The enhancement of macrophage bacteriostasis by products of activated lymphocytes

It was reported previously that the incubation of normal guinea pig macrophages with partially purified products of activated lymphocytes resulted in altered macrophage function including increased cell adherence to culture vessels, spreading, phagocytosis, and glucose carbon-1 oxidation. Studies reported here demonstrate that such macrophages also exhibit enhanced bacteriostasis. Lymphocytes were stimulated with concanavalin A, the culture supernatant was chromatographed over Sephadex G-100 and the fraction of mol wt 25,000-55,000, rich in lymphocyte mediators, was cultured with normal guinea pig macrophages for 1-3 days. Macrophages incubated with fractions from unstimulated lymphocyte cultures served as controls. The resulting macrophage monolayers were infected with Listeria monocytogenes. Macrophages incubated with mediator-rich fractions exhibited 2- to 10-fold enhanced bacteriostasis compared to controls. Further studies indicate that this enhancement was attributable to intrinsic changes in the macrophages and not simply a consequence of the number of macrophages on the monolayers. The studies support the concept that macrophage bacteriostasis can be enhanced by lymphocyte mediators. However, macrophages, which have been preincubated directly with sensitive lymphocytes and antigen exhibit even greater bacteriostasis and sometimes bactericidal capacity, suggesting that either a labile lymphocyte factor or direct lymphocyte macrophage interaction may also be involved in bactericidal activity.