Arg1 expression defines immunosuppressive subsets of immune infiltrating cells in cancer

Tumor-associated macrophages (TAM) have attracted attention as they can modulate key cancer-related activities, yet TAM represent a heterogenous group of cells that remain incompletely characterized. In growing tumors, TAM are often referred to as M2-like macrophages, which are cells that display immunosuppressive and tumorigenic functions and express the enzyme arginase 1 (Arg1). Here we combined single cell intravital imaging with scRNA seq to uncover the topography and molecular profiles of Arg1+ macrophages in mice. We further assessed how immunotherapeutic interventions impact these cells directly in vivo. We show that: i) Arg1+ macrophages are more abundant in tumors compared to other organs; ii) there exist two morphologically distinct subsets of Arg1 TAM defined by previously unknown markers (Gbp2b, Bst1, Sgk1, Pmepa1, Ms4a7); iii) anti-Programmed Cell Death-1 (aPD-1) therapy decreases the number of Arg1+ TAM while increasing Arg1– TAM; iv) accordingly, pharmacological inhibition of arginase 1 does not synergize with aPD-1 therapy. Overall, this research defines subsets of immunosuppressive myeloid cells with powerful complementary single cell analytical approaches that pave the way for a more intricate understanding of TAM behavior. Overall design: We report the application of 10x Genomics single-cell RNAseq of live CD45+ cells sorted from MC38 tumors injected in C57BL/6J mice. Tumors were injected intradermally and grown for 7 days before treatment with either control injection or antiPD-1 antibody for 3 days.