoglobulins to characterize the antibody response to the SARS-CoV-2 spike protein in vaccinated and infected individuals. We recombinantly expressed abundant plasma IgG lineages and identified nine antibodies targeting the highly conserved S2 spike subunit. Most of these S2- reactive plasma mAbs (7 of 9) exhibited neutralizing activity against pangolin coronavirus, while one mAb (SC45) demonstrated potent (IC50 < 1 μg/mL) neutralizing activity against both pangolin and bat sarbecoviruses RsSHC014 and WIV1. Additionally, SC45 exhibited potent prophylactic efficacy in the K18-hACE2 mouse model challenged with Pangolin-Guangdong CoV MP789 (Pg- CoV), significantly reducing lung viral replication at day 4 post-infection. The cryo-EM structure of SC45 bound to the prefusion-stabilized S2 domain of Pg-CoV reveals that SC45 targets a novel, conserved epitope in the connector domain. Strikingly, even though SC45 was the most abundant antibody (~13%) of anti-spike plasma IgG in an infected patient and binds to both SARS-CoV-2 and Pg-CoV spike with comparable affinities, it fails to neutralize early (D614G and Omicron BA.1) SARS-CoV-2 variants. Together, these findings identify a conserved site of vulnerability in the spike S2 subunit which informs on potential mechanisms of antibody immunity to zoonotic coronaviruses.

Epstein-Barr virus (EBV) causes over 200,000 cancers annually, including immunoblastic lymphomas in immunosuppressed hosts. Most transformed cells arrest, yet survive when deprived of the essential amino acid methionine. We instead find that EBV transformed lymphoblastoid cell lines (LCLs), which model the EBV latency III program-driven B-cell lymphoproliferative diseases of immunosuppressed hosts, rapidly die upon methionine restriction. Methionine restriction elevated LCL lipid reactive oxygen species and triggered ferroptosis. Whereas methionine restriction hypomethylates the EBV genome and triggers viral reactivation in latency I Burkitt cells by lowering the cellular methylation potential, the LCL latency III program instead redirected methionine toward redox defense, without altering the SAM/SAH ratio. Stable-isotope tracing revealed that latency III strongly induces transsulfuration, synthesizing cysteine de novo to support glutathione pools. The EBV oncoprotein LMP2A, which mimics B-cell receptor signaling, supported newly infected human B cell cystathionine-β-synthase and cystathionine-γ-lyase expression and methionine dependence, phenocopied by immunoglobulin crosslinking. In vivo , dietary methionine restriction impaired LCL xenograft outgrowth and depleted tumor cystine. Combined methioninase and cyst(e)inase administration blocked both cysteine sources, collapsed tumor glutathione levels, and triggered ferroptosis. Our results define methionine metabolism as a targetable ferroptosis vulnerability of EBV-transformed B cells. Highlights:Methionine restriction triggers EBV-transformed lymphoblastoid B cell ferroptosisEBV latency III induces transsulfuration to sustain LCL cysteine and glutathione Methioninase or dietary methionine restriction strongly impair LCL growth in vivo Methioninase plus cyst(e)inase collapses xenograft GSH levels and drives ferroptosis.

Metastatic disease remains a major cause of cancer-related mortality. Recent studies suggest that dissemination to other organs comes with metabolic changes that allow the metastasizing cancer cells to adapt to new microenvironments. A deeper knowledge of these specific metabolic features and associated vulnerabilities could lead to the development of more effective therapies against metastasis. We used in vivo and ex vivo models of MYC-driven breast tumorigenesis to explore the key metabolic pathways that change when mammary gland tumor cells metastasize to the lung. Stable isotope-resolved metabolomics, mass spectrometry imaging, and single-cell RNA sequencing demonstrated that mammary gland tumor-derived lung metastases have increased synthesis of glutathione fueled by increased cystine uptake. Metastatic cells relied heavily on the availability of extracellular cysteine or cystine, possibly due to downregulated intracellular cysteine synthesis through the transsulfuration pathway. When combined with focal radiotherapy, the amino acid degrader cyst(e)inase effectively reduced metastatic burden in the lungs. Together, these findings show that targeting cystine/cysteine exploits a metabolic dependency that is unique to metastatic cells and acts as a sensitizer to radiotherapy-induced oxidative stress, offering a promising targeted strategy.

