Structural and functional analysis of interhelical interactions in the human immunodeficiency virus type 1 gp41 envelope glycoprotein by alanine-scanning mutagenesis.

Membrane fusion by human immunodeficiency virus type 1 (HIV-1) is promoted by the refolding of the viral envelope glycoprotein into a fusion-active conformation. The structure of the gp41 ectodomain core in its fusion-active state is a trimer of hairpins in which three antiparallel carboxyl-terminal helices pack into hydrophobic grooves on the surface of an amino-terminal trimeric coiled coil. In an effort to identify amino acid residues in these grooves that are critical for gp41 activation, we have used alanine-scanning mutagenesis to investigate the importance of individual side chains in determining the biophysical properties of the gp41 core and the membrane fusion activity of the gp120-gp41 complex. Alanine substitutions at Leu-556, Leu-565, Val-570, Gly-572, and Arg-579 positions severely impaired membrane fusion activity in envelope glycoproteins that were for the most part normally expressed. Whereas alanine mutations at Leu-565 and Val-570 destabilized the trimer-of-hairpins structure, mutations at Gly-572 and Arg-579 led to the formation of a stable gp41 core. Our results suggest that the Leu-565 and Val-570 residues are important determinants of conserved packing interactions between the amino- and carboxyl-terminal helices of gp41. We propose that the high degree of sequence conservation at Gly-572 and Arg-579 may result from selective pressures imposed by prefusogenic conformations of the HIV-1 envelope glycoprotein. Further analysis of the gp41 activation process may elucidate targets for antiviral intervention.

Structural and functional analysis of the HIV gp41 core containing an Ile573 to Thr substitution: implications for membrane fusion.

The envelope glycoprotein of HIV-1 consists of the surface subunit gp120 and the transmembrane subunit gp41. Binding of gp120 to target cell receptors induces a conformational change in gp41, which then mediates the fusion of viral and cellular membranes. A buried isoleucine (Ile573) in a central trimeric coiled coil within the fusion-active gp41 ectodomain core is thought to favor this conformational activation. The role of Ile573 in determining the structure and function of the gp120-gp41 complex was investigated by mutating this residue to threonine, a nonconservative substitution in HIV-1 that occurs naturally in SIV. While the introduction of Thr573 markedly destabilized the gp41 core, the three-dimensional structure of the mutant trimer of hairpins was very similar to that of the wild-type molecule. A new hydrogen-bonding interaction between the buried Thr573 and Thr569 residues appears to allow formation of the trimer-of-hairpins structure at physiological temperature. The mutant envelope glycoprotein expressed in 293T cells and incorporated within pseudotyped virions displayed only a moderate reduction in syncytium-inducing capacity and virus infectivity, respectively. Our results demonstrate that the proper folding of the gp41 core underlies the membrane fusion properties of the gp120-gp41 complex. An understanding of the gp41 activation process may suggest novel strategies for vaccine and antiviral drug development.

Antibody binding and neutralization of primary and T-cell line-adapted isolates of human immunodeficiency virus type 1.

The relative resistance of human immunodeficiency virus type 1 (HIV-1) primary isolates (PIs) to neutralization by a wide range of antibodies remains a theoretical and practical barrier to the development of an effective HIV vaccine. One model to account for the differential neutralization sensitivity between Pls and laboratory (or T-cell line-adapted [TCLA]) strains of HIV suggests that the envelope protein (Env) complex is made more accessible to antibody binding as a consequence of adaptation to growth in established cell lines. Here, we revisit this question using genetically related PI and TCLA viruses and molecularly cloned env genes. By using complementary techniques of flow cytometry and virion binding assays, we show that monoclonal antibodies targeting the V3 loop, CD4-binding site, CD4-induced determinant of gp120, or the ectodomain of gp41 bind equally well to PI and TCLA Env complexes, despite large differences in neutralization outcome. The data suggest that the differential neutralization sensitivity of PI and TCLA viruses may derive not from differences in the initial antibody binding event but rather from differences in the subsequent functioning of the PI and TCLA Envs during virus entry. An understanding of these as yet undefined differences may enhance our ability to generate broadly neutralizing HIV vaccine immunogens.

Turning a corner on HIV neutralization?

HIV vaccine development has been hampered by the inability of conventional immunogens to elicit antibodies capable of neutralizing primary isolates of the virus. Recent studies using 'fusion-competent' immunogens that capture transitional intermediate structures of the functioning envelope protein suggest that this goal may now be achievable.

Fusion-competent vaccines: broad neutralization of primary isolates of HIV.

