Abstract 902: Augmentation of anti-melanoma response by combining radiation-based in situ vaccination with GIFT4 B cell therapy

We have recently demonstrated the effectiveness of combining external beam radiation therapy (RT), intratumoral immunocytokine (IT-IC), and anti-CTLA4 (αCTLA4) in eliminating large (500mm3) B78 melanomas (MEL), prolonging survival and invoking T cell immune memory in mice. This highly translational therapy is currently being investigated in a phase I clinical trial. Despite the success of this regimen against large single subcutaneous MEL tumors, many mice with large tumors and intravenously (IV) injected metastatic MEL, receiving this same regimen, still die from metastatic disease. A recent report has shown that fusokine GIFT4, a recombinant chimera of GMCSF and IL4 programs B cells (GIFT4BC) to acquire a unique phenotype with augmented capacity for antigen presentation and anti-MEL activity. Herein, we report the preliminary results of studies combining RT+IC+αCTLA4 with intratumoral (IT) GIFT4BCs therapy. We hypothesized that GIFT4BCs enhance the anti-tumor and anti-metastatic efficacy of RT+IC+αCTLA4. C57BL/6 mice were engrafted with 2 × 106 B78 MEL intradermally in the flank. At a tumor volume of ~250mm3, mice were intravenously injected with 3.5 × 105 B16 MEL (a metastatic parental line of B78 MEL) to induce metastases. Mice were treated with 12 Gy RT on day 1 (D1), 1 × 107 IT- GIFT4BC on D4, IC (IT, 50ug) on D6-D10, and αCTLA4 (ip, 200ug) on D3, 6, and 9 (designated RT+IC+αCTLA4+GIFT4BC). Control groups received no treatment, GIFT4BCs alone, or RT+IC+αCTLA4. Tumor growth and survival were monitored. At the time of euthanasia for tumor burden or illness, necropsy was performed to quantify metastases. A pre-determined cohort of mice from each treatment group were euthanized on D12. From these, tumor-draining lymph nodes were fixed for histology and tumors were harvested for qPCR, histology, and flow cytometry. Preliminary results indicate that the addition of GIFT4BCs to RT+IC+αCTLA4 improves survival. Lungs from treated mice showed reduced metastatic burden in RT+IC+αCTLA4+GIFT4BC mice compared to controls. GIFT4BCs were identified in the tumor-draining lymph nodes using confocal microscopy in RT+IC+αCTLA4+GIFT4BC mice. qPCR analysis of tumor samples revealed an upregulation of interferon gamma (Ifny) in RT+IC+αCTLA4+GIFT4BC mice compared to control groups. Flow cytometry analysis show increased CD45+ tumor infiltrating lymphocytes in RT+IC+αCTLA4+GIFT4BC compared to RT+IC+αCTLA4 groups. Combination of RT+IC+αCTLA4+GIFT4BC displays anti-MEL activity that reduces metastatic burden and prolongs survival in murine MEL models. These preliminary findings support published hypotheses which postulate that GIFT4BC-mediated mechanisms may augment the T cell response through B cell antigen presentation and chemotactic cytokine production in vivo. Future studies will elucidate the mechanisms of anti-tumor efficacy of RT+IC+αCTLA4+GIFT4BC. Citation Format: Elizabeth G. Sumiec, Claire C. Baniel, Amber M. Bates, Sarah Emma, Erin Nystuen, Alexander L. Rakhmilevich, Amy K. Erbe, Pradyut Paul, Jacques Galipeau, Paul M. Sondel, Zachary S. Morris. Augmentation of anti-melanoma response by combining radiation-based in situ vaccination with GIFT4 B cell therapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 902.