The classical and alternate pathways of T cell activation are impaired in leprosy.

The proliferative response of circulating T lymphocytes from bacterial index-positive lepromatous patients to mitogenic anti-CD3 and pairs of anti-CD2 monoclonal antibodies was significantly reduced. In these patients, the CD2 but not CD3 receptor expression was down-regulated. Further, the CD2 modulation and the associated suppression of proliferative response to monoclonals was brought about in T cells of healthy subjects by prior incubation of mononuclear cells in vitro with Mycobacterium leprae. Thus, the T cell activation pathways through the CD3 and CD2 receptors are impaired in lepromatous leprosy patients and the impairment appears to be due to the modulation of the CD2 receptor specifically by M. leprae.