The structure of CD4

CD4 and BST-2/tetherin proteins retro-translocate from endoplasmic reticulum to cytosol as partially folded and multimeric molecules

CD4 and BST-2/Tetherin are cellular membrane proteins targeted to degradation by the HIV-1 protein Vpu. In both cases proteasomal degradation following recruitment into the ERAD pathway has been described. CD4 is a type I transmembrane glycoprotein, with four extracellular immunoglobulin-like domains containing three intrachain disulfide bridges. BST-2/Tetherin is an atypical type II transmembrane glycoprotein with an N-terminal transmembrane domain and a C-terminal glycophosphatidylinositol anchor, which dimerizes through three interchain bridges. We investigated spontaneous and Vpu-induced retro-translocation of CD4 and BST-2/Tetherin using our novel biotinylation technique in living cells to determine ER-to-cytosol retro-translocation of proteins. We found that CD4 retro-translocates with oxidized intrachain disulfide bridges, and only upon proteasomal inhibition does it accumulate in the cytosol as already reduced and deglycosylated molecules. Similarly, BST-2/Tetherin is first exposed to the cytosol as a dimeric oxidized complex and then becomes deglycosylated and reduced to monomers. These results raise questions on the required features of the putative retro-translocon, suggesting alternative retro-translocation mechanisms for membrane proteins in which complete cysteine reduction and unfolding are not always strictly required before ER to cytosol dislocation.

Cell-type specific requirements for thiol/disulfide exchange during HIV-1 entry and infection

BACKGROUND

The role of disulfide bond remodeling in HIV-1 infection is well described, but the process still remains incompletely characterized. At present, the data have been predominantly obtained using established cell lines and/or CXCR4-tropic laboratory-adapted virus strains. There is also ambiguity about which disulfide isomerases/reductases play a major role in HIV-1 entry, as protein disulfide isomerase (PDI) and/or thioredoxin (Trx) have emerged as the two enzymes most often implicated in this process.

RESULTS

We have extended our previous findings and those of others by focusing on CCR5-using HIV-1 strains and their natural targets--primary human macrophages and CD4+ T lymphocytes. We found that the nonspecific thiol/disulfide exchange inhibitor, 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), significantly reduced HIV-1 entry and infection in cell lines, human monocyte-derived macrophages (MDM), and also phytohemagglutinin (PHA)-stimulated peripheral blood mononuclear cells (PBMC). Subsequent studies were performed using specific anti-PDI or Trx monoclonal antibodies (mAb) in HIV-1 envelope pseudotyped and wild type (wt) virus infection systems. Although human donor-to-donor variability was observed as expected, Trx appeared to play a greater role than PDI in HIV-1 infection of MDM. In contrast, PDI, but not Trx, was predominantly involved in HIV-1 entry and infection of the CD4+/CCR5+ T cell line, PM-1, and PHA-stimulated primary human T lymphocytes. Intriguingly, both PDI and Trx were present on the surface of MDM, PM-1 and PHA-stimulated CD4+ T cells. However, considerably lower levels of Trx were detected on freshly isolated CD4+ lymphocytes, compared to PHA-stimulated cells.

CONCLUSIONS

Our findings clearly demonstrate the role of thiol/disulfide exchange in HIV-1 entry in primary T lymphocytes and MDM. They also establish a cell-type specificity regarding the involvement of particular disulfide isomerases/reductases in this process and may provide an explanation for differences among previously published studies. More importantly, from an in vivo perspective, the preferential utilization of PDI may be relevant to the HIV-1 entry and establishment of virus reservoirs in resting CD4+ cells, while the elevated levels of Trx reported in the chronic stages of HIV-1 infection may facilitate the virus entry in macrophages and help to sustain high viremia during the decline of T lymphocytes.

Indoleamine 2,3-dioxygenase (IDO) downregulates the cell surface expression of the CD4 molecule

Indoleamine 2,3-dioxygenase (IDO) has been implicated in preventing the fetus from undergoing maternal T cell-mediated immune responses, yet the mechanism underlying these kinds of IDO-mediated immune responses has not been fully elucidated. Since the CD4 molecule plays a central role in the onset and regulation of antigen-specific immune responses, and T cell is sensitive in the absence of tryptophan, we hypothesize that IDO may reduce cell surface CD4 expression. To test this hypothesis, an adenoviral vector-based construct IDO-EGFP was generated and the effect of IDO-EGFP on CD4 expression was determined on recombinant adenoviral infected C8166 and MT-2 cells, by flow cytometry and/or Western blot analysis. The results revealed a significant downregulation of cell membrane CD4 in pAd-IDOEGFP infected cells when compared to that of mock-infected cells or infection with empty vector pAd-EGFP. Further experiments disclosed that either an addition of tryptophan or IDO inhibitor could partly restore CD4 expression in pAd-IDOEGFP infected C8166 cells. Our findings suggest that downregulation of CD4 by IDO might be one of the mechanisms through which IDO regulates T cell-mediated immune responses.

