Xenopus, a unique comparative model to explore the role of certain heat shock proteins and non-classical MHC class Ib gene products in immune surveillance

Abstract

The heat shock proteins (HSPs) gp96 and hsp70 can elicit potent anti-tumor responses and as such have significant clinical potential. Besides cytotoxic CD8 T cell (CTLs) effectors, evidence suggests that natural killer (NK) cells and other less well-characterized cell types also play a critical role in HSP-mediated anti-tumor responses. Owing to their high degree of phylogenetic conservation, we have proposed that HSPs are ancestral agents of immune surveillance; and postulated that their immunological properties, if important, should have been conserved during evolution. We are investigating this issue using a unique non-mammalian comparative tumor-immunity model in the frog Xenopus, which allows us to focus on the relationship between HSPs, classical MHC class Ia, and non-classical MHC class Ib molecules. In addition to a transplantable lymphoid tumor in genetically defined cloned Xenopus, we are generating transgenic frogs with inducible or knocked-down (RNAi) gene expression.

Fig. 1

Generation of transgenic Xenopus with the “Sleeping Beauty” transposase. Dejellied eggs from outbred (OB) or LG-6 isogenetic cloned animals were co-injected with 10⁻² ng of transposase mRNA and 15 pg of vector containing a GFP reporter transgene under the control of the elongation factor α1 promoter in 10nL volume. Control eggs (C) were dejellied but not injected. Live larvae were screened for EGFP expression with a fluorescence stereomicroscope at pre-metamorphic stage (st. 56, 1-month old). Arrows: thymus

Fig. 2

Model of the involvement of HSPs and class Ib molecules in immune surveillance and anti-tumor responses. A During the priming phase (adapted from [2]), HSP-peptide complexes released in the extracellular compartment from infected or stressed cells (e.g., apoptosis, cell lysis) are internalized by APCs through receptor-mediated endocytosis (e.g., CD91). (1) Antigenic peptides channeled into the class Ia presentation pathway specifically activate CD8 T cells. (2) HSPs internalized by the same receptors or interacting with other receptors (e.g., Toll-like receptors) stimulate production of inflammatory cytokines and chemokines and up-regulate co-stimulatory molecules. (3) HSPs are proposed to also stimulate class Ib-mediated response by a yet unknown mechanism that is likely to be Ag-specific, and involve a CD8 T cell subset. B During the effector phase, tumor targets that have down-regulated surface class Ia molecules to escape recognition and lysis by conventional CTL, can still be killed by NK cells unless they express surface class Ib molecule that can bind inhibitory NK receptors. We propose that HSPs generate CCU-CTL effectors that can recognize and kill tumors expressing these class Ib molecules