Apoptosis and CD4+ lymphocyte depletion following feline immunodeficiency virus infection of a T-lymphocyte cell line.

An interleukin-2-dependent feline T-lymphocyte cell line (FCD4-D), of which 65% of cells express CD4, was inoculated with the NCSU-1 isolate of feline immunodeficiency virus (FIV(NCSU-1)) and subsequently monitored for percentage of viable cells, percentage of apoptotic cells, percentage of CD4-expressing cells, and virus production. A decrease in viability from 91% to 12% over an 11-day postinoculation period was associated with an increase in the percentage of cells with nuclear morphology suggestive of apoptosis from < 5% to 97% based on ethidium bromide and acridine orange fluorescence. These changes were associated with a 24% reduction in the percentage of viable CD4-expressing cells at 7 days postinoculation. The relative amount of low-molecular-weight nuclear DNA was greater in FIV-infected cultures than in uninfected cultures from day 7 to day 15 postinoculation. This DNA was characterized by cleavage into fragments differing in size by approximately 180 base pairs. Ultrastructurally, nuclear chromatin and cytoplasm were condensed into discrete electron-dense bodies, and cell membrane projections were lost. Syncytia were occasionally present in FIV-inoculated cultures. Cytologic changes were associated with a logarithmic rise in Mg+2-dependent reverse transcriptase levels in culture supernatants on days 4-7 postinoculation. Supplementation of FIV-inoculated culture medium with 1 mM ZnCl2 enhanced viability, decreased the percentage of cells undergoing apoptosis, and prevented the loss of CD4+ lymphocytes at 7 days postinoculation. These data suggest that feline CD4+ lymphocytes die by apoptosis following in vitro infection with FIV(NCSU-1). The feline/FIV model may be a suitable system to investigate the mechanisms of lentivirus-associated CD4+ lymphocyte depletion in vivo.