Supporting our in vitro studies, we demonstrate in vivo that GBM cell proliferation and, therefore, tumor progression, is assisted by the interaction with pericytes. Our results strongly suggest that GBM cell interaction with pericytes is required to maintain immunotolerance in the immunocompetent mouse brain . Indeed, our results analysing GBM+Pc grafts compared to control grafts, show high levels of anti-inflammatory cytokines in perivascular areas where grafted GBC-PC were found. Perivascular detection of less intense anti-inflammatory cytokine levels in GBM grafts might correspond to expression by endogenous pericytes.