The amphibian Xenopus laevis is to date the only species outside of mammals where a MHC class I-like (MHC-like) restricted innate-like (i) T cell subset (iVα6 T cells) reminiscent of CD1d-restricted iNKT cells has been identified and functionally characterized. This provides an attractive in vivo model to study the biological analogies and differences between mammalian iT cells and the evolutionarily antecedent Xenopus iT cell defense system. Here, we report the identification of a unique iT cell subset (Vα45-Jα1.14) requiring a distinct MHC-like molecule (mhc1b4.L or XNC4) for its development and function. We used two complementary reverse genetic approaches: RNA interference by transgenesis to impair expression of either XNC4 or the Vα45-Jα1.14 rearrangement, and CRISPR/Cas9-mediated disruption of the Jα1.14 gene segment. Both XNC4 deficiency that ablates iVα45T cell development and the direct disruption of the iVα45-Jα1.14 T cell receptor dramatically impairs tadpole resistance to Mycobacterium marinum (Mm) infection. The higher mortality of Mm-infected tadpoles deficient for iVα45T cells correlates with dysregulated expression responses of several immune genes. In contrast, iVα45-Jα1.14-deficient tadpoles remain fully competent against infection by the ranavirus FV3, which indicates a specialization of this unique iT cell subset toward mycobacterial rather than viral pathogens that involve iVα6 T cells. These data suggest that amphibians, which are evolutionarily separated from mammals by more than 350 My, have independently diversified a prominent and convergent immune surveillance system based on MHC-like interacting innate-like T cells.
Keywords: MHC evolution; innate-like T cells; ranavirus; reverse genetics; unconventional T cells.