Rab11 family interacting proteins as regulators of T cell recognition and infiltration in triple negative breast cancer.

2555 Background: Triple negative breast cancer (TNBC) accounts for 10-15% of all breast cancer cases and has the worst prognosis (5 year relative survival rate of 78%). TNBC is defined by absent estrogen and progesterone receptors and lack of HER2 amplification. First-line treatment for metastatic TNBC has evolved in the past several years to include immunotherapy for PD-L1+ patients. Recent NCCN guidelines recommend antibody-drug conjugate (ADC) therapy as first line treatment for metastatic TNBC regardless of PD-L1 positivity. ADCs target cell surface proteins that are abundant on tumors and sparse on healthy cells, delivering cytotoxic payloads selectively to the tumor. ADC efficacy depends on surface localization of target antigens, whose localization is dictated by endosomal trafficking via Rab GTPases. We showed that Rab11b-mediated endosomal trafficking is required for breast cancer brain metastasis. Here we investigate how trafficking-mediated control of the cell surface dictates immune recognition and response. Methods: Using a CRISPR screen we found that Rab11 family interacting proteins, Rab11fip2 and Rab11fip3, are required for immunogenic breast cancer cells to be recognized by CD8+ T cells. Flow cytometry revealed that the loss of recognition is not due to changes in surface PD-L1 or MHC/antigen presentation. Knockout of either Rab11fip did not change cell proliferation or tumor burden in immunocompromised animals, but led to significantly increased tumor growth in immunocompetent animals. This suggests that knockout of either Rab11fip causes loss of immune recognition to allow faster tumor growth while control tumors are restrained by the immune system. IHC analysis revealed a significant decrease of tumor infiltrating T cells in Rab11fipKO tumors. Interestingly, immune disruption is systemic, as splenic architecture is disrupted in Rab11fipKO tumor bearing mice. Results: Since the primary function of Rab11fips is to mediate trafficking of Rab11 cargo proteins, our results suggest that surface localization of these cargo proteins is required for immune cell recruitment, recognition, or activation. Conclusions: We are currently focused on identifying these cargo proteins, determining whether Rab11 controls trafficking of Trop-2 the target of several current ADCs, and characterizing Rab11 control of the T cell-cancer cell interaction in human breast cancer cell lines. Our work holds the promise of finding novel targets for ADCs and biomarkers for responsiveness to immunotherapy.