Psoriasis and the MAITing game: a role for IL-17A+ invariant TCR CD8+ T cells in psoriasis?

Abstract

Recent findings have indicated that the majority of IL-17A+ CD8+ T cells in the blood belong to a subset of innate T cells named mucosa-associated invariant T cells (MAITs). In this issue, Teunissen and colleagues (2014) demonstrate that, although MAIT cells are found in psoriatic skin, they are not increased in abundance and that the majority of IL-17A+CD8+ T cells in plaques of psoriasis are devoid of MAIT cell characteristics.

Figure 1. Mucosa-associated invariant T-cells (MAIT) join the ranks of the unconventional T-cells

Characterized by their limited TCR rearrangements, unconventional T-cells respond more quickly than their conventional counterparts, stationed in peripheral tissues poised to act. These cells expand the repertoire of antigens recognized by T-cells; with the invariant natural killer (iNK) T-cells responding to lipid antigens presented by CD1d, and several clones of γδ T-cells recognizing a diverse array of structures including phospho-antigens and sphingolipids apparently independent of MHC-antigen presentation. Like conventional αβ T-cells and, MAIT cells recognize antigens presented in the context of an MHC-like molecule, MR1, on the surface of antigen presenting cells (green) or epithelia (brown). Thus far, antigens derived from the structures of vitamins B2 and B9 containing a ribityl carbohydrate group have been identified to activate MAIT cells. MAIT cells express many of the markers associated with Th17 cells (RORC, CD161, CCR6) and account for the majority of IL17-producing CD8+ T-cells in the blood, but only a fraction of epidermal IL-17+CD8+ cells.