The Use of Myeloid Suppressor Cells to Inhibit Experimental Autoimmune Encephalomyelitis: a Potential Immunotherapy for Multiple Sclerosis

Abstract

The ideal treatments for multiple sclerosis (MS) are ones that specifically target the disease causing autoreactive T cells without compromising the immune system's ability to respond to pathogens and infections. However, the current treatments for MS are antigen non-specific and there is a need for the development of antigen-specific therapies that do not induce global immunosuppression. Thus, this thesis aims to investigate the potential of using the body's own suppressor cells to develop an antigen-specific immunotherapy to inhibit experimental autoimmune encephalomyelitis (EAE), the murine model for MS. In our laboratory, there are two versions of mutated superantigens, SMEZ-2-M1 (SM) and double mutant SMEZ-2 (DM). SM is defective at its TCR binding site, but retains its ability to bind to MHCII molecules. Based on previous findings from our laboratory that administration of a SM conjugate with myelin oligodendrocyte glycoprotein (MOG35-55) peptide in incomplete Freund's adjuvant (IFA) suppressed EAE in a CD25+ regulatory T cell (Treg)-dependent manner, it was hypothesised that the administration of SM-MOG35-55/IFA expanded and/or activated MOG35-55 specific Tregs in vivo. In the first part of this thesis, I tested this hypothesis. The experimental results showed that neither the Foxp3+ nor CD25+ Tregs primed in vivo by SM-MOG35-55/IFA could inhibit EAE and surprisingly, treating mice with SM-MOG35-55/IFA did not significantly suppress EAE as previously described. Nevertheless, the administration of SM-MOG35-55 into mice using various methods repeatedly showed minor suppression of EAE, suggesting an in vivo suppressive capability of SM-MOG35-55. Interestingly, after being injected into mice intravenously, SM was captured by a blood MHCII-CD11b+F4/80+Gr-1+ cell population in an MHCII-independent manner. Cells expressing the same surface markers have been reported in the literature to be myeloid derived suppressor cells (MDSCs), suggesting that the SM+MHCII-CD11b+F4/80+Gr-1+ cells may be suppressor cells, i.e. a subpopulation of MDSCs, and play a role in SM-MOG35-55 mediated EAE suppression. In the second part of this thesis, I went on to test the blood MHCII-CD11b+F4/80+Gr-1+ cells' suppressive potential using DM. Unlike SM, DM is defective at both MHCII and TCR binding sites, and possessed an enhanced binding capability to the blood MHCIICD11b+ F4/80+Gr-1+ cells. The experimental results demonstrated that the blood MHCII-CD11b+F4/80+Gr-1+cells are potent suppressors of T cell responses, and were subsequently named as blood MDSCs (bMDSCs). bMDSCs suppressed T cell proliferation in vitro in a cell contact-dependant manner, and nitric oxide played an important role in this suppression. In the third part of this thesis, I investigated the potential of using DM for EAE suppression via bMDSCs. When DM was conjugated to MOG35-55 and administered subcutaneously into mice, EAE was suppressed in a MOG35-55-specific manner. Moreover, the adoptive transfer of bMDSCs from the DM-MOG35-55 treated mice transferred EAE suppression, confirming that bMDSCs play an important role in this suppression. Taken together, these results reveal a previously unknown role of bMDSCs in limiting immune responses. Moreover, the use of DM to direct the activity of bMDSC may prove to be a unique antigen-specific immunotherapy for EAE, which has great potential to be developed into a treatment of MS and other autoimmune diseases.