MAIT cells promote inflammatory monocyte differentiation into dendritic cells during pulmonary intracellular infection

Mucosa-associated invariant T (MAIT) cells are a unique innate T cell subset that is necessary for rapid recruitment of activated CD4+ T cells to the lungs after pulmonary F. tularensis LVS infection. Here, we investigated the mechanisms behind this effect. We provide evidence to show that MAIT cells promote early differentiation of CCR2-dependent monocytes into monocyte-derived DCs (Mo-DCs) in the lungs after F. tularensis LVS pulmonary infection. Adoptive transfer of Mo-DCs to MAIT cell-deficient mice (MR1-/- mice) rescued their defect in the recruitment of activated CD4+ T cells to the lungs. We further demonstrate that MAIT cell-dependent GM-CSF production stimulated monocyte differentiation in vitro, and that in vivo production of GM-CSF was delayed in the lungs of MR1-/- mice. Finally, GM-CSF-deficient mice exhibited a defect in monocyte differentiation into Mo-DCs that was phenotypically similar to MR1-/- mice. Overall, our data demonstrate that MAIT cells promote early pulmonary GM-CSF production, which drives the differentiation of inflammatory monocytes into Mo-DCs. Further, this delayed differentiation of Mo-DCs in MR1-/- mice was responsible for the delayed recruitment of activated CD4+ T cells to the lungs. These findings establish a novel mechanism by which MAIT cells function to promote both innate and adaptive immune responses.