Envelope glycoprotein determinants of neutralization resistance in a simian-human immunodeficiency virus (SHIV-HXBc2P 3.2) derived by passage in monkeys.

The simian-human immunodeficiency virus SHIV-HXBc2 contains the envelope glycoproteins of a laboratory-adapted, neutralization-sensitive human immunodeficiency virus type 1 variant, HXBc2. Serial in vivo passage of the nonpathogenic SHIV-HXBc2 generated SHIV KU-1, which causes rapid CD4(+) T-cell depletion and an AIDS-like illness in monkeys. A molecularly cloned pathogenic SHIV, SHIV-HXBc2P 3.2, was derived from the SHIV KU-1 isolate and differs from the parental SHIV-HXBc2 by only 12 envelope glycoprotein amino acid residues. Relative to SHIV-HXBc2, SHIV-HXBc2P 3.2 was resistant to neutralization by all of the antibodies tested with the exception of the 2G12 antibody. The sequence changes responsible for neutralization resistance were located in variable regions of the gp120 exterior envelope glycoprotein and in the gp41 transmembrane envelope glycoprotein. The 2G12 antibody, which neutralized SHIV-HXBc2 and SHIV-HXBc2P 3.2 equally, bound the HXBc2 and HXBc2P 3.2 envelope glycoproteins on the cell surface comparably. The ability of the other tested antibodies to achieve saturation was less for the HXBc2P 3.2 envelope glycoproteins than for the HXBc2 envelope glycoproteins, even though the affinity of the antibodies for the two envelope glycoproteins was similar. Thus, a highly neutralization-sensitive SHIV, by modifying both gp120 and gp41 glycoproteins, apparently achieves a neutralization-resistant state by decreasing the saturability of its envelope glycoproteins by antibodies.

Apelin, the natural ligand of the orphan seven-transmembrane receptor APJ, inhibits human immunodeficiency virus type 1 entry.

In addition to the CCR5 and CXCR4 chemokine receptors, a subset of primary human immunodeficiency virus type 1 (HIV-1) isolates can also use the seven-transmembrane-domain receptor APJ as a coreceptor. A previously identified ligand of APJ, apelin, specifically inhibited the entry of primary T-tropic and dualtropic HIV-1 isolates from different clades into cells expressing CD4 and APJ. Analysis of apelin analogues demonstrated that potent and specific antiviral activity was retained by a 13-residue, arginine-rich peptide. Antiviral potency was influenced by the integrity of methionine 75, which contributes to APJ-binding affinity, and by the retention of apelin residues 63 to 65. These studies demonstrate the ability of a small peptide ligand to block the function of APJ as an HIV-1 coreceptor, identify apelin sequences important for the inhibition, and provide new reagents for the investigation of the significance of APJ to HIV-1 infection and pathogenesis.

Adaptation of a CCR5-using, primary human immunodeficiency virus type 1 isolate for CD4-independent replication.

The gp120 envelope glycoprotein of the human immunodeficiency virus type 1 (HIV-1) promotes virus entry by sequentially binding CD4 and chemokine receptors on the target cell. Primary, clinical HIV-1 isolates require interaction with CD4 to allow gp120 to bind the CCR5 chemokine receptor efficiently. We adapted a primary HIV-1 isolate, ADA, to replicate in CD4-negative canine cells expressing human CCR5. The gp120 changes responsible for the adaptation were limited to alteration of glycosylation addition sites in the V2 loop-V1-V2 stem. The gp120 glycoproteins of the adapted viruses bound CCR5 directly, without prior interaction with CD4. Thus, a major function of CD4 binding in the entry of primary HIV-1 isolates can be bypassed by changes in the gp120 V1-V2 elements, which allow the envelope glycoproteins to assume a conformation competent for CCR5 binding.

Tyrosine sulfation of the amino terminus of CCR5 facilitates HIV-1 entry.

Chemokine receptors and related seven-transmembrane-segment (7TMS) receptors serve as coreceptors for entry of human and simian immunodeficiency viruses (HIV-1, HIV-2, and SIV) into target cells. Each of these otherwise diverse coreceptors contains an N-terminal region that is acidic and tyrosine rich. Here, we show that the chemokine receptor CCR5, a principal HIV-1 coreceptor, is posttranslationally modified by O-linked glycosylation and by sulfation of its N-terminal tyrosines. Sulfated tyrosines contribute to the binding of CCR5 to MIP-1 alpha, MIP-1 beta, and HIV-1 gp120/CD4 complexes and to the ability of HIV-1 to enter cells expressing CCR5 and CD4. CXCR4, another important HIV-1 coreceptor, is also sulfated. Tyrosine sulfation may contribute to the natural function of many 7TMS receptors and may be a modification common to primate immunodeficiency virus coreceptors.