Current recombinant human immunodeficiency virus (HIV) gp120 protein vaccine candidates are unable to elicit antibodies capable of neutralizing infectivity of primary isolates from patients. Here, "fusion-competent" HIV vaccine immunogens were generated that capture the transient envelope-CD4-coreceptor structures that arise during HIV binding and fusion. In a transgenic mouse immunization model, these formaldehyde-fixed whole-cell vaccines elicited antibodies capable of neutralizing infectivity of 23 of 24 primary HIV isolates from diverse geographic locations and genetic clades A to E. Development of these fusion-dependent immunogens may lead to a broadly effective HIV vaccine.

Continued utilization of CCR5 coreceptor by a newly derived T-cell line-adapted isolate of human immunodeficiency virus type 1.

The differential use of CC chemokine receptor 5 (CCR5) and CXC chemokine receptor 4 (CXCR4) may be intimately involved in the transmission and progression of human immunodeficiency virus infection. Changes in coreceptor utilization have also been noted upon adaptation of primary isolates (PI) to growth in established T-cell lines. All of the T-cell line-adapted (TCLA) viruses studied to date utilize CXCR4 but not CCR5. This observation had been suggested as an explanation for the sensitivity of TCLA, but not PI, viruses to neutralization by recombinant gp120 antisera and V3-directed monoclonal antibodies, but recent studies have shown coreceptor utilization to be independent of neutralization sensitivity. Here we describe a newly isolated TCLA virus that is sensitive to neutralization but continues to utilize both CXCR4 and CCR5 for infection. This finding further divorces coreceptor specificity from neutralization sensitivity and from certain changes in cell tropism. That the TCLA virus can continue to utilize CCR5 despite the changes that occur upon adaptation and in the apparent absence of CCR5 expression in the FDA/H9 T-cell line suggests that the interaction between envelope protein and coreceptor may be mediated by multiple weak interactions along a diffuse surface.

Coreceptor utilization by human immunodeficiency virus type 1 is not a primary determinant of neutralization sensitivity.

We have examined the relationship between coreceptor utilization and sensitivity to neutralization in a primary isolate of human immunodeficiency virus type 1 and its T-cell line-adapted (TCLA) derivative. We determined that adaptation of the primary-isolate (PI) virus 168P results in the loss of the unique capacity of PI viruses to utilize the CCR5 coreceptor and in the acquisition by the TCLA 168C virus of sensitivity to neutralization by V3-directed monoclonal antibodies (MAbs). In experiments wherein infection by 168P is directed via either the CCR5 or the CXCR4 pathway, we demonstrate that the virus, as well as pseudotyped virions bearing a molecularly cloned 168P envelope protein, remains refractory to neutralization by MAbs 257-D, 268-D, and 50.1 regardless of the coreceptor utilized. This study suggests that coreceptor utilization is not a primary determinant of differential neutralization sensitivity in PI and TCLA viruses.

Synthetic peptides from P. falciparum sexual stage 25-kDa protein induce antibodies that react with the native protein: the role of IL-2 and conformational structure on immunogenicity of Pfs25.

To identify B-cell epitopes of the Plasmodium falciparum 25-kDa ookinete protein, Pfs25, 41 overlapping synthetic peptides spanning the entire length of the protein were used individually to immunize CAF1 (F1 hybrid of BALB/c female and A/J male) mice. Antipeptide sera were tested for reactivity to live intact zygote/early ookinete (post-fertilization stage) by immunofluorescence, and by Western blot analysis under nonreducing and reducing conditions, immunoprecipitation of 35S-cysteine-labeled antigen, and ELISA using a vaccinia recombinant Pfs25 antigen. Fourteen B-cell epitopes were identified. These peptides were immunogenic only when administered with high-dose recombinant interleukin-2. Antibodies to 11 peptides recognized only the native conformational structure, one peptide induced antibodies that recognized both reduced and native protein, and two other peptides, after primary immunization, made antibodies to denatured Pfs25, but after boosting the antibodies reacted to both denatured and native Pfs25. Anti-sera to peptides in the first (peptide 7) and fourth (peptide 34) epidermal growth factor-like domains of Pfs25 reacted most strongly with zygotes/ookinetes by immunofluorescence assay. The antibodies elicited by immunization with peptide 34 suppressed infectivity of the parasite to mosquitoes. We further observed that the secondary structure of Pfs25 may be important for immunogenicity because monoclonal antibodies (MAbs) 1C7 and 1D2, both transmission-blocking MAbs, protected enzyme cleavage sites in Pfs25 from proteolysis, suggesting that discontinuous segments of Pfs25 may come together to form immunogenic epitopic sites. Thus, definition of B- and T-cell epitopes may be required to construct a Pfs25 vaccine for optimum immunogenicity.

