[7, 37] (Supporting Information Fig. 2B and 3C). Although expression of some alternative activation markers in CD11bloF4/80hi TAMs was affected by Stat1 deficiency, a parallel upregulation of M2 transcripts could be observed in the Stat1-null CD11bhiF4/80lo subset. We hypothesize that this may represent MAPK Inhibitor Library price a compensatory mechanism that should guarantee expression of M2 proteins of potentially vital importance for the tumor such as IL-10 implicated in blunting antitumor

T-cell response [38]. Our findings expand the existing knowledge on the impact of CSF1R signaling on TAM homeostasis [5, 6, 22] by documenting its profound influence on proliferation and/or survival of CD11bloF4/80hi macrophages (Fig. 6). CSF1, postulated to act as a strong M2-polarization factor [5], may also trigger the M2 transcriptional response in these cells (Supporting Information Fig. 2B). In contrast, the transient effects of the CSF1R blockage

on the CD11bhiF4/80lo TAM population (Fig. 6) Selleck CP-673451 may point toward a redundancy between CSF1/CSF1R and other signaling pathways (e.g., IL-4R [17] or GM-CSFR [5, 11, 13]) in the homeostasis of this subset. Here, we report that STAT1 interacts with the promoter of the Csf1 gene and stimulates its expression in tumor cells (Fig. 6). By this means, STAT1 could regulate via CSF1 the expansion of CD11bloF4/80hi TAMs and their M2 phenotype. Despite lower CSF1 levels in Stat1-null tumors, Stat1-deficient TAMs possessed similar proliferation capabilities and did not display enhanced apoptosis when compared with Stat1+/+ infiltrating macrophages (Fig. 5). Furthermore, monocyte recruitment was apparently not controlled by STAT1

(Supporting Information Fig. 5C, 10B and C). We consider the possibility that STAT1-dependent CSF1 fosters the maturation of CD11bhiF4/80lo Etomidate cells into CD11bloF4/80hi TAMs and by this means accounts for the higher numbers of the later in Stat1-proficient tumors. This hypothesis needs to be further explored. It is also conceivable that the diminished, but still substantial amounts of CSF1 in Stat1-deficient tumors (Fig. 7A and B) can suffice for the development and maintenance of a smaller and less M2-polarized CD11bhiF4/80lo TAM population (Fig. 1 and Supporting Information Fig. 2B) by means of in situ proliferation. However, a more profound interference with the CSF1R signaling through pharmacological inhibition resulted in a depletion of the CD11bloF4/80hi population (Fig. 6). In summary, we provide here a novel insight into ontogeny and homeostasis of TAMs with potential clinical implication, stressing the role of their heterogeneity and their local proliferation and survival fostered by CSF1 production.