GVHD is associated with significant shifts in the composition of the intestinal microbiota in human and mouse models; manipulating the microbiota can alter the severity of GVHD in mice.

Key Points PD-L1 and PD-L2 expression were upregulated during GVHD, whereas PD-1/PD-L1 was more critical than PD-1/PD-L2 in downregulating GVHD. Our data provide new insight into the differential roles of host PD-L1 and PD-L2 and associated mechanisms in controlling GVHD.

R-spondin1 stimulates the proliferation of intestinal stem cells through the Wnt signaling pathway and protects against graft-versus-host disease.

Dendritic cells (DCs) can be classified into 2 distinct subsets: conventional DCs (cDCs) and plasmacytoid DCs (pDCs). cDCs can prime antigen-specific T-cell immunity, whereas in vivo function of pDCs as antigen-presenting cells remains controversial. We evaluated the contribution of pDCs to allogeneic T-cell responses in vivo in mouse models of graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation by an add-back study of MHCexpressing pDCs into major histocompatibility complex-deficient mice that were resistant to GVHD. Alloantigen expression on pDCs alone was sufficient to prime alloreactive T cells and cause GVHD. An inflammatory environment created by host irradiation has the decisive role in maturing pDCs for T-cell priming but this process does not require Toll-like receptor signaling. Thus, functional outcomes of pDC-T-cell interactions depend on the immunologic context of encounter. To our knowledge, these results are the first to directly demonstrate an in vivo pathogenic role of pDCs as antigenpresenting cells in an antigen-specific T cell-mediated disease in the absence of other DC subsets and to provide important insight into developing strategies for tolerance induction in transplantation. IntroductionThe interaction of naive T cells and dendritic cells (DCs) is essential for initiating primary immune responses. DCs can be divided into 2 distinct subsets: conventional DCs (cDCs) and plasmacytoid DCs (pDCs) according to their immunophenotype and functional properties. 1-3 pDCs represent a CD11c int B220 ϩ DC subset that differs from the CD11c high B220 Ϫ major histocompatibility complex (MHC) class II high cDCs, commonly viewed as the classic stimulators of naive T cells. One distinctive feature of pDCs is their capacity to rapidly produce high levels of type I interferon (IFN) in response to viral and bacterial stimuli, highlighting the importance of pDCs in innate immune responses. 2-8 pDCs express low levels of surface MHC and classical costimulatory molecules; therefore, they are poor T-cell stimulators. [5][6][7][8][9][10][11] In contrast, pDCs matured with CD40 ligands or Toll-like receptor (TLR) ligands are potent antigen-presenting cells (APCs), capable of stimulating naive T-cell proliferation and differentiation to helper, killer, memory, and regulatory T cells in vitro. 7,12,13 In vivo, injection of pDCs activated by synthetic oligodeoxynucleotides containing unmethylated cytosine-guanine motifs (CpG), but not immature pDCs, is capable of eliciting antigen-specific CD8 ϩ T-cell responses. 10,14 On the other hand, OVA-pulsed pDCs protected mice against OVA-induced asthma development. 15 pDCs in the tumordraining lymph nodes express indole 2, 3-dioxygenase, and suppress antitumor T-cell responses. 16 In patients with ovarian cancer, large numbers of pDCs, which induced interleukin 10 (IL10)-producing regulatory T cells, were found in ascites. 17 pDCs mediate tolerance and prolong survival of cardiac allografts. 11,18,19 Several recent clinical observations...