Abstract
Mucosa-associated invariant T (MAIT) cells are abundant antimicrobial T cells in humans and recognize antigens derived from the microbial riboflavin biosynthetic pathway presented by the MHC-Ib-related protein (MR1). However, the mechanisms responsible for MAIT cell antimicrobial activity are not fully understood, and the efficacy of these mechanisms against antibiotic resistant bacteria has not been explored. Here, we show that MAIT cells mediate MR1-restricted antimicrobial activity against Escherichia coli clinical strains in a manner dependent on the activity of cytolytic proteins but independent of production of pro-inflammatory cytokines or induction of apoptosis in infected cells. The combined action of the pore-forming antimicrobial protein granulysin and the serine protease granzyme B released in response to T cell receptor (TCR)-mediated recognition of MR1-presented antigen is essential to mediate control against both cell-associated and free-living, extracellular forms of E. coli. Furthermore, MAIT cell-mediated bacterial control extends to multidrug-resistant E. coli primary clinical isolates additionally resistant to carbapenems, a class of last resort antibiotics. Notably, high levels of granulysin and granzyme B in the MAIT cell secretomes directly damage bacterial cells by increasing their permeability, rendering initially resistant E. coli susceptible to the bactericidal activity of carbapenems. These findings define the role of cytolytic effector proteins in MAIT cell-mediated antimicrobial activity and indicate that granulysin and granzyme B synergize to restore carbapenem bactericidal activity and overcome carbapenem resistance in E. coli.
Highlights
Granulysin and granzyme B synergize to restore carbapenem bactericidal activity and overcome carbapenem resistance in E. coli
We developed a method for short-term peripheral blood (PB) Mucosa-associated invariant T (MAIT) cell expansion to provide the cell numbers with high purity required to perform such assays (S1A and S1B Fig)
Similar or greater response was observed following stimulation with the positive control MR1 ligand 5-OP-RU (S2B and S2C Fig). These newly activated MAIT cells still failed to efficiently control bacterial loads within infected cells, compared to the bacterial control exerted by MAIT cells that had been cultured for 15 d (Fig 1I)
Summary
Granulysin and granzyme B synergize to restore carbapenem bactericidal activity and overcome carbapenem resistance in E. coli. Potent antimicrobial activity of human MAIT cells overcomes carbapenem resistance in control of E. coli activation in HeLa cells infected with EC241 in an MR1-dependent manner (Fig 3E), consistent with results using the drug-sensitive strain EC120S (Fig 1). These findings suggest that cytolytic proteins, Gnly and GrzB, may significantly contribute to the MAIT cell secretome antimicrobial activity against extracellular CREC in the presence of imipenem.
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