Abstract
Mucosal Associated Invariant T (MAIT) cells are innate-like T cells that play an important role in health and disease. MAIT cells are associated with rapid and strong response to infection, and primarily target bacterially infected cells. MAIT cells are characterised by the expression of the semi-invariant TCR (Vα7.2-Jα12/20/33) and are restricted to antigen presentation by the major histocompatibility complex class I-related gene protein MR1. MR1 is expressed on the surface of antigen presenting cells (APCs) and presents 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU), a pyrimidine derivative formed by the reaction of 5-amino-6- D-ribitylaminouracil (5-A-RU), an intermediate of the bacterial riboflavin synesis pathway, and methylglyoxal, a by-product of glycolysis. Activated MAIT cells rapidly produce multiple proinflammatory cytokines and cytotoxic molecules.
During bacterial infection, MAIT cell activation can be influenced by various innate signalling pathways in APCs, Toll-Like Receptors (TLRs) being the most studied so far. However, the effect of other pattern recognition receptor (PRR) signalling in APC and its impact on MAIT cell activation is yet to be explored. Given that NOD-Like receptor (NLR) signalling and TLR signalling share many similarities and have been previously observed to synergise, NLR signalling might also play an important role in the regulation of MAIT cell response. This study aims to understand the effect of the NLRs, NOD1 and NOD2 activation in APCs, alone and in combination with TLRs, on early MAIT cell activation by 5-OP-RU.
Human PBMCs (isolated from healthy blood donors and NZ blood) were treated 5-A-RU or 5-OP-RU to assess MAIT cell activation; expression of CD69 and CD107a and intracellular cytokines (IFNγ and TNFα) and cytotoxic molecules (perforin and grzB) were analysed by flow cytometry. NOD1 signalling was stimulated by agonist C12-iE-DAP, and NOD2 signalling was stimulated by agonists L18 MDP and N-Glycolyl MDP alone or in combination with NOD1 agonists. I found that NOD1 and NOD2 enhanced MAIT cell activation in response to TCR stimulation but did not have an additive effect. Co-stimulation with NOD1 or NOD2 together with human TLR (1-9) agonists alongside TCR stimulation was also compared to NOD1or NOD2 and TCR stimulation to assess the additive effect between the innate signalling pathways.
Interestingly, NLR and TLR pathways did have an additive effect to enhance MAIT cell activation. NOD1 had an additive effect with TLR3 and TLR4, and NOD2 had an additive effect with TLR6/2 and TLR8, enhancing MAIT cell activation beyond that of TLR or NOD1/2 agonism alone. Type I Interferon (T1-IFN) neutralisation revealed that the enhancement of MAIT cell activation by 5-OP-RU following NOD1/2 and TLR stimulation suggested T1-IFNs are the key co-stimulatory molecule for MAIT cell activation.
This study found that stimulation of NOD1 and NOD2 in APCs, either alone or through an additive effect with TLR signalling, enhances early MAIT cell activation in response to TCR stimulation and that this enhancement is mediated via production of T1-IFNs in the PBMC system.