THE GLYCOSYL-INOSITOL-PHOSPHATE AND DIMYRISTOYLGLYCEROL MOIETIES OF THE GLYCOSYLPHOSPHATIDYLINOSITOL ANCHOR OF THE TRYPANOSOME VARIANT-SPECIFIC SURFACE GLYCOPROTEIN ARE DISTINCT MACROPHAGE-ACTIVATING FACTORS

The TNF-alpha-inducing capacity of different trypanosome components wa s analyzed in vitro, using as indicator cells a macrophage cell line ( 2C11/12) or peritoneal exudate cells from LPS-resistant C3H/HeJ mice a nd LPS-sensitive C3H/HeN mite. The variant-specific surface glycoprote in (VSG) was identified as the major TNF-alpha-inducing component pres ent in trypanosome-soluble extracts. Both soluble (sVSG) and membrane- bound VSG (mfVSG) were shown to manifest similar TNF-alpha-inducing ca pacities, indicating that the dimyristoylglycerol (DMG) compound of th e mfVSG anchor was not required for TNF-alpha triggering. Detailed ana lysis indicated that the glycosyl-inositol-phosphate (GIP) moiety was responsible for the TNF-alpha-inducing activity of VSG and that the pr esence of the GIP-associatcd galactose side chain was essential for op timal TNF-alpha production. Furthermore, the results showed that the r esponsiveness of macrophages toward the TNF-alpha-inducing activity of VSG was strictly dependent on the activation state of the macrophages , since resident macrophages required IFN-gamma preactivation to becom e responsive, Comparative analysis of the ability of both forms of VSG to activate macrophages revealed that mfVSG but not sVSG stimulates m acrophages toward IL-1 alpha secretion and acquisition of LPS responsi veness. The priming activity of mfVSG toward LPS responsiveness was al so demonstrated in vivo and may be relevant during trypanosome infecti ons, since Trypanosoma brucei-infected mice became gradually LPS-hyper sensitive during the course of infection. Collectively, the VSG of try panosomes encompasses two distinct macrophage-activating components: w hile the GIP moiety of sVSG mediates TNF-alpha induction, the DMG comp ound of the mfVSG anchor contributes to IL-1 alpha induction and LPS s ensitization.