Perivascular Accumulation Of Immunosuppressive Cells In The Stroma Of Human Triple Negative Breast Carcinomas: Implications For Immunotherapy.

Abstract A distinct subset of tumor-associated macrophages (TAMs) accumulate around blood vessels in mouse tumors where they stimulate tumor angiogenesis and various steps in the metastatic pathway. They also limit tumor responses to frontline anti-cancer therapies like irradiation and chemotherapy, and dampen anti-tumor immunity by recruiting regulatory T cells and suppressing the proliferation of T cells. However, the presence and phenotype of such perivascular (PV) TAMs have yet to be investigated in human tumors. In this study we have used MultiOmyx multiplex immunofluorescence coupled to advanced analytics to compare the distribution and phenotype of TAMs, CD4+ and CD8+ T cells, and CD4+FOXP3+ regulatory T cells (Tregs) in PV vs non-PV areas in the stroma and tumor cell islands (TCIs) of 40 human triple negative breast carcinomas (TNBCs) - and whether this was altered after neoadjuvant chemotherapy. Tregs along with distinct subsets of TAMs and CD4+ and CD8+ T cells were found to preferentially accumulate in PV areas (ie. within 50um of CD31+ blood vessels), especially in the tumor stroma. CD163+ TIM3+ TAMs often made direct contact with the abluminal surface of blood vessels in these sites, and at increased numbers in in chemotherapy-treated TNBCs. This suggests they may regulate the formation/function of blood vessels in relapsing tumors. In both untreated and chemotherapy-treated tumors, a major subset of CD163+ TAMs lacked PDL1 expression and also accumulated preferentially in stromal PV areas, where many made direct contact with PD1- CD4+ or PD1- CD8+ T cells as well as FOXP3+CD4+ Tregs. We are currently investigating whether such close contact of these subsets of T cells with two immunosuppressive cell types (ie. CD163+ TAMs and Tregs) inhibits their subsequent anti-tumor functions. Taken together, our study shows that, as T cells cross blood vessels into TNBCs, they encounter a high density of at least two immunosuppressive cell types in the perivascular niche. This could lead to the inactivation of T cells before they migrate further into tumors, thereby reducing anti-tumor immunity. It could also limit the efficacy of cancer immunotherapies mediated by activated T cells. Identifying and targeting the mechanism causing these suppressive cells to accumulate and interact in such sites could remove this inhibition. Citation Format: Mohammed Ridha Moamin, Richard J. Allen, Nicholas Stavrou, Mate Nagy, Anna Juncker-Jensen, Claire E. Lewis. Perivascular accumulation of immunosuppressive cells in the stroma of human triple negative breast carcinomas: implications for immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2720.