Background: Recent studies indicate that soluble Fas (sFas) may modulate T-cell apoptosis, since it inhibits Fas-ligand (Fas-L)-mediated cytotoxicity in vitro. Here, we explored whether the soluble receptor and its major immunogenic domain, namely VEINCTR-N, interfered with apoptosis of T cells from human immunodeficiency virus-type 1 (HIV-1)+ subjects showing serum elevations of both the soluble receptor and anti-Fas antibodies, and with that of several T-cell lines.
Materials and methods: Both proliferation and apoptosis extent of T cells from 16 HIV-1+ patients showing serum anti-VEINCTR-N immunoglobulin G (IgG) and 15 controls were tested after incubation with sFas and three 8-mer peptides of its first consensus sequence that included VEINCTR-N. Several cell lines were also investigated by flow cytometry for their expression of Ki-67, the APO2.7-related mitochondrial protein, and the annexin-V. In addition, we evaluated the expression of Fas-L and caspases FLICE, CPP32 and ICE either by flow cytometry, immunoblotting, and/or reverse transcription polymerase chain reaction (RT-PCR).
Results: Cell proliferation in cultures from both patients and controls was affected significantly by sFas and VEINCTR-N. However, a prevalent increase of the subdiploid DNA-containing cell population occurred within these cultures. Similarly, Jurkat, CEM cells, and a mouse WR19L transformant overexpressing native human Fas underwent prompt apoptosis, which was detected as enlargement of APO2.7-reactive and annexin-V-positive populations. By exploring the Fas pathway in Jurkat cells, we found that both apoptosis inducers acted through Fas, since Fas-L, as well as CPP32 and FLICE were activated. By contrast, ICE was up-regulated only in control cells treated with tumor necrosis factor alpha (TNFalpha).
Conclusions: These data suggest that the soluble molecular forms of Fas prime cell death in Fas-positive cells. Therefore, the shedding of high amounts of sFas in HIV- 1 disease is possibly entrusted with amplification of the death execution program by cells functionally exhausted and committed to die. It is conceivable that the appearance of anti-Fas antibodies reflects an attempt by the immune system to neutralize these effective forms of the receptor and its structurally degraded domains, such as VEINCTR-N.