pathogenesis ͉ biochemistry ͉ glycolipid ͉ sulfation T he thick Mycobacterium tuberculosis (M. tb) cell wall consists of numerous glycolipids that are distinctive to the mycobacterial genus, including phosphatidylinositol mannosides, trehalose mycolates, and lipoarabinomannans (1). These molecules are essential for many of the characteristics that distinguish mycobacterial pathogenesis, such as the inhibition of phagosomal maturation, drug resistance, and alteration of the host immune response (2-6). A family of cell surface sulfated lipids (dubbed sulfatides) were identified in M. tb extracts and correlated to strain virulence (7-9). The most abundant sulfatide, termed Sulfolipid-1 (SL-1), consists of a trehalose core, four fatty acyl groups, and a sulfate ester (Fig. 1A) (10-13). Despite the discovery of SL-1 nearly 50 years ago, the biological function of the molecule is not known. Conflicting reports suggest a role for SL-1 in superoxide (O 2 Ϫ ) release from human neutrophils or monocytes, alteration of trehalose dimycolate toxicity, and inhibition of trehalose dimycolate-induced macrophage recruitment (14-19). The relevance of these studies to the physiological role of SL-1 in M. tb infection is debatable.Although the role of SL-1 remains elusive, advances in genetics and metabolite analysis have sped the discovery of genes, proteins, and intermediates associated with SL-1 biosynthesis (20). Currently, three proteins are known to be involved in SL-1 assembly: Stf0, Pks2, and MmpL8. The sulfotransferase Stf0 sulfates trehalose at the 2-position, forming trehalose-2-sulfate (T2S), thereby initiating SL-1 biosynthesis (21). Meanwhile, the polyketide synthase Pks2 synthesizes the phthioceranoyl and hydroxyphthioceranoyl lipids that occupy the 6-, 6Ј-, and 3Ј-positions of SL-1 (Fig. 1 A) (22). The proteins responsible for transfer of the Pks2 products and the palmitoyl group to the T2S core, and the order in which these lipids are added, have not yet been defined.Insight into the order of lipid addition came from characterization of the putative lipid transporter MmpL8 (23,24). A mutant strain, ⌬mmpL8, lacks SL-1 but accumulates the diacylated intermediate SL 1278 (named for its observed mass) inside the cell (Fig. 1B). This intermediate possesses two of the four SL-1-associated lipids: a hydroxyphthioceranoyl group at the 3Ј-position and a palmitoyl group at the 2Ј-position (24). SL 1278 was recently found to be an immunostimulant in human tuberculosis patients (25). The glycolipid is presented on the surface of M. tb-infected antigen-presenting cells by CD1b, a member of the MHC class I-like CD1 family. Intriguingly, the ⌬mmpL8 mutant, which lacks SL-1 but accumulates SL 1278 , shows attenuated virulence in mice (23,24). By contrast, a ⌬pks2 mutant, which lacks both SL-1 and SL 1278 , is indistinguishable from WT M. tb in mice and guinea pigs (23,26). These observations suggest that SL 1278 , and possibly other SL-1 intermediates, modulate M. tb pathogenesis.In our effort to define the functions of M. tb sulf...
