Cell-Permeable Peptide Blocks TLR4 Signaling and Improves Islet Allograft Survival

Toll-like receptor 4 (TLR4) activation in pancreatic β cells activates aberrant islet graft cellular pathways and contributes to immune rejection in allogeneic islet transplantation. As an approach to overcoming this problem, we determined the capacity of a 33-amino acid peptide consisting of a protein transduction domain (PTD) from the Hph-1 virus and a fragment of the intracellular domain of TLR4 from the C3H mice (PTD-dnTLR4) to block TLR4 signaling and improve allogeneic islet survival in vitro and after transplantation. The efficacy of PTD-dnTLR4 in blocking TLR4 signaling was assessed in the Raw264.7 macrophage line, in the islets, and the βTC3 cell line. In Raw264.7 cells, preculture with the peptide reduced LPS-induced NF-κB activation and production of proinflammatory cytokines (IL-1β, TNF-α, iNOS, and IL-6). In islets and β cells, preincubation with PTD-dnTLR4 suppressed LPS-induced TNF-α expression via inhibition of NF-κB activation and protected them from stress-induced cell death. In vivo, preincubation of BALB/c (H-2d) islets with PTD-dnTLR4 resulted in significantly longer survival than control islets in a streptozotocin-induced diabetes model (two of seven grafts survived long term >100 days). PTD-dnTLR4-treated grafts exhibited reduced expression of TNF-α and iNOS and reduced macrophage infiltration posttransplant. The data indicate that PTD-dnTLR4 blocked TLR4 signaling in both macrophages and β cells, and prolonged allograft survival at least in part by suppressing inflammation and macrophage infiltration. This strategy for blocking TLR4 activity has potential utilization in the treatment of diseases where excessive TLR4 activation contributes to the pathologic cellular pathways such as islet transplantation.