Mucosa-associated invariant T (MAIT) cells are a novel subset of innate-like T cells. They are the most abundant T cell in human liver tissue, representing as much as 50% of all hepatic T cells, and found mainly around the portal tract. Recently MAIT cells have been dubbed “new guardians of the liver”, with potential roles in protecting against infection, cancer and autoimmunity. However, these liver-abundant cells have not been described in the setting of liver transplantation. It is unknown whether these cells have the potential to induce graft acceptance or rejection in three types of major liver grafts from DBD, DCD and living related donors (LRD).

Using flow cytometry we defined MAIT cells by assessing the expression of CD3, CD4, CD8, CD161 and the Vα7.2 invariant TCR chain on intrahepatic immune cells isolated from perfusates of 22 DBD, 16 DCD and 6 LRD livers. Peak levels of peripheral blood aspartate aminotransferase (AST), a surrogate marker for organ rejection, and histological signed of acute rejection post transplant were recorded.

Following transplantation, livers displaying early signs of acute rejection had significantly higher proportions of MAIT cells than those without any signs of rejection (P = 0.026). Furthermore, peak AST levels during the 7-day period post transplantation, correlated positively with CD4^posCD8^pos (r = 0.443, P = 0.01) MAIT cells and negatively with CD4^negCD8^neg MAIT cells (r = - 0.435, P = 0.01). Furthermore, CD4^negCD8^neg MAIT cells were significantly reduced in DCD compared to DBD and LRD (P < 0.05), and CD4^negCD8^neg MAIT cells were enriched in DCD grafts.

We have found a link between MAIT cells and allograft function, where high proportions of MAIT cells, in particular CD4^posCD8^pos MAIT cells, is associated with a higher risk of organ rejection. However, such a risk is reduced with higher proportions of CD4^negCD8^neg MAIT cells.