Retinoic Acid Regulates Donor Myeloid Immune Reconstitution after Allogeneic Hematopoietic Stem Cell Transplantation in Mice

Graft-versus-host disease (GVHD) remains a major obstacle for the success of allogeneic hematopoietic stem cell transplantation (HSCT). Donor hematopoietic progenitor cells reconstitute the recipient's immune system after transplant, and newly generated donor derived immune cells actively participate in propagating alloimmunity. It has been demonstrated that donor antigen presenting cells (APCs) can amplify GVHD by cross presenting alloantigen. Besides mediating a detrimental effect on GVHD, myeloid immune cells may also play an immunoregulatory role. Specifically, myeloid-derived suppressor cells (MDSCs) can reduce GVHD upon adoptive transfer into recipient mice. Retinoic acid (RA), the active metabolite of vitamin A, has been implicated in GVHD pathogenesis. Given the well-known effects of RA on the differentiation and proliferation of myeloid progenitor cells, we hypothesized that RA signaling regulates donor myeloid immune reconstitution after allogeneic HSCT.

We found that bone marrow (BM) cells from Rag-1 KO mice (no T cells or B cells) constitutively expressed retinoic acid receptors (RARs), rendering them responsive to RA mediated effects. Treating BM cells with RA for 24 hours significantly increased gene expression of retinoic acid receptor (RAR)-alpha, -gamma, and retinoid X receptor (RXR)-alpha. MDSCs were generated from BM cells in the presence of cytokines GM-CSF and IL-6. We found that adding RA to the culture promotes the differentiation of a subpopulation of ly6C^-ly6G⁺ granulocytic MDSCs. To examine the effects of RA on MDSCs in vivo, we used a C57BL/6 → Balb/c murine allogeneic HSCT model. Gr-1⁺Mac-1⁺ cells accumulated in the spleen of recipient mice early after BMT and showed potent immunosuppressive function. Treating recipient mice with RA resulted in an increased GVHD-associated mortality. Notably, the absolute number of MDSCs was significantly reduced in RA-treated mice compared with DMSO-treated mice. Furthermore, MDSCs isolated from RA-treated HSCT recipient mice showed decreased immunosuppressive function in mixed lymphocyte reactions (MLRs). Thus, enhancing RA signaling causes both quantitative and qualitative defects of MDSCs after allogeneic HSCT.

Recent studies have demonstrated an important role of donor intestinal CD103⁺ DCs in propagating GVHD. Using the same GVHD model, we found that RA-treated recipient mice had significantly increased colonic CD103⁺ DCs compared with DMSO-treated mice. On the contrary, vitamin A deficiency was associated with a significant reduction in the percentage of CD103⁺ DCs. To study the effects of RA on DCs in vitro, BM cells were cultured in the presence of GM-CSF. There was a significant increase in the percentage of CD11c⁺CD103⁺ mucosal DCs in cultures exposed to RA. Furthermore, the expression of MHC-II was significantly increased in RA-exposed DCs, indicating an enhanced antigen presenting function of these cells.

In summary, our study demonstrates that RA signaling participates in regulating donor myeloid immune reconstitution after allogeneic HSCT. Targeting this pathway in donor myeloid cells may reduce alloimmunity and improve transplant outcomes.

No relevant conflicts of interest to declare.