Immune checkpoint blockade augments lymphodepleting chemotherapy-induced antitumor immunity by expanding effector CD8+ T cell clones in preclinical models of cancer

Cancer treatment using immune checkpoint blockade (ICB) with anti-programmed cell death protein (PD)-1 and anti-cytotoxic T lymphocyte associated protein (CTLA)-4 has been successful. However, primary and acquired resistance limits clinical benefit. To improve the effectiveness of ICB therapies, strategies that reorchestrate anti-tumor immunity through mechanism-based drug combinations are being actively explored. The alkylating chemotherapeutic agent, cyclophosphamide (CTX), has direct tumoricidal and immunomodulatory properties, including the induction of homeostatic proliferation of T cells. Since ICB suppresses inhibitory signals in T cells, we hypothesized that ICB could augment CTX-induced homeostatic proliferation of antigen-specific T cells, thereby resetting the T cell receptor (TCR) repertoire in favor of tumor-specific T cells. Here, we show that a single dose of CTX one day prior to starting aPD-1+aCTLA-4 is sufficient to delay tumor progression in established melanoma and prolong survival in tumor-bearing mice in preclinical models. These effects extended to other lymphodepleting treatments, such as Gemcitabine and radiation therapy. The anti-tumor immune response was mainly driven by the clonal expansion of activated/effector CD8+ tumor infiltrating lymphocytes. Furthermore, combined CTX and aPD-1+aCTLA-4 treatment demonstrated efficacy across additional preclinical tumor models, including colorectal cancer and triple negative breast cancer. Overall, our findings highlight that the combination of CTX and aPD-1+aCTLA-4 represents a clinically relevant approach in the treatment of ICB-refractory tumors.