Immunosuppressive activity of an extract of Listeria monocytogenes.

The effects of treatment of mice with an aqueous extract of Listeria monocytogenes on lymphoid populations and on immune responsiveness were studied. Thymus weight was decreased to a third of the control, whereas spleen weight was proportionally increased as the number of treatments was increased. The total numbers of cells in the thymus and bone marrow were diminished, whereas the number in the spleen was increased. Serum hemolysin production and plaque-forming cell response to sheep erythrocytes were suppressed in treated mice when the extract was given before administration of antigen. Spleen cells from treated mice failed to mount an antibody response upon transfer to syngeneic gamma-irradiated recipients, suggesting that potential antibody-producing cells were impaired by the treatment.