Activation and proliferation of PD-1 ⁺ kidney double-negative T Cells is dependent on nonclassical MHC proteins and IL-2

Background CD4 ^- CD8 ^- double-negative (DN) αβ T cells with innate-like properties represent a significant component of T cells in human andmouse kidneys. They spontaneously proliferate in the steady state and protect against ischemic AKI. However, the mechanisms regulating DN T cell homeostasis and responses to external danger signals from "sterile" inflammation remain poorly understood. MethodsWe used knockoutmice, functional assays, and an established ischemic AKI model to investigate the role of various MHC class I and II molecules in regulating kidney DN T cells. We also studied human nephrectomy samples. Results Deficiency of β2m-dependent MHC class I (but not MHC class II) molecules led to significant reduction in frequency or absolute numbers of kidney DN T cells due to impaired activation, proliferation, increased apoptosis, and loss of anNK1.1 ⁺ subset ofDNT cells. The remaining DNT cells in β2mknockout mice mainly comprised a programmed cell death protein-1 receptor (PD-1 ⁺ ) subset that depends on IL-2 provided by conventional T cells for optimal homeostasis. However, this PD-1 ⁺ subset remained highly responsive to changes inmilieu, demonstrated by responses to infused lymphocytes. It was also themajor responder to ischemic AKI; theNK1.1 ⁺ subset and CD8 ⁺ T cells hadminimal responses.We found bothDN T cell subsets in normal and cancerous human kidneys, indicating possible clinical relevance. Conclusions DN T cells, a unique population of kidney T cells, depend on nonclassical β2m molecules for homeostasis and use MHC-independent mechanisms to respond to external stimuli. These results have important implications for understanding the role these cells play during AKI and other immune cell-mediated kidney diseases.