Changes in human immunodeficiency virus type 1 envelope glycoproteins responsible for the pathogenicity of a multiply passaged simian-human immunodeficiency virus (SHIV-HXBc2).

In vivo passage of a poorly replicating, nonpathogenic simian-human immunodeficiency virus (SHIV-HXBc2) generated an efficiently replicating virus, KU-1, that caused rapid CD4(+) T-lymphocyte depletion and AIDS-like illness in monkeys (S. V. Joag, Z. Li, L. Foresman, E. B. Stephens, L.-J. Zhao, I. Adany, D. M. Pinson, H. M. McClure, and O. Narayan, J. Virol. 70:3189-3197, 1996). The env gene of the KU-1 virus was used to create a molecularly cloned virus, SHIV-HXBc2P 3.2, that differed from a nonpathogenic SHIV-HXBc2 virus in only 12 envelope glycoprotein residues. SHIV-HXBc2P 3.2 replicated efficiently and caused rapid and persistent CD4(+) T-lymphocyte depletion in inoculated rhesus macaques. Compared with the envelope glycoproteins of the parental SHIV-HXBc2, the SHIV-HXBc2P 3.2 envelope glycoproteins supported more efficient infection of rhesus monkey peripheral blood mononuclear cells. Both the parental SHIV-HXBc2 and the pathogenic SHIV-HXBc2P 3.2 used CXCR4 but none of the other seven transmembrane segment receptors tested as a second receptor. Compared with the parental virus, viruses with the SHIV-HXBc2P 3.2 envelope glycoproteins were more resistant to neutralization by soluble CD4 and antibodies. Thus, changes in the envelope glycoproteins account for the ability of the passaged virus to deplete CD4(+) T lymphocytes rapidly and specify increased replicative capacity and resistance to neutralization.

The orphan seven-transmembrane receptor apj supports the entry of primary T-cell-line-tropic and dualtropic human immunodeficiency virus type 1.

Human immunodeficiency virus type 1 (HIV-1) enters target cells by sequential binding to CD4 and specific seven-transmembrane-segment (7TMS) coreceptors. Viruses use the chemokine receptor CCR5 as a coreceptor in the early, asymptomatic stages of HIV-1 infection but can adapt to the use of other receptors such as CXCR4 and CCR3 as the infection proceeds. Here we identify one such coreceptor, Apj, which supported the efficient entry of several primary T-cell-line tropic (T-tropic) and dualtropic HIV-1 isolates and the simian immunodeficiency virus SIVmac316. Another 7TMS protein, CCR9, supported the less efficient entry of one primary T-tropic isolate. mRNAs for both receptors were present in phytohemagglutinin- and interleukin-2-activated peripheral blood mononuclear cells. Apj and CCR9 share with other coreceptors for HIV-1 and SIV an N-terminal region rich in aromatic and acidic residues. These results highlight properties common to 7TMS proteins that can function as HIV-1 coreceptors, and they may contribute to an understanding of viral evolution in infected individuals.

CD40 preferentially costimulates activation of CD4+ T lymphocytes.

CD40 is a membrane differentiation antigen constitutively expressed on B cells that induces B cell growth and Ig synthesis after ligation with anti-CD40 mAb or with the recently identified CD40 ligand (CD40L). CD40L is rapidly induced on T cells after activation with anti-CD3 mAb or mitogens. While CD40-CD40L interactions are clearly beneficial to B cells, we speculated that a reciprocal costimulation of T cells might also occur. We have used genetic transfection to demonstrate that interactions between human small, resting T cells and CD40+ murine transfectants substantially augmented anti-CD3 induced T cell proliferation and resulted in the generation of CTL. T cell proliferation costimulated by CD40 was IL-2 dependent. The ability of CD40+ transfectants to costimulate T cell proliferation was specific in that VCAM-1+, CD54+, CD72+, CD56+, CD31+, and fas+ transfectants in the same host cells were inactive. CD4+ T cells preferentially responded to CD40 costimulation, whereas CD8+ T cells were substantially less reactive. By contrast, costimulation with B7 transfectants induced equivalent proliferation in the CD4+ and CD8+ T cell subsets. In addition, adult naive and memory T cells, as well as cord blood T cells, were responsive to CD40. These findings suggest that the CD40-CD40L costimulation pathway may allow for selective expansion of CD4+ T cells after interaction with CD40-bearing APC. The relatively restricted expression of CD40 on APC, as well as on medullary and cortical thymic epithelium, indicates a possible role for this interaction in T cell differentiation and activation.

Requirements for CD28-dependent T cell-mediated cytotoxicity.

