Despite a lack of unifying drivers in Triple-Negative Breast Cancer (TNBC), our lab and others have uncovered that these cancers have elevated expression of inflammatory genes and immunosuppressive molecules (i.e. PD-L1), as well as elevated numbers of infiltrating immune cells (including CD8+ T-cells and Foxp3+ T-regulatory cells) which suggests the therapeutic potential for single and combinations of checkpoint blockade antibodies. While early trials with PD-1 inhibitors have been encouraging for TNBC, only a fraction of treated patients respond to this therapy. To test and define the mechanisms that govern responses, we explored the utility and mechanistic basis of both PD-1 and CTLA-4 inhibition in generating tumor-specific immunity in an established murine model of TNBC.

Consistent with patient samples, we found TNBC tumors from our model exhibited elevated PD-1+ expressing CD8+ T-cell infiltrates, Foxp3+ T-regulatory cell infiltrates (~66% of CD4+ TILs), as well as highly elevated tumor cell expression of PD-L1. We also found that while TNBC cells were easily killed by T-cell in vitro, TNBC tumors were highly immuno-suppressive and resistant to antigen-specific T-cell attack in vivo, even after adoptive transfer of up to 5x10E6 tumor-specific T-cells.

However, we found that both CTLA-4 and PD-1 antibodies could curtail this immunosuppression to different degrees and through alternate mechanisms. Specifically, we found that CTLA-4 antibody mediated anti-tumor immunity through the elimination and blockade of Foxp3+ T-regulatory cells in the tumor microenvironment, which allow for potent T-cell expansion. Conversely, PD-1 antibodies elicited anti-tumor immunity through blockade of PDL1/PD1 signaling between tumor cells and T-cells in the TNBC tumor microenvironment that allowed for a more modest expansion of individually tailored T-cell specific clones in vivo.

Strikingly, the combination of these antibodies and their alternate mechanisms of action resulted in greatly enhanced anti-tumor responses and led to regression of ~80% of tumors. This was accompanied by an augmented infiltration of T-cells into the tumor microenvironment and significantly enhanced systemic tumor-specific T-cell responses, which appear to be emergent properties of dual CTLA-4/PD-1 antibody treatment.

However, we found that these treatments did not expand a common tumor-specific T-cell clone, despite adoptive transfer of identical tumor-specific immunodominant T-cells into mice after tumor implantation. Thus, despite our use of a highly homogeneous model utilizing genetically identical mice implanted with an identical tumor line bearing a unique tumor antigen under identical conditions, the tumor-specific T-cell responses were highly unique for each individual tumor. Collectively, our study suggest that dual blockade could be an effective therapeutic clinical strategy against TNBC and further suggest the utility of monitoring systemic immune response and TCR expansion of TILs as the most useful correlates in clinical studies utilizing CTLA-4 and PD-1 antibodies.

Citation Format: Hartman ZC, Crosby EJ, Wei J-P, Yang X-Y, Lei G-J, Wang T, Liu C-X, Agarwal P, Morse MS, Lyerly HK. CTLA-4 and PD-1 checkpoint inhibitors enhance individually tailored adaptive anti-tumor immune responses to overcome tumor immunosuppression and effectively treat triple-negative breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P2-04-27.