Jayanthi Ganesan

The success of allogeneic hematopoietic cell transplantation is limited by acute graft-versus-host disease (GvHD), a severe complication accompanied by high mortality rates. Yet, the molecular mechanisms initiating this disease remain poorly defined. In this study, we show that, after conditioning therapy, intestinal commensal bacteria and the damage-associated molecular pattern uric acid contribute to Nlrp3 inflammasome-mediated IL-1β production and that gastrointestinal decontamination and uric acid depletion reduced GvHD severity. Early blockade of IL-1β or genetic deficiency of the IL-1 receptor in dendritic cells (DCs) and T cells improved survival. The Nlrp3 inflammasome components Nlrp3 and Asc, which are required for pro-IL-1β cleavage, were critical for the full manifestation of GvHD. In transplanted mice, IL-1β originated from multiple intestinal cell compartments and exerted its effects on DCs and T cells, the latter being preferentially skewed toward Th17. Compatible with these mouse data, increased levels of active caspase-1 and IL-1β were found in circulating leukocytes and intestinal GvHD lesions of patients. Thus, the identification of a crucial role for the Nlrp3 inflammasome sheds new light on the pathogenesis of GvHD and opens a potential new avenue for the targeted therapy of this severe complication.

Sensitization to contact allergens requires activation of the innate immune system by endogenous danger signals. However, the mechanisms through which contact allergens activate innate signaling pathways are incompletely understood. In this study, we demonstrate that mice lacking the adenosine triphosphate (ATP) receptor P2X(7) are resistant to contact hypersensitivity (CHS). P2X(7)-deficient dendritic cells fail to induce sensitization to contact allergens and do not release IL-1β in response to lipopolysaccharide (LPS) and ATP. These defects are restored by pretreatment with LPS and alum in an NLRP3- and ASC-dependent manner. Whereas pretreatment of wild-type mice with P2X(7) antagonists, the ATP-degrading enzyme apyrase or IL-1 receptor antagonist, prevents CHS, IL-1β injection restores CHS in P2X(7)-deficient mice. Thus, P2X(7) is a crucial receptor for extracellular ATP released in skin in response to contact allergens. The lack of P2X(7) triggering prevents IL-1β release, which is an essential step in the sensitization process. Interference with P2X(7) signaling may be a promising strategy for the prevention of allergic contact dermatitis.

Extracellular ATP and adenosine have immunoregulatory roles during inflammation. Elevated extracellular ATP is known to exacerbate GVHD, and the pharmacologic activation of the adenosine A2A receptor is protective. However, the role of endogenous adenosine is unknown. We used gene-targeted mice and a pharmacologic inhibitor to test the role of adenosine generated by CD73/ecto-5'-nucleotidase in GVHD. In allogeneic transplants, both donor and recipient CD73 were protective, with recipient CD73 playing the dominant role. CD73 deficiency led to enhanced T-cell expansion and IFN-γ and IL-6 production, and the migratory capacity of Cd73-/- T cells in vitro was increased. However, the number of regulatory T cells and expression of costimulatory molecules on antigen-presenting cells were unchanged. A2A receptor deficiency led to increased numbers of allogeneic T cells, suggesting that signaling through the A2A receptor via CD73-generated adenosine is a significant part of the mechanism by which CD73 limits the severity of GVHD. Pharmacologic blockade of CD73 also enhanced graft-versus-tumor activity. These data have clinical implications, as both the severity of GVHD and the strength of an alloimmune antitumor response could be manipulated by enhancing or blocking CD73 activity or adenosine receptor signaling depending on the clinical indication.

Abstract Abstract 1336 Poster Board I-358 Conditioning prior allogeneic hematopoietic cell transplantation causes local tissue injury. Cellular molecules that are released upon cell stress, injury or cell death can act as potent danger associated molecular patterns (DAMP). Nucleotides like ATP are ideal candidates to serve as such indicators of cell damage, since they are present in the extracellular environment in the nanomolar range under physiological conditions but are released upon unphysiological cell death which causes activation of purinergic receptors. The role of purinergic signaling in alloantigen driven immune responses such as graft-versus-host disease (GvHD) is unclear. We found that ATP levels in the peritoneal fluid increased when GvHD was present in human and mice. Detection of extracellular ATP with a luciferase based in vivo detection system demonstrated early ATP release from the gastrointestinal tract. Blockade of multiple purinergic receptors on T cells or ATP neutralization during GvHD development reduced disease severity and proinflammatory cytokine production significantly. Gene expression analysis of different P2R in GvHD target organs identified the P2X7(R)receptor to be increasingly expressed. P2X7R−/− recipients developed less severe GvHD as compared to wildtype mice. This was paralleled by the induction of STAT5 signaling with increased numbers of Foxp3 positive regulatory T cells while STAT1 phosphorylation and IFN-γ production was reduced. Despite P2X7R blocking, T cells were still able to reconstitute the lymphoid compartment with a polyclonal intact TCR repertoire analyzed by spectratyping DNA sequence analyses of the CDR3 region and to reject established B-cell lymphoma. Therefore, DAMPs such as endogenous nucleotides may not be required for anti-tumor activity but dominantly trigger acute GvHD. Blockade or genetic deficiency of P2X7R in BMT recipients conferred GvHD protection without loss of GvL effects, which has important clinical implications given the availability of P2X7R inhibitors. Disclosures No relevant conflicts of interest to declare.