LOSER LIGHT SUICIDE OF PROLIFERATING VIRUS-SPECIFIC CD8(-CELLS IN AN IN-VIVO RESPONSE() T)

Virus-specific CD8(+) CTL precursor (CTLp) frequencies ave a function of rates of clonal expansion and loss, both of which are difficult to assess in vivo. Proliferating T cells incorporate the nucleoside analo gue 5-bromo-2'-deoxyuridine (BrdU), making them sensitive to light-ind uced apoptotic cell death (suicide). Mice were infected with an influe nza A virus, then given 50 mg of BrdU 24 h before sampling. Exposure o f freshly isolated, CD8-labeled T cells to the laser beam of the flow cytometer resulted in the elimination of >90% of the responding CTLp, The effect was obvious for the regional lymph node from day 6 to day 1 2 after priming, indicating continued cycling over a 7-day interval. T hus, as CTLp frequencies remain fairly constant from 7 to 30 days afte r infection, the persistent increase (>30x) in numbers must be accompa nied by a very substantial loss of virus-specific CD8(+) T cells.