Abstract Introduction Humoral immunity depends on a dynamic interplay between durable, circulating polyclonal IgG antibodies and long-lived memory B cells (MBC). However, the clonal relationship between these long-lived compartments remains poorly defined, particularly decades after childhood vaccination. Methods To investigate the durability of immunological memory to poliovirus (PV), we analyzed the molecular features of the recall response to inactivated poliovirus vaccine (IPV) boosting in healthy adults who had received their primary immunizations 24—55 years earlier during childhood in the United States. Using IgG immunoproteomics (Ig-Seq) coupled with ex vivo activation of MBCs via the Modular Immune In vitro Construct (MIMIC) system, we distinguished the respective contributions of LLPCs (as proxied by secreted plasma IgG proteomics) and MBCs to the anti-PV antibody response. Results IPV immunization elicited highly skewed plasma IgG recall repertoires, dominated by a small subset (∼8%) of total antibody lineages that were detectable prior to boosting (i.e., “pre-existing”). These IgG lineages spiked logarithmically in abundance (∼102-104 fold-increase) and potently cross-neutralized multiple PV serotypes in vitro (titers ≥ 1:105). Notably, these dominant plasma IgG lineages were clonally linked to class-switched MBCs that circulated in blood 21 days post-boost and bore heavily mutated VH and VL variable region genes. Importantly, IPV-primed ex vivo B-cell cultures yielded clonal expansions that mirrored the dominant post-boost plasma IgG lineages in vivo, providing an in vitro mechanistic correlate of recall immunity. Conclusion Together these findings demonstrate a decades-long interconnectivity between persistent MBC clones and the serological IgG repertoire. They further highlight the central role of immunological imprinting, whereby cross-reactive MBC clones–originally primed in childhood–can be reactivated to dominate the circulating IgG recall response more than 50 years after initial exposure. Funding Source DARPA Topic Categories Vaccines and Immunotherapy (VAC)

The Fc region of therapeutic IgG antibodies is often engineered to remove or "silence" Fc effector functions, but it remains unclear whether these mutations eliminate all Fc-mediated effector activity. Human Fc receptor-like 5 (FCRL5/FcRH5) is a low-affinity IgG Fc receptor expressed on B cells and is an actively pursued antibody target in multiple myeloma. Here, we show that common Fc function-silencing mutations do not impair FCRL5-mediated activity and therefore attenuate, rather than eliminate, Fc effector function. The crystal structure of the FCRL5-IgG1 Fc complex, solved at 3.4 Å resolution, revealed that FCRL5 binds IgG1 Fc in a 1:1 complex through a binding mode distinct from that of classical Fcγ receptors, explaining why mutations that attenuate Fc effector function spare FCRL5 binding. Fc-engineered antibodies that selectively engage FCRL5 inhibited B cell receptor-induced Ca2+ flux in FCRL5-expressing B cells. These findings demonstrate that Fc-attenuated therapeutic IgG retains the ability to engage FCRL5, identifying an unappreciated pathway for B cell modulation.

Protection against human norovirus correlates with attachment ligand blockade antibody titers, fecal IgA titers, and serum IgA titers. IgA responses in serum comprise approximately 80 to 95% of monomeric IgA (mIgA) and 5 to 20% of dimeric IgA (dIgA). Using serum LC-MS/MS proteomics, we established clonal relationships between circulating IgG and IgA, as well as between dIgA and mIgA. We observed a modest degree of clonal overlap between circulating IgG and IgA at steady state and found that more than 80% of antigen-specific mIgA was also detectable as dIgA. We biochemically characterized neutralizing epitopes on norovirus GII.4 virus-like particles (VLPs) targeted by serum IgA clonotypes and demonstrated that dIgA markedly enhances neutralization potency relative to mIgA and IgG in an epitope-specific manner. Cryoelectron microscopy and cryoelectron tomography revealed that whether IgA dimerization enhances viral neutralization depends on epitope accessibility on the VLP and antibody binding orientation. Together, these findings provide molecular-level resolution of the serum IgA response and define the structural basis by which dIgA enhances neutralization potency in an epitope-specific manner.

