Stock vs. Passage 24 SIV CD4 dependence and infection of CD4+ and CD14+ depleted hPBMCs. A. Day 7 growth of stock SIV mac239 CEMx174 and SIV 17E-Fr in human PBMCs was robust (> 8,000 pg/ml p27) whether or not cells were pretreated with CD4 blocking concentrations of Leu3a. Limited supernatant volumes prevented endpoint dilutions, but contained p27 levels that remained off-scale (*) at 1:10 dilution. ImmunodepletedhPBMCs containing fewer than 1% CD4+ or CD14+ cells still supported readily detectable levels of viral replication, but at significantly lower levels (** p < 0.0001 vs. 8,000 pg/ml). B. By the 24th passage in allogeneic hPBMCs, both SIV mac239CEMx174 and SIV 17E-Fr had lost the ability to infect PBMCs pretreated with anti-CD4. Moreover, no growth of either strain was detectable above EIA sensitivity (dashed line) in day 7 supernatants of hPBMC depleted of CD4+ and CD14+ cells.