Serological response to purified mycobacterial phosphatidylinositol mannoside in healthy controls and in patients with tuberculosis and leprosy.

The serological response to a monoclonal antibody-defined phosphatidylinositol mannoside (L4-PIM) present in all mycobacteria was examined in patients with various mycobacterial diseases and healthy subjects from different populations. IgG but not IgM antibodies were detected in most patients with untreated lepromatous (84%) or borderline lepromatous (65%) leprosy, but in only a minority of those with disease at the tuberculoid end of the leprosy spectrum (< 17% positive). The response to L4-PIM was correlated with the IgM response to disaccharide octyl-bovine serum albumin (dBSA), and decreased with successful treatment. On the other hand, the test proved to be of little value in the diagnosis of untreated tuberculosis (4/15 positive) or atypical mycobacterial infection in patients with AIDS (0/11 positive). IgG antibodies to L4-PIM were also found in a significant proportion of healthy individuals, irrespective of their Mantoux status. These antibodies were shown to be specific for L4-PIM on immunoblotting, and their incidence increased with age in random donors from both urban Australia and rural Papua New Guinea. Despite the limited value of the assay in diagnosis of any particular mycobacterial disease, the presence of antibodies to L4-PIM appears to be a sensitive indicator of subclinical infection with environmental mycobacteria in subjects with an intact immune system.