Rapid NKT cell responses are self-terminating during the course of microbial infection

NKT cells play a protective role in immune responses against infectious pathogens. However, when the NKT cell response to infection is initiated and terminated is unknown. In this study, we demonstrate that NKT cells become activated, proliferate, and exert their effector function before MHC-restricted T cells during infection with Mycobacterium bovis bacillus Calmette-Guérin in mice. After a cell expansion phase, NKT cells underwent cell death, which contracts their numbers back to baseline. Surprisingly, despite ongoing infection, the remaining NKT cells were profoundly unresponsive to TCR stimulation, while MHC-restricted T cells were vigorously proliferating and producing IFN-gamma. Similarly, we show that NKT cells became unresponsive in uninfected mice after receiving a single exposure to a TLR agonist LPS, suggesting that NKT cell unresponsiveness may be a major mechanism of terminating their response in many infectious conditions. This characterization of the NKT cell response in antimicrobial immunity indicates that rapid NKT cell activation contributes to the innate phase of the response to the infectious pathogen, but then, the NKT cell response is shut down by two mechanisms; apoptotic contraction and marked unresponsiveness to TCR stimulation, as a synchronized hand off to MHC-restricted T cells occurs.