Demonstration of Mycobacterium leprae antigen in nerves of tuberculoid leprosy.

Twenty nerve biopsies of tuberculoid leprosy patients who showed no acid-fast bacilli in their skin smears or in tissue biopsies, were stained for mycobacterial antigens using anti-bacille Calmette-Gúerin (BCG) by the peroxidase-antiperoxidase method. Adjacent parts of some of these nerves were examined for the presence of osmiophilic bacilli under the transmission electron microscope. Eight of the 20 nerves were both clinically and histologically uninvolved. All the 20 involved nerves showed presence of antigen located, mainly intracellularly, in the cytoplasm of epithelioid cells and to a lesser degree in Schwann, endothelial and plasma cells. A few nerves with caseated nerve abscesses showed clusters of antigen deposits in both the caseous mass as well as the wall of the abscess. In six of the nine nerves processed for electron microscopy, electron-dense bacilli were noted within the cytoplasm of Schwann cells but not within the infiltrating cells. The uninvolved nerves showed neither antigen deposits nor osmiophilic bacilli despite fine ultrastructural changes. Our observations indicate that (a) the specificity of the immune response in paucibacillary nerve lesions is probably against bacterial components. (b) There is a differing antigen handling by Schwann cell and the inflammatory epithelioid cell. (c) Plasma cells may play a role in presenting antigen. (d) Mycobacterium leprae may be acting as an adjuvant in causing damage to uninvolved nerves at distal sites.