TH1 Immune Responses to Fully MHC Mismatched Allografts are Diminished in the Absence of MyD88, a Toll‐Like Receptor Signal Adaptor Protein

Toll‐like receptors (TLRs) are innate immune receptors that are critical for recognizing conserved microbial motifs by inducing TH1 immunity. The majority of TLRs utilize the adaptor protein MyD88 for signal transduction, although other adaptors have been recently described. As the role of innate immunity in transplantation is unclear, we examined the importance of the MyD88 pathway in acute rejection of fully MHC‐mismatched murine allografts and specifically investigated whether MyD88 signaling is important for DC (dendritic cell) function and TH1 alloimmune responses. Our results demonstrate that acute rejection of both fully allogeneic skin and cardiac allografts occurs in the absence of MyD88. However, priming of naïve recipient T cells by allogeneic DCs and TH1 immune responses were diminished in the absence of MyD88, although TH2 immunity remained intact. Thus, these results demonstrate that MyD88 signaling is important for DC function and TH1 responses during fully MHC‐mismatched solid‐organ transplantation, although graft rejection occurs independently of MyD88.