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

In this report we have inspected whether difficulties in controlling viral infections negatively impacts the generation of CD127high memory T cells. Using both MHC class I and II tetramers we reveal that CD127low T cells are not necessarily rapidly deleted but can persist in a pseudoeffector state in which they display the hallmarks of activated effector cells but are functionally inferior. CD127high cells can, however, emerge if the infection is contained. We also show that in the absence of CD4 T cell help significant populations of CD127high CD8 T cells fail to emerge. Analyses of cytokine-producing activities by both mouse and human CD8 T cells further document that the extended maintenance of T cells in a CD127low state has functional consequences which manifest as an impairment of IL-2 production.

Regulatory T (Treg) cells contribute to the anti-inflammatory response during atherogenesis. Here we show that during atherogenesis Treg cells lose Foxp3 expression and their immunosuppressive function, leading to the conversion of a fraction of these cells into T follicular helper (Tfh) cells. We show that Tfh cells are pro-atherogenic and that their depletion reduces atherosclerosis. Mechanistically, the conversion of Treg cells to Tfh cells correlates with reduced expression of IL-2Rα and pSTAT5 levels and increased expression of IL-6Rα. In vitro, incubation of naive T cells with oxLDL prevents their differentiation into Treg cells. Furthermore, injection of lipid-free Apolipoprotein AI (ApoAI) into ApoE−/− mice reduces intracellular cholesterol levels in Treg cells and prevents their conversion into Tfh cells. Together our results suggest that ApoAI, the main protein in high-density lipoprotein particles, modulates the cellular fate of Treg cells and thus influences the immune response during atherosclerosis.

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