Circulating immune complexes in human tuberculosis sera: demonstration of specific antibodies against Mycobacterium tuberculosis glycolipid (DAT, PGLTb1, LOS) antigens in isolated circulating immune complexes.

An enzyme-linked immunosorbent assay (ELISA) for IgG using three glycolipid antigens from Mycobacterium tuberculosis in 65 tuberculosis (TB) patients and 50 healthy control subjects was performed. The circulating immune complexes (CICs) were isolated by precipitation with polyethylene glycol 6000 (PEG). This method associated to ELISA measured the specific antibodies present in these CICs. PEG [optical density (OD) 280] was shown to be significantly elevated (P < 0.001) in tuberculous samples. The concentrations of IgG antibodies complexed to the three glycolipid antigens were shown to be higher in patient with tuberculosis than in normal control subjects (P < 0.001). No correlation was observed between levels of free and CIC-bound antibodies. These antibodies isolated from CICs were responsible for almost all of the false-negative serological results. However, great heterogeneity was noticed depending on the antigen used, showing a more positive ELISAs against DAT (77%) than against LOS (71%) or PGLTb1 (18.5%). No correlation was established between the presence of specific CIC-complexed IgG and the bacteriological load or the tuberculosis localization (pulmonary vs. extrapulmonary). The sensitivity of ELISA for CIC-complexed IgG to DAT and LOS was lower in HIV-infected TB patients. From these results, we conclude that detection of complexed IgG DAT and LOS glycolipid antigen will be useful as a complementary technique for the serodiagnosis of tuberculosis.