Abstract IgG1 immune complexes bind to all the Fcγ receptors (FcγR) expressed on myeloid cells, making it challenging to determine the precise role of each FcγR on Fc effector phenotypes. Here we report the engineering of Fc2 KG , an aglycosylated human IgG1 Fc domain that binds with near physiological affinity to FcγRIIa/b with no detectable binding to any other FcγRs. Crystallographic analysis elucidated the structural basis of how mutations in the Fc domain compensate for the absence of the N297 glycan and enable selective binding. Using single-cell phagocytosis assays, we show that particles opsonized with Fc-engineered antibodies formatted with Fc2 KG or with an Fc domain that binds only FcγRI (Fc5), are ingested by THP-1 cells with near identical kinetics. We find that with CD16 + primary human monocytes, selective FcγRII engagement is a major contributor to the ADCP mediated by wild-type IgG1. Additionally, we showed that Fc2 KG formatted antibodies induce high levels of GM-CSF. With M1-like monocyte-derived human macrophages, trastuzumab formatted with wild-type IgG1 Fc, with Fc2 KG or Fc5 were all equally proficient in the trogocytotic killing of opsonized SK-BR-3 HER2 + cells but only FcγRI engagement led to secretion of proinflammatory cytokines. Collectively, our results highlight how precisely tuned, Fc-engineered antibodies can be deployed to answer long-standing questions regarding the precise effector functions mediated by human FcγRs, information which is key for the optimization of therapeutic antibodies.

Human Fc receptor-like 5 (FCRL5) is a low-affinity IgG Fc receptor expressed on various B cell subsets and a potential therapeutic target. We discovered that commonly used Fc-silencing mutations, designed to prevent interactions between the Fcγ receptors on immune cells and the Fc domain of therapeutic IgG, do not prevent binding to FCRL5. As a result, unintended interactions between Fc-silent therapeutic IgG and human B cells may occur. We isolated a well-expressed variant of the Fc-binding portion of human FCRL5 by directed evolution and used structural modeling to guide the engineering of a human IgG1 Fc variant with approximately 100-fold higher affinity for FCRL5, enabling us to produce FCRL5:Fc complexes in solution. Native mass spectrometry, size exclusion chromatography, and the crystal structure of the FCRL5-IgG1 Fc complex solved at 3.4 Å indicate that the two proteins bind in a 1:1 stoichiometry. Furthermore, the structure revealed that FCRL5 binds to IgG1 Fc in a manner completely distinct from that of previously characterized Fc-binding proteins, such as Fcγ receptors, explaining why most Fc-silencing mutations do not disrupt FCRL5 binding. We demonstrate that selective cross-linking of FCRL5 with the B cell receptor (BCR) in cis, using Fc-engineered antibodies with either physiological or enhanced FCRL5 affinity, inhibits Ca2+ flux in FCRL5-expressing B cells. We compare this effect with the selective co-ligation of FcγRIIb with the BCR. Our work demonstrates that FCRL5 interacts with human IgG Fc in a distinctive manner and that engagement of FCRL5 by Fc-silent therapeutic IgG could influence B cell function.

Immunotherapy has revolutionized cancer treatment; however, only limited aspects of the tumor-immune interaction have been exploited successfully. While it is well-known that trafficking of T cells is often impaired during cancer, there are no available therapies that address this aspect of antitumor immunity. Clinically, expression levels of the sphingosine-1-phosphate producing enzyme sphingosine kinase 1 negatively correlates with survival of melanoma patients after immune checkpoint inhibitor (ICI) therapy. We therefore hypothesized that aberrant S1P signaling in tumors causes immune exclusion by impeding lymphocyte egress from the tumor draining lymph node (TDLN). We tested the efficacy of an engineered S1P-degrading enzyme (S1PL) on established murine tumors and performed immune profiling of TDLN and intratumoral T cell populations. S1PL increased both trafficking of lymphocytes out of TDLNs and T cell infiltration of tumors. Mice treated with S1PL evinced a higher proportion of S1P1+ circulating T cells relative to controls, suggesting that liberation of T cells from the TDLN occurred through the S1P/S1P1 axis. Increased lymphocyte trafficking correlated with strong inhibition of tumor growth and enhanced efficacy of ICI therapy. Administration of S1PL may be an attractive strategy for improving therapies that rely on antitumor adaptive immunity. This approach may thus be potent in settings wherein tumor-mediated immune exclusion is a dominant method of immune escape. Texas Biologics & CPRIT RP240454 Tumor Immunology: Cellular Responses and Tumor Microevironment (TIME)

