Lipids from Mycobacterium tuberculosis are presented through CD1 proteins to T lymphocytes in humans, but the accessory molecules required for antigen loading and presentation remain unidentified. Here we show that fibroblasts deficient in sphingolipid activator proteins (SAPs) transfected with CD1b failed to activate lipid-specific T cells. However, the T cell response was restored when fibroblasts were reconstituted with SAP-C but not other SAPs. Lipid antigen and SAP-C colocalized in lysosomal compartments, and liposome assays showed that SAP-C efficiently extracts antigen from membranes. Coprecipitation demonstrated direct molecular interaction between SAP-C and CD1b. We propose a model in which SAP-C exposes lipid antigens from intralysosomal membranes for loading onto CD1b. Thus, SAP-C represents a missing link in antigen presentation of lipids through CD1b to human T cells.
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We thank J.T. Belisle sharing reagents and J. Enders for technical assistance. Supported by the Deutsche Forschungsgemeinschaft (SFB 421, U.E.S. and S.H.E.K.; SPP 1131, U.E.S.).
The authors declare no competing financial interests.
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Winau, F., Schwierzeck, V., Hurwitz, R. et al. Saposin C is required for lipid presentation by human CD1b. Nat Immunol 5, 169–174 (2004). https://doi.org/10.1038/ni1035
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