Peritoneal cells (PC), from nonimmunized mice, incubated in a carboxymethyl cellulose gum containing sheep red blood cells (SRBC) and guinea pig complement (following the technique of Ingraham and Bussard) start to produce plaques of hemolysis 20 hr after the beginning of incubation at 37°C.
In contrast, spleen cells from immunized mice complete their plaque-forming activity in 6 hr. The fact that formation of plaques by previously uncommitted cells is related to the life of the leukocytes, and is complement dependent brings evidence that we are dealing with an immunological phenomenon. Puromycin suppresses the formation of plaque. Previous incubation of the PC with SRBC in liquid medium, before incorporation in the detection system, reduces the lag in the production of plaques. This indicates that a phase of stimulation precedes the phase of expression (manifested by plaque formation).
The immunological activity of the peritoneal cell suspension is highly dependent on the concentration of the suspension, which indicates that this activity results from a cooperative process. Actinomycin D, which does not suppress the production of plaques by cells from immunized animals, stops completely the in vitro induction. It is concluded that we have probably observed a primary immune response induced in vitro.