First-in-class immunomodulatory anti-heat shock protein 70 (HSP70) antibody demonstrates broad antitumor activity across myeloma, lymphoma, and solid tumor models

Abstract Background: Treatment resistance and relapse in myeloma and lymphoma are driven by diverse immunosuppressive mechanisms within the tumor microenvironment (TME), including expansion of myeloid-derived suppressor cells (MDSCs) and regulatory T cells, upregulation of immune checkpoints, and suppression of antigen presentation, which collectively impair cytotoxic T and natural killer (NK) cell function. Heat shock protein 70 (HSP70), highly expressed in many hematologic malignancies, including multiple myeloma and lymphoma, plays a central role in these processes by stabilizing oncoproteins, inhibiting apoptosis, and promoting immune escape. Extracellular HSP70 released from tumor cells, often bound to tumor-derived antigens, can induce pro-tumorigenic inflammation and facilitate recruitment of immunosuppressive cells, and interfere with antigen processing and stress-induced cell death. Prior attempts at HSP70–based vaccination showed limited clinical efficacy due to challenges in purifying material and efficiently targeting APCs, highlighting the need for novel strategies to leverage this pathway in myeloma and lymphoma immunotherapy. Methods: We developed a panel of murine monoclonal antibodies to HSP70 by immunizing BALB/c mice with murine fibroblast L-cells expressing human HSP70 fused to green fluorescent protein. This panel was then screened in immune-competent BALB/c mice injected with luciferase (luc)-expressing MOPC315.BM murine myeloma cells, selected due to the 95% homology between murine and human HSP70. Clone 77A was identified from this screen as showing the most robust in vivo anti-tumor activity and subjected to further in vitro and in vivo testing. Results: Clone 77A enhanced antigen-presenting cell (APC) uptake of HSP70 tumor antigen complexes, promoted cross-priming of antigen-specific T cells, and engaged FcγRIII on NK cells and antigen presenting cells and thus modulated both innate and adaptive immunity. In immunocompetent myeloma models (MPC11 and Vk*MYC), 77A remodeled the immunosuppressive TME by inhibiting myeloid derived suppressor cells (MDSCs) and tumor associated macrophages (TAMs). Also, 77A promoted dendritic cell (DC) infiltration and expanded central memory CD4+ and CD8+ T- and NK-cell populations, subsequently generating a potent anti-tumor activity. Bulk RNA-seq and pathway analysis confirmed activation of antigen processing/presentation and NK cell-mediated cytotoxicity pathways. In the EG7.OVA (chicken ovalbumin) lymphoma model, 77A reshaped the TME to favor tumor-specific immune activation, including expansion of OVA presenting DCs and proliferation of OVA323–339-specific CD4+ T cells and OVA257-264- specific CD8+ T cells with enhanced effector function. These antigen specific T cells from 77A-treated tumor bearing mice exerted significant ex vivo cytotoxicity against EG7.OVA cells. Moreover, 77A augmented proliferation and effector function of OVA257-264- specific CD8+ T cells administered into the EG7.OVA lymphoma bearing mice, supporting its potential for a combined regimen with adoptive T cell therapy (ACT). We also found that HSP70 subcutaneous co-administration into established lymphoma tumors enhanced 77A's therapeutic efficacy in the EG7.OVA model. Additionally, 77A markedly enhanced anti-tumor activity of proteosome inhibitors (bortezomib and carfilzomib), radiation, and STING agonists (diABZI and IACS-8803) in MPC11 and Vk*MYC models. Its activity extended to solid tumors, including 4T1 breast cancer, and was further potentiated by tumor expression of immunogenic antigen hemagglutinin. or PD-1 immune checkpoint blockade. In the MM1.S xenograft model, 77A retained activity under NK cell depletion, likely due to myeloid polarization toward an M1 phenotype. The activity was more modest compared with that seen in more immune competent models, highlighting its long-term efficacy relied on T cell priming and effector function. Finally, 77A directly enhanced human immune function by boosting MART-1 specific CD8+ T cell expansion and enhancing NK cell cytotoxicity in healthy donor samples. Conclusion: 77A is a first-in-class anti-HSP70 antibody that uniquely leverages HSP70 neoantigen biology to bridge innate and adaptive immunity. It generates robust and durable anti-tumor activity in hematologic and solid tumor models, and synergizes with multiple immunomodulatory and standard therapies, supporting its clinical translation as a novel immunotherapy platform.