Glycosylphosphatidylinositol-anchored CD4 supports human immunodeficiency virus type 1 replication, but not cytopathic effect, in T-cell transfectants

Signaling Through the Lymphotoxin-β Receptor Stimulates HIV-1 Replication Alone and in Cooperation with Soluble or Membrane-Bound TNF-α

The level of ongoing HIV-1 replication within an individual is critical to HIV-1 pathogenesis. Among host immune factors, the cytokine TNF-α has previously been shown to increase HIV-1 replication in various monocyte and T cell model systems. Here, we demonstrate that signaling through the TNF receptor family member, the lymphotoxin-β (LT-β) receptor (LT-βR), also regulates HIV-1 replication. Furthermore, HIV-1 replication is cooperatively stimulated when the distinct LT-βR and TNF receptor systems are simultaneously engaged by their specific ligands. Moreover, in a physiological coculture cellular assay system, we show that membrane-bound TNF-α and LT-α1β2 act virtually identically to their soluble forms in the regulation of HIV-1 replication. Thus, cosignaling via the LT-β and TNF-α receptors is probably involved in the modulation of HIV-1 replication and the subsequent determination of HIV-1 viral burden in monocytes. Intriguingly, surface expression of LT-α1β2 is up-regulated on a T cell line acutely infected with HIV-1, suggesting a positive feedback loop between HIV-1 infection, LT-α1β2 expression, and HIV-1 replication. Given the critical role that LT-α1β2 plays in lymphoid architecture, we speculate that LT-α1β2 may be involved in HIV-associated abnormalities of the lymphoid organs.

The nature of membrane anchorage determines kinase association and detergent solubility of CD4

The presence of a glycosylphosphatidylinositol (GPI) anchor on a membrane protein is thought to influence aspects of the protein's biochemistry. While it has been demonstrated that a GPI-anchor is sufficient for altering the detergent solubility of integral membrane proteins, it has not been shown that the anchor is sufficient for changing the phosphoprotein associations of membrane proteins. In order to define the influence of GPI-anchors on the biochemistry of membrane proteins we compared the phosphoprotein associations and detergent solubility of wild-type and GPI-anchored CD4 expressed on HSB cell transfectants. While wild-type CD4 was mostly associated with lck kinase, GPI-anchored CD4 was associated with the 'GPI-anchored pattern of phosphoproteins'. The Triton X-100 solubilities of the two forms of CD4 were also distinct: wild-type CD4 was > 95% soluble, whereas GPI-anchored CD4 was only 65% soluble. These results underscore the deterministic role of the GPI-anchor in the properties associated with GPI-anchored proteins.

Postbinding events mediated by human immunodeficiency virus type 1 are sensitive to modifications in the D4-transmembrane linker region of CD4

Evidence from both structural and functional studies of the CD4 molecule suggests that several domains, including the transmembrane (TM) domain and the adjoining extracellular region (D4-TM linker), contribute to the post-gp12O-binding events leading to human immunodeficiency virus-mediated membrane fusion. To investigate such a role in syncytium formation and cell-free infectivity, we generated several deletion and substitution mutations in the TM and D4-TM linker regions of the CD4 molecule. We found that while the TM domain of CD4 was dispensable for cell-cell and virus-cell interactions, modifications in the D4-TM linker led to perturbations in both processes. Deletion of the five amino acid residues linking D4 to the TM domain resulted in a delayed and reduced capacity to form syncytia, whereas replacement of the residues with the heterologous sequence from the CD8 molecule restored the kinetic profile to wild-type CD4 levels. On the other hand, both mutants of the CD4 D4-TM linker demonstrated delayed cell-free human immunodeficiency virus type 1 infectivity profiles. The defective fusion capacity may be linked to structural perturbations identified with anti-CD4 monoclonal antibodies in the D1-D2 interface and D3 domain of the deletion mutant yet absent in D1 and D4. While all cells were found to bind comparable levels of gp120, both D4-TM linker mutants appeared to induce a decrease in the V3 loop exposure of bound gp120. This underexposure may explain the delays in cell-free infectivities observed for both of these mutants. Together, these findings confirm a role for regions of the CD4 molecule located outside D1 in post-gp120-binding events and suggest that the D4-TM interface contributes to the conformational changes that direct the fusion process.

Potential roles of apoptosis in viral pathogenesis

Apoptosis is a physiologic mechanism of cell death that is induced during development or reversible tissue expansion or after tissue damage to eliminate unwanted cells. Recently, viral gene products have been shown to either specifically induce or specifically inhibit apoptosis. In this review, our current understanding of the regulation of apoptosis is outlined. The mechanisms by which viruses can inhibit or induce apoptosis are described. The role of apoptosis in modulating the immune response to viral infection is discussed. Finally, the hypothesis is advanced that the balance between induction and inhibition of apoptosis is modulated by viruses to optimize the course of viral infection.

Human immunodeficiency virus type 1 Nef increases the efficiency of reverse transcription in the infected cell

We have analyzed the replication of Nef+ and Nef- isogenic human immunodeficiency virus in CEM, HUT78, MT4 lymphoid, and U937 monocytic cell lines. At each passage of infected cells, we have assessed the relative infectivity of the virus particles released in culture media by measuring the number of infections units per nanogram of p24 protein. Values appeared to be 3- to 10-fold higher for the Nef+ virus than for the Nef- number The positive effect of Nef was observed regardless of the cell line, the multiplicity of infection, and the number of virus replication cycles achieved. We showed, by using cells expressing glycosylphosphatidylinositol-linked CD4, that the enhancement of virion infectivity could be dissociated from the down-regulation of cell surface CD4 also induced by Nef. The gp120-to-p24 ratio and the RNA content of virus particles produced in the presence or in the absence of Nef were equivalent. Virions bound to cell surface CD4 receptors with equal efficiencies. Equivalent reverse transcriptase activities were measured both on exogenous substrate and on particle genomic RNAs. In contrast, reverse transcription in infected cells generated 5- to 10-fold less DNA when the virions were produced in the absence of Nef, indicating that these particles performed reverse transcription in a suboptimal environment. These data suggest that the expression of Nef in virus-producing cells is required for efficient processing of the early stages of virus replication in target cells, including the internalization in an appropriate cell compartment and the uncoating of the particle.