The Pathogenic Function of Bystander-activated Th17-like Memory Cells in Autoimmune Encephalomyelitis

Abstract

Multiple sclerosis is a human autoimmune disease caused by the induction of inflammation in the central nervous system (CNS) by myelin-specific Th1 and Th17 cells. Recently, however, the attention has expanded to the role of innate-like lymphocytes including gamma delta (γδ) T cells, invariant natural killer T (iNKT) cells and innate lymphoid cells (ILCs) in this disease. Unlike myelin-specific Th1 and Th17 cells, innate-like lymphocytes can produce innate IL-17 in response to pro-inflammatory cytokines. Interestingly, correlating with innate-like lymphocytes, CD4+ effector T lymphocytes have been reported to have an innate immunological function by responding to cytokine stimulation. However, the role of these antigen-independent T cells in autoimmune encephalomyelitis is largely unknown. Despite the importance of antigen-specific T cells, here we show that antigen non-related, bystander memory-like CD4+ T cells are also significant contributors to autoimmune pathogenesis.

Firstly, we demonstrated memory-like, but not naïve, CD4+ T cells can produce TCR-independent IL-17A and IFN- in response to IL-1β and IL-23. Transcriptome analysis demonstrated that IL-1β and IL-23-primed T cells express pathogenic Th17 signature genes such as RORγt, CCR6, and GM-CSF. Importantly, when co-transferred with myelin-specific, 2D2 TCR-transgenic naïve T cells, unrelated OT-II TCR-transgenic memory-like Th17 cells infiltrate the spinal cord and produce IL-17A, IFN-γ and GM-CSF, thereby increasing, in an IL-1 receptor-dependent manner, the susceptibility of the recipients to experimental autoimmune encephalomyelitis. In human, IL-1R1 high memory CD4+ T cells are the major producers of IL-17A and IFN-γ in response to IL-1β and IL-23.

Secondly, we further distinguished the heterogeneous population of memory CD4+ T cells responding to IL-1β and IL-23. We performed single-cell RNA sequencing (scRNA-seq) of splenic memory-like CD4+ T cells at steady state and after in vitro stimulation with pro-inflammatory cytokines IL-1β and IL-23. Memory-like CD4+ T cells were transcriptionally heterogeneous, with subpopulations distinguished by transcription factors, effector cytokines, and chemokine receptors. IL-1β and IL-23 induced distinct Th17 populations in memory-like CD4+ T cells. We further identified that CCR6high memory-like CD4+ T cells, which exhibit a Th17 phenotype, were the major responder of IL-1β and IL-23 producing IL-17A, IFN-γ, and GM-CSF. IL-1β induced antigen-independent activation and proliferation, while IL-23 synergistically facilitated the pathogenicity of bystander-activated TH17-like memory cells.

Collectively, our findings reveal an innate-like pathogenic function of antigen non-related Th17-like memory cells that contribute to the development of autoimmune disease. Our findings suggest that the majority of antigen-independent bystander-activated Th17-like memory cells would have a significant role in contributing to disease pathogenesis along with just a few antigen-specific T cells. This study opens up a new era in understanding the function of antigen non-specific memory T cells, and also suggests a new direction for the treatment of autoimmune diseases.