Donor lymphocyte infusion (DLI) induces donor-specific hypo responsiveness or immune-tolerance among allo-reactive T cells in different organ transplantation models. However, the optimal cell source or subset for DLI effects remains to be elucidated. B-cells are ideal candidates as they constitute a complex system of antigen-presenting cells (APCs) and exist as distinct subsets that differ in their lineage affiliation, surface molecule expression, and biological function, potentially regulating the immune response. In this study, we investigated the immune-regulatory roles of murine B cell subsets as tolerogeneic APCs targeting allo-reactive T cells.

We showed that MHC class II⁺CD80⁺CD86⁺PD-L1⁺PD-L2⁺ B cell subclass are exclusively part of the CD5⁺ B-1a cells that are located in the peritoneal cavity (PerC). These PD-L1/L2 B-1a cells expressed surface molecules necessary for efficient Ag-presentation to T cells together with the apoptosis-inducing ligand, potentially imparting their tolerogenic potential upon alloantigen recognition of T cells. When the purified PD-L1/L2 B-1a cells isolated from Balb/c mice were adoptively transferred into PerC of NOD-scid IL-2Rγ^null mice, the inoculated B cells were phenotypically invariant over 2 weeks, indicating that the PD-L1/L2 B-1a cells are well-differentiated steady cells.

After intravenous injection of either splenic B cells, PerC B cells, or non B cells from Balb/c mice into C57BL/6 (B6) mice, serum levels of anti-Balb/c antibodies (IgG) in the recipients of PerC B cells were significantly lower than those in the recipients of splenic B cells and PerC non B cells during 4 weeks after the injection (P<0.05, n=4 in each group). Two weeks after the injection, splenocytes from the B6 mice were harvested and used for mixed-lymphocyte reaction (MLR) assay. In the recipients of PerC B cells, the stimulation index of anti-Balb/c CD4⁺ and CD8⁺ T cells was significantly lower than that in the recipients of splenic B cells and PerC non B cells (P<0.05, n=6-7). The pre-incubation with anti-PD-L1/PD-L2 mAbs and PerC B cells from Balb/c mice prior to the injection into B6 mice abrogated their immune-regulatory effects on anti-Balb/c CD4⁺ and CD8⁺ T cells in MLR assay (P<0.05, n=4-5). These findings indicate that the PerC B cells, including the MHC class II⁺CD80⁺CD86⁺PD-L1⁺PD-L2⁺ B-1a cell subpopulation, have the potential to suppress T cells responding to allostimulation and could possibly be an optimal candidate for DLI.

CITATION INFORMATION: Hirose T, Tanaka Y, Ohdan H. PD-L1/PD-L2-Expressing B-1a Cells Inhibit Allo-Reactive T Cells in Mice. Am J Transplant. 2016;16 (suppl 3).

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