Myeloid-Derived Suppressor Cells in Immune-Related Bone Marrow Failure: Novel Therapeutic Implication

Immune-related bone marrow failure (IBMF) is characterized by pancytopenia and bone marrow (BM) hypoplasia. Abnormally activated CD8⁺ T cells are important in its pathogenesis and majority of the patients having this abnormity are diagnosed as aplastic anemia (AA). The exploration of intra-BM immune network will help to conduct novel treatments to this disease.

BM mesenchymal stromal cells (MSCs), regulatory T cells (Tregs), natural killer (NK) cells, myeloid-derived suppressor cells (MDSCs) are all immune-regulators within BM microenvironment. Among them, lymphocytes including CD8⁺ T cells, Tregs, NKs were indicated relating to IBMF pathogenesis with either stimulating or suppressive function. However, the relationship within BM between activated CD8⁺ T cells and other immune-regulating component remains largely unknown. In this study, we investigated MDSCs' characterization in IBMF models and AA patients to look into the intra-BM immune response during IBMF occurrence and progression.

IBMF mice models were produced by lymphocyte infusion as previously described (Chen J, et al. Blood. 2004). To determine the major T cell population responsible for attacking BM environment, we injected H2 ^b/b -CD3⁺CD4⁺ T cells or H2 ^b/b -CD3⁺CD8⁺ T cells in sublethally irradiated hybrid F1 recipients of H2 ^b/b and H2 ^d/d mice. The recipients infused with CD3⁺CD8⁺ T cells were manifested BMF as pancytopenia, promoted BM-MSC adipogenesis and shorter life span. To further clarify why BM is the major targeted organ of CD8⁺ T cell in IBMF models, we detected homing receptors and proliferative potential of CD8⁺ T cells in BM, spleen and lymph nodes. CXCR4, CXCR6, CD49d, TRAIL as well as CD25, EDU were increasingly expressed on intra-BM CD8⁺ T cells, suggesting that CD8⁺ T cells reside more sufficiently in BM than in other lymph organs.

MDSCs, subgrouped as mMDSCs and gMDSCs by a panel of surface markers, are a heterogeneous population of immature myeloid cells with immunoregulatory function (Gabrilovich DI, et al. Nat Rev Immunol. 2009). The numbers of either circulating MDSCs (CD11b⁺CD33⁺HLA-DR^-/low) in AA patients or intra-BM MDSCs (CD11b⁺Gr1⁺) in IBMF models were decreased compared to normal controls (p=0.0016 and 0.03, respectively), indicating that MDSCs are impacted in the presence of over-activated CD8⁺ T cells. In vitro co-cultivation of MDSC subpopulation with T cells showed that it was not gMDSCs (CD11b⁺Ly6G⁺Ly6C^low, p=0.112) but mMDSCs (CD11b⁺Ly6G^-/lowLy6C^hi, p=0.017) capable of reducing anti-CD3/CD28-stimulated T cell proliferation. A nitric oxide synthase (NOS) inhibitor, L-N^G-Nitroarginine Methyl Ester (L-NAME), significantly reversed mMDSC-induced T cell hypoproliferation (38.7%±17.5% vs 72.3%±3.5%, p=0.031), suggesting that inducible NOS pathway is an important modulator in the interaction of mMDSC and activated CD8⁺ T cells.

To further investigate whether mMDSCs exert immune regulating function in vivo, freshly sorted H2 ^b/b -mMDSCs were transfused in sublethally irradiated H2 ^b/d with CD3⁺ T cells or lately in IBMF models every three days. The earlier mMDSCs transfusion was able to reduce the severity of pancytopenia and intra-BM infiltration of CD8⁺ T cells. The overall survival (OS) of repeatedly transfusion group was significantly longer than that of non-mMDSCs transfusion controls (26 days vs 17days, p=0.025).

Taking these together, mMDSCs serve as an important regulator with other immune suppressive parts on one side of the teeter-totter, the other side of which the overactivated CD8⁺ T cells are the major counterpart attacking hematopoietic components in IBMF. In BM microenvironment, immune balance together with orderly multi-differentiation of MSCs and hematopoietic stem cells are all essential to orchestrate normal hematopoiesis.