Neem leaf glycoprotein salvages T cell functions from Myeloid-derived suppressor cells-suppression by altering IL-10/STAT3 axis in melanoma tumor microenvironment

Myeloid-derived suppressor cells (MDSCs) suppress antitumor immune functions. We have observed that an immunomodulator, neem leaf glycoprotein (NLGP), inhibits tumor-resident MDSCs and enhances antitumor CD8+ T cell immunity. NLGP inhibits the number as well as functions of tumor-resident MDSCs (Gr1 +/- CD11b +/-) and enhances antitumor CD8 +/- T cell immunity by downregulating arginase 1 and inducible nitric oxide synthase production in MDSCs. Accordingly, decreased T cell anergy and helper to regulatory T cell conversion have been observed in the presence of NLGP, which ultimately augments T cell functions. Mechanistically, NLGP-mediated rectification of T cell suppressive functions of MDSCs was primarily associated with downregulation of the interleukin (IL)-10/signal transducer and activator of transcription 3 (STAT3) signaling axis within the tumor microenvironment, as confirmed by knockdown of STAT3 (by STAT3-siRNA) and using IL-10(-/-) mice. Thus, NLGP-mediated suppression of MDSC functions in tumor hosts is appeared to be another associated effective mechanism for the eradication of murine melanoma by NLGP.