The immune function of pericytes in response to tumor cells remains unknown. To characterize if brain pericytes may acquire an anti-inflammatory function in response to their interaction with GBM cells (glioblastoma-conditioned-pericytes: GBC-PC) during tumor progression, we determined first if changes in the expression levels of cytokines would occur in response to the interaction of pericytes with GBM cells in vitro. Purification of brain pericytes from transgenic mice that express actin fusioned to GFP (C57Bl/6-Tg(ACTB-EGFP)1Osb/J), was confirmed by immunofluorescence after five passages [9, 26, 27], using well characterized pericyte markers [28, 29], such as NG2, PDGFR-β, and RGS-5 (Figure 1A). Staining for control markers of astrocytes, microglia and endothelial cells was negative (not shown), which ensures that none other cell type without stem cells properties survived in a culture without specific growth factors [26, 27]. Analyses of cytokines secreted from pericytes co-cultured with a GBM human cell line (GBC-PC), revealed a significant increase in the production of IL-10 and TGF-β after 72 hours compared to basal levels of control pericytes (Figure 1B). The expression of pro-inflammatory cytokines, such as IL-1, IL-23 and IL-12 was, however, hardly detectable (not shown). GBC-PC also produced the pro-inflammatory cytokine TNF-α, no significant differences were detected compared to the control pericytes, until late time points of co-culture with GBM cells (Figure 1B). Importantly, the increase in the production of TNF-α, which showed just a doubling of its expression at 72 hours of co-culture, was much lower than the one detected when assessing the anti-inflammatory cytokines TGF-β and IL-10 (Figure 1B). These data correlated with similar changes in the levels of mRNA expression in GBC-PC compared to controls (Figure 1C). Analysis of cytokine gene expression in GBC-PC reflected upregulation of the mRNA of the anti-inflammatory cytokine genes Tgfb and Il10. Il1b and Il12 gene expression was not detected even in control pericytes (not shown), and the mRNA level of Tnfa, Il4 and Il23 in pericytes was not significantly affected by GBM cells. Surprisingly, mRNA and protein expression of the angiogenic cytokine IL-6 was clearly increased in GBC-PC compared to control pericytes after 72 hours of coculture (Figure 1C, Supplementary Figure 1) [30, 31]. No cytokine mRNA or protein were detected in control GBM cells (not shown). To determine if the marked rise of expression of TGF- β and IL-10 in GBC-PC requires direct cell-cell interaction or is mediated by soluble molecules expressed by GBM cells [32, 33], we incubated pericytes with sequential dilutions of supernatants from different lines of GBM cells. Our results showed the same levels of cytokine expression in supernatant-treated pericytes as in control pericytes, supporting that the acquired immunomodulatory phenotype in pericytes in response to GBM likely requires cell-to-cell interaction (Figure 1D).