Small, resting human peripheral blood T cells are able to mediate anti-CD3 redirected lysis against murine P815 cells transfected with human B7, a ligand of CD28. We demonstrate that cytotoxicity is mediated by preexisting cytotoxic effectors within the small, resting "memory" T cell population and by the de novo generation of additional CTL within both the "memory" and "virgin" T subsets. This conclusion is based on analysis of the kinetics of the response and the effects of metabolic inhibitors on the generation of CTL function. Memory CD45RO+ T cells demonstrated cytotoxicity within 4 h of coculture with anti-CD3 mAb and B7+ P815 cells and cytolysis was only partially prevented by inhibitors of protein synthesis. By contrast, virgin CD45RO- T cells demonstrated anti-CD3-induced lysis against B7+ P815 targets only after 6 or 8 h of coculture and cytotoxicity was completely prevented by inhibiting protein synthesis. Induction of cytotoxicity was B7 dependent in that parental P815 cells and P815 cells transfected with CD72 and vascular cell adhesion molecule-1, ligands for T cell-associated membrane receptors CD5 and very late activation antigen-4, respectively, did not initiate cytotoxicity. Our studies also revealed cooperation between the CD28/B7 and lymphocyte function-associated Ag-1/intercellular adhesion molecule-1 pathways in the generation of CTL from small, resting T cells. However, after CTL generation, the CD28-B7 interaction was not required for cytotoxic effector cell function. These observations may have important physiologic implications because this would permit activated CTL to lyse targets in vivo that do not express B7, after the CTL were generated by APC that do express B7 or possibly other costimulatory molecules.

Involvement of CD28 in MHC-unrestricted cytotoxicity mediated by a human natural killer leukemia cell line.

NK cells and certain CTL can recognize and lyse targets without restriction by the MHC. NK cells do not express CD3/TCR complexes and the membrane receptors participating in MHC-unrestricted cytotoxicity are largely unknown. We demonstrate that YT2C2, a human NK leukemia cell line, expresses the CD28 differentiation Ag and can spontaneously lyse both murine and human cell lines expressing B7, a B cell- activation Ag that is a ligand for CD28. The participation of CD28/B7 interactions in MHC-unrestricted cytotoxicity mediated by YT2C2 cells was demonstrated by correlation of target sensitivity with levels of B7 expression, inhibition of cytotoxicity by anti-CD28 or anti-B7 mAb, and by making both murine and human cell lines susceptible to YT2C2-mediated lysis by genetic transfection with expression vectors containing B7 cDNA. However, CD28/B7 interactions alone were insufficient to initiate cytotoxicity. mAb inhibition experiments and selection of CD54- (intercellular adhesion molecule-1) deficient B cell targets indicated that CD11a/18 (lymphocyte function-associated Ag-1) also cooperated in CD28/B7-dependent cytotoxicity. The requirement for both CD28/B7 and lymphocyte function-associated Ag-1/intercellular adhesion molecule-1 interactions in YT2C2-mediated MHC-unrestricted cytotoxicity was confirmed by demonstrating that efficient lysis of murine L cells required cotransfection with both B7 and intercellular adhesion molecule-1. These findings support the concept that MHC-unrestricted cytotoxicity may not be due to a unique receptor, but may result from interactions between an appropriate array of "adhesion" molecules with their ligands.

CD28 interaction with B7 costimulates primary allogeneic proliferative responses and cytotoxicity mediated by small, resting T lymphocytes.

Engagement of the CD3/T cell antigen receptor complex on small, resting T cells is insufficient to trigger cell-mediated cytotoxicity or to induce a proliferative response. In the present study, we have used genetic transfection to demonstrate that interaction of the B7-BB1 B cell activation antigen with the CD28 T cell differentiation antigen costimulates cell-mediated cytotoxicity and proliferation initiated by either anti-CD2 or anti-CD3 monoclonal antibody (mAb). Moreover, a B7-negative Burkitt's lymphoma cell line that fails to stimulate an allogeneic mixed lymphocyte response is rendered a potent stimulator after transfection with B7. The mixed leukocyte reaction proliferative response against the B7 transfectant is inhibited by either anti-CD28 or B7 mAb. We also demonstrate that freshly isolated small, resting human T cells can mediate anti-CD3 or anti-CD2 mAb-redirected cytotoxicity against a murine Fc receptor-bearing mastocytoma transfected with human B7. These preexisting cytotoxic T lymphocytes in peripheral blood are present in both the CD4 and CD8 subsets, but are preferentially within the CD45RO+ "memory" population. While small, resting T cells apparently require costimulation by CD28/B7 interactions, this requirement is lost after T cell activation. Anti-CD3 initiates a cytotoxic response mediated by in vitro cultured T cell clones in the absence of B7 ligand. The existence of functional cytolytic T cells in the small, resting T cell population may be advantageous in facilitating rapid responses to immune challenge.