Specificity study of PPD-reactive human T cell line and clones

Peripheral blood mononuclear cells from a PPD-sensitive human subject, in vitro stimulated by PPD, could be maintained for several weeks in interleukin 2-containing medium, with regular reactivation with PPD and irradiated autologous mononuclear cells used as antigen-presenting cells. The proliferative response of this T cell PPD-reactive bulk culture could be stimulated to proliferate by PPD itself as well as by BCG and other mycobacteria. This T cell line comprised a majority of T cells expressing the OKT4+, OKT8- phenotype, and a minor proportion of T cells (8%) showing the OKT4-, OKT8+ phenotype. Two PPD-reactive clones, both OKT4+, were obtained from this line by the limiting dilution technique. Clone C9 was found to be highly specific of PPD and BCG, whereas clone A9 could be activated by four other species of mycobacteria. These data demonstrate the existence of species-specific and cross-reactive epitopes within PPD and prompt for the use of PPD-reactive clones as a tool for identification and purification of the molecules bearing these epitopes.