T cells in atherogenesis

Abstract

The work of this thesis aims to answer the following questions: (1) Which role do CD4+ cells play in atherosclerosis? (2) Is the expression of CD4 essential for the athero-protective effect mediated by immunization with oxidized LDL by adjuvant? (3) How does the antigenspecific T cell response to oxidized LDL influence lesion development? (4) Is there a general shift to Th2 responses in severe hypercholesterolemia? and finally (5) Which role does transforming growth factor-B (TGF-B)-signaling in T cells play in atherogenesis? ApoE-/-CD4-/- mice were used to study the effect of CD4 deficiency in atherosclerosis. The development of atherosclerosis was decreased with attenuated inflammation in apoE-/- mice lacking CD4+ cells compared to immunocompetent apoE-/-.

Immunization with oxidized LDL protected against atherosclerosis in the absence of CD4+ T cells but adjuvant-induced protection against atherosclerosis was extinguished in CD4/apoE double knockout mice. Thus, CD4+ cells play a significant role in the development of atherosclerosis and our findings further indicate that the adjuvant-induced atheroprotection is CD4-dependent. In contrast, immunization with oxLDL may confer disease protection through mechanisms operating also in CD4 deficient mice. To study the importance of T cell specificity in atherogenesis, we transferred CD4+ T cells from mice immunized with oxidized LDL or with the unrelated antigen keyhole limpet hemocyanine (KLH) to apoE-/-scid/scid mice. The mice receiving T cells from mice immunized with oxidized LDL developed more atherosclerosis than the mice receiving cells from KLH-immunized mice. There was a more substantial T cell infiltrate in lesions from the former group, suggesting homing to the lesions by antigen-specific T cells. These results indicate an antigen-specific immune activation by CD4+ cells specific for oxidized LDL. ApoE-/- mice with different levels of hypercholesterolemia were immunized with KLH.

Immune responses in mice with severe hypercholesterolemia were skewed to Th2 as shown by decreased titers of KLH antibodies of the IgG2a subclass and increased IgE antibodies, increased production of interleukin-4 (IL-4) and IL-10 and decreased production of IFN-y. Lipid metabolites may therefore affect T cell function, which could be of relevance in local environments with high lipid levels, for example in the inflamed arterial intima. Finally, we crossed the atherosclerosis-prone apolipoprotein E knockout (apoE-/-) mice with mice carrying dominant negative receptors for TGF-B on T cells, thus rendering the T cells insensitive to the dampening effect of TGF-B The formation of atherosclerotic lesions was dramatically increased in the mice with abrogated TGF-B signaling in T cells, with lesions of a vulnerable phenotype. Therefore, TGF-B exerts important anti-atherosclerotic effects by acting on T cells.

In conclusion, we have studied T cells in atherosclerosis and severe hypercholesterolemia. CD4+ T cells play a major role in the proatherogenic inflammatory process, and cell-mediated immunity to oxLDL promotes atherogenesis. In addition, T cells play differential roles in diseaseprotecting immunization strategies and regulated by lipid components in their environment as well as by anti-inflammatory cytokines, with TGF-B probably playing major role as an antiatherosclerotic mediator.