Apolipoprotein E (apoE) is synthesized in the liver and in macrophages, and it has antiatherogenic properties that are mediated, at least in part, through the regulation of plasma cholesterol homeostasis. Previous data suggest that apoE also has antiinflammatory properties that may contribute to protection against atherosclerosis independent of its role in lipid metabolism. In this study, apoE knockout and C57BL/6 mice were stimulated with low-dose lipopolysaccharide (LPS) and other Toll-like receptor (TLR) agonists. We show that apoE modulates the systemic type I inflammatory response in vivo. The proinflammatory cytokines tumor necrosis factor alpha, interleukin (IL)-6, IL-12, and interferon-gamma were upregulated to a significantly greater extent in apoE-deficient mice than in wild-type mice at both the mRNA and protein levels following administration of LPS. In contrast, hypercholesterolemic low-density lipoprotein receptor/apobec-1 double knockout mice had a similar cytokine response as wild-type mice, eliminating hypercholesterolemia as a cause for the exaggerated cytokine response. Importantly, reconstitution of apoE expression in the liver of apoE-deficient mice normalized the LPS-induced plasma protein levels of IL-12p40. Furthermore, there was selective upregulation of plasma IL-12 in apoE knockout mice by a TLR3 agonist, poly I:C, but not by other TLR agonists, CpG oligonucleotide or Toxoplasma gondii antigen. This implies that apoE selectively regulates TLR4- and TLR3-mediated signaling of IL-12 production. These results indicate that apoE modulates the T helper-1-type immune response in vivo by modulating IL-12 production.