The immunological events leading to the "in vitro" response to PPD

In normal individuals the 5/9 monoclonal antibody (5/9 MAb) recognizes a T-cell fraction that includes all T lymphocytes with inducer activities. Here, circulating 5/9+ and 5/9- T lymphocytes were isolated from tuberculim skin-positive subjects and the proliferative response induced by PPD was investigated. The results show that the total PPD-induced lymphocyte DNA synthesis is confined to the 5/9+ T cell fraction. PPD was unable to induce a direct or indirect (through the 5/9+ cells) proliferation of T8+ cells. Whether 5/9 antigen, expressed on the PPD responsive T-cell subset, is involved in PPD-induced cell proliferation, was also analyzed. No significant effect was observed adding 5/9 MAb, whereas complete inhibition of antigen-induced blastogenesis was observed upon addition of a MAb (D1.12) directed to common determinants of Ia antigens. The supernatant fluids of 5/9+ and 5/9- cells stimulated with PPD were studied for the presence of IL-2 and IFN-gamma. Production of both these lymphokines was detected in the 5/9+ T-cell supernatants, whereas neither IL-2 or IFN-gamma were found in supernatants of 5/9- cells. In addition, preliminary experiments suggested that only the supernatants of PPD-stimulated 5/9+ T cells were able to activate a macrophage cell line, inducing a remarkable release of hydrogen peroxide. These results indicate that the lymphocyte responsiveness to PPD is confined in a small T cell fraction, expressing the 5/9 antigen; only these T cells are able to release soluble factors (i.e., IL-2 and IFN-gamma); those or other soluble factor(s) produced in PPD-stimulated cultures seem able to activate the macrophages.