Apolipoprotein E modulates immune activation by acting on the antigen-presenting cell

Apolipoprotein E (ApoE) is synthesized by a variety of cells including macrophages. These cells activate T lymphocytes by antigen presentation, while the T-cell cytokine, interferon-gamma, inhibits macrophage ApoE expression. ApoE inhibits T-cell proliferation in culture but its role in immune responses has been unclear. The ApoE-deficient (E0) mouse permits an evaluation of the immunological role of ApoE. We have analysed T-cell responses to an exogenous antigen (ovalbumin) and polyclonal mitogen (concanavalin A) in E0 and ApoE+/+ mice. Macrophages of E0 mice stimulated T-cell activation more effectively as antigen-presenting cells than macrophages from ApoE+/+ mice. Both proliferation and interferon-gamma secretion were enhanced in T cells activated in the context of antigen-presenting cells from E0 mice. Since the macrophage-T-cell interaction depends on interactions between cell surface molecules, we assessed the expression of such molecules after in vivo stimulation with interferon-gamma. This treatment caused an increased expression of the co-stimulatory surface proteins CD40 and CD80, and also of the major histocompatibility complex class II molecules I-Ab on macrophages of E0 mice compared with ApoE+/+. Our data suggest that ApoE inhibits T-cell activation by reducing the density of immune stimulatory proteins on antigen-presenting cells.