Regulation of T follicular helper cells by CD8+ regulatory T cells reduces pro-atherogenict tertiary lymphoid organ formation in apolipoprotein E KO mice

Abstract

Tertiary lymphoid organs (TLOs), composed of B cell follicles, develop in the adventitia of atherosclerotic arteries. However, it is not known whether TLOs play an effective role in atherogenesis. In order to address this issue it is necessary to modulate the formation/function of these ectopic B cell follicles within the adventitia. Recent studies have shown that the function of germinal center B cells in secondary lymphoid organs (SLO) is tightly controlled by the Qa-1-restricted CD8+ T cells (CD8+ Treg) which regulate T follicular helper (TfH) cells function. We therefore evaluated whether regulation of TfH by CD8+ Treg can control the development of TLOs and, in turn, of atherogenesis in hypercholesterolemic (ApoE°) mice. We firstly found that the frequency of CD8+ Treg in ApoE° mice significantly decreases with age in the spleen and peripheral lymph nodes of this model. We therefore crossed ApoE° mice with the D227K-Tg mice in which, due to a Qa-1 point mutation, the CD8+ Treg are unable to control the development/function of TfH cells. ApoE° were compared with ApoE°/D227K-Tg littermates (n=5-12/group) at 8, 15 and 35 weeks of age. Qa-1 mutation increased the TfH cell population in SLO of ApoE°/D227K-Tg as compared to ApoE° mice. This effect was accompanied by an increase in the percentage of activated/memory CD4+ and CD8+ T cells and of B cells in lymph nodes of ApoE°/D227K-Tg, at 15 and 35 weeks of age. In parallel, lesion development was accelerated: atherosclerotic plaques in the aortic root of ApoE°/D227K-Tg mice were 2-fold greater than those of ApoE° mice. Immunofluorescent analysis of adventitial whole mounts revealed a greater incidence of TLOs (organized B cell follicles) in ApoE°/D227K-Tg than in ApoE° mice. Our data therefore suggest that CD8+ Treg control the development of adventitial pro-atherogenic TLOs in ApoE° mice through their regulatory action on TfH cells.