Stimulation of monocyte production by an endogenous mediator induced by a component from Listeria monocytogenes.

A monocytosis-producing activity (MPA) is present in a saline-extractable material (SE) from Listeria monocytogenes. The mechanism of stimulation of monocyte production by SE was studied. Serum obtained from mice at appropriate times after injection of SE induced monocytosis in normal recipients. The monocytosis-inducing activity present in serum differed from SE with respect to timing of the monocytosis, fractionation pattern on a Sephadex G-200 column, and thermolability. The minimum dose of SE capable of producing a monocytosis was 100 micrograms. Antibody to SE capable of detecting SE at a concentration of greater than 5 micrograms/ml failed to detect SE in samples of active serum. Therefore it seemed highly unlikely that activity in serum was due to the presence of trace amounts of SE. The activity present in serum was thermolabile and had a molecular weight of about 38,000. The data are consistent with the hypothesis that injection of SE caused the production or release of an endogenous mediator capable of stimulating monocytosis.