Protection against human norovirus infection, which is the major cause of acute gastroenteritis, is strongly correlated with humoral immunity, particularly with IgA binding titers in serum and mucosal surfaces. Serum IgA is mostly monomeric (mIgA), with 10-20% present as dimeric (dIgA). Using serum antibody proteomics, we determined the clonal composition of the serum IgG, mIgA, and dIgA repertoires specific to GII.4 virus-like particles (VLPs) following oral vaccination with an adenoviral vectored norovirus vaccine candidate, which phenocopies the route of administration and the higher IgA to IgG serum titers associated with natural infection. We detected a low level of clonal overlap between circulating IgG and IgA at steady state, which, however, increased up to 65% following vaccination. Importantly, > 80% of serum mIgA specific to VLP was also found as dIgA, yet the mIgA:dIgA abundance ratio for different clonotypes varied by up to 250-fold (0.4190). We observed a similar congruence of the influenza hemagglutinin-binding dIgA and mIgA repertoires in serum following intramuscular flu vaccination. For antibodies targeting the apex, but not the lateral cleft, of the protruding (P) domain on human norovirus VLPs, IgA dimerization conferred markedly improved ligand blockade titers, resulting in higher potency towards the neutralization of live virus in the human intestinal enteroid system. Cryo-EM structures of prototypical apical and lateral cleft binding antibodies, coupled with cryo-ET imaging of VLP cross-linking mediated by dIgA, revealed that epitope accessibility, antibody binding orientation, and proximity to the attachment ligand binding site required for virus entry into host cells all influence the epitope-dependent differential potency of dIgA. Our findings elucidate key features of the molecular nature of the IgA serological response and the structural basis for the high neutralization potency of dIgA in an epitope-specific manner.

The efficacy of antibody responses is inherently linked to paratope diversity, as generated through V(D)J recombination and somatic hypermutation. Despite this, it is unclear how genetic diversification mechanisms evolved alongside codon optimality and affect antibody expression. Here, we analyze germline immunoglobulin (IG) genes, natural V(D)J repertoires, serum IgG, and monoclonal antibody (mAb) expression through the lens of codon optimality. Germline variable genes (IGVs) exhibit diverse optimality that is inversely related to mutability. Hypermutation deoptimizes heavy-chain (IGH) VDJ repertoires within human tonsils, bone marrow, lymph nodes (including SARS-CoV-2-specific clones), blood (HIV-1-specific clones), mice, and zebrafish. Analyses of mutation-affected codons show that targeting to complementarity-determining regions constrains deoptimization. Germline IGHV optimality correlates with serum variable fragment (VH) usage after influenza vaccination, while synonymous deoptimization attenuated mAb yield. These findings provide unanticipated insights into an antagonistic relationship between diversification mechanisms and codon optimality. Ultimately, the need for diversity takes precedence over that for the most optimal codon usage.

IgA antibodies are key to mucosal immunity; however, the identity, abundance, and oligomeric states of individual IgA clonotypes comprising the polyclonal repertoire remain poorly understood. To determine the molecular nature and protective mechanism of IgA clones elicited by mucosal vaccination, we performed serum proteomics analysis (Ig-Seq) in three subjects immunized with an oral norovirus vaccine designed for gut delivery. Serum IgA titers specific to the GII.4 norovirus vaccine strain were higher than IgG titers both on day 0 pre- and day 28 post-vaccination. Ig-Seq analysis of GII.4-specific serum IgA and IgG repertoires revealed that oral vaccination increased IgG+IgA clonal overlap by 2.4-fold, with 43 ± 17% of IgA clonotypes on day 28 also detectable in IgG. Mass separation of monomeric (mIgA) and dimeric IgA (dIgA) showed that >75% of antigen-specific IgA clonotypes co-circulate in both forms, with some clonotypes 5-times more abundant as dIgA. Monoclonal dIgAs targeting surface-exposed, immunodominant GII.4 epitopes significantly enhanced neutralization potency compared to mIgA or IgG. Conversely, dIgAs targeting partially occluded epitopes showed minimal potency gains, suggesting that epitope accessibility may impact the neutralization mechanism of dIgA and that dIgA titers to highly exposed epitopes could provide robust protection at mucosal sites. Our study provides molecular insights into oligomeric serum IgA repertoires, guiding future mucosal vaccine design. National Institute of Allergy and Infectious Disease R01 AI148260. Vaccines and Immunotherapy (VAC)