Reduced monomeric CD4 is the preferred receptor for HIV

CD4 is a co-receptor for binding of T cells to antigen-presenting cells and the primary receptor for the human immunodeficiency virus type 1 (HIV). CD4 exists in three different forms on the cell surface defined by the state of the domain 2 cysteine residues: an oxidized monomer, a reduced monomer, and a covalent dimer linked through the domain 2 cysteines. The disulfide-linked dimer is the preferred immune co-receptor. The form of CD4 that is preferred by HIV was examined in this study. HIV entry and envelope-mediated cell-cell fusion were tested using cells expressing comparable levels of wild-type or disulfide bond mutant CD4 in which the domain 2 cysteines were mutated to alanine. Eliminating the domain 2 disulfide bond increased entry of HIV reporter viruses and enhanced HIV envelope-mediated cell-cell fusion 2-4-fold. These observations suggest that HIV enters susceptible cells preferably through monomeric reduced CD4, whereas dimeric CD4 is the preferred receptor for binding to antigen-presenting cells. Cleavage of the domain 2 disulfide bond is possibly involved in the conformational change in CD4 associated with fusion of the HIV and cell membranes.

Docking of peptide-T onto the D1 domain of the CD4 receptor

Peptide T (pepT) is a segment of the human immunodeficiency virus (HIV) envelope protein gp120. The peptide competitively binds to the CD4 receptor of a subset of peripheral T lymphocytes and inhibits binding of gp120. Previous studies of this laboratory allowed the assessment of a bioactive form of the peptide and a pharmacophore for the peptide-receptor interaction. In the present study the proposed bioactive form of pepT and its (4-8) segment, the smallest pepT fragment shown to retain full activity, were docked onto the D1 domain of the CD4 receptor. The bioactive conformation of the peptides complements well a cleft on the surface of the CD4 receptor, shown to be the attachment site of gp120 from site directed mutagenesis experiments. These studies provide an improved description of the ligand-receptor pharmacophore.

Ocular disease in children with vertically acquired human immunodeficiency virus infection

BACKGROUND

Clinical reports suggest that ocular disease in infants and children vertically infected with human immunodeficiency virus (HIV) is different from that in adults. Pediatric patients with acquired immunodeficiency syndrome (AIDS) and HIV infection are being treated more aggressively and are living longer, but current literature on the incidence of AIDS-related ocular disease in vertically acquired HIV infection is limited.

METHODS

Thirty-three children with culture-positive, vertically acquired HIV infections were prospectively followed with ophthalmic examinations between September 1991 and August 1996 at the University of Massachusetts. Patients were categorized as having symptomatic or asymptomatic HIV disease according to the U.S. Centers for Disease Control and Prevention guidelines. Absolute CD4 counts and other measures of immune function were obtained.

RESULTS

The average length of follow-up was 30 months, and the average number of ophthalmic examinations per patient was 4.8. Ten patients developed ophthalmic findings. Nine of 18 (50%) patients with symptomatic AIDS disease developed ophthalmic findings. One of 15 asymptomatic HIV-infected patients developed ocular findings. Two patients with absolute CD4 counts less than 10 developed cytomegalovirus retinitis.

CONCLUSIONS

These results suggest that AIDS-related ophthalmic disease is less common in vertically infected children than in adult AIDS patients. It also supports intensified clinical surveillance for cytomegalovirus retinitis in children with end-stage disease and very low CD4 counts.

The leukocyte/neuron cell surface antigen OX2 binds to a ligand on macrophages

The OX2 membrane glycoprotein contains two immunoglobulin superfamily (IgSF) domains and seems likely to interact with other cell surface proteins. A soluble chimeric protein with the two IgSF domains of OX2 engineered onto domains 3 + 4 of rat CD4 antigen was expressed. To detect possible weak interactions, the chimeric protein was coupled to fluorescent covaspheres to provide a highly avid display of OX2. The OX2 covaspheres bound macrophages but not other cell types. The specificity of the interaction was demonstrated by blocking with Fab fragments of the OX2 monoclonal antibody (mAb). A new mAb, MRC OX88, was raised against macrophages which also blocked the interaction and presumably recognizes the ligand. The epitope for the MRC OX2 mAb and a site for ligand binding were mapped to domain 1 by site-directed mutagenesis. The OX2 antigen is present on thymocytes, some lymphocytes, neurons and endothelial cells; thus, it has the potential to mediate interactions between these cell types and macrophages.