Islet‐expressed TLR2 and TLR4 sense injury and mediate early graft failure after transplantation

Abstract

Although islet transplantation is an effective treatment for Type 1 diabetes, primary engraftment failure contributes to suboptimal outcomes. We tested the hypothesis that islet isolation and transplantation activate innate immunity through TLR expressed on islets. Murine islets constitutively express TLR2 and TLR4, and TLR activation with peptidoglycan or LPS upregulates islet production of cytokines and chemokines. Following transplantation into streptozotocin-induced diabetic, syngeneic mice, islets exposed to LPS or peptidoglycan had primary graft failure with intra-and peri-islet mononuclear cell inflammation. The use of knockout mice showed that recipient CD8 1 T cells caused engraftment failure and did so in the absence of islet-derived DC. To mimic physiological islet injury, islets were transplanted with exocrine debris. Transplantation of TLR2/4 À/À islets reduced proinflammatory cytokine production and improved islet survival. Stressed islets released the alarmin high-mobility group box protein 1 (HMGB1) and recombinant HMGB1 (rHMGB1) induced NFkB activation. NFkB activation was prevented in the absence of both TLR2 and TLR4. rHMGB1 pretreatment also prevented primary engraftment through a TLR2/4-dependent pathway. Our results show that islet graft failure can be initiated by TLR2 and TLR4 signaling and suggest that HMGB1 is one likely early mediator. Subsequent downstream signaling results in intra-islet inflammation followed by T-cellmediated graft destruction.Key words: Chemokines . Diabetes . Innate immunity . Islet cell transplantation IntroductionIslet transplantation holds promise as a definitive therapy for Type 1 diabetes, but the long-term results have been disappointing, as progressive loss of graft function is observed in the majority of patients [1]. The islet mass is already marginal shortly after transplantation and thus susceptible to become insufficient when subsequently exposed to negative local influences. Recent estimates indicate that less than 30% of islets stably engraft, a result that explains the requirement for infusing large numbers of islets and for repeat islet infusions to maintain insulin-free euglycemia [2]. Mechanisms underlying early islet loss following à These authors have contributed equally to this work. 2914transplantation remain poorly defined but apoptotic cell islet cell death associated with peri-and intra-islet graft inflammation have been described previously [3,4].TLR are a family of pattern recognition receptors that bind to PAMP or to endogenous ligands released by damaged cells (damage-associated molecular patterns, DAMP). Among the latter group are HSPs, high-mobility group box protein 1 (HMGB1), heparan sulfate, hyaluronan fragments, and fibronectin [5]. Regardless of the source of the specific ligand, TLRtransmitted signals activate innate immunity by inducing chemokine and cytokine release and through upregulating costimulatory molecule expression, among a multitude of other effects [6]. Recent studies revealed the importance of islet-expressed TLR...

Control of In Vivo Collateral Damage Generated by T Cell Immunity

“…Consistent with true collateral damage rather than prevention of engraftment by CD4 T cells, CD4 TCR Tg recipients usually took 2-3 wk to reject the islets even though engraftment/revascularization of islets is normally evident within the first week after transplantation (24). This conclusion is also supported by a recent study showing CD8 T cells but not CD4 T cells have a substantial role in preventing islet engraftment (25). It is intriguing that the bystander injury observed in this study is not compatible with an entirely nonspecific form of tissue injury.…”

Islet inflammation in type 2 diabetes and physiology

“…Indeed, saturated FFAs have been reported to induce chemokine (Cxcl1 and Ccl3) expression within islets in vitro (12,46). Additionally, HMGB1 has been shown to activate the NF-κB pathway in isolated murine islets via TLR2 and TLR4 pathways (65). Furthermore, Tlr2 -/-mice fed a HFD for 20 weeks were protected from HFD-induced β cell dysfunction and insulin resistance (66), indicating a critical role for the TLR2 pathway in islet inflammation.…”

Clinical islet transplantation

“…In addition, varied results have identified some critical donor-derived injury signals that promote early inflammatory islet loss. Several studies have shown that agonists of Toll-like receptors (TLRs), especially TLR2 and TLR4 plus stimulation of the receptor for advanced glycation endproducts (RAGEs) can trigger rapid inflammation following islet transplant and result in islet injury [30][31][32]. Related results show a significant contribution to this innate activation because of the release of the alarmin high-mobility group box 1 (HMGB1) that incites inflammation.…”