The non-essential amino acid L-serine (serine) is the second most consumed amino acid by cancer cells, supporting their survival and proliferation by contributing to the biosynthesis of nucleotides, lipids, and proteins. Serine-auxotrophic tumors such as luminal breast cancers have compromised serine synthesis pathways, thus rely on extracellular serine, making them vulnerable to serine depletion therapies. While dietary serine restriction, the only current method to reduce serine availability for tumors in vivo, has shown limited success in reducing tumor growth, its application is hindered by poor patient compliance and an inadequate reduction of circulating serine. To overcome these limitations, we developed an engineered human serine/threonine dehydratase (eSDH) enzyme that irreversibly depletes over 95% of circulating serine and threonine without requiring dietary changes. Preclinical studies show that eSDH treatment significantly inhibits the growth of serine-auxotrophic tumors. To enhance the therapeutic potential of eSDH and reduce off-target effects, particularly the depletion of the essential amino acid threonine, we focused on improving the specificity of eSDH for serine. Through iterative structure-based engineering, we developed a new SDH variant with over 20 times higher selectivity for serine toward threonine. Both in vitro and in vivo studies have confirmed that this novel SDH variant depletes serine while preserving threonine in cell culture media and serum. Current efforts are focused on improving the stability of the new SDH variant to extend its half-life in serum and optimize dosing for greater therapeutic efficiency. This advancement allows us to target serine-auxotrophic cancers more precisely while minimizing the impact on normal cells. The broader impact of this work lies in establishing enzymatic serine depletion as a more effective and feasible alternative to dietary restriction for cancer therapy. This approach could provide a new therapeutic strategy for treating various serine-auxotrophic tumors, offering better patient compliance and fewer side effects. Ladan Mashouri, Ebru Cayir, Sarita Bhetawal, Daniel J Diaz, Vipin Rawat, Jonathan Coloff, George Georgiou, Everett Stone. Engineering a novel serine-degrading enzyme for targeted cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4725.

The World Health Organization (WHO) recently recommended the programmatic use of the R21/Matrix-M vaccine for Plasmodium falciparum malaria prevention in children living in malaria-endemic areas. To determine its effects on humoral immunity, we conducted a proteomic analysis of polyclonal IgG antibodies directed against the NANP tetrapeptide of the circumsporozoite protein (CSP), which comprises the vaccine's core immunogen. In 10 malaria-naïve adult volunteers, R21/Matrix-M induced polarized IgG anti-NANP repertoires, heavily skewed for IGHV3-30/3-33 genes bearing minimal somatic mutation, which remained static in composition following a controlled human malaria infection challenge. Notably, these vaccine-generated antibodies cross-reacted with another protective CSP epitope, the N-terminal junction region, despite its absence from the R21 construct. NANP-specific IGHV3-30/3-33 mAbs mined from polyclonal IgG repertoires blocked sporozoite invasion in vitro and prevented parasitemia in vivo. Overall, R21/Matrix-M elicits polarized, minimally mutated, polyclonal IgG responses that can target multiple protective CSP epitopes, offering molecular insight into the serological basis for its demonstrated efficacy against P. falciparum malaria.

