4D and E), demonstrating that the CD11bhiF4/80lo TAM CD11bloF4/80hi TAM differentiation takes place in intact tumors. The noticed expansion of grafted macrophages in tumors lesions (Fig. 4C) prompted us to test whether local proliferation of TAMs present in MMTVneu tumors could compensate the relatively inefficient monocyte differentiation into CD11bloF4/80hi macrophages (Fig. 3, 4D and E). Both TAM types in MMTVneu tumors, irrespectively of the Stat1 status, were found to express Ki67, a marker of G1/S/G2 phases of cell cycle

[28] (Fig. 5A). The percentage of cycling cells measured by this method was markedly higher in the CD11bloF4/80hi TAM subset than in the CD11bhiF4/80lo HDAC inhibitor population and comparable with the CD11b− tumor fraction. We investigated the cell cycle distribution in TAM populations by pulsing tumor-bearing mice with BrdU for 3 h and analyzing genome incorporation of the BrdU label and total DNA content. The BrdU signal was absent from blood leukocytes at this time point, which allowed us to assess the rate of macrophage proliferation without superimposition of blood cell recruitment (Supporting Information Fig. 12). Both TAM subsets incorporated the label, thus demonstrating local proliferation. In line with the higher Ki67 positivity, the frequency of S phase cells

was significantly higher in the CD11bloF4/80hi subset relative to CD11bhiF4/80lo TAMs (Fig. 5B, and Supporting Information Fig. 12A), indicating more rapid proliferation of the predominant macrophage subset. Additionally, the CD11bhiF4/80lo population displayed

an ABT-263 molecular weight elevated extent Phloretin of cell death discerned by abundance of sub-G1 events. The genotype status had only a slight influence on the cell cycle phase distribution in the main macrophage subset (Fig. 5A) and no impact on the amount of actively cycling cells as determined by Ki67 positivity (Fig. 5A). Hence, it is unlikely that the difference in rate of proliferation are able to explain the lowered abundance of CD11bhiF4/80lo TAM in Stat1-null animals. As reported previously, therapeutic application of the DNA-damaging agent doxorubicin [29] in tumor-bearing MMTVneu mice leads to a dropdown of CD11b+F4/80+ tumor-infiltrating cells [4]. In both TAM subsets, cell cycle progression was stalled upon doxorubicin treatment (Supporting Information Fig. 13A) simultaneously to the inhibition of CD11b− tumor cell replication (Supporting Information Fig. 13B). This notion suggests that cytotoxic cancer therapeutics may lower TAM content through direct interference with their in situ cell division. Since CSF1 levels were linked to macrophage marker expression in human breast carcinoma tissue (Table 1) and TAMs in MMTVneu lesions expressed CD115/CSF1R (Fig. 1B), we investigated the potential role of CSF1/CSF1R signaling in fostering accumulation of TAMs.