ARF6 is a Novel Target for Immunotherapy in Triple Negative Breast Cancer

Triple negative breast cancer (TNBC) is one of the most aggressive breast cancer subtypes with poor clinical outcomes due to lack of effective treatments owing to its hormone receptor negative status. Immune checkpoint blockade (ICB) therapy, which prevents the exhaustion of CD8+ T cells, has shown promise in treating these patients. However, only a small proportion respond, possibly due to resistance and immune evasion mechanisms by the tumor cells. A primary mechanism by which tumor cells evade immune surveillance is by reduced tumor antigen presentation, as indicated by a decreased level of antigen-MHC-I (major histocompatibility complex-I) on the surface of tumor cells. The dynamics of tumor antigens on the cell surface and how cell endocytosis contributes to antigen presentation and their recycling is little known. Here we sought to study the roles of two proteins: clathrin and ADP-Ribosylation Factor 6 (ARF6) which are essential for clathrin-mediated endocytosis and clathrin-independent endocytosis respectively, on the surface turnover of fluorophore-conjugated antigenic peptide bound to MHC-I. We employed Total Internal Reflection Fluorescence Microscopy (TIRFM) and Single Molecule Tracking (SMT) to determine the dynamics of tumor antigen endocytosis on the surface of EO771 murine TNBC cells. We found that the inhibition of ARF6 remarkably impaired the endocytosis of the antigen-MHC-I foci, leading to extended stay of the foci on the cell membrane, while inhibition of clathrin did not, suggesting that clathrin-independent endocytosis is the primary route for MHC-I-mediated antigen endocytosis. Consistent with this finding, reduced ARF6 levels promoted in vitro tumor cell killing by CD8+ T cells and suppressed tumor growth in mice when combined with ICB therapy. We further investigated the effect of pharmacological inhibition of ARF6 in murine TNBC cells and splenic CD8+ T cells using a commercially available ARF6 inhibitor NAV-2729. Treatment with NAV-2729 increased surface MHC-I levels and enhanced the secretion of T cell functional markers such as IFN-, TNF- and IL-2, suggesting the possibility of in vivo administration of ARF6 inhibitors in combination with ICB therapy. Collectively, these data suggest that ARF6 is a novel target for the combined treatment with ICB therapy to improve its efficacy in TNBC patients.