[27] This may explain the proangiogenesis potential by SIRPα-KD Mψ in tumor-bearing models. Moreover, it is reported that SIRPα is required in T- and NK-cells homeostasis in vivo, indicating an important role of SIRPα in immunomodulation.[28, 29] Nevertheless, further investigation is required to determine the role of SIRPα in Mψ crosstalking with other immune cells in the tumor microenvironment. The mechanisms contributing to the reduction of SIRPα on monocytes/Mψ in response to tumor are not yet well elucidated. Although soluble factors in the tumor microenvironment, such as TNFα, could decrease SIRPα expression GDC-941 on Mψ, specific anti-TNFα neutralization antibody only

partially reversed Hepa1-6-induced reduction of SIRPα with a concentration of antibody that effectively neutralized TNFα in the coculture system (Supporting Fig. 6A). To determine whether tumor PLX4032 cell-induced SIRPα reduction was attributable to an increased

rate of protein degradation, we examined the effects of inhibitors of protein degradation by lysosomes or the proteasome. MG132, a reagent that blocks proteasome activity, had no marked effect on the ability of tumor cells to suppress SIRPα expression. In contrast, preincubation of macrophages with dexamethasone, chloroquine, or NH4Cl, all of which inhibit lysosomal function, markedly attenuated the effect of tumor cell-induced SIRPα reduction Rucaparib manufacturer (Supporting Fig. 6B). Confocal assay showed that, in response to Hepa1-6 stimulation, SIRPα proteins translocated from cell membrane to the cytosolic fraction and mainly localized on lysosomes (Supporting Fig. 6C). Taken together, these data suggest that, in addition to transcriptional repression of SIRPα expression (Supporting Fig. 2A), tumor cells

also induce SIRPα protein degradation mainly by lysosome, which contributes to the loss of SIRPα proteins in macrophages after tumor cells stimulation. SIRPα-CD47 interactions serve as a “don’t eat me” and “self-recognized” signaling.[30] Many reports have demonstrated that the SIRPα-CD47 signaling pathway could be a therapeutic target for human tumors.[18, 30, 31] This may due to enhanced phagocytic capability of Mψ against tumor cells when treated with anti-CD47 antibodies. One problem with these series researches is that CD47-IgG antibody may not only disrupt SIRPα-CD47 interaction but also target the tumor cells by antibody-dependent cellular cytotoxicity (ADCC) or antibody-dependent phagocytosis.[32] Another problem is that most of the experiments were performed on a xenograft tumor model, which may not exactly meet the role of SIRPα in syngeneic models. It was demonstrated that additional activating signals, such as Fcγ or complement-receptor ligation, are required for Mψ-mediated phagocytosis of target cells in the absence of SIRPα-dependent inhibition.