W184 - foxp3+ Regulatory T Cells Co-localize with CD8+ T Cells in Tumors Responding to PD-1 Blockade

Tho-Alfakar Al-Aubodah; Fernando Alvarez; Sonia Del Rincon; Fan Huang; Roman Istomine; Tamiko Nishimura; Ciriaco Piccirillo; Constantin Polychronakos; Yasser Riazalhosseini

Abstract Text: PD-1 blockade has been a breakthrough in cancer therapy, inducing durable remission in patients. Yet, 2/3 of patients do not respond to treatment in melanoma, the solid tumor type with maximal efficacy. Objective responses are associated with higher degrees of pre-existing inflammation in the tumor, while regulatory T cells (Treg) play a dominant role in suppressing anti-tumor responses. However, it is still unclear how PD-1 blockade impacts Treg cell localization and how Treg cells themselves impact treatment outcome.
To dissect this, we used the highly immunogenic YUMMER1.7 melanoma model which, following anti-PD-1 treatment, yields a bimodal distribution of tumor volumes indicating high and low responders. By flow cytometry, we identified that responsiveness to anti-PD1 was correlated with the presence of IFNγ+ CD8+ cells. Immunofluorescence studies established that in both high and low responder tumors, Treg cells preferentially localize in zones of dense CD8 infiltration. Furthermore, tumor-infiltrating Treg cells from both groups displayed potent suppressive capacity in vitro, suggesting that CD8+ cells are evading Treg cell suppression locally in high responders.
Further characterisation revealed that, exclusively in high responders, tumor-infiltrating Treg cells progressively acquire Th1-like characteristics, namely T-bet and IFNγ expression, which has been hypothesized to dampen their suppressive function within tissues. To assess Treg cell suppressive capacity in situ, we are using spatial proteomics to compare their distribution and phenotypic heterogeneity across regions of high and low tumor cytotoxicity. Taken together, these data suggest a potential mechanism through which PD-1 blockade relieves Treg cell suppression locally in hot tumors.