Partially purified protein antigen which was prepared from cells of Nocardia asteroides was used to assess the immune responsiveness of guinea pigs inoculated subcutaneously with living N. asteroides. Cell-mediated reactivity, as measured by in vitro lymphocyte transformation of and blastogenic factor production by lymph node cells as well as a delayed skin reaction, appeared clearly at 1 week after N. asteroides infection, when an early abscess was formed at the inoculated site. The in vitro proliferative response of lymph node cells reached a maximum at 2 weeks at which time the lesion of the inoculated site showed central suppuration surrounded by epithelioid and giant cells. The lymphocyte transformation response then declined rapidly but a significant response continued until 6 weeks after infection. The production of blastogenic factor was maximum at 3 weeks, but continued till 6 weeks. A high level of skin reactivity was maintained until 6 weeks. By 6 weeks after infection the lesion at the inoculated site usually resolved and disappeared. Thus, enhancement of cell-mediated reactivity coincided with the time of granulomatous conversion of an early suppurative abscess at the site of inoculation of Nocardia organisms. In studies on the lymphocyte proliferative response in vitro of N. asteroides-infected animals, purified lymph node T cell populations failed to respond to Nocardia antigen, but they could be induced to proliferate in the presence of autologous macrophages or B cells. Purified B lymphocytes, though unresponsive to Nocardia antigen by themselves or following the addition of macrophages, were stimulated by Nocardia antigen in the presence of T cells. The proliferative response of lymph node cells to Nocardia antigen seems to be the result of a collaborative event that occurs between T cells and macrophages or T and B cells.
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