The role of humoral factors in the initiation of in vitro primary immune responses. II. Effects of lymphocyte mitogens.

Lymphocyte mitogens were used to analyze the induction of in vitro primary immune responses. Bacterial lipopolysaccharide (LPS), concanavalin A (Con A), pokeweed mitogen (PWM) and phytohemagglutinin (PHA) stimulated DNA synthesis but inhibited primary immune responses to sheep erythrocytes (SRBC) in normal mouse spleen cultures. In the same concentration range LPS, Con A and PWM stimulated immune responses to SRBC in spleen cultures depleted of thymus-derived cells by treatment with anti-θ C3H serum. Spleen cultures prepared from congenitally athymic (nude) mice did not support in vitro primary immune responses to SRBC. LPS, but not Con A or PWM, stimulated both DNA synthesis and primary immune responses to SRBC in spleen cultures from athymic mice. Con A and PWM appear to require the presence of thymus-derived cells to exert their inhibitory or stimulatory effects on immune responses. In contrast LPS replaces the requirement for thymusderived cells in the induction of primary immune responses.