Sequential expression of T cell activation (Tac) antigen and Ia determinants on circulating human T cells after immunization with tetanus toxoid.

Recent studies have indicated that a monoclonal antibody, termed anti-Tac, may recognize the receptor sites or closely associated structures for interleukin 2 on activated human T cells. The Tac antigen, definable by anti-Tac antibody and usually found on mitogen- or alloantigen-stimulated T cells, was not expressed to any appreciable extent on normal circulating T cells. In the present study, we showed that an increase in circulating T cells expressing Tac antigen as well as Ia determinants occurred in normal individuals after immunization with tetanus toxoid. The expression of Tac antigen and Ia determinants on T cells was evaluated by the rosette method with Staphylococcal protein A-(SPA) coated bovine red blood cells (BRBC) or the indirect immunofluorescence method with monoclonal anti-Tac and anti-Ia antibodies. An increase in Tac-positive or Ia-positive T cells was more evident with the use of the rosette method with SPA-coated BRBC than with conventional immunofluorescence. The percentage of Ia-positive T cells showed a peak between 24 and 48 hr after toxoid injection, and remained at high levels until 2 wk after immunization. In contrast to Ia-positive T cells, the appearance of Tac-positive T cells was transient and at a rather early period of toxoid immunization. The maximum increase of Tac-positive T cells was apparent around 12 hr after toxoid injection, and Tac-positive T cells disappeared abruptly from circulation by 24 hr after inoculation. Ia-positive T cells were induced in both Leu-2 suppressor/cytotoxic and Leu-3 helper/inducer subsets, whereas Tac-positive T cells were generated only within the Leu-3 subset. The fact that induction of Tac-positive and Ia-positive T cells might occur at different stages of T cell activation and in different subsets of T cells seemed to be important for elucidating their roles in the in vivo T cell proliferation and differentiation.