Mucosal associated invariant T-cells (MAIT) are a population of CD161⁺⁺ T-cells expressing the semi-invariant T-cell receptor Vα7.2-Jα33/Jα12/Jα20, restricted by the monomorphic MHC-related molecule 1, MR1. The development of MR1-antigen tetramers have shown unequivocally that MAIT cells can be divided into CD8+, CD4+, and CD4-CD8- (double-negative; DN) subsets. However the functional relationship between the different subsets, especially in humans, is poorly understood. 

We have therefore performed a comprehensive study on the differences between the CD8, DN, and CD4-MAIT cells in human tissues and the role of the CD8 coreceptor on MAIT cell function.  Defining MAIT cells as CD161⁺⁺Vα7.2+T-cells, we show that all subsets of MAIT cells share a RORgt^high, T-bet^dim, PLZF^high, and Thpok^dim phenotype but CD4-MAIT cells express significantly less EOMES. In line with this, all three subsets can produce IFNγ, IL-17, IL-2, TNFα, and MIP1B in an MR1-dependent manner, but CD4 MAIT cells were unable to degranulate, and produced significantly less IFNγ, TNFα and MIP1B. In turn, CD4-MAIT cells were uniquely able to produce IL-4 and IL-13, while CD8 and DN-MAIT cells showed very little Th2 cytokine production. A detailed analysis of surface phenotype also showed that CD4 MAIT cells were unique in expressing CCR4, CD62L and CCR7, and were highly enriched in tonsils. Finally, DN-MAIT cells were more susceptible to activation-induced cell death (AICD) compared to CD8 MAIT cells, and blockade of CD8 coreceptor on MAIT cells significantly reduced MR1-dependent activation.

Thus we have identified two key differences between MAIT cell subsets: firstly, CD4-MAIT cells have lymphoid-tissue homing potential, are enriched in tonsils and can secrete IL-4 and IL-13; and secondly, CD8 coreceptor may play a fine-tuning role for MAIT cells which protects them from AICD. Understanding the functional differences between subsets will help us understand the distinct role each subset may play in health and disease.