Early T cell infiltration correlates with anti-CTLA4 treatment response in the P815 mastocytoma model

Abstract Immune checkpoint blockade (ICB) antibodies are a revolutionary class of cancer treatment, but only approximately 30% of patients receive a lasting benefit from therapy. Pre-clinical studies using animals from the same genetic backgrounds, challenged with the same cancer models, also show non-uniform responses. Most mouse studies that have evaluated tumour-infiltrating leukocytes (TILs) after ICB therapy cannot directly correlate their findings with treatment outcomes because terminal methods were used to acquire immune infiltrate data. In the present study, we used fine-needle aspiration (a non-terminal sampling method) to collect multiple aspirates over several days from subcutaneously implanted P815 mastocytomas treated with ICB antibodies. These aspirates were then analysed with flow cytometry to directly correlate nine TIL populations from both the innate and adaptive arms of immunity with treatment success. We found that the P815 model responds well to anti-CTLA4 but not anti-PD1. Mice that regressed following anti-CTLA4 therapy showed a significant increase in CD8+ T cells on days 3, 5, and 7 and CD4+ T cells on days 5 and 7, and a simultaneous decrease in macrophages and monocytes on days 3, 5, and 7 post-treatment. We also found that P815 tumours sometimes regressed in response to non-specific isotype control antibodies, likely due to their binding with Fc receptors on the cancer cells. Our study is the first to directly correlate early tumour infiltration of T cells with anti-CTLA4 treatment success, thus providing a mechanistic clue toward understanding why alloidentical mice challenged with identical tumours do not respond uniformly to ICB therapies. This project was funded by the Australian Government Research Training Program, the Lea Chapuis Memorial Fund, The Lionel & Yvonne Spencer and William G Maxwell Trusts administered through Perpetual’s IMPACT Philanthropy, and the generosity of Tim and Margaret Bourke.