Phylogenetic conservation of gp96-mediated antigen-specific cellular immunity: New evidence from adoptive cell transfer in Xenopus

In vertebrates from man to frogs, the heat shock protein (hsp) gp96 elicits T-cell responses against antigenic peptides that it chaperones. In Xenopus, immunization with gp96 purified from normal tissues accelerates rejection of MHC identical, minor histocompatibility (H) antigen-disparate skin grafts in vivo and induces MHC-restricted CTL responses in vitro. Also in Xenopus, gp96 derived from MHC class I-negative tumors elicits peptide-specific responses against these tumors in vivo and MHC-unrestricted CD8 killing in vitro. We have developed an adoptive cell transfer protocol to further characterize these gp96-stimulated Xenopus effectors in vivo. Carboxyfluorescein diacetate succinimidyl ester (CFSE)-stained splenocytes from cloned LG-6 donor frogs immunized with gp96 purified from minor H-antigen-disparate LG-15 tissues were transferred into LG-6 recipients bearing a LG-15 minor H antigen (ag)-disparate skin graft. Primed anti-LG-15 but not naive CFSE T cells accumulated and divided in the spleen of allografted recipients to a greater extent than in those of autografted recipients. Similar accumulation and division occurred when CD8 T cells primed by 15/0 tumor-derived gp96 were transferred to an isogeneic recipient bearing the same MHC class I-negative tumor. Furthermore, the transfer of such primed antitumor splenocytes into naive recipients before tumor challenge delayed the appearance of tumors. These data provide new in vivo evidence that in frogs as in mammals, gp96 can prime CD8 T cells against antigens they chaperone. In addition, at least in Xenopus, gp96 can prime CD8(+) T-cell effectors that are not MHC restricted.