Mucosal-associated invariant T (MAIT) cells are innate-like T cells comprising up to 10% of the peripheral blood T cells in humans. During ontogeny, MAIT cells can first be detected in the cord blood in low amounts, but rise steadily after birth. In this population-based study, we show that their counts continue to increase, reaching maximal levels (4.5% of CD3 + cells, 65 cells/ll) in the third and fourth decenniums. At this age, the amounts of MAIT cells exhibit the highest interindividual variability. The values then dramatically decline; subjects 80 years old and older have on average 10 times less MAIT cells, both absolutely and as a percentage among CD3 + T cells, than subjects in fertile age. The senescence of MAIT cells is associated with decreased CD8/double negative (DN) ratio. Finally, we observed significantly higher amounts of MAIT cells in women of reproductive age than in men of the same age. Our data suggest that further studies aimed at elucidating a role of MAIT cells in human pathologies must recruit age-and gender-matched controls.

Immune reconstitution after high-dose chemotherapy and stem cell transplantation plays a key role in restoring immunocompetence including defense against infection, immune regulation, and onco-immune surveillance. In this work, we examined the recovery of mucosal-associated invariant T (MAIT) cells, recently discovered innate-like T cells, after various types of myeloablative chemotherapy and autologous peripheral blood stem cell transplantation in 29 patients. We show that MAIT cells are relatively resistant to myeloablative conditioning. The median amount of MAIT cells rises to 43 % around day +30 and is sustained through further measurements on days +60 and +100. Moreover, MAIT cell recovery reaches 100 % of pre-treatment values in 33 % of patients already by day +60. The only factor affecting recovery of MAIT cells is age, younger age being associated with earlier MAIT cell recovery. The pre-treatment quantity of MAIT cells carries a prognostic impact on the early post-transplantation course. Patients with high levels of MAIT cells pre-treatment have significantly lower peak CRP levels (79.45 vs. 150 mg/L) post-treatment, reflecting a clinical trend of less severe infectious complications (less febrile days and less days on intravenous antibiotics). Altogether these data suggest that a high proportion of MAIT cells survive myeloablative chemotherapy and maintain their capacity to fight against infections probably on mucosal surfaces.

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