GaJpin/DA/N2 IIIBenv 
The CTL assay is based on a complex biological system, with consequent variability 
in the reproducibility of the results. Therefore, studies have been undertaken to 
determine the variability of the human HIV-1 env -^Cr-release assay. Preliminary data 
indicate that the coefficient of variation from individual measurements is between 
10 - 20%, depending on the effector to target cell ratio that is being evaluated. This 
makes the assay possible to work with, but the variability and the relatively high level of 
HIV-1 env-specific CTL detected following restimulation of patient PBMC point to the 
potential advantage of a more HIV-IT (V)-specific marker of CTL activity. One 
approach would be to measure the HIV-1 env-specific lysis without the in vitro 
stimulation step resulting in lower background lysis (but also lower HIV-1 env-specific 
lysis). In 5 patients studied thus far, only one patient showed direct lysis above 10% 
(Figure 18), whereas all 5 exhibited levels of lysis considerably above 10% after in vitro 
stimulation. Because measurable CTL in the direct assay is a rare event, administration 
of HIV-IT (V) might boost the level of CTL activity measured directly to significant 
levels of lysis above that obtained prior to HIV-IT (V) administration. 
An additional approach to examining responses to HIV-IT (V) involves use of 
peptide coated targets. Studies were initiated to examine the lysis of HIV-1 IIIBenv 
peptide (P18IIIB) coated target cells. Because HIV-1 IIIB is under-represented in the 
United States population of HIV-1 infected patients, and the HIV-IT (V) incorporates the 
IIIBenv gene, responses against the P18IIIB peptide in clinical trial patients would likely 
be due to HIV-IT (V) stimulation. In pilot experiments, HIV-1 gag peptides employed as 
positive controls yielded greater than 50% specific lysis in CTL assays using in vitro 
stimulation of PBMC. However, none of the 7 patients tested for P18IIIB specific lysis 
have exhibited lysis above 5%. Four of these 7 patients were also tested for P18MN 
specific lysis with only one showing P18MN specific cell lysis (Figure 19). These data 
suggest that P 1 8 IIIB specific peptide lysis may be a useful indicator for HIV-IT (V) 
induced reactivity. 
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