The hallmark of Mycobacterium-induced pathology is granulomatous inﬂammation at the site of infection. Mycobacterial lipids are potent immunomodulators that contribute to the granulomatous response and are released in appreciable quantities by intracellular bacilli. Previously we investigated the granulomagenic nature of the peripheral cell wall lipids of Mycobacterium bovis bacillus Calmette-Gue´rin (BCG) by coating the lipids onto 90 m diameter microspheres that were mixed into Matrigel matrix with syngeneic bone marrow-derived macrophages and injected i.p. into mice. These studies demonstrated that BCG lipids elicit proinﬂammatory cytokines and recruit leukocytes. In the current study we determined the lipids responsible for this proinﬂammatory effect. BCG-derived cell wall lipids were fractionated and puriﬁed by liquid chromatography and preparative TLC. The isolated fractions including phosphatidylinositol dimannosides, cardiolipin, phosphatidylglycerol, phosphatidylethanolamine, trehalose monomycolate, trehalose dimycolate, and mycoside B. Trehalose dimycolate, when delivered to bone marrow-derived murine macrophages, induced the greatest secretion of IL-1ß, IL-6, and TNF-α
in vitro. Trehalose dimycolate similarly induced the greatest secretion of these proinﬂammatory cytokines in ex vivo matrices over the course of 12 days. Trehalose monomycolate and dimycolate also induced profound neutrophil recruitment in vivo. Experiments with TLR2 or TLR4 gene-deﬁcient mice revealed no defects in responses to trehalose mycolates, although MyD88-deﬁcient mice manifested signiﬁcantly reduced cell recruitment and cytokine production. These results demonstrate that the trehalose mycolates, particularly trehalose dimycolate, are the most bioactive lipids in the BCG extract, inducing a proinﬂammatory cascade that inﬂuences granuloma formation. The Journal of Immunology, 2005, 174: 5007–5015.