Involvement of CD91 and scavenger receptors in Hsp70‐facilitated activation of human antigen‐specific CD4+ memory T cells

Hsp70 plays several roles in the adaptive immune response. Based on the ability to interact with diverse peptides, extracellular Hsp70:peptide complexes exert profound effects both in autoimmunity and in tumor rejection by evoking potent T cell responses to the chaperoned peptide. The interaction with receptors on APC represents the basis for the immunological functions of Hsp70 and a critical point where the immune response can be regulated. Various surface proteins (e.g. CD91, scavenger receptors (SR)) have been implicated in binding of Hsp70. In this study, antigenic peptides from tetanus toxin and influenza hemagglutinin complexed to human stress‐inducible Hsp70 were found to enhance the proliferation and cytokine production of human antigen‐specific CD4⁺ T cells. This was demonstrated in proliferation experiments using human monocytes as APC. Proliferated antigen‐specific cells were detected combining HLA‐DRB1^*0401 or HLA‐DRB1^*1101 tetramer and CFSE staining. Treating monocytes with CD91 siRNA diminished these effects. Additional blocking of SR by the SR ligand fucoidan completely abolished enhanced proliferation and production of Th1 and Th2 cytokines. Taken together, our data indicate that in the human system, CD91 and members of the SR family efficiently direct Hsp70:peptide complexes into the MHC class II presentation pathway and thus enhance antigen‐specific CD4⁺ T cell responses.