Purpose T-cell depletion by immunosuppressive antibodies has been increasingly used in treatment and prevention of allograft rejection. However, the impact of T-cell depletion on B-cell homeostasis remains poorly understood. This work evaluated the B-cell pool changes caused by anti T-cell antibodies.

Method C57BL/6 mice sensitized with skin allografts from a HLA.A2 transgenic mouse were T-cell depleted with intraperitoneal injections of anti-CD3e mAb. B lineage cell development in the central (bone marrow, BM) and peripheral (spleen) lymphoid organs were studied.

Results B cells expressing an immature phenotype (B220^losIgM⁺ sIgD^–) accumulate in the spleens following T-cell depletion by anti-CD3. The surge of immature B-cells in the spleen is anti-CD3 induced, but not alloantigen dependent. FACS analysis of the central (BM) and peripheral (spleen) B-cells harvested at 72 hours after T-cell depletion revealed that B220^losIgM⁺ CD93⁺ pre-B-cell population in the BM were reduced while the B220^losIgM⁺ CD23^–CD93^+/-sIgD^– immature B-cells in the spleens were increased. Immunofluorescent microscopy confirmed B-cell proliferation and T-cell depletion in the spleen following anti-CD3 treatment. To further study the impact of anti-CD3 on BM T-cells, marrow cells were harvested from the femurs at day 3 after injection of anti-mouse CD3 mAb. Flow cytometric analysis demonstrated that (1) different subsets of T-cells existed in the naive BM; and (2) anti-CD3 significantly reduced T-cells, including CD8+, CD4+ and FoxP3+ Treg. Most importantly, T-cell depletion by anti-CD3 is associated with a reduction in B220+ cells in the BM and an increase B220^lo cells in the spleens.

Conclusion Following systemic T-cell depletion, including BM T-cells the peripheral B-cell pool was repopulated with immature B-cells (B220^loCD23^–CD93⁺sIgM⁺sIgD^–). The BM immature B-cell subset was significantly reduced, suggesting these BM immature B-cells have migrated into the periphery. The association of T-cell depletion to immature B-cells migration to the periphery indicates that T-cells may play a critical role in controlling release of immature B-cells from the BM to the periphery. Further studies to determine the mechanism by which T-cells regulate B-cell homeostasis would allow for designing new immunosuppression strategies to improve transplant survival.

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