Abstract Introduction: Immunotherapy has revolutionized the landscape of cancer treatment; however, to date, only limited aspects of the tumor-immune interaction have been exploited successfully, e.g., relief of immune checkpoints, increase of tumor-associated antigen presentation. Although it is well-known that trafficking and entry of T cells from the circulation into tumors is often impaired, leading to an immune “cold”, checkpoint inhibitor-refractory state, there are no available therapies that address this important aspect of antitumor immunity. Sphingosine-1-phosphate (S1P) is a pleiotropic, bioactive lipid that has been shown to regulate lymphocyte egress from lymph nodes through its interaction with one of its several receptors, S1P1. Clinically, tumor expression levels of the sphingosine-1-phosphate-producing enzyme sphingosine kinase 1 (SPHK1) strongly and negatively correlates with survival of melanoma patients after immune checkpoint inhibitor (ICI) therapy, and pre-clinically correlates with reduced tumor infiltrating lymphocytes (TILs). We therefore hypothesize that aberrant S1P signaling in tumors and in their draining lymph nodes (TDLN) causes immune exclusion by impeding lymphocyte egress from the TDLN, ultimately restricting immune cell infiltration of tumors. Methods: We tested the therapeutic efficacy of an engineered S1P-degrading enzyme, S1P lyase (S1PL), on established (~100mm^3) syngeneic B16, CT26, and MC38 murine tumors. Using flow cytometry, we performed immune profiling of systemic, TDLN, and intratumoral T cell populations, assessing both activation status and expression of the S1P1 receptor. Results: Treatment of tumor-bearing animals with S1PL depleted systemic S1P, and dramatically increased both trafficking of lymphocytes out of TDLNs and T cell infiltration of tumors relative to control-treated animals. Mice treated with S1PL evinced a higher proportion of S1P1+ circulating and intratumoral T cells relative to control-treated mice, suggesting that the increase in liberation of T cells from the TDLN was effected through the S1P/S1P1 axis. This increased lymphocyte trafficking correlated with strong inhibition of tumor growth when S1PL was administered as a single agent. Conclusions: Therapeutic administration of S1PL may represent an attractive strategy for modulating lymphocyte trafficking to improve the efficacy of therapies that rely on the adaptive immune system to exert antitumor activity. This approach may be particularly potent in settings wherein tumor-mediated immune exclusion is a dominant method of immune escape. Citation Format: Alessandra Araujo, Max Rodnick-Smith, Ranya Al-Khaledy, George Georgiou, Mark Badeaux, Everett Stone. Therapeutic administration of an engineered sphingosine-1-phosphate lyase improves T cell trafficking to tumors and enhances antitumor response [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2659.

The domestic ferret is the preferred model organism for the study of influenza A infection and responses to vaccination, because its respiratory tract architecture and sialic acid receptor type and distribution are similar to those in humans. Despite this, the ferret has remained underutilized in antibody-omics research, which is increasingly critical to inform vaccine design. The molecular analysis of antibody responses is predicated on precise knowledge of the germline V(D)J segments, and is an active area of research for this species. To define a reference immunoglobulin repertoire for the ferret, we used a curated set of V(D)J genes from human and closely related carnivores to BLAST the ferret genome. Non-overlapping BLAST hits were annotated and vetted for recombination signal sequences as well as segment-specific functionality as defined by IMGT. Immunoglobulin transcript expression was analyzed for both variable and constant region genes, and we identified two functional IGHG genes in the ferret. We report a publicly available workflow for annotating immunoglobulin genes in any species, as well as a complete ferret immunoglobulin gene set. We include the genomic sequences for 409 ferret immunoglobulin genes of both the heavy and light chains. This reference data set establishes a critically important foundation for future BCR and antibody repertoire studies in this established preclinical vaccine model.

The clinical benefits of tumor-targeting antibodies (tAb) are modest in solid human tumors. The efficacy of many tAbs is dependent on Fc receptor (FcR)–expressing leukocytes that bind Fc fragments of tAb. Tumor-associated macrophages (TAM) and neutrophils (TAN) represent the majority of FcR+ effectors in solid tumors. A better understanding of the mechanisms by which TAN and TAM regulate tAb response could help improve the efficacy of tAbs. Here, we found that myeloid effectors interacting with tAb-opsonized cancer cells used antibody-dependent trogocytosis (ADT). During this process, myeloid cells “nibble off” tumor cell fragments containing tAb/targeted antigen (tAg) complexes. ADT is only tumoricidal when tumor cells express high levels of tAg and effectors are present at high effector-to-tumor ratios. If neither of these conditions are met, which is typical for solid tumors, ADT is sublethal. Sublethal ADT, mainly mediated by TAM, leads to two outcomes: (i) removal of surface tAg/tAb complexes from the tumor surface thereby facilitating tumor cell escape; and (ii) acquisition of bystander tAgs by TAM with subsequent cross-presentation and stimulation of tumor-specific T-cell responses. TAN stimulate tumor cell growth in the presence of IgG1 tAb; however, IgA tAbs trigger the tumoricidal activity and negate the stimulatory effect of TAN. Overall, this study provides mechanistic insight by which myeloid effectors mediate tumor cell killing and resistance during tAb therapy This work was supported by the NIH/NCI RO1 CA187392–01 and Janssen Pharmaceuticals Inc. Tumor Immunology: Cellular Responses and Tumor Microevironment (TIME)