Liposomal microparticle injection can induce myeloid-derived suppressor cells (MDSC)-like cells in vivo

Context: Myeloid-derived suppressor cells (MDSCs) are a subset of immature myeloid cells that function as immunosuppressive cells in various pathological conditions. Membrane-derived microvesicles are thought to be involved in MDSC induction. Earlier reports have described that injection of considerable amount of liposome into rat can suppress Con A-induced splenic T-cell proliferation. Liposome-internalized cells expressing CD11b/c suppress T-cell proliferation. Nitric oxide (NO) appears to be involved in the suppression. We speculated that, similarly to membrane-derived microvesicles, liposomal microparticles can induce MDSC-like cells in vivo. Objectives: To confirm our speculation we investigated dose-dependency of the suppressive effect, the effect of liposome on the induction of inducible NO synthase (iNOS), and anti-CD3 antibody-stimulated T-cell proliferation and cytokine production. Materials and methods: Liposome particles of 250nm diameter were prepared and suspended in saline. Then, various amounts of liposomal suspension were injected intravenously into rats. After 24h, rat spleens were removed and concanavalin A (or anti-CD3 antibody) stimulated-splenic T-cell proliferation and the production of iNOS, NO and cytokines were evaluated. Results: T-cell proliferation was suppressed dose-dependently by liposome injection. The immunosuppressive cell exerts its suppressive activity in a dose-dependent manner. The suppression was eliminated by iNOS inhibitor. iNOS was detected in liposome-loaded splenocytes. Anti-CD3 antibody-stimulated T-cell proliferation was also inhibited. Enhanced production of IL-10 was observed. Conclusions: Liposomal microparticles can induce MDSC-like cells in vivo. The lipids which comprise liposomes might serve an important role in the induction of MDSCs in vivo.