Introduction: Atherosclerosis is an inflammatory disease, where innate and adaptive immune cells play an important role. Mucosal-associated invariant T-cells (MAIT-cells) are innate-like T cells that are involved in various immune-mediated diseases. To date the role of MAIT cells in atherosclerosis is unknown.

Objective: We aim to investigate the role of MAIT cells in atherosclerosis.

Methods: Low-density lipoprotein receptor knockout (Ldlr^-/-) mice susceptible to developing atherosclerosis were crossed with mice overexpressing MAIT cells (CAST Ldlr^-/-) as well as mice deficient in MAIT cells (CAST MR1^-/- Ldlr^-/-), and control littermates (B6 Ldlr^-/-). First, we characterized MAIT cells in atherosclerosis. Therefore, CAST Ldlr^-/- mice were fed either a chow diet (CD) or a high cholesterol diet (HC) for 8 weeks. MAIT cells were isolated from the liver, small intestine, and spleen and processed for flow cytometry. Second, we investigated the influence of MAIT cells on the development of atherosclerosis. Male and female CAST Ldlr^-/-, CAST MR1^-/- Ldlr^-/-, and B6 Ldlr^-/-  mice were fed HC for 8 weeks. Plaque size and composition were analyzed in the aortic root, and the thoracic aorta. Third MAIT cells were characterized in human carotid plaques and blood by flow cytometry.

Results: CAST Ldlr^{-/ -}mice fed HC showed a decrease of MAIT cells in the spleen and an increase in the liver while there was no difference in the small intestine compared to CD. They also showed a higher activation state as assessed by the expression of CD69 and polarization towards a pro-atherogenic phenotype, by expressing the transcription factor T-bet (T-box expressed in T cells). We could not observe any differences in plaque size in male mice in the aortic root whereas there was a significant decrease in plaque size in female CAST MR1^-/- Ldlr^-/- mice. Interestingly, both males and females showed an increase in plaque size in the thoracic aorta of CAST Ldlr^-/- mice compared to CAST MR1^-/- Ldlr and B6 Ldlr^-/-  mice, without significant differences in plasma cholesterol levels between the groups. Regarding plaque composition, there was no difference in the size of necrotic cores and T-cell infiltration while there was a trend (p=0.056) to a decreased monocyte/macrophage infiltration in CAST MR1^-/- Ldlr^-/- mice. In human carotid plaques, MAIT cells also showed a higher activation state in terms of CD69 expression and a high expression of the pro-atherogenic interferon (IFN-γ) compared to circulating MAIT cells.

Conclusion: Here we showed that MAIT cells are more activated and tend to have a more pro-atherogenic phenotype under HC diet. They contribute to a bigger plaque size in male and female mice and are more activated in human carotid plaques than circulating MAIT cells.