Technical Abstract: Toll-like receptors (TLRs) provide critical signals to induce innate immune responses in antigen presenting cells such as dendritic cells (DCs) that in turn link to adaptive immune responses. Results from our previous studies demonstrated that saturated fatty acids activate TLRs, whereas n-3 polyunsaturated fatty acids (PUFAs) inhibit agonist-induced TLR activation. These results raise a significant question as to whether fatty acids differentially modulate immune responses mediated through TLR activation. The results presented here demonstrate that the saturated fatty acid, lauric acid, upregulates the expression of costimulatory molecules (CD40, CD80, and CD86) and cytokines (IL-12p70 and IL-6) in bone marrow-derived DCs. The dominant-negative mutant of TLR4 or it downstream signaling components inhibits lauric acid-induced expression of CD86 promoter-reporter gene. In contrast, a n-3 PUFA, docosahexaenoic acid, inhibits TLR4 agonist(LPS)-induced upregulation of the costimulatory molecules and cytokine production. Similarly, DCs treated with lauric acid show increased T cell activation capacity, whereas docosahexaenoic acid inhibits T cell activation induced by LPS-treated DCs. Together, our results demonstrate that the reciprocal modulation of both innate and adaptive immune responses by saturated fatty acids and n-3 PUFAs is mediated at least in part through TLRs. These results imply that TLRs are involved in sterile inflammation and immune responses induced by non-microbial endogenous molecules. These results shed new light in understanding how types of dietary fatty acids differentially modulate immune responses that could alter the risk of many chronic diseases.