Localization of B-cell stimulatory activity of HIV-1 to the carboxyl terminus of gp41

Patients with AIDS are known to have B-cell hyperactivity. We have previously demonstrated that an extract of HIV-1 could induce differentiation of peripheral blood B lymphocytes of healthy volunteers into immunoglobulin-secreting cells. In an attempt to delineate the B-cell stimulatory subregion in HIV-1, we have investigated the influences of native glycoproteins and recombinant proteins of the envelope. The complete envelope glycoprotein, gp160 and a recombinant protein in the carboxyl terminal region of gp41 termed PE-8 were effective in inducing terminal differentiation of normal peripheral blood B lymphocytes and did so in a T-lymphocyte-dependent manner. The activity was not present in the native exterior envelope glycoprotein, gp120 and several other recombinant proteins, viz PE-2 an PE-3, which are in the amino terminal region of gp120 or in env-9, a protein in the junctional region of gp120 and gp41. Polyclonal and monoclonal antibodies directed to diverse regions of the envelope abrogated the influence of gp160. The PE-8-induced B-cell differentiation was abrogated by goat anti-gp160 antibody but not by goat anti-gp120 antibody or monoclonal antibody to the amino terminal of gp41. These studies suggest that a putative polyclonal B-cell stimulatory epitope of HIV-1 is located in the carboxyl end of the envelope glycoprotein.

Characterization of the secreted, native gp120 and gp160 of the human immunodeficiency virus type 1

We have previously shown that the cell line 6D5(451) chronically infected with the HIV-1 isolate HTLV-III(451), secretes the HIV-1 envelope glycoproteins gp120 and gp160 in the extracellular medium. The HTLV-III(451) gp120 and gp160 were purified by sequential affinity chromatographic steps using a monoclonal antibody to HIV-1 gp41 and an anti-HIV-1-positive human serum. Amino acid sequence analysis of gp120 and gp160 showed the loss of the signal peptide. Digestion of the purified gp120 and gp160 with endoglycosidases revealed that both proteins are heavily glycosylated and contain complex carbohydrates, in contrast to the intracellular form of gp160 which has been shown to contain mannose-rich immature sugars. Competitive binding analysis showed that while both gp120 and gp160 bind CD4, the affinity of gp160 was five times lower than that of gp120. Both gp120 and gp160 inhibited syncytia formation by HIV-1-infected cells when mixed with CD4+ cells. Furthermore, both gp120 and gp160 had strong mitogenic effects on the T cells from HIV-1-infected gibbons but not on cells from uninfected gibbons.

The HIV protein, GP120, activates nuclear protein kinase C in nuclei from lymphocytes and brain

Nuclear pool(s) of protein kinase C (PKC) may be a common target for hormones and growth factors which affect the trophic state of cells. The data presented demonstrate a time and dose-dependent activation of nuclear PKC by the HIV coat protein, gp120, in isolated nuclei from rat spleen and hippocampus. This gp120-stimulated PKC response was blocked by specific PKC inhibitors, a monoclonal antibody to PKC, and a monoclonal antibody directed against the murine T4 analog, L3T4. It is suggested that the gp120 interaction with the nuclear trophic factor-PKC system may impair normal gene expression, and thus result in the clinical symptoms associated in AIDS infection.

The CD4 molecule transmits biochemical information important in the regulation of T lymphocyte activity

The role of the CD4 molecule in the transmission and regulation of the biochemical signals involved in T cell activation was investigated using an anti-CD4 monoclonal antibody termed 6B10. 6B10 immunoprecipitated the 55-kDa CD4 molecule and detected an epitope of CD4 that overlapped with that detected by OKT4A, B, and D. 6B10, 6B10 Fab fragments and recombinant HIV envelope glycoprotein (gp120) induced calcium mobilization in PBMC. 6B10 stimulation also resulted in calcium mobilization in murine L cells expressing transfected CD4 gene products, indicating that CD4-mediated calcium mobilization occurred independently of the CD3/T cell receptor (TCR) complex. 6B10 induced a phosphatidylinositol response, but the response resulted in reduced inositol phosphate production compared to levels obtained using OKT3. Though 6B10 caused calcium mobilization and a phosphatidylinositol response, 6B10 did not induce DNA synthesis. The amount of inositol phosphates produced by 6B10 may be below the threshold necessary for cell cycle progression. We hypothesized that 6B10-mediated calcium mobilization is important in the regulation of T cell proliferation. 6B10, but not 6B10 Fab fragments, inhibited OKT3-induced DNA synthesis. Furthermore, 6B10 but not 6B10 Fab fragments inhibited OKT3-induced calcium mobilization, suggesting that crosslinking of CD4 may be an important factor determining whether signals result in both the up- and down-regulation of CD3/TCR complex function. The implication of this work is that signals generated via the CD4 molecule are important in the regulation of T cell function and that the signals generated as a result of HIV gp120 binding to CD4 can contribute to the mechanism by which HIV inhibits T cell function.

