Inhibition of mixed lymphocyte reactions in humans after immunization with tetanus toxoid.

Immunization of humans with tetanus toxoid (TT) results in an inhibition of the reactivity of peripheral lymphocytes to TT as well as to one-way mixed lymphocyte reaction. The strongest inhibition was observed in cells taken 3 or 7 days after immunization. These reactivities reappeared on the 14th day and reached their initial levels by the 21st day after immunization. The role of the various cell types was studied by culturing separately purified T or B lymphocytes taken on day 7 after immunization. B cells alone are not stimulated by TT or allogeneic cells. Purified T cells are stimulated much more than the initial total cell population. When B cells were mixed and cultured along with T cells, a suppressor effect upon stimulation appeared which reduced 3H-thymidine incorporation to the levels of the initial population. Treatment with 5-bromodeoxyuridine followed by exposure to light of cells stimulated with TT abrogated their ability to respond to subsequent stimulation with TT; whereas the responsiveness of treated cells to allogeneic lymphocytes was not significantly affected and vice versa. These results suggest that each stimulator activated a separate population of T cells but that in vivo immunization provoked the development of both a specific and nonspecific suppressor activity. Most probably this suppressor activity was brought about by adherent, surface Ig-bearing (B) cells.