Inhibition of RIP1 improves immune reconstitution and reduces GVHD mortality while preserving graft-versus-leukemia effects

Graft-versus-host disease (GVHD) remains the major cause of morbidity and nonrelapse mortality (NRM) after hematopoietic cell transplantation (HCT). Inflammatory cytokines mediate damage to key GVHD targets such as intestinal stem cells (ISCs) and also activate receptor interacting protein kinase 1 (RIP1; RIPK1), a critical regulator of apoptosis and necroptosis. We therefore investigated the role of RIP1 in acute GVHD using samples from HCT patients, modeling GVHD damage in vitro with both human and mouse gastrointestinal (GI) organoids, and blocking RIP1 activation in vivo using several well-characterized mouse HCT models. Increased phospho-RIP1 expression in GI biopsies from patients with acute GVHD correlated with tissue damage and predicted NRM. Both the genetic inactivation of RIP1 and the RIP1 inhibitor GNE684 prevented GVHD-induced apoptosis of ISCs in vivo and in vitro. Daily administration of GNE684 for 14 days reduced inflammatory infiltrates in three GVHD target organs (intestine, liver, and spleen) in mice. Unexpectedly, GNE684 administration also reversed the marked loss of regulatory T cells in the intestines and liver during GVHD and reduced splenic T cell exhaustion, thus improving immune reconstitution. Pharmacological and genetic inhibition of RIP1 improved long-term survival without compromising the graft-versus-leukemia (GVL) effect in lymphocytic and myeloid leukemia mouse models. Thus, RIP1inhibition may represent a nonimmunosuppressive treatment for GVHD.

Graft-versus-host disease: establishing IL-33 as an important costimulatory molecule

Approximately half of patients with hematologic malignancy who are treated with allogeneic hematopoietic stem cell transplantation (alloHCT) experience graft-versus-host disease (GVHD), which has high mortality rates despite immunosuppressive therapy. IL-12 is known to drive donor T cells toward an inflammatory Th1 lineage in GVHD, but other mechanisms also promote pathological Th1 alloimmune responses. In this issue of the JCI, Dwyer et al. report on their use of transgenic mice and alloHCT models of GVHD to demonstrate that IL-33 acts directly on donor T cells to increase Tbet expression independently of IL-12. Notably, IL-33 amplified T cell receptor–signaling pathways and inhibited production of regulatory molecules. These findings firmly establish IL-33 as an important costimulatory molecule for Th1 cells during GVHD and provide a target for reducing GVHD, especially in the gastrointestinal (GI) tract, where damage drives mortality.

Mannan binding lectin promotes murine graft versus host disease by amplifying lipopolysaccharide-initiated inflammation

Conditioning regimens used for hematopoietic stem cell transplantation (HCT) can escalate the severity of acute T cell-mediated graft-versus-host disease (GVHD) by disrupting gastrointestinal integrity and initiating lipopolysaccharide (LPS)-dependent innate immune cell activation. Activation of the complement cascade has been associated with murine GVHD, and previous work has shown that alternative pathway complement activation can amplify T cell immunity. Whether and how mannan-binding lectin (MBL), a component of the complement system that binds mannose as well as oligosaccharide components of LPS and lipoteichoic acid, affects GVHD is unknown. In this study, we tested the hypothesis that MBL modulates murine GVHD and examined the mechanisms by which it does so. We adoptively transferred C3.SW bone marrow (BM) cells ± T cells into irradiated wild type (WT) or MBL-deficient C57Bl/6 (B6) recipients with or without inhibiting MBL-initiated complement activation using C1-esterase inhibitor (C1-INH). We analyzed the clinical severity of disease expression and analyzed intestinal gene and cell infiltration. In vitro studies assessed MBL expression on antigen-presenting cells (APCs) and compared LPS-induced responses of WT and MBL-deficient APCs. MBL-deficient recipients of donor BM ± T cells exhibited significantly less weight loss over the first 2 weeks post-transplantation weeks compared with B6 controls (P < .05), with similar donor engraftment in the 2 groups. In recipients of C3.SW BM + T cells, the clinical expression of GVHD was less severe (P < .05) and overall survival was better (P < .05) in MBL-deficient mice compared with WT mice. On day-7 post-transplantation, analyses showed that the MBL-deficient recipients exhibited less intestinal IL1b, IL17, and IL12 p40 gene expression (P < .05 for each) and fewer infiltrating intestinal CD11c⁺, CD11b⁺, and F4/80⁺ cells and TCRβ⁺, CD4⁺, CD4⁺IL17⁺, and CD8⁺ T cells (P < .05 for each). Ovalbumin or allogeneic cell immunizations induced equivalent T cell responses in MBL-deficient and WT mice, demonstrating that MBL-deficiency does not directly impact T cell immunity in the absence of irradiation conditioning. Administration of C1-INH did not alter the clinical expression of GVHD in preconditioned WT B6 recipients, suggesting that MBL amplifies clinical expression of GVHD via a complement-independent mechanism. WT, but not MBL-deficient, APCs express MBL on their surfaces. LPS-stimulated APCs from MBL-deficient mice produced less proinflammatory cytokines (P < .05) and induced weaker alloreactive T cell responses (P < .05) compared with WT APCs. Together, our data show that MBL modulates murine GVHD, likely by amplifying complement-independent, LPS-initiated gastrointestinal inflammation. The results suggest that devising strategies to block LPS/MBL ligation on APCs has the potential to reduce the clinical expression of GVHD.