Association of TLR-4 Mutations in Host and Donor Cells and the Risk for Acute GVHD after Allogeneic Hematopoietic Stem Cell Transplantation.

Lipopolysaccharide (LPS) has been implicated in the pathogenesis of acute graft-versus-host disease (GVHD). The toll-like receptor (TLR)-4 has been recently identified as a major receptor for LPS. It is possible that mutations of TLR-4 have been associated with LPS hyporesponsiveness, thereby reducing the risk of acute GVHD in allogeneic hematopoietic stem cell transplantation (HSCT). However, stimulation of TLRs may also initiates a signaling cascade for induction of various pro-inflammatory cytokines, leading to microbial eradication. Therefore, mutations of TLR-4 may increase risk of microbial infection, thereby accelerating acute GVHD in allogeneic HSCT recipients. Retrospective studies to determine the frequency of TLR-4 mutations and their possible association with acute GVHD contradict each other. In the present study, we examined the association of TLR-4 mutations in host and donor cells and the risk for acute GVHD after allogeneic HSCT. To examine the role of TLR-4 in host cells, we analyzed TLR-4 expression in the intestine by immunohistochemistry. Lethally irradiated C3H/HeN mice received allogeneic HSCT from C57BL/6 donors. TLR-4 expression in the intestinal epithelial cells was significantly increased after GVHD induction. Next, we used a mouse strain (C3H/HeJ) that has a single amino acid substitution in its TLR-4 as recipients in an allogeneic HSCT. Lethally irradiated either C3H/HeN or C3H/HeJ hosts received HSCT from C57BL/6 donors. C3H/HeN recipients receiving C57BL/6 donors developed significantly less GVHD as measured by mortality and intestinal histopathology compared with C3H/HeJ recipients receiving C57BL/6 donors. TNF-a mRNA expression in C3H/HeJ recipients was stronger in the intestine but weaker in the spleen and the liver comparing to that in C3H/HeN recipients. These results suggest that TLR-4 mutation in host cells reduces the expression of pro-inflammatory cytokines in the spleen and the liver, while it also reduces microbial eradication in the intestine, increasing the risk of LPS stimulation of macrophages in the intestine, thereby augmenting TNF-a expression and damages of the intestine. Next, we examined the role of TLR-4 in donor cells. Lethally irradiated C57BL/6 hosts received allogeneic HSCT from either C3H/HeN or C3H/HeJ donors. Mice receiving C3H/HeJ donors developed significantly less GVHD as measured by mortality and intestinal histopathology compared with mice receiving C3H/HeN donors. Taken together, these results suggest that TLR-4 in host cells is crucial in the microbial eradication in the intestine and the protection of intestinal GVHD, while responsiveness of donor cells to LPS may be an important risk factor for acute GVHD.