Tim-3 regulates the immunosuppressive function of decidual MDSCs via the Fyn-STAT3-C/EBPβ pathway during Toxoplasma gondii infection

<p><span>Myeloid-derived suppressor cells (MDSCs) play a key role in maintaining maternal-fetal tolerance for a successful pregnancy, but the role of MDSCs in abnormal pregnancy caused by <em>Toxoplasma</em> <em>gondii</em> infection is unknown. Herein, we revealed a distinct mechanism by which T-cell immunoglobulin domain- and mucin domain-containing protein-3 (Tim-3), an immune checkpoint receptor that balances maternal-fetal tolerance during pregnancy, contributes to the immunosuppressive function of MDSCs during <em>T. gondii</em> infection. The expression of Tim-3 in decidual MDSCs was significantly </span><span>downregulated</span><span> following <em>T. gondii </em>infection. The proportion of monocytic </span><span>MDSC</span><span> population, the inhibitory effect of MDSCs on T-cell proliferation, the levels of STAT3 phosphorylation, and the expression of functional molecules (Arg-1 and IL-10) in MDSCs were all decreased in <em>T. gondii</em>-infected pregnant Tim-3 gene knockout (Tim-3KO) mice compared with infected pregnant WT mice. After treatment with Tim-3-neutralizing Ab <em>in vitro</em>, the expression levels of Arg-1, IL-10, C/EBPβ, and p-STAT3 were decreased, the interaction between Fyn and Tim-3 or between Fyn and STAT3 </span><span>was</span><span> weakened,</span><span> and</span><span> the binding ability of C/EBPβ to the promoters of <em>ARG1</em> and <em>IL10</em> </span><span>was</span><span> decreased in human decidual MDSCs with <em>T. gondii</em> infection, while opposite results were observed following treatment with galectin-9 (a ligand for Tim-3). </span><span>Inhibitors</span><span> of Fyn and STAT3 also </span><span>downregulated</span><span> the expression of Arg-1 and IL-10 in decidual MDSCs and exacerbated adverse pregnancy outcomes caused by <em>T. gondii</em> infection in mice. Therefore, our studies discovered that the decrease of Tim-3 after <em>T. gondii</em> infection could downregulate the functional molecules of Arg-1 and IL-10 expression in decidual MDSCs through the Fyn-STAT3-C/EBPβ signaling pathway and weaken their immunosuppressive function, which eventually contributes to the development of adverse pregnancy outcomes.</span></p>