Background:
In atherosclerosis, CD4
+
T helper cells recognize auto-antigens including ApoB, the main protein in low-density lipoprotein (LDL). However, atherosclerosis-specific, auto-reactive CD4
+
T cells have not been detected
in vivo
, and their function is unknown.
Methods and Results:
We have previously identified peptides derived from mouse ApoB that bind with high affinity to the MHC class II molecule of C57BL/6 mice (I-A
b
). We designed and validated a new multimer of a recombinant MHC-II molecule fused to one ApoB auto-epitopes, P6 (TGAYSNASSTESASY, P6:I-A
b
), that enabled detection of low-affinity, P6-reactive CD4
+
T cells. Using this P6:I-A
b
multimer, we identified ApoB-reactive CD4
+
T cells in healthy, young C57BL/6 mice that were predominately differentiated T-regulatory cells (T
regs
) and expressed IL-10, a known atheroprotective cytokine. This population was detectable in lymph nodes and already showed a memory phenotype in young animals without atherosclerosis. In
Apoe
-/-
mice, adoptively transferred ApoB P6-specific T
regs
accumulated in the aorta and draining lymph nodes and gave rise to pathogenic T
H
1 and T
H
17 cells. This phenotypic switch was caused by enhanced plasticity of antigen-specific T
regs
as evidenced by multiple clusters of intermediate T
reg
-T
eff
phenotypes in single cell RNA sequencing of 4485 antigen-specific CD4
+
T cells. In the plaque, many T cells were ex-T
regs
as identified by a FoxP3 lineage tracker mouse, suggesting that atherosclerosis-specific CD4
+
T cells lost their regulatory capacity. Vaccination with P6 maintained a protective phenotype in antigen-specific T
regs
and protected from atherosclerosis. In humans, ApoB-specific CD4
+
T cells from atherosclerotic patients showed the same cytokine patterns found in mouse CD4
+
T cells, suggesting that autoimmunity to ApoB is protective first, but later gives rise to a pathogenic CD4
+
T cell response that aggravates atherosclerosis.
Conclusion:
Protective T-regulatory cells recognizing peptide antigens of ApoB exist in naïve mice, protect against atherosclerosis, but convert into pathogenic T
H
1 and -17 cells during the natural course of disease in mice and humans. These results call for immunomodulatory therapies to maintain protective autoimmunity.
