CD11b⁺gr-1⁺ myeloid Derived Suppressor Cells (MDSC) In Normal Bone Marrow Suppress T Cell Proliferation and Enhance T-Regulatory Function Via a IL10-, IL4- and IDO-Independent Mechanism

Abstract 4801

MDSC have been described as an important negative regulators of autologous anti-cancer immune responses. Considering the important role of MDSC in immune regulation in allogenic stem cell and organ transplantation, we undertook an investigation of the mechanism(s) by which MDSC inhibit T–cell activation and proliferation, and tested the hypothesis that local cytokine secretion or IDO activity is required for suppression of T-cell proliferation.

Two separate populations CD11b^hiGr-1^hi and CD11b^hi Gr-1^int were isolated by high-speed FACS from lineage- BM antigen presenting cells (C57 & BALB/c mice). Both MDSC subsets had potent capacity for in–vitro suppression of CD4+ and CD8+ T cells proliferation in response to anti-CD3/anti-CD28 beads and Con A. A ratio of 0.5/1 MDSC: T-cells were sufficient to inhibit >66% control levels of T-cell proliferation. MDSC isolated from transgenic mice that had been “knocked-out” for IFN-γ and IDO had equivalent suppressive activity as MDSC from wild-type donors. Addition of saturating concentrations of anti IL-10 and IL-4 MAb, or in combination with anti- IFN-γ MAb did not abrogate MDSC-suppressive activity. Ex-vivo culture of MDSC with mitogen-activated T-cells generated two—fold more Fox-p3 T-reg compared with cultures of T cell plus mitogen.

Data will be presented regarding the novel role of MDSC involving in the homeostasis regulation of normal T-cell activation and proliferation in non-tumor-bearing mice.