Foxp3⁺ regulatory T Cells Maintain Bone Marrow Microenvironment for B Cell Differentiation from Hematopoietic Stem Cells

Background: Foxp3⁺regulatory T cells (Treg) are a subpopulation of T cells, which regulate the immune system, maintain self-tolerance and enhance immune tolerance after transplantation. It was also reported that recipient derived Treg could provide immune privilege niche to allogeneic hematopoietic stem cells (HSC) after transplantation. However, the precise role of Treg in hematopoiesis has not been fully elucidated.

Methods: We used Foxp3-DTR mice (B6, CD45.2) for in vivo depletion of Treg through diphtheria toxin (DT) injection and investigated whether Treg depletion would affect hematopoiesis derived from HSC. To investigate whether Treg depletion affects the function of the bone marrow microenvironment, we transplanted wild type bone marrow cells into lethally irradiated Foxp3-DTR mice after Treg depletion.

Results: We found 1) a significant defect on B cell progenitors including mature B cells (IgM⁺B220⁺, P<0.001), pre-B cells (IgM^-B220⁺CD19⁺cKit^-, P<0.001) and pro-B cells (IgM^-B220⁺CD19⁺cKit⁺, P<0.05), 2) LT-HSC population (CD34-/lowFlit3-cKit+Sca1+Lin-) was significantly expanded (p<0.01) and entered into cell cycle, 3) the residual Foxp3-CD4+ or CD8+ T cells in the bone marrow had an activated immune phenotype and clustered at sinusoids when bone marrow cells from Treg depleted mice were analyzed. Expanded LT-HSC from Treg depleted mice had reduced long-term reconstitution capacity when we performed competitive repopulation experiments using purified LT-HSC from Foxp3-DTR mice with or without Treg depletion (100 cells/mice, CD45.2), total bone marrow cells (2x10e5/mice, B6-F1, CD45.1/CD45.2) and congenic recipient mice (lethally irradiated B6, CD45.1). B cell reconstitution was also severely abrogated following transplantation using Treg depleted mice as recipients (p<0.01). In those mice, we observed a significant reduction of IL-7 production (p<0.01). Interestingly, we found that a subpopulation of CD45^-TER119^-CD31^- ICAM1⁺ perivascular stromal cells are a major source of IL-7 in the bone marrow. ICAM1+ perivascular stromal cells also secrete SCF and CXCL12, which is crucial for the maintenance of LT-HSC. In Treg depleted BM cells, a significant reduction in IL-7 secretion from ICAM1+ perivascular stromal cells was observed, suggesting that this population is the target of activated T cells after Treg depletion.

Conclusions: These data demonstrate that Treg play a key role in B cell differentiation from HSCs by maintaining the immunological homeostasis in the bone marrow microenvironment. These data provide new insights into Treg biology and function in normal and stress hematopoiesis.