MyD88 in Donor Bone Marrow Cells Is Critical for Protection from Acute Intestinal Graft-Versus-Host Disease

Allogeneic hematopoietic stem cell transplantation is an important therapeutic modality used to treat malignancies of hematopoietic origin, such as leukemia and lymphoma. However, development of graft-versus-host disease (GVHD) causes non-relapse mortality and substantial morbidity of recipients. Myeloid differentiation factor 88 (MyD88), a major adaptor mediating TLR signaling, is also known to deliver pro-inflammatory signals. Activation of inflammatory signaling through MyD88 plays a key role in the expansion of myeloid-derived suppressor cells (MDSC) which are a heterogeneous population of immature myeloid ells with anti-inflammatory activity. To explore the contribution of MyD88 expressed by donor bone marrow (BM) cells to development of GVHD, we induced GVHD using T-cell-depleted BM (TCD-BM) isolated from MyD88-deficient (MyD88KO) mice and T cells isolated from wild-type (WT) mice. We employed C57BL/6 (H-2^b) → B6D2F1 (H-2^b/d) mouse model of GVHD, which differ at major and minor histocompatibility loci. Lethally irradiated B6D2F1 recipient mice were transplanted with either T cell-depleted bone marrow (TCD BM, 5 x 10⁶) from either WT or MyD88KO mice together with WT spleen T cells (1 x 10⁶). Transplantation with MyD88KO TCD BM aggravated GVHD; serious gut damage was evident, with infiltration of T cells specifically into the intestines of recipients. GVHD hosts transplanted with MyD88KO TCD BM exhibited markedly reduced expansion of MDSC. GVHD aggravation after transplantation with MyD88KO TCD-BM, associated with high-level T cell infiltration into the intestine and insignificant expansion of MDSC, was reproduced in another minor histocompatibility mismatch model (C57BL/6 → BALB.B). We next examined allogeneic T cells in the spleens of GVHD hosts in terms of the expression levels of CCR9, which are known to be associated with T cell migration to the intestinal mucosa and the proportion of CCR9 positive cells in CFSE low CD8⁺ T cells was higher in recipients of MyD88KO TCD BM than WT controls. In parallel, the levels of CCL25 were more highly expressed in the gut of MyD88KO recipients than WT controls. Mixed leukocyte cultures of CFSE- labeled C57BL/6 T cells and irradiated B6DF1 feeder cells were prepared in the presence of MDSC isolated from MyD88KO or WT mice. T cells, co-incubated with MDSC isolated from MyD88KO BM, exhibited a greater extent of CFSE dilution and less Annexin V staining, compared to T cells co-incubated with cells from WT BM. Moreover, MDSC from recipients of MyD88KO TCD BM exhibited a reduced suppressive function, compared to their WT counterparts. Next, we determined whether insufficient expansion of and ineffective suppression by MDSC caused severe GVHD in recipients of MyD88KO TCD BM. Supplementation of transplanted mice with MDSC from WT mice, not from MyD88KO mice, attenuated the severity of GVHD and reduced intestinal T cell infiltration in recipients of MyD88KO TCD BM. To verify the importance of MyD88-mediated signaling by MDSC in protection against severe GVHD, we determined if transplantation with TCD-BM cells containing high levels of MDSC attenuated the severity of GVHD. Pre-treatment of BM donors with lipopolysaccharide increased the frequencies of MDSC and the amounts of MyD88 transcripts in TCD-BM transplant, and alleviated the severity of GVHD and intestinal T-cell infiltration. To explore whether MDSC expansion levels could be used to predict the severity of intestinal GVHD, the T/MDSC ratios were calculated in blood of patients at the time of engraftment and were significantly higher in patients with intestinal GVHD ≥ grade 2. In conclusion, we have shown that MyD88-dependent MDSC expansion from donor BM is critical for protection against fatal acute intestinal GVHD.