Defects intheImmuneSystemofMiceInfected with Lymphocytic Choriomeningitis Virus

effect. Colony-formingstemcells, whichmay actas theprecursors ofthelymphoidcells, were temporarily inhibited during thefirst period oftheinfection. Presumably this inhibition alsoaffected thethymuscells, whichdecreased dramatically atthe same time. Atalater stageofinfection, defects developed within thepopulation ofimmunocompetent cells, andthis was mostprobably a consequence ofthe preceding suppression oftheprecursorcells. Thedefects intheimmunocompetentcells were temporally related totheimmunosuppression andseemedtobe theultimate causeofthisphenomenon. Atallevents, antibody-forming cells were notdamaged bythevirus. Instudies ofneonatally infected babymice,it was found that thedevelopment ofimmunological responsiveness was completely abolished forthefirst 2weeks oflife. Itistherefore probable thatthegeneration ofimmunocompetent cells was alsoaffected inthebabies. Evidence was obtained supporting thehypothesis thatthis effect played an important role for theinduction oftolerance tothevirus intheneonatally infected mice.