Slow growing tumors are characterized by increased immune cell infiltration and a more differentiated tumor histology. (A) Representative micrographs of primary tumors from three immune-mediated slow growers (#69, #55, #238) and fast growers (#252, #234, #29) sectioned and stained with H&E and anti-CD45 antibody. (B) Number of tumor-infiltrating CD45+ cells per high-power field (HPF) in primary tumors from three immune-mediated slow growers (#69, #55, #238) vs. five fast growers (#252, #234, #29, #235, #256). Ten HPF were counted per tumor sample. (C) Number of tumor infiltrating CD45+ cells per HPF in primary tumors from four PyMT/Rag1−/− (#69, #55, #29, #31) vs. four PyMT WT (#252, #234, #256, #238) cell lines. Ten HPF were counted per tumor sample. The percent of cystic changes as a morphological indication of tumor differentiation was examined on H&E stained sections in the primary tumor of three immune-mediated slow growers (#69, #55, #238) vs. five fast growers (#252, #234, #29, #235, #256).