Clonal deletion of tumor-specific T cells by IFN-g confers therapeutic resistance to combination immune checkpoint blockade

Combination immunotherapy is being developed with the goal of enhancing anti-tumor immunity. Despite remarkable clinical responses, most patients do not experience complete responses from combination checkpoint blockade treatments. Therefore, a need exists to further explore the mechanisms of resistance to these therapies. Here, we investigate whether tumor burden could be an important factor in determining the treatment outcomes of combination checkpoint blockade. We found that while combined anti-CTLA-4 and anti-PD-1 improves control of established tumors, this combination can paradoxically compromise anti-tumor immunity in the low tumor burden (LTB) state in pre-clinical models as well as in melanoma patients. This paradoxical outcome results from treatment-induced deletion of tumor-specific T cells that is mediated through IFN-γ. Activated tumor-specific T cells express higher levels of IFN-γ receptor and are more susceptible to apoptosis. As a result, combination treatment alters the T cell repertoire landscape, skewing the distribution of T cells toward lower frequency clonotypes. Deficiency of the IFN-γ receptor on immune cells restores anti-tumor activity. Additionally, tumor-specific T cells lacking the IFN-γ receptor demonstrate a significant survival advantage compared to their wild-type counterparts in tumor-bearing mice receiving combination therapy. Finally, we show that combination therapy induces significantly higher levels of IFN-γ in the low versus high tumor burden state on a per cell basis, reflecting their less exhausted immune status. This elevated IFN-γ secretion in the LTB state therefore contributes to the development of an immune-intrinsic mechanism of resistance to combination checkpoint blockade. Our report underscores the importance of achieving the optimal magnitude of immune stimulation for successful immunotherapy strategies. Spas-1+ CD8 cell RNA profiles from the draining lymph nodes of Tramp-C2 injected B57BL/6 wild type (WT) Mice were examined. There were four groups of mice with five mice per group and were treated with their respective checkpoint inhibitor on day 3 6, and 8 after tumor injection. Draining lymph nodes were harvested 28 days after tumor injection and for reach group, the cells were pooled and split into triplicates to be analyzed.