Abstract 6354: A new tumor-bearing humanized mouse model to evaluate the efficacy of bispecific T cell engager and monoclonal checkpoint antibodies

Abstract T cell-based immunotherapies such as bispecific T- cell engagers (BiTE) and checkpoint inhibitors (CIs) have been developed rapidly for cancer treatment, but each has their own limitations in the clinic. Combination therapies are an emerging approach in ongoing clinical trials to enhance the drug efficacy of monotherapy. Here we described a new preclinical animal model using immunodeficient NSG™-SGM3-IL15 mice engrafted with human CD34+ hematopoietic stem cells (HSC) to evaluate the combination treatment of EGFRXCD3 BiTE and two CIs (anti-PD-1 pembrolizumab and anti-PD-L1 avelumab). NSG™-SGM3-IL15 mice, which were created by crossing NSG™-SGM3 with NSG™-IL15, develop a more complete humane immune system after HSC engraftment. Humanized NSG™-IL15 mice have a higher peripheral NK cell frequency, and NK cells are functional with ADCC activity when compared to NSG™ (Aryee et al., 2022 FASEB). Here we show that humanized NSG™-SGM3-IL15 mice show improved human CD33+ myeloid progenitor cells and near physiological levels of NK cells and Tregs in peripheral blood when compared to humanized NSG™-SGM3. Triple negative breast cancer MDA-MB-453 cell line is PD-L1 positive, expresses low levels of EGFR and has a low baseline tumor immune cell infiltration in xenograft using humanized NSG™ mice. Previously we have shown BiTE efficacy on this tumor model. Here tumor-bearing humanized NSG™-SGM3-IL15 mice from two donors were used to evaluate the efficacy and immune responses from the BiTE treatment in combination with CIs. For donor 0348, monotherapy BiTE and avelumab didn’t result in significant tumor growth reduction when compared to PBS control whereas the combination treatment significantly reduced tumor growth. BiTE treatment alone resulted in a significant increase in CD3+ T cell infiltration into tumor and higher frequency of CD69+ CD4 T and CD8 T cells. PD-1 expression on the tumor infiltrating lymphocytes and PD-L1 expression on tumor cells increased significantly. BiTE and avelumab combination treatment further increased human CD45+ and CD3+ T cell infiltrations with activated T cell phenotypes in tumor. For the second donor 0313, BiTE alone or in combination with the CIs didn’t show anti-tumor growth activity. Flow cytometry analysis showed that the immune response to BiTE treatment was less pronounced in this donor, especially there were little change in lymphocyte infiltration and CD4+ T cell population in tumor. In this study, we demonstrated donor-dependent response to combination treatment of BiTE and avelumab and found that the responding donor showed an increase in T cell infiltration and activation in tumor after the treatment. The data suggested beneficial effects from inhibition of PD-1/PD-L1 axis when used in combination with an immunotherapy targeting tumor antigen. The potential involvement of NK cells in this combination therapy remained to be examined. Citation Format: Li-Chin Yao, Danying Cai, Destanie Rose, Guoxiang Yang, Mingshan Cheng, Michael A. Brehm, Dale L. Greiner, Leonard D. Shultz, James G. Keck. A new tumor-bearing humanized mouse model to evaluate the efficacy of bispecific T cell engager and monoclonal checkpoint antibodies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6354.