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Background: Autologous stem cell transplantation (ASCT) after induction therapy improves disease-free survival for patients with multiple myeloma (MM). While the goal of ASCT is to render a minimal disease state, it is also associated with eradication of immunosuppressive cells, and we hypothesize that early introduction of immunotherapy post-ASCT may provide a window of opportunity to boost treatment efficacy., Methods: We conducted a phase 1 clinical trial to investigate the application of autologous lymphocyte infusion and anti-SLAMF7 monoclonal antibody, elotuzumab, after ASCT in patients with newly diagnosed MM previously treated with induction therapy. In addition to CD34+ stem cells, peripheral blood mononuclear cells were harvested prior to transplant and infused on day 3 after stem cell infusion to accelerate immune reconstitution and provide autologous natural killer (NK) cells that are essential to the mechanism of elotuzumab. Elotuzumab was administered starting on day 4 and then every 28 days after until 1 year post-ASCT. Cycles 4-12 were administered with standard-of-care lenalidomide maintenance., Results: All subjects were evaluated for safety, and 13 of 15 subjects completed the treatment protocol. At 1 year post-ASCT, the disease status of enrolled subjects was as follows: five stringent complete responses, one complete response, six very good partial responses, one partial response, and two progressive diseases. The treatment plan was well tolerated, with most grade 3 and 4 AEs being expected hematologic toxicities associated with ASCT. Correlative analysis of the immune microenvironment demonstrated a trend toward reduced regulatory T cells during the first 3 months post-transplant followed by an increase in NK cells and monocytes in patients achieving a complete remission., Conclusions: This phase 1 clinical trial demonstrates that early introduction of immunotherapy after ASCT is well tolerated and shows promising disease control in patients with MM, accompanied by favorable changes in the immune microenvironment., Trial Registration Number: NCT02655458., Competing Interests: Competing interests: DGC, KO, AA, SK-S, SB, and SK declare no conflict of interest. DC reports participation in the speakers’ bureau for Bristol Myers Squibb (BMS). SP reports consulting fees from Foundation Medicine and research funding from BMS (Celgene), Karyopharm, and Amgen. SG reports current and past research funding from Regeneron Pharmaceuticals, Boehringer Ingelheim, BMS (Celgene), Genentech, EMD Serono, Pfizer, and Takeda, unrelated to the current work. SJ reports consulting fees from Janssen, BMS, Regeneron, Sanofi, Takeda, and Karyopharm and data monitoring committee participation for Genmab and Sanofi. HJC reports employment by the Multiple Myeloma Research Foundation and receives research funding from BMS and Takeda, Inc, for work unrelated to this study., (© Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)

Background: Cancer-associated neoantigens (neoAg) derived from tumor genomic sequencing and predictive algorithms for mutated peptides are a promising basis for therapeutic vaccines under investigation. Although these are generally designed to bind major histocompatibility complex class I and induce CD8 cytolytic T lymphocyte (CTL) activity, results from preclinical and clinical studies demonstrate that the majority of neoAg vaccines efficiently induce CD4 T helper (Th) responses but not CTL. Despite this, these vaccines have demonstrated clinical efficacy. Therefore, understanding the mechanisms of CD4 + T cell-mediated tumor protection is critical to optimizing this immunotherapeutic strategy., Methods: We investigated this phenomenon in the mineral oil-induced plasmacytoma (MOPC).315.BM (MOPC315) mouse model of multiple myeloma, a malignancy of plasma cells. MOPC315 cells express in their lambda chain a unique tumor-specific neoAg, an idiotypic (Id) peptide. We generated a vaccine formulated with this Id peptide fused to a heat shock protein HSC70 binding (HSB) motif co-delivered with poly (I:C). The immunogenicity of the Id-vaccine was measured in splenocytes by ELISpot. Mice were challenged with MOPC315 cells and antitumor immunity was assessed by co-incubating splenocytes and bone marrow mononuclear cells derived from vaccinated mice and controls, with the Id antigen and irradiated MOPC315 cells. The frequency of activated CD4 and CD8 T cells and their phenotype were characterized by flow cytometry., Results: Id-vaccine efficiently induced antigen-specific CD4 Th activity and antitumor immunity, protecting mice from MOPC315 tumor growth. CD4 cytolytic activity was not detected under these conditions. Polyfunctional CD8 T cells homed to the bone marrow microenvironment of protected mice and preferentially expanded only when restimulated ex vivo with both Id peptide and MOPC315 cells. Protective activity was abrogated by depletion of either CD4 or CD8 lymphocytes., Conclusion: These results demonstrate that Id-HSB +poly (I:C) vaccine protects against MOPC315 growth by priming Id-specific CD4 Th cells that confer protection against tumor but are not directly cytotoxic. These data indicate that activation of CD8 CTL against MOPC315-associated antigens not present in the vaccine is one of the major mechanisms of tumor immunity., Competing Interests: Competing interests: DL is an employee of Agenus and JC was an employee of Agenus at the time the research was being conducted. All other authors do not have relevant competing interests., (© Author(s) (or their employer(s)) 2022. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)