Cross-reactive and species specific Mycobacterium tuberculosis antigens in the immunoprofile of Schaumann bodies: a major clue to the etiology of sarcoidosis.
Sarcoidosis, once thought to be a variant of tuberculosis, is currently listed as a disease of unknown etiology. The present study was initiated by unpublished observations that Schaumann bodies-the laminated inclusions often encountered in sarcoid granulomas-cross-reacted with commercial polyclonal antibodies to Mycobacterium bovis, Mycobacterium duvalii and Mycobacterium paratuberculosis. Given the broad cross-reactivity of many mycobacterial antigens, those findings lacked specificity but warranted in depth probing of the immunoprofile of the bodies, particularly for specific mycobacterial antigens. Formalin-fixed tissue from eight patients with an established diagnosis of sarcoidosis was studied with panels of antibodies against both common cytoplasmic proteins and various mycobacterial antigens, using a labeled streptavidin-biotin-alkaline phosphatase technique. Our findings indicate that Schaumann bodies are indeed residual bodies of heterophagic mycobacterial derivation. They immunostained intensely for the lysosomal proteins muramidase and CD68, variably for some cytoskeletal proteins (tubulin, desmin, vimentin) and not at all for cytokeratin, muscle actin, alpha-1-antichymotrypsin and ferritin. Both cross-reactive and species specific antigenic determinants of M. tuberculosis complex were shown to be present. Affinity absorption with killed intact bacilli H37 Rv resulted in virtually equal loss of binding by all polyclonal antimycobacterial antibodies to cross-reactive ligands in Schaumann bodies. In addition, the bodies were clearly labeled with the monoclonal antibodies TB68 and TB71, known to recognize species specific epitopes of Mycobacterium tuberculosis complex. Although obtained on a small number of cases, our findings uphold Schaumann's original postulate that the laminated calcific inclusions represent remnants of "transformed tubercle bacilli".