A critical role of nonclassical MHC in tumor immune evasion in the amphibian Xenopus model

Abstract

Nonclassical class Ib (class Ib) genes are found in all jawed vertebrates, including the amphibian Xenopus, which possesses at least 20 distinct Xenopus nonclassical class Ib genes (XNCs). As an immune evasion strategy, tumors often down-regulate surface expression of classical MHC class Ia molecules. In contrast, cancers commonly express class Ib molecules, presenting an alternative for tumor immune recognition. We characterized a novel XNC gene, XNC10, functionally similar to CD1d from a class Ia-deficient thymic lymphoid tumor (15/0), which grows aggressively in Xenopus LG15 cloned animals. To investigate the roles of XNC10 in antitumor immunity, we generated stable 15/0-transfectants with silenced XNC10 mRNA and protein expression. Notably, XNC10 silencing resulted in acute tumor rejection by naturally class Ia-deficient syngeneic tadpoles, with greater potency of rejection in tumors with more efficient XNC10 knockdown. In vivo killing assays shows that the rejection of XNC10-deficient tumors is due to a cell-mediated cytotoxic immune response elicited by the tadpole host. Importantly, priming enhances XNC10-deficient tumor rejection. Flow cytometry reveals that XNC10-deficient tumor rejection is associated with an accumulation of XNC10-restricted invariant T cells and conventional CD8 T cells as well as other leukocytes. Similarly, semi-solid tumor grafts in tadpoles also exhibit leukocytes infiltration. These findings suggest that XNC10 allows the 15/0-tumor to escape immune recognition and class Ia-independent cytotoxicity, thus emphasizing the critical roles of class Ib's in tumor immunity.