Limiting dilution analysis of the human T cell response to mycobacterial antigens from BCG vaccinated individuals and leprosy patients.

The number of peripheral blood T lymphocytes responding to soluble mycobacterial antigens from Mycobacterium tuberculosis purified protein derivative (PPD) and M. leprae (MLS) was estimated by limiting dilution analysis. Antigen-induced lymphocyte activation was measured by means of [3H]TdR incorporation on day 10 of culture in the presence of suboptimal concentrations of interleukin 2 (IL-2). In the peripheral blood of BCG-vaccinated individuals from the UK, the frequency of T lymphocytes responding to PPD was 1.5 to 4 times greater than to MLS. Frequencies between 1/1970 and 1/13, 982 were observed in response to PPD and between 1/4097 and 1/24, 717 in response to MLS. A proportion of cells in the peripheral blood were also observed to respond to IL-2 only. The frequency of cells observed in limiting dilution analysis for PPD and MLS reflected the relative amounts of proliferation to these two antigens in bulk culture lymphocyte transformation tests. Use of PPD-specific T cell lines suggested that the responsiveness observed to M. leprae antigens in BCG-vaccinated individuals was due to cross-reactivity with antigens shared with M. bovis BCG. In tuberculoid leprosy, the frequency of peripheral blood T lymphocytes responding to M. leprae antigens was either greater than or similar to the frequency of T cells responding to PPD. In contrast, limiting dilution analysis of T lymphocytes from the peripheral blood of lepromatous leprosy patients revealed the complex regulatory heterogeneity of this group. In some patients M. leprae responsive T cells were detected in the presence of exogenous IL-2.