The aim of the present study was to assess the role of Toll-like-receptor 4 (TLR4) in mediating the immune response to allogeneic pancreatic islets.

We first assessed the impact of TLR4 blockade on human Β-cell apoptosis using anti-TLR4 monoclonal antibody (mAb). Human or mouse islets were then co-cultured with allogeneic lymphocytes ± anti-TLR4 mAbs. T-cell proliferation and IFNΓ secretion were assessed. In vivo, we used both a syngeneic (B6-to-B6)marginal mass islet transplant model and an allogeneic (DBA1-to-B6) model. Islets and/or recipients were treated with anti-TLR4 mAb. Islets were transplanted under the kidney capsule of chemically diabetic mice.

TLR4 blockade in vitro inhibited LPS-mediated human Β-cell apoptosis (p=0.014), T-cell proliferation against human (79±2% p<0.0001) and murine (67±16% p=0.004) islets, and IFNΓ-secreting-cell number (human: 62±9% (p=0.0049); mouse: 64±10% (p<0.0001)). In vivo, marginal mass syngeneic islet transplantation resulted in 100% recipients reversing diabetes in a median time of 7 days, as compared to 75% in 20.5 days in controls, indicating better engraftment in the absence of immune phenomena. Anti-TLR4 treatment of both islets and recipients led to indefinite graft survival (>100 days) in 63% of animals (p=0.02 vs controls), while treatment of either islets or recipient mice only, was associated with median graft survivals of 41.5 days and 14 days, respectively (as compared to 12.5 control animals). Donor-specific skin graft rechallenge in non-rejecting animals resulted in rejection of both skin and islets, but without accelerated rejection as compared to naive animals.

Taken together, our data indicate that TLR4 blockade leads to enhanced syngeneic marginal mass islet engraftment and indefinite allogeneic islet graft survival. TLR4 blockade is as critical on donor islets as it is on host immune cells. Mechanisms are likely to involve graft accommodation with concurrent inhibition of donor-specific immune memory.

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