Background:  Since HIV infection can be considered an acquired genetic disease, gene therapy has been proposed as a potential addition to current antiretroviral regimens in the treatment of HIV-infected patients.  However, data regarding the long-term safety and in vivo cell persistence of genetically modified cells in HIV-infected patients are limited.  We examined these factors retrospectively in 2 cohorts of monozygotic twins discordant for HIV infection that received syngeneic peripheral blood lymphocytes virally transduced with either the neomycin phosphotransferase (NPT) gene or a chimeric receptor gene (CD4/CD3-z). 

Methods:  A retrospective chart review was performed on all patients involved in NIH studies 93-I-0110 and 94-I-0206.  Data were collected on the quantity of infusions, genetically modified CD4+ T lymphocytes, total CD4+ T lymphocytes, HIV viral load, and adverse events.  Prevalence of virally transduced cells was determined using semiquantitative PCR, Southern blotting, and phosphorimaging.  Copy number or proportion of circulating gene-modified cells was calculated from band intensities using a standard curve from experimental samples with known quantities.

Results:  24 of 27 patients who received genetically modified cells had data available regarding the proportion of circulating gene-modified CD4+ T lymphocytes.  As of their most recent evaluation, all 24 patients continue to demonstrate persistence of genetically modified cells.  Mean in vivo cell persistence at the time of this analysis was 5.15 + 1.82 years for NPT gene-transduced cells and 3.32 + 1.17 years for CD4/CD3-z gene-transduced cells.  3 of 9 patients who received intermittent subcutaneous IL-2 in addition to their CD4/CD3-z gene-transduced cell infusions were able to sustain a stable proportion of gene modified CD4+ T lymphocytes in the interval since their last genetically modified cell infusion. The mean CD4+ gene-modified T lymphocytes/10⁶ CD4+ T lymphocytes in these 3 patients was 1.32 x 10⁴, 2.87 x 10⁴ and 4.14 x 10⁴.  Despite receiving similar numbers of genetically modified cells, the remaining 21 patients demonstrated decreasing proportions of genetically modified CD4+ T lymphocytes over time.  In both studies, long-term follow-up has revealed no significant adverse events or changes in virologic and immunologic parameters related to the genetically modified cells.

Conclusions:  Genetically altered cells have long-term in vivo persistence with an excellent safety profile in an HIV-infected population.  These findings provide a rational basis for future clinical trials of genetic strategies, possibly in combination with other immune-based therapies such as IL-2, in the treatment of HIV-infected patients.