The majority of immune therapies for cancer have focused on generating tumor-specific CD8+ T cells that directly engage and kill tumor cells. However, CD4+ T cells play a critical yet poorly understood role in tumor immunity. Vaccines composed of MHC class II epitopes derived from mutated neoantigens (NeoAg) conferred protection against different mouse tumor types. Clinical studies of neoAg vaccines demonstrated antitumor activity, with the majority of neoAg-specific immune responses being CD4+ T cells. These tumor Ag-specific CD4+ T cells conferred protective antitumor immunity even in the absence of CD8+ cells directed against the same antigen. These findings are even more striking because most tumor cells do not express MHC class II and CD4+ T cells cannot directly kill them. In order to understand how CD4+ T cells protect against tumors in the absence of direct cytolytic activity, we used an animal model of multiple myeloma, the MOPC315.BM (MOPC) cell line derived from BalB/C background. MOPC cells do not express MHCII and produce an IgA with a unique, mutated neoAg in the lambda-2 light chain called Idiotype lambda-2.315 (Id). This Id antigen is well characterized and known to prime CD4+ T cells; however, when used as a peptide vaccine alone, it is weakly immunogenic and fails to elicit protective immune responses. We used an Id peptide sequence fused with a high-affinity HSP70 binding site delivered with poly(I:C) that elicited strong, Id-specific CD4+ T cell immunity and protection against MOPC tumor growth. No Id-specific CD8+ T cell or B cell responses were detected, and Id-specific CD4+ cells did not directly lyse MOPC cells in vitro. These findings led to the hypothesis that Id-specific CD4+ T cells confer protective immunity by promoting cross-priming of CD8+ T cells against non-Id, MOPC-associated antigens. To investigate this hypothesis, splenocytes and bone marrow cells were harvested from mice either challenged with MOPC cells only, vaccinated with the Id peptide only, or Id vaccinated and challenged with MOPC cells with protection against tumor growth. CD4+ and CD8+ T cell activity was assessed by measuring IFNg and TNFa production by intracellular cytokine staining (ICS) after eight days of in vitro restimulation with Id peptide and irradiated MOPC cells. In the absence of Id vaccination, very little Id-specific activity was observed in the spleen and bone marrow. Id vaccination alone induced CD4+ T cells in both compartments. In mice that received both Id vax and MOPC challenge, there was induction of CD8+ T cell, with a notable trend towards high levels of CD8+ activity in the tumor microenvironment (bone marrow). These CD8+ T cells were not detected when cells from the same animals were restimulated with the Id peptide alone, indicating that these CD8+ responses were against non-Id, MOPC-associated Ags. These data support a model in which Id-specific CD4+ T cells protect against tumor by providing critical helper activity to promote cross-priming of CD8+ T cell against MOPC-associated antigens.

Citation Format: Selma Bekri, Reunet Rodney-Sandy, Diana Gruenstein, Bjarne Bogen, Daniel Levey, Hearn Jay Cho. Mechanisms of CD4 T-cell tumor immunity in a preclinical model of multiple myeloma [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B37.