The role of mitogens and antigens in the generation of antibody-producing human B lymphocytes

Satisfactory experimental systems with which to study the antigen specific humoral immune response of human peripheral blood lymphocytes have not been available until recently. A commonly used method for the study of antibody production by human lymphocytes is that developed by Fauci and Pratt. This system is considered to be antigen nonspecific since the antigen against which the determined antibody is directed is not added to the cultures. We show here that the assumption of the Fauci-Pratt system being antigen nonspecific is not justified. An essential ingredient of this culture system is human serum that has been exhaustively absorbed with antigen (sheep red blood cells). This absorption procedure causes shedding of highly immunogenic antigenic fragments whose immunogenic activity we demonstrated using a recently developed antigen-dependent culture system. In the latter system, we have shown that the control of suppressor cells is a critical factor for the successful induction of antibody responses, particularly in view of the fact that lymphocyte mitogens must be added to cultured human lymphocytes to support their responsiveness. Appropriate timing of mitogen addition to the cultures was found to be an effective means of preferentially stimulating helper or suppressor activity. Pokeweed mitogen, a mitogen known to act on B and on T lymphocytes, stimulates B-cell responses readily but abrogates them prematurely by the simultaneous activation of suppressor cells. When pokeweed mitogen is added to an ongoing response with a delay of 48 hr, it enhances antibody responses markedly, presumably by providing additional help to B cells at a time when they have lost susceptibility to suppressor-cell effects.