Alloreactive T cells deficient of the short-chain fatty acid receptor GPR109A induce less graft-versus-host disease.

The intestinal microbiota is essential for the fermentation of dietary fiber into short-chain fatty acids (SCFA) such as butyrate, acetate, and propionate. SCFAs can bind to the G-protein-coupled receptors GPR43 and GPR109A (HCAR2), with varying affinities to promote cellular effects in metabolism or changes in immune function. We explored the role of GPR109A as the main receptor for butyrate in mouse models of allogeneic hematopoietic cell transplantation (allo-HCT) and graft-versus-host disease (GVHD). Deletion of GPR109A in allo-HCT recipients did not affect GVHD, but transplantation of T cells from GPR109A knockout (KO) (Gpr109a-/-) mice into allo-HCT recipient mice significantly reduced GVHD morbidity and mortality compared with recipients of wild-type (WT) T cells. Recipients of Gpr109a-/- T cells exhibited less GVHD-associated target organ pathology and decreased proliferation and homing of alloreactive T cells to target tissues. Although Gpr109a-/- T cells did not exhibit immune deficits at a steady state, following allo-activation, Gpr109a-/- T cells underwent increased apoptosis and were impaired mitochondrial oxidative phosphorylation, which was reversible through antioxidant treatment with N-acetylcysteine (NAC). In conclusion, we found that GPR109A expression by allo-activated T cells is essential for metabolic homeostasis and expansion, which are necessary features to induce GVHD after allo-HCT.

Production of BMP4 by endothelial cells is crucial for endogenous thymic regeneration.

The thymus is not only extremely sensitive to damage but also has a remarkable ability to repair itself. However, the mechanisms underlying this endogenous regeneration remain poorly understood, and this capacity diminishes considerably with age. We show that thymic endothelial cells (ECs) comprise a critical pathway of regeneration via their production of bone morphogenetic protein 4 (BMP4) ECs increased their production of BMP4 after thymic damage, and abrogating BMP4 signaling or production by either pharmacologic or genetic inhibition impaired thymic repair. EC-derived BMP4 acted on thymic epithelial cells (TECs) to increase their expression of Foxn1, a key transcription factor involved in TEC development, maintenance, and regeneration, and its downstream targets such as Dll4, a key mediator of thymocyte development and regeneration. These studies demonstrate the importance of the BMP4 pathway in endogenous tissue regeneration and offer a potential clinical approach to enhance T cell immunity.

Autophagy protein ATG16L1 prevents necroptosis in the intestinal epithelium.

A variant of the autophagy gene ATG16L1 is associated with Crohn's disease, an inflammatory bowel disease (IBD), and poor survival in allogeneic hematopoietic stem cell transplant recipients. We demonstrate that ATG16L1 in the intestinal epithelium is essential for preventing loss of Paneth cells and exaggerated cell death in animal models of virally triggered IBD and allogeneic hematopoietic stem cell transplantation. Intestinal organoids lacking ATG16L1 reproduced this loss in Paneth cells and displayed TNFα-mediated necroptosis, a form of programmed necrosis. This cytoprotective function of ATG16L1 was associated with the role of autophagy in promoting mitochondrial homeostasis. Finally, therapeutic blockade of necroptosis through TNFα or RIPK1 inhibition ameliorated disease in the virally triggered IBD model. These findings indicate that, in contrast to tumor cells in which autophagy promotes caspase-independent cell death, ATG16L1 maintains the intestinal barrier by inhibiting necroptosis in the epithelium.

Increased GVHD-related mortality with broad-spectrum antibiotic use after allogeneic hematopoietic stem cell transplantation in human patients and mice.

Intestinal bacteria may modulate the risk of infection and graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Allo-HSCT recipients often develop neutropenic fever, which is treated with antibiotics that may target anaerobic bacteria in the gut. We retrospectively examined 857 allo-HSCT recipients and found that treatment of neutropenic fever with imipenem-cilastatin and piperacillin-tazobactam antibiotics was associated with increased GVHD-related mortality at 5 years (21.5% for imipenem-cilastatin-treated patients versus 13.1% for untreated patients, P = 0.025; 19.8% for piperacillin-tazobactam-treated patients versus 11.9% for untreated patients, P = 0.007). However, two other antibiotics also used to treat neutropenic fever, aztreonam and cefepime, were not associated with GVHD-related mortality (P = 0.78 and P = 0.98, respectively). Analysis of stool specimens from allo-HSCT recipients showed that piperacillin-tazobactam administration was associated with perturbation of gut microbial composition. Studies in mice demonstrated aggravated GVHD mortality with imipenem-cilastatin or piperacillin-tazobactam compared to aztreonam (P < 0.01 and P < 0.05, respectively). We found pathological evidence for increased GVHD in the colon of imipenem-cilastatin-treated mice (P < 0.05), but no difference in the concentration of short-chain fatty acids or numbers of regulatory T cells. Notably, imipenem-cilastatin treatment of mice with GVHD led to loss of the protective mucus lining of the colon (P < 0.01) and the compromising of intestinal barrier function (P < 0.05). Sequencing of mouse stool specimens showed an increase in Akkermansia muciniphila (P < 0.001), a commensal bacterium with mucus-degrading capabilities, raising the possibility that mucus degradation may contribute to murine GVHD. We demonstrate an underappreciated risk for the treatment of allo-HSCT recipients with antibiotics that may exacerbate GVHD in the colon.