No T-cell tyrosine protein kinase signalling or calcium mobilization after CD4 association with HIV-1 or HIV-1 gp120

Replication of an acutely lethal simian immunodeficiency virus activates and induces proliferation of lymphocytes

A variant of simian immunodeficiency virus from sooty mangabey monkeys (SIVsmm), termed SIVsmmPBj14, was previously identified and shown to induce acute disease and death within 1 to 2 weeks of inoculation of pig-tailed macaques and mangabey monkeys (P. N. Fultz, H. M. McClure, D. C. Anderson, and W. M. Switzer, AIDS Res. Hum. Retroviruses 5:397-409, 1989). SIVsmmPBj14 differed from its parent virus, SIVsmm9, not only in pathogenicity but also in multiple in vitro properties. As a first approach to understanding the biological and molecular mechanisms responsible for the acute disease and death induced by this variant, virus-host cell interactions of SIVsmmPBj14 and SIVsmm9 were studied. Initial rates of replication of the two viruses were identical in primary peripheral blood mononuclear cells (PBMC) from normal pig-tailed macaques and mangabey monkeys, but SIVsmmPBj14 infection always resulted in higher yields of virus than did SIVsmm9 infection, as assessed by levels of reverse transcriptase activity in culture supernatants. Surprisingly, despite its cytopathicity for macaque and mangabey CD4+ cells, replication of SIVsmmPBj14 was accompanied by up to 10-fold increases in number of viable cells compared with cell numbers in uninfected or SIVsmm9-infected cultures. Furthermore, SIVsmmPBj14 was shown to infect and replicate in resting PBMC just as efficiently as in mitogen-stimulated PBMC, irrespective of whether exogenous interleukin-2 (IL-2) or antibodies that neutralized IL-2 were added to culture media. Accumulation of virus in culture supernatants of resting PBMC preceded by several days the appearance of activated cells which expressed the IL-2 receptor alpha subunit (CD25), suggesting that activation of cells was not essential for replication. The ability to activate and to induce simian PBMC to proliferate appeared specific for the acutely lethal variant because incorporation of [3H]thymidine by PBMC from naive animals was observed only upon incubation with concentrated, heat-inactivated SIVsmmPBj14 and not with other viruses. Both CD4(+)- and CD8(+)-enriched cell populations proliferated in response to SIVsmmPBj14. These results are consistent with in vivo observations and suggest that the abilities both to replicate in resting cells and to induce lymphocytes to proliferate may contribute to the extreme virulence of SIVsmmPBj14.

Anti-immunoglobulin stimulation of B lymphocytes activates src-related protein-tyrosine kinases

Stimulation of resting B lymphocytes with antibodies to surface immunoglobulin (sIgD or sIgM) induces protein tyrosine phosphorylation, implicating one or more B-cell protein-tyrosine kinases (PTKs) in sIg signal transduction. We have evaluated whether members of the src family of PTKs are involved in this process. Our results show that addition of antibodies to IgD or to IgM can stimulate the PTK activity of the blk, fyn, and lyn gene products. Additionally, all three PTKs were found to coimmunoprecipitate with sIg in digitonin lysates from resting B cells. In all stimulatory conditions, whether initiated through sIgD or sIgM, the blk gene product p56blk displayed the strongest activation index. The kinetics of activation of these kinases, particularly that of p56blk, paralleled the early appearance of newly tyrosine-phosphorylated B-cell proteins, suggesting that this group of kinases may account for some portion of the tyrosine kinase activity in sIg-activated B cells. These observations demonstrate a functional and possible physical association between the members of the src family of PTKs and the B-cell antigen receptors.

CD4-gp120 interactions

The three-dimensional structure of the binding domain of the CD4 molecule has been determined and extensive mutational analyses of the respective binding sites on gp120 and CD4 have been completed. The consequences of gp120-CD4 binding with respect to secondary changes in the virion, or the cell, that may be required for infection or that may interfere with cellular function are current active areas of investigation.

Biochemical identification of a direct physical interaction between the CD4:p56lck and Ti(TcR)/CD3 complexes

The CD4 and CD8 antigens function in synergy with the TcR/CD3 complex in the generation of intracellular signals leading to T cell proliferation. The association of the protein-tyrosine kinase p56lck with CD4 and CD8 provides a potential mechanism in the generation of intracellular signals. Several studies have shown that CD4 can co-modulate with TcR/CD3 suggesting that these receptor complexes may associated on the surface of the T cell. Nevertheless, it has proven difficult to formally demonstrate a direct physical interaction between the CD4 and TcR/CD3 complexes using biochemical techniques. In this study, we have used the sensitivity of the in vitro kinase assay to show a direct physical linkage between the CD4:p56lck complex and various CD3 subunits. Immunoprecipitation of CD4 from cell lysates derived from the T lymphoblastoid line HPB-ALL results in the co-purification of p56lck with an additional polypeptide at 20 kDa. Re-precipitation analysis and isoelectric focusing demonstrated that this band corresponds to the CD3 epsilon chain. An alternative approach which involves the labeling of microsomal membranes with [gamma-32P]ATP revealed the presence of CD3 epsilon and zeta chains in anti-CD4 immunoprecipitates. By contrast, we were unable to demonstrate the association of the CD4:p56lck and TcR/CD3 complex in resting peripheral blood lymphocytes. These data indicate that the CD4:p56lck and TcR/CD3 complexes have the ability to form stable complexes on the surface of certain T cell lines.

Selective elimination of HIV-1-infected cells with an interleukin-2 receptor-specific cytotoxin

Infection by human immunodeficiency virus type 1 (HIV-1) is associated with cellular activation and expression of the interleukin-2 (IL-2) receptor. A genetically engineered fusion toxin, DAB486 IL-2, that contains the enzymatic site and translocation domain of diphtheria toxin and the receptor binding domain of IL-2 specifically kills cells that express high-affinity IL-2 receptors. This toxin selectively eliminated the HIV-1-infected cells from mixed cultures of infected and uninfected cells and inhibited production of viral proteins and infectious virus. Thus, cellular activation antigens present a target for early antiviral intervention.

Positive signal transduction via surface CD4 molecules does not need coexpression of the CD3/TcR complex

We previously reported that the human CD4 molecule is capable of transducing a positive signal when activated by an anti-CD4 mAb B66. This antibody, in contrast to many other anti-CD4 mAb, induced IL2 production and proliferation of resting CD4+ peripheral blood T lymphocytes in the absence of any other signal. We further reported that anti-CD4 mAb B66 was able to induce IL2 production in murine T-cell hybridoma cells transfected with full-length human CD4 cDNA. In the present study, we extend these findings by demonstrating that anti-CD4 mAb B66 was able to induce Ca2+ mobilization and IL2 production in a CD3/TcR- variant 31-13, of the CD3/TcR+ Jurkat cell line. We further showed that anti-CD4 mAb B66 was able to activate CD4+ cells from the promonocytic cell line U937. In these cells, mAb B66 induced Ca2+ mobilization when cross-linked with a second antibody and, in addition, the production of large quantities of IL1 beta was measured. In essence, our findings provide direct evidence that cross-linking of CD4 may cause T-cell activation in the absence of the coexpression of the CD3/TcR molecular complex and that, in addition, CD4 might transduce a positive signal in CD4+ cells of the myeloid lineage.