*Purpose: We previously demonstrated that anti-CD137 agonist prevent acute graft-versus-host disease (GVHD) and chronic GVHD in the parent-into-unirradiated F1 acute or chronic GVHD model. Although activation-induced dell death of donor T cells is one mechanism of anti-CD137-mediated suppression of GVHD, its detailed process remains to be clarified.So we investigated the role of DC with anti-CD137 in GVHD suppression.

*Methods: Induction of cGVHD Single-cell suspensions in PBS were prepared from the spleens and lymph nodes of female bm12 mice. Chronic GVHD was induced by transfer of 8 x 10⁷ of bm12 cells into the tail vein of female wild-type and CD137^-/- mice. Immediately thereafter, 200 μg of anti-CD137 mAb (3E1) or control Ig was intraperitoneally administered.MACS purification and FACS sorting of DC subsets Splenocytes were prepared by digestion. For purification of CD11b⁺ DCs, CD8α^– DCs were first removed by a negative selection using anti-CD8α-microbeads . The remained cells were positively selected using anti-CD11c-microbeads . For purification of CD8α⁺ DCs, a negative selection of splenocytes with anti-CD11b-microbeads and a positive selection with anti-CD11c-microbeads were sequentially performed. For FACS sorting of DC subsets, DCs were first enriched from splenocytes using anti-CD11c-microbeads. The enriched cells were then stained with anti-CD11 and anti-CD8α⁺ mAb at 4 °C for 30 min. After the cells were washed twice with PBS buffer containing 1% BSA, CD8α⁺ and CD11b⁺ DCs were sorted using FACSAria III.

*Results: . Here, we report that in vivo engagement of CD137 in chronic GVHD mice massively induced the generation of PD-L1^hiPD-L2⁺OX40⁺IL-10⁺CD11b⁺ dendritic cells (DCs) and such regulatory DCs could prevent either chronic or acute GVHD. IL-10 is required for these regulatory DCs to expand conventional Treg cells. Interestingly, a lower number of Treg cells suppressed acute GVHD compared to chronic GVHD, suggesting that autoantibody production is governed by less stringent immunoregulation. In some GVHD models, however, anti-CD137-primed DCs potently induced activation-induced cell death of donor T cells.

*Conclusions: . In conclusion, our study indicates that anti-CD137 antibody suppresses acute and chronic GVHD by different mechanisms involving distinct DCs subsets.

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