Abstract
Thioglycollate‐stimulated macrophages are known to release a plasminogen activator (PA) into the medium. In this study it was investigated whether macrophages could be activated to release PA after exposure to lymphokines. Macrophage monolayers obtained by 24 h culture of proteose peptone‐elicited murine exudate cells were incubated with lymphocyte culture supernatants. After 48 h the supernatants were replaced by serum‐free medium and the macrophages were incubated for another 24–48 h. These supernatants were assayed for PA as measured by the lysis of ^125^I‐labeled fibrin. The following results were obtained:
(a) Supernatants of antigen or mitogen‐stimulated spleen cells induced PA secretion by macrophages whereas control supernatants were ineffective. The same was found with supernatants of mitogen‐stimulated lymph node cells.
(b) PA secretion by macrophages seems to be induced by a rather narrow concentration range of lymphokines.
(c) Lymphokine‐induced PA secretion by macrophages is enhanced after phagocytosis of latex beads. The results show that PA secretion by activated macrophages can be considered as a parameter of immunoactivation.