Mucosal‐associated invariant T cells: A cryptic coordinator in HIV‐infected immune reconstitution

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Abstract
Human immunodeficiency virus type 1 (HIV‐1) infection causes considerable morbidity and mortality worldwide. Although antiretroviral therapy (ART) has largely transformed HIV infection from a fatal disease to a chronic condition, approximately 10%–40% of HIV‐infected individuals who receive effective ART and sustain long‐term viral suppression still cannot achieve optimal immune reconstitution. These patients are called immunological nonresponders, a state associated with poor clinical prognosis. Mucosal‐associated invariant T (MAIT) cells are an evolutionarily conserved unconventional T‐cell subset defined by expression of semi‐invariant αβ T‐cell receptor (TCR), which recognizes metabolites derived from the riboflavin biosynthetic pathway presented on major histocompatibility complex‐related protein‐1. MAIT cells, which are considered to act as a bridge between innate and adaptive immunity, produce a wide range of cytokines and cytotoxic molecules upon activation through TCR‐dependent and TCR‐independent mechanisms, which is of major importance in defense against a variety of pathogens. In addition, MAIT cells are involved in autoimmune and immune‐mediated diseases. The number of MAIT cells is dramatically and irreversibly decreased in the early stage of HIV infection and is not fully restored even after long‐term suppressive ART. In light of the important role of MAIT cells in mucosal immunity and because microbial translocation is inversely associated with CD4+ T‐cell counts, we propose that MAIT cells participate in the maintenance of intestinal barrier integrity and microbial homeostasis, thus further affecting immune reconstitution in HIV‐infected individuals.

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