Stimulation of polymorphonuclear leukocyte bactericidal activity by supernatants of activated human mononuclear cells

Supernatants of phytohemagglutinin-activated human mononuclear cells stimulated polymorphonuclear leukocyte (PMN) activity against the gram-negative organism Serratia marcescens. In the absence of serum opsonins, when control PMN could not impede bacterial growth, stimulated PMN averaged more than 0.6-log kill of the original bacterial inoculum. In the presence of optimal amounts of serum opsonins, when control PMN were significantly bactericidal, stimulated PMN killed, on the average, at least 0.6 log more of bacteria. Stimulation was not found when PMN were preincubated with supernatants for 1 h or less. The data strongly suggested that the action of the PMN stimulating factor was independent of and different from classically described serum opsonins. PMN stimulating activity may be an additional lymphokine-mediated immune defense mechanism enabling hosts to kill invading microorganisms.