CD4 and infection: Analysis of apoptosis in circulating T cells by Annexin-V binding did not show significant elevations, compared to baseline, in either untreated or IL-7-treated animals throughout the acute phase of SIV infection (Figure 4B), suggesting that the peripheral blood compartment is largely spared from in vivo apoptosis during primary infection; however, IL-7-treated animals exhibited significant and sustained increases in the intracellular levels of the anti-apoptotic protein Bcl-2 in both CD4+ and CD8+ T cells during the first three weeks of treatment (Figure 4C).