Abstract 4981: HER2-targeted antibody drug conjugates (ADCs) induce host immunity against cancer stem cells and are enhanced by anti-PD-L1

HER2-targeted antibody drug conjugate (ADC) was only tested in immunocompromised (SCID) host, which doesn’t allow the evaluation of the ADC-induced host immune responses. In addition, no potential impact of ADC on cancer stem cells (CSCs) has been reported. Furthermore, tumor cells with PD-L1 expression may escape immune attack by converting PD-1 positive immune effector cells into anergy. We hypothesized that HER2-targeted ADC could modulate host immune response, target HER2high CSCs, and the efficacy of ADC could be enhanced by anti-mPD-L1. To test this hypotheses, we used two immunocompetent murine models: mouse breast tumor D2/F2 and TNBC 4T1 with enforced expression of HER2, D2F2/E2 and HER2-4T1 respectively. Both D2F2/E2 and HER2-4T1 could generate tumor in immunocompetent Balb/c mice, which allowed us to test the efficacy and specificity of HER2-targeted ADC, and to evaluate host immune responses. In D2F2/E2 mouse model, we found that HER2-targeted ADC significantly inhibited D2F2/E2 tumor growth, but not the parental D2F2 tumors, in a dose dependent manner. The specificity was confirmed in HER2-4T1 model. In parallel, we observed that HER2-targeted ADC treatment reduced the number of HER2 positive cells and ALDHhighCSCs. We found that anti-mPD-L1 mAb significantly augmented the therapeutic efficacy of HER2-targeted ADC in both D2F2/E2 and HER2-4T1 tumor models. Mechanistically, HER2-targted ADC combined with anti-PD-L1 significantly modulated the gene signature of multiple immune factors in tumor microenvironment. At the cellular level, anti-mPD-L1 plus HER2-targeted ADC increased the number of CD3+ and CD19+ tumor infiltrating lymphocytes (TILs). Expanded CD3+ TILs from the tumor subjected to combined treatment killed tumor cells significantly more than mono-treatment. Expanded CD19+ TILs from tumor subjected to the combined therapy produced more IgG than all the controls. Importantly, the IgG could specifically bind to tumor cells, resulting in cytotoxicity of the tumor cells via ADCC. As a result, ADC plus anti-PD-L1 further reduced the number of HER2 positive cells. Importantly, the combined therapy not only reduced the number of ALDHhigh CSCs in the residual tumor, but also significantly paralyzed their tumoriginicity. Together, our data indicate that HER2-targeted ADC could induce significant host immune responses, which was evidenced by the regulation of gene signature and detection of anti-tumor CD3+ and CD19+ TILs. We first show that HER2-