Pathogenic Autoimmunity in Atherosclerosis Evolves From Initially Protective Apolipoprotein B ₁₀₀ –Reactive CD4 ⁺ T-Regulatory Cells

<jats:sec> <jats:title>Background:</jats:title> <jats:p> Throughout the inflammatory response that accompanies atherosclerosis, autoreactive CD4 <jats:sup>+</jats:sup> T-helper cells accumulate in the atherosclerotic plaque. Apolipoprotein B <jats:sub>100</jats:sub> (apoB), the core protein of low-density lipoprotein, is an autoantigen that drives the generation of pathogenic T-helper type 1 (T <jats:sub>H</jats:sub> 1) cells with proinflammatory cytokine secretion. Clinical data suggest the existence of apoB-specific CD4 <jats:sup>+</jats:sup> T cells with an atheroprotective, regulatory T cell (T <jats:sub>reg</jats:sub> ) phenotype in healthy individuals. Yet, the function of apoB-reactive T <jats:sub>regs</jats:sub> and their relationship with pathogenic T <jats:sub>H</jats:sub> 1 cells remain unknown. </jats:p> </jats:sec> <jats:sec> <jats:title>Methods:</jats:title> <jats:p> To interrogate the function of autoreactive CD4 <jats:sup>+</jats:sup> T cells in atherosclerosis, we used a novel tetramer of major histocompatibility complex II to track T cells reactive to the mouse self-peptide apo B <jats:sub>978-993</jats:sub> (apoB <jats:sup>+</jats:sup> ) at the single-cell level. </jats:p> </jats:sec> <jats:sec> <jats:title>Results:</jats:title> <jats:p> We found that apoB <jats:sup>+</jats:sup> T cells build an oligoclonal population in lymph nodes of healthy mice that exhibit a T <jats:sub>reg</jats:sub> -like transcriptome, although only 21% of all apoB <jats:sup>+</jats:sup> T cells expressed the T <jats:sub>reg</jats:sub> transcription factor FoxP3 (Forkhead Box P3) protein as detected by flow cytometry. In single-cell RNA sequencing, apoB <jats:sup>+</jats:sup> T cells formed several clusters with mixed T <jats:sub>H</jats:sub> signatures that suggested overlapping multilineage phenotypes with pro- and anti-inflammatory transcripts of T <jats:sub>H</jats:sub> 1, T helper cell type 2 (T <jats:sub>H</jats:sub> 2), and T helper cell type 17 (T <jats:sub>H</jats:sub> 17), and of follicular-helper T cells. ApoB <jats:sup>+</jats:sup> T cells were increased in mice and humans with atherosclerosis and progressively converted into pathogenic T <jats:sub>H</jats:sub> 1/T <jats:sub>H</jats:sub> 17-like cells with proinflammatory properties and only a residual T <jats:sub>reg</jats:sub> transcriptome. Plaque T cells that expanded during progression of atherosclerosis consistently showed a mixed T <jats:sub>H</jats:sub> 1/T <jats:sub>H</jats:sub> 17 phenotype in single-cell RNA sequencing. In addition, we observed a loss of FoxP3 in a fraction of apoB <jats:sup>+</jats:sup> T <jats:sub>regs</jats:sub> in lineage tracing of hyperlipidemic <jats:italic>Apoe</jats:italic> <jats:sup>–/–</jats:sup> mice. In adoptive transfer experiments, converting apoB <jats:sup>+</jats:sup> T <jats:sub>regs</jats:sub> failed to protect from atherosclerosis. </jats:p> </jats:sec> <jats:sec> <jats:title>Conclusions:</jats:title> <jats:p> Our results demonstrate an unexpected mixed phenotype of apoB-reactive autoimmune T cells in atherosclerosis and suggest an initially protective autoimmune response against apoB with a progressive derangement in clinical disease. These findings identify apoB autoreactive T <jats:sub>regs</jats:sub> as a novel cellular target in atherosclerosis. </jats:p> </jats:sec>