Endogenous neoantigen-specific antibodies mediate antitumor responses and determine vaccine efficacy

BACKGROUND: The contribution of B cells to antitumor immunity remains controversial, with studies reporting varied effects across cancer types. Even less is known about the role of the endogenous humoral response, including when tumor-elicited antibodies are protective, when they are deleterious, and how they might be modulated to influence antitumor immunity. Critically, it remains unclear whether specific antigenic features govern the efficacy of humoral immune responses against cancer. We aim to define the conditions under which endogenous antibodies mediate antitumor immunity and to leverage these principles to improve neoantigen-directed immunotherapies, including cancer vaccines. METHODS: We performed a pan-cancer analysis of The Cancer Genome Atlas (TCGA) to assess associations between intratumoral IgG and clinical outcomes. Using syngeneic mouse tumor models expressing membrane or cytoplasmic neoantigens, we examined how antigen localization influences endogenous antibody responses. We evaluated strategies to enhance humoral immunity via cytokine and chemokine modulation and assessed antibody responses in the context of cancer vaccination. RESULTS: Our pan-cancer TCGA analysis revealed marked heterogeneity in the prognostic impact of intratumoral IgG. In vivo, expression of membrane-localized, but not cytoplasmic, neoantigens drove potent class-switched IgG responses, activated myeloid cells, and restricted tumor growth independently of CD8 T cells. Notably, membrane-restricted antigen expression was sufficient to convert tumor types such as colorectal cancer, where IgG correlates with poor prognosis, into settings in which antibodies mediate tumor suppression. These effects required CD4 T-cell help, antigen-specific IgG, and Fc receptor engagement. Enhancing B-cell recruitment (C-X-C motif chemokine ligand 13) or B-cell help (interleukin (IL)-21) further amplified antibody-mediated tumor control, with co-expression providing the strongest benefit. Finally, we establish that antigen localization dictates vaccine efficacy: the point-mutated trophoblast cell-surface antigen 2 (Trop2) T256R, which exhibits impaired membrane localization and reduced antibody binding relative to wild-type Trop2, failed to elicit comparable vaccine-induced tumor control. CONCLUSIONS: These findings identify antigen subcellular localization as a key regulator of endogenous antibody-mediated antitumor immunity and cancer vaccine efficacy, providing a mechanistic framework for leveraging humoral immunity in immunotherapeutic strategies. CXCL13 and IL-21 emerge as candidate approaches to selectively enhance antibody-mediated tumor control in settings where membrane-bound antigens are present.