The immunosuppressive cells existing in the tumor microenvironment lead to tumor immune tolerance and treatment failure [12,13]. Inhibiting the activity of these immunosuppressive cells is the key to improving the efficacy of immunotherapy [14,15]. The infiltration of PMN-MDSCs in tumor microenvironment is closely related to poor prognosis . PMN-MDSCs exert immunosuppressive functions via multiple signal pathways, such as secreting inhibitory cytokines (IL-1, IL-6, IL-10, and TGF-β) , inducing the expression of Arg-1 and iNOS , expressing a series of reactive oxygen species ROS , and inducing the production of CD25+ FOXP3+ Treg cells . Inhibiting the immunosuppressive activity of PMN-MDSCs may be an effective way to improve anti-tumor immunity . Therefore, it is of great significance to further explore the related mechanisms by which PMN-MDSCs exerts immunosuppressive functions.
In recent years, several studies have shown that both FATP2 and RIPK3 play a key role in promoting the suppressive activity of PMN-MDSCs [7,8]. Both FATP2 and RIPK3 enhance the suppressive activity of PMN-MDSCs by promoting the synthesis of PGE2, which is an important immunosuppressive molecule in maintaining the immunosuppressive activity of PMN-MDSCs . In the study, we also found that a large number of PMN-MDSCs existed in the tumor microenvironment, and PMN-MDSCs infiltration significantly correlated with FATP2 and RIPK3 expression. This phenomenon also exists in the tumor microenvironment of human bladder cancer. Moreover, FATP2 and RIPK3 expression was correlated with clinical stage, which indicated that overexpressed FATP2 and underexpressed RIPK3 were associated with poor prognosis.
Despite the fact that PMN-MDSCs and neutrophils share same origin and the same differentiation pathways, PMN-MDSCs have distinct genomic and biochemical features and are immunosuppressive . When compared with PMNs, FATP2 expression was significantly up-regulated in PMN-MDSCs, while RIPK3 expression was significantly down-regulated. More interestingly, PMN-MDSCs isolated from tumor tissue showed higher FATP2 expression and lower RIPK3 expression than PMN-MDSCs isolated from spleen. This phenomenon has also been reported by a previous study . At present, the specific mechanism by which this phenomenon occurs in the tumor microenvironment remain largely unknown.
FATP2 is a key molecule that regulates lipid metabolism in PMN-MDSCs . Abnormal lipid metabolism could contribute to pathological activation of PMN-MDSCs . An accumulation of lipids in cancer has been shown in macrophages , dendritic cells (DC) , and total population of MDSCs where it was associated with immunosuppressive activity . A previous study have reported that FATP2 enhanced the suppressive activity of PMN-MDSCs by promoting arachidonic acid utilization and PGE2 synthesis . In this study, FATP2 KO significantly decreased arachidonic acid and arachidonic acid metabolite PGE2 in PMN-MDSCs and seriously impaired the immunosuppressive effect of PMN-MDSCs on CD8+ T cells. At present, although STAT5 has been reported to be one of the key molecules regulating FATP2 expression , the mechanism that regulates FATP2 still not fully understood.
RIPK3, as a key regulator of programmed cell necrosis, plays an important role in both inflammation and tumors . RIPK3 was negatively correlated with tumor stage and is an important prognostic factor for colon cancer . Recently, a previous study revealed the regulatory mechanism of a RIPK3-PGE2 circuit in PMN-MDSCs . Lacking RIPK3 PMN-MDSCs had a stronger immunosuppressive activity. RIPK3 reduction activated the NF-κB-COX2-PGE2 signaling pathway and promoted PGE2 synthesis. The PGE2 further inhibited the expression of RIPK3 and promoted the NF-kB/COX-2 and Arg-1 expression through the cAMP/PKA-CREB signaling pathway, thus forming the RIPK3-PGE2 circuit. In the tumor microenvironment, PMN-MDSCs significantly promoted the PGE2 synthesis through the RIPK3-PGE2 circuit, thereby enhancing the immunosuppressive activity of PMN-MDSCs. As shown by the results of this study, breaking the RIPK3-PGE2 circuit by up-regulating RIPK3 significantly reduced the synthesis of PGE2 and further inhibited the immunosuppressive activity of PMN-MDSCs.
In PMN-MDSCs, both FATP2 and RIPK3 pathways could synthesize the PGE2 [8,30]. The PGE2 synthesized by the FATP2 pathway could further promote the synthesis of PGE2 through the RIPK3-PGE2 circuit, so the RIPK3-PGE2 circuit might be an amplifier of the FATP2 pathway in terms of promoting PGE2 synthesis. In vitro, FATP2 KO significantly up-regulated the RIPK3 expression and reduced PGE2 synthesis in PMN-MDSCs. However, after exogenous PGE2 stimulation, FATP2 KO PMN-MDSCs could also synthesize PGE2. In vivo, PGE2 is derived not only from MDSCs but also from tumor cells or other cells . Blocking FATP2 partially reduced the PGE2 synthesis of PMN-MDSCs, but it cannot completely inhibit the PGE2 synthesis in vivo. There was a synergy between FATP2 and RIPK3 pathways in promoting the PGE2 synthesis of PMN-MDSCs. Therefore, blocking the FATP2 and RIPK3 pathways at the same time can successfully minimize the synthesis of PGE2 in PMN-MDSCs. Our in vitro experiments also confirmed that blocking the FATP2 combined with up-regulating the RIPK3 minimized the PGE2 synthesis of PMN-MDSCs even adding exogenous PGE2.
In this study, there are some shortcomings. This study initially proved that there were a large number of PMN-MDSCs in tumor tissues, and the expression of FATP2 was down-regulated, and RIPK3 was down-regulated. However, this study has not yet elucidated the molecular mechanism that causes this phenomenon in the tumor microenvironment. Elucidating the molecular mechanism is likely to help discover more effective therapeutic targets to overcome PMN-MDSCs-mediated immune escape. In addition, the clinical sample size included in this study is small, and a larger sample is needed to verify the conclusions of the study.
In summary, our results demonstrated that the synergy between FATP2 and RIPK3 pathways in PMN-MDSCs significantly promoted the synthesis of PGE2, which severely impaired the CD8+ T cell function and IFN-γ secretion. Combination therapy with targeting FATP2 and RIPK3 might be considered for rational treatment for inhibiting the immunosuppressive activity of PMN-MDSCs and enhancing the antitumor immunity. These findings provided the molecular basis and potential ideas for the immunotherapy of bladder cancer.