The Microbiota Promotes Skin Allograft Rejection.

Abstract

The kinetics of transplant rejection are determined both by genetic and environmental factors. One potential environmental factor that is gaining wide attention as modulating immune responses is the microbiota, the collection of microbes that inhabit body surfaces; but whether the microbiota influences alloresponses is not known. Moreover, commensal bacteria have been shown to promote both effector and regulatory T cell responses locally and systemically such that whether the microbiota would exacerbate or dampen alloreactivity cannot be anticipated. To address this question, we transplanted minor mismatched skin grafts bearing H-Y antigens into female mice, using donors and recipients that were either both germ-free (GF) or both conventional mice treated with broad spectrum antibiotics prior to transplantation. Whereas GF mice lacked microbial colonization, antibiotic-treated mice showed a marked reduction in bacterial diversity of their intestinal and cutaneous flora, with relative expansion of Lactobacillales, but no decrease in bacterial load. Mean skin allograft survival more than doubled in either GF or antibiotic-treated conventional mice compared to controls. Colonization of GF mice with the microbiota of conventional mice through gavage of fecal material restored faster rejection kinetics, confirming the causal role of the commensal flora in promoting graft rejection. Adoptive transfer of CFSE-labeled CD4+ anti-HY Marilyn T cells into antibiotic- and control-treated mice revealed reduced proliferation of graft-specific T cells in the draining lymph nodes and graft of antibiotic-treated animals. In addition, the proportion of IFN-g producing T cells in the graft was also reduced. Together, these data suggest decreased priming of allogeneic T cells when the diversity of resident bacterial communities is diminished. This effect is not due to a reduction in bacterial load and associated reduction in microbe-associated molecular pattern signaling; rather, the effect is caused by loss of taxon-specific host-microbe interactions. This study illustrates the profound systemic effects of the microbial environment in transplant recipients and points to the microbiota as a potential therapeutic target for enhancing graft acceptance.