Syncytium formation induced by human immunodeficiency virus type 1 isolates correlates with affinity for CD4.

Different strains of human immunodeficiency virus type 1 (HIV-1) show considerable divergence in genetic content and biological properties. One property that has been closely correlated with clinical prognosis is the ability to induce syncytia formation in susceptible cells. This ability had been correlated with the V3 loop sequence of major envelope glycoprotein, gp120, but recent reports have questioned this connection. We investigated the contributions of different regions of the env gene to syncytia induction using chimeric viruses that contain part of the genome of a strain that lacks this ability (HIV-1(Ba-L)) within the genome of a virus that can form syncytia (HIV-1(HXB-2)). When tested in two cell lines susceptible to both parental viruses, as well as in primary cells, these chimeric viruses demonstrated that the ability to induce syncytia formation was determined by regions of env outside the V3 loop, which encompass residues that contribute to the binding of CD4 by gp120. Further investigation failed to show any difference in the expression of gp120 on the cell surface or cell adhesion molecules by cells infected with SI or NSI variants that would explain the observed differences in the ability to form syncytia. Assays of relative affinity for CD4 indicated that gp120 from SI variants showed a significantly higher affinity for CD4 than gp120 from NSI variants. These observations suggest that areas of the HIV-1 env gene contributing to the CD4 binding site may also contribute to the determination of syncytium-inducing (SI) and non-syncytium-inducing (NSI) phenotypes.

Impaired antigen presentation to CD4+ T-cells by HIV-infected monocytes is related to down-modulation of CD4 expression on helper T-cells: possible involvement of HIV-induced cellular factors.

Defective antigen presentation by HIV-infected monocytes is related to severe immune dysfunction in patients with AIDS, although the mechanism by which this process occurs is not well defined. Here we report that reduced capacity by HIV-infected monocytes to stimulate or present antigen to CD4+ T-cells was mediated by cellular factors associated with the plasma membranes of HIV-infected monocytes. In contrast, soluble factors secreted by HIV-infected monocytes had little or no effect on T-cell stimulation. Reduced T-cell stimulation by HIV-infected monocytes was related to down-modulation of CD4 expression on helper T-cells and was not affected by the inclusion of anti-HIV-gpl20 Ab, indicating the involvement of soluble or cell-associated viral envelope protein to be less likely. Exposure of CD4+ T-cells, that had been in co-culture with HIV-infected monocytes, to uninfected monocytes partially restored impaired T-cell stimulation. Thus, for the first time we report that altered capacity of HIV-infected monocytes to stimulate and present antigen to CD4+ T-cells is related to down-modulation of CD4 expression on T-cells, and appears to occur via membrane-associated cellular factors on HIV-infected monocytes.

Expression of CD26 does not correlate with the replication of macrophage-tropic strains of HIV-1 in T-cell lines.

Strains of human immunodeficiency virus type 1 (HIV-1) differ significantly in both genetic content and biological properties. One of the earliest discovered differences between HIV-1 strains was divergence in the relative ability of different strains to replicate in either T-cell lines or monocytes/macrophages. This observation has led to the suggestion that molecules present on the surface of HIV-susceptible cells other than CD4 may interact with gp120 in facilitating the entry of HIV-1 into host cell populations. Several reports have suggested that CD26, a cell surface protease expressed on many cells of the immune system including some CD4+ T-cells and macrophage, may be an accessory molecule for HIV-1 entry. Recently, it has also been reported that the expression of high levels of CD26 correlates with the entry and replication of macrophage-tropic strains of HIV-1 in a T-cell line. In this report, we demonstrate that replication of macrophage-tropic strains of HIV-1 in T-cell lines is independent of CD26 expression. From this observation, we conclude that CD26 plays no role in the entry of HIV-1 into these cells.

Model for studying virus attachment. II. Binding of biotinylated human T cell leukemia virus type I to human blood mononuclear cells potential targets for human T cell leukemia virus type I infection.

Purified human T cell leukemia virus type I (HTLV-I) was biotinylated and used to study its attachment to human PBMC. The use of biotinylated HTLV-I (biot-HTLV-I) in conjunction with mouse mAb specific for selected cell-surface molecules and flow cytometric analysis allowed us to positively identify virus-binding cells among a heterogeneous blood mononuclear cell population. Biot-HTLV-I efficiently bound not only to T cells, but also to B cells and monocytes. Preincubation of monocytes with excess of unlabeled HTLV-I significantly reduced the attachment of biot-HTLV-I. HTLV-I not only bound to, but also infected, B cells, as suggested by: i) in situ hybridization of a 35S-labeled full length HTLV-I DNA probe with EBV-transformed B cells, previously cocultured with HTLV-I-producing (G11MJ) T cells, and ii) hybridization of the same nick-translated 32P-labeled DNA probe with blotted DNA from similar HTLV-I-infected EBV-transformed B cells. HTLV-I infection did not affect the ability of B cells to secrete IgG. These findings suggest that HTLV-I cannot only infect cells of the T lineage, but can also infect B cells.

Identification of human immunodeficiency virus in the heart of a patient with acquired immunodeficiency syndrome.

A 35-year-old homosexual white male diagnosed as having human immunodeficiency virus (HIV) infection presented with recurrent pericardial effusion, and HIV was isolated from the pericardial fluid. Autopsy revealed an extensive gross infiltrative lesion in the left ventricle of the heart. Microscopic examination of the myocardium demonstrated multifocal atypical lymphoid cell infiltrations associated with scattered foci of myocardial necrosis. The atypical lymphoid cells were immunoreactive to UCHL-1, a T cell antigen antibody. HIV was immunocytochemically identified in the cardiac lymphocytic infiltrate with anti-p24 monoclonal antibodies. This finding was supported by ultrastructural examination and in situ hybridization study.

Evaluation of two in vitro assays to screen for potential developmental toxicants.

To evaluate two in vitro assays for their ability to detect known developmental toxicants and nontoxicants, a series of 44 coded compounds were assayed by two independent laboratories using standardized protocols. The two test systems were (1) the human embryonic palatal mesenchymal cell growth inhibition assay and (2) the mouse ovarian tumor cell attachment inhibition assay. After all compounds were tested, they were decoded and ranked according to the minimum IC50 value (the millimolar concentration of compound which inhibits growth or attachment by 50% compared to the solvent control) from either test. The in vitro test result concordance with established in vivo animal and human test results was examined over a wide range of concentration levels (above which the in vitro results were called positive and below which they were considered negative). A positive response from either test was defined as a positive in vitro response. Concordance was defined as the number of correct responses divided by the number of chemicals tested. At the 1 mM level, the concordance of data from the combined in vitro assays with the in vivo data was 66% in one laboratory and 58% in the other. The maximum agreement between the combined in vitro and in vivo data was reached at the 20 mM level, where there was a 73 and 74% concordance of results in the two laboratories. At that level there was a 16 and 10% incidence of false negative results, and a 54 and 77% incidence of false positive results. A portion of these false negative compounds may require metabolic activation. The use of either assay alone was not as accurate as using a positive result from either test. Agreement of the in vitro data at the 10 mM level with available human data was 71 and 75% for each laboratory. The data indicate that the two assays are complimentary and as such the combination of these assays could be useful as a preliminary screen to establish priorities for in vivo developmental toxicity testing.