Human immunodeficiency virus (HIV) type 2-mediated inhibition of HIV type 1: a new approach to gene therapy of HIV-infection.

Human immunodeficiency virus (HIV) type 2, the second AIDS-associated human retrovirus, differs from HIV-1 in its natural history, infectivity, and pathogenicity, as well as in details of its genomic structure and molecular behavior. We report here that HIV-2 inhibits the replication of HIV-1 at the molecular level. This inhibition was selective, dose-dependent, and nonreciprocal. The closely related simian immunodeficiency provirus also inhibited HIV-1. The selectivity of inhibition was shown by the observation that HIV-2 did not significantly downmodulate the expression of the unrelated murine leukemia virus; neither did the murine leukemia virus markedly affect HIV-1 or HIV-2 expression. Moreover, while HIV-2 potently inhibited HIV-1, the reverse did not happen, thus identifying yet another and remarkable difference between HIV-1 and HIV-2. Mutational analysis of the HIV-2 genome suggested that the inhibition follows a complex pathway, possibly involving multiple genes and redundant mechanisms. Introduction of inactivating mutations into the structural and regulatory/accessory genes did not render the HIV-2 provirus ineffective. Some of the HIV-2 gene defects, such as that of tat and rev genes, were phenotypically transcomplemented by HIV-1. The HIV-2 proviruses with deletions in the putative packaging signal and defective for virus replication were effective in inducing the suppressive phenotype. Though the exact mechanism remains to be defined, the inhibition appeared to be mainly due to an intracellular molecular event because it could not be explained solely on the basis of cell surface receptor mediated interference. The results support the notion that the inhibition likely occurred at the level of viral RNA, possibly involving competition between viral RNAs for some transcriptional factor essential for virus replication. Induction of a cytokine is another possibility. These findings might be relevant to the clinical-epidemiological data suggesting that infection with HIV-2 may offer some protection against HIV-1 infection.

Human naive and memory T lymphocytes differ in telomeric length and replicative potential.

The present study has assessed the replicative history and the residual replicative potential of human naive and memory T cells. Telomeres are unique terminal chromosomal structures whose length has been shown to decrease with cell division in vitro and with increased age in vivo for human somatic cells. We therefore assessed telomere length as a measure of the in vivo replicative history of naive and memory human T cells. Telomeric terminal restriction fragments were found to be 1.4 +/- 0.1 kb longer in CD4+ naive T cells than in memory cells from the same donors, a relationship that remained constant over a wide range of donor age. These findings suggest that the differentiation of memory cells from naive precursors occurs with substantial clonal expansion and that the magnitude of this expansion is, on average, similar over a wide range of age. In addition, when replicative potential was assessed in vitro, it was found that the capacity of naive cells for cell division was 128-fold greater as measured in mean population doublings than the capacity of memory cells from the same individuals. Human CD4+ naive and memory cells thus differ in in vivo replicative history, as reflected in telomeric length, and in their residual replicative capacity.

Telomere length and replicative aging in human vascular tissues.

Because repeated injury of the endothelium and subsequent turnover of intimal and medial cells have been implicated in atherosclerosis, we examined telomere length, a marker of somatic cell turnover, in cells from these tissues. Telomere lengths were assessed by Southern analysis of terminal restriction fragments (TRFs) generated by HinfI/Rsa I digestion of human genomic DNA. Mean TRF length decreased as a function of population doublings in human endothelial cell cultures from umbilical veins, iliac arteries, and iliac veins. When endothelial cells were examined for mean TRF length as a function of donor age, there was a significantly greater rate of decrease for cells from iliac arteries than from iliac veins (102 bp/yr vs. 47 bp/yr, respectively, P < 0.05), consistent with higher hemodynamic stress and increased cell turnover in arteries. Moreover, the rate of telomere loss as a function of donor age was greater in the intimal DNA of iliac arteries compared to that of the internal thoracic arteries (147 bp/yr vs. 87 bp/yr, respectively, P < 0.05), a region of the arterial tree subject to less hemodynamic stress. This indicates that the effect is not tissue specific. DNA from the medial tissue of the iliac and internal thoracic arteries showed no significant difference in the rates of decrease, suggesting that chronic stress leading to cellular senescence is more pronounced in the intima than in the media. These observations extend the use of telomere size as a marker for the replicative history of cells and are consistent with a role for focal replicative senescence in cardiovascular diseases.

In vivo estimates of division and death rates of human T lymphocytes.

We present data on the decay, after radiotherapy, of naive and memory human T lymphocytes with stable chromosome damage. These data are analyzed in conjunction with existing data on the decay of naive and memory T lymphocytes with unstable chromosome damage and older data on unsorted lymphocytes. The analyses yield in vivo estimates for some life-history parameters of human T lymphocytes. Best estimates of proliferation rates have naive lymphocytes dividing once every 3.5 years and memory lymphocytes dividing once every 22 weeks. It appears that memory lymphocytes can revert to the naive phenotype, but only, on average, after 3.5 years in the memory class. The lymphocytes with stable chromosome damage decay very slowly, yielding surprisingly low estimates of their death rate. The estimated parameters are used in a simple mathematical model of the population dynamics of undamaged naive and memory lymphocytes. We use this model to illustrate that it is possible for the unprimed subset of a constantly stimulated clone to stay small, even when there is a large population of specific primed cells reverting to the unprimed state.