HIV-gp120 can block CD4-class II MHC-mediated adhesion

A possible component of the immune dysfunction associated with infection by HIV is the inhibition of CD4 function resulting from the avid binding of soluble HIV envelope glycoprotein (gp120) to cell surface CD4. We assessed CD4 function by measuring the ability of CD4+ T cells to form conjugates with cell size lipid vesicles, artificial target cells (ATC), bearing the natural ligand for CD4, MHC class II proteins. Conjugate formation was a transient process with the greatest number of specific cell to ATC conjugates found after approximately 30 min of incubation at 37 degrees C. Addition of gp120 specifically blocked conjugates between CD4+ cells and class II ATC in a concentration-dependent manner. These data indicate that T lymphocyte adhesion mediated by CD4 is a dynamic event and that binding of gp120 to CD4 is able to disrupt the normal progression of the interaction between CD4+ T lymphocytes and class II+ APC.

Inhibition of functional properties of tetanus antigen-specific T-cell clones by envelope glycoprotein GP120 of human immunodeficiency virus

We investigated mechanisms by which the soluble native envelope glycoprotein gp120 of the human immunodeficiency virus (HIV-1) suppresses antigen-driven T cell responses. For this study, exogenous interleukin-2 (IL-2)-independent, antigen-specific, CD4 positive, human T-cell clones were developed by cyclic restimulation with soluble tetanus toxoid antigen. In the presence of soluble antigen and antigen-presenting cells (APC), T-cell clones proliferated and secreted IL-2. Purified gp120 suppressed the proliferative responses of the T-cell clones with concomitant suppression of IL-2 secretion; proliferative responses of CD8+ T cells preincubated with gp120 were not inhibited. A short pulse of 20 minutes with gp120 was sufficient to inhibit the proliferative response of the T-cell clones. Anti-CD3 monoclonal antibody (MoAb)-driven proliferation of the T-cell clones was also suppressed by gp120, but responses elicited by mitogens, phorbol myristate acetate (PMA) plus calcium ionophore, ionomycin, anti-CD2 MoAbs, and a combination of anti-CD3 plus anti-CD28 MoAb driven responses remained unaffected. Investigation of signal transduction events showed that antigen-driven early activation signals via translocation of protein kinase C (PKC), increase in intracellular inositol phosphates, and increase in intracellular calcium were suppressed in gp120 pretreated, tetanus toxoid antigen-stimulated T-cell clones. One mechanism of immune suppression by gp120 may involve interference with the initiation of signal transduction through the T-cell receptor complex.

Cytokine augmentation of human immunodeficiency virus type 1 (HIV-1) gp120-specific cellular cytotoxicity

Currently available anti-human immunodeficiency virus type 1 (HIV-1) agents such as azidothymidine can prevent de novo virus infection in vitro but lack significant activity against chronically infected cells. Our laboratory has recently described glycoprotein (gp)120-specific cell mediated cytotoxicity (CMC) present in HIV-1-seropositive individuals that is capable of destroying virally infected cells. As a means of potentially eliminating persistent reservoirs of HIV-1, we examined the ability of various cytokines to augment preexisting gp120-specific CMC activity of peripheral blood mononuclear cells obtained from early disease patients. We found that interferon-gamma alone had no effect on gp120 cellular reactivity; however, the combination of interferon-gamma plus IL-2 produced enhancement beyond that of IL-2 alone.