Adaptation to persistent growth in the H9 cell line renders a primary isolate of human immunodeficiency virus type 1 sensitive to neutralization by vaccine sera.

Seven diverse primary isolates of human immunodeficiency virus type 1 (HIV-1) were examined and found to be refractory to neutralization by antisera to recombinant gp120 (rgp120) protein from HIV-1 MN. This stands in marked contrast to the sensitivity exhibited by certain laboratory-adapted viruses. To understand the difference between primary and laboratory-adapted viruses, we adapted the primary virus ACH 168.10 to growth in the FDA/H9 cell line. ACH 168.10 was chosen because the V3 region of gp120 closely matches that of MN. After 4 weeks, infection became evident. The virus (168A) replicated in FDA/H9 cells with extensive cytopathic effect but was unchanged in sensitivity to antibody-mediated neutralization. Thus, growth in cell lines is not sufficient to render primary virus sensitive to neutralization. The 168A virus was, however, partially sensitive to CD4 immunoadhesin (CD4-Ig). Adaptation was continued to produce a persistently infected FDA/H9 culture that displayed minimal cytopathic effect. The virus (168C) was now sensitive to neutralization by MN rgp120 vaccine sera and by MN-specific monoclonal antibodies and showed increased sensitivity to HIVIG and CD4-Ig. 168C encoded three amino acid changes in gp120, including one within the V3 loop (I-166-->R, I-282-->N, G-318-->R). MN-specific monoclonal antibodies bound equally to the surface of cells infected with either neutralization-resistant or -sensitive virus. The coincidence of changes in neutralization sensitivity with changes in cell tropism and cytopathic effect suggests a common underlying mechanism(s) acting through the whole of the envelope protein complex.

Immunogenicity and HIV-1 virus neutralization of MN recombinant glycoprotein 120/HIV-1 QS21 vaccine in baboons .

The effect of adjuvant and immunization schedule on the immunogenicity of HIV-1 envelope glycoprotein, MN rgp120, was optimized by using baboons. The novel adjuvant QS21 elicited earlier seroconversion than alum adjuvant, and the antibody titers to MN rgp120 for animals treated with QS21 were significantly greater than the titers obtained in animals treated with alum. The use of QS21 shifted the dose-response curve, resulting in less MN rgp120 required to achieve equivalent titers to those in the alum formulations. The in vitro virus neutralizing (VN) titers from animals treated with QS21 were 3- to 10-fold higher than with alum. The data presented herein point to the superiority of QS21 as adjuvant in primates for MN rgp120.

Development of a single-shot subunit vaccine for HIV-1.

The successful development of an AIDS vaccine will require formulations that not only invoke the desired immunological response, but also are stable and easy to administer. A single shot MN rgp120 vaccine formulation comprised of MN rgp120 encapsulated in poly (lactic-coglycolic) acid (PLGA) microspheres was developed to provide an in vivo autoboost of antigen. These formulations were designed to yield an in vivo autoboost at 1, 2, 3 or 4-6 months. In addition, PLGA microspheres containing the adjuvant, QS21, were also prepared to provide an in vivo autoboost concomitant with antigen. In guinea pigs, these formulations yielded higher anti-MN rgp120 and anti-V3 loop antibody titers than alum formulations that were administered at higher antigen doses. Different doses of encapsulated MN rgp120 provided a clear and well-defined dose response curve for both anti-MN rgp120 and anti-V3 loop antibody titers. When soluble QS21 was mixed with the encapsulated MN rgp120, the antibody titers were increased by a factor of 5 over the titers with encapsulated MN rgp120 alone. An additional fivefold increase in antibody titers was observed for guinea pigs immunized with encapsulated MN rgp120 and QS21 on the same microspheres. These results suggest that the adjuvant properties of QS21 can be increased by microencapsulation in PLGA. Furthermore, antibodies induced by these preparations neutralized the MN strain of HIV-1. The neutralization titers for sera from animals immunized with MN rgp120-PLGA and soluble QS21 were greater than the titers obtained from guinea pigs that were treated with MN rgp120 and soluble QS21 at the same dose. Overall, these studies validate the in vivo autoboost concept, reveal a method for improving the adjuvant properties of QS21, and indicate the potential of future single shot vaccine formulations.

L-696,229 specifically inhibits human immunodeficiency virus type 1 reverse transcriptase and possesses antiviral activity in vitro.

L-696,229 (3-[2-(benzoxazol-2-yl)ethyl]-5-ethyl-6-methyl-pyridin-2 (1H)-one) is a specific inhibitor of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) activity that possesses antiviral activity in cell culture (W.S. