Distinct Host-Mycobacterial Pathogen Interactions between Resistant Adult and Tolerant Tadpole Life Stages of Xenopus laevis

Mycobacterium marinum is a promiscuous pathogen infecting many vertebrates, including humans, whose persistent infections are problematic for aquaculture and public health. Among unsettled aspects of host-pathogen interactions, the respective roles of conventional and innate-like T (iT) cells in host defenses against M. marinum remain unclear. In this study, we developed an infection model system in the amphibian Xenopus laevis to study host responses to M. marinum at two distinct life stages, tadpole and adult. Adult frogs possess efficient conventional T cell-mediated immunity, whereas tadpoles predominantly rely on iT cells. We hypothesized that tadpoles are more susceptible and elicit weaker immune responses to M. marinum than adults. However, our results show that, although anti-M. marinum immune responses between tadpoles and adults are different, tadpoles are as resistant to M. marinum inoculation as adult frogs. M. marinum inoculation triggered a robust proinflammatory CD8(+) T cell response in adults, whereas tadpoles elicited only a noninflammatory CD8 negative- and iT cell-mediated response. Furthermore, adult anti-M. marinum responses induced active granuloma formation with abundant T cell infiltration and were associated with significantly reduced M. marinum loads. This is reminiscent of local CD8(+) T cell response in lung granulomas of human tuberculosis patients. In contrast, tadpoles rarely exhibited granulomas and tolerated persistent M. marinum accumulation. Gene expression profiling confirmed poor tadpole CD8(+) T cell response, contrasting with the marked increase in transcript levels of the anti-M. marinum invariant TCR rearrangement (iV alpha 45-J alpha 1.14) and of CD4. These data provide novel insights into the critical roles of iT cells in vertebrate antimycobacterial immune response and tolerance to pathogens.