BACKGROUND: Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of naturally occurring, immunosuppressive cells that may be exploitable in transplantation.There are only a few incomplete studies suggesting that immunosuppressive drugs may affect MDSC, although mechanisms are undefined. Anti-thymocyte globulin (ATG) is a commonly used T cell depleting agent, yet the effect of ATG on MDSCs has not been investigated. We hypothesized that ATG could bind to the MDSC cell surface and alter functional characteristics.

METHODS: MDSCs were expanded and purified from mouse splenocytes using a Lewis Lung Carcinoma 1 (LLC1) tumor xenograft model. MDSCs were characterized by expression of CD11b, Gr-1, Ly6-C, or Ly6G. MDSC suppressive activity was assessed by CD4+ T cell proliferation co-culture assay. In vivo MDSC development and function were assessed after administration of ATG (1mg/kg/day). In vitro binding and co-culture suppression assays determined if ATG directly bound and functionally inhibited MDSCs. qPCR was used to measure expression of transcripts for the suppressive molecules Arg-1, iNOS, and PD-L1.

RESULTS: Spleen size and weight increased 4-fold and CD11b+ Gr-1+ MDSCs expanded 7-fold in splenocytes of LLC1 treated mice. The majority (80%) of MDSC were CD11b+Ly6C-Ly6G+ polymorphonuclear MDSC (PMN-MDSC) while 20% were CD11b+Ly6ChighLy6G- monocytic MDSC (M-MDSC). In the absence of pharmacologic immunosuppression, CD11b+Gr-1+MDSCs were highly suppressive of alloreactive CD4+ T cell proliferation. In contrast, ATG-treated LLC1 mice developed 50% fewer MDSCs (p<0.05), and these MDSCs were no longer suppressive of alloreactive CD4+ T cells. In vitro, ATG directly bound 92.6% of purified MDSCs versus only 0.3% of rabbit-serum treated controls. Arg-1 and PD-L1 expression were decreased 40% after ATG treatment in vivo and in vitro. iNOS expression was unaffected by ATG treatment.

CONCLUSION: MDSCs are capable of controlling alloreactive T cell responses. However, commonly administered T cell depleting agents bind to and functionally inhibit MDSCs. Targeted suppression of alloreactive T cells and simultaneous optimization of the inhibitory effects of naturally occurring MDSCs may provide a pathway towards immunosuppression minimization or avoidance.

CITATION INFORMATION: Lee Y., Davila E., Bromberg J., Scalea J. Anti-Thymocyte Globulin Directly Binds and Functionally Inhibits Allo-Regulatory Myeloid Derived Suppressor Cells Am J Transplant. 2017;17 (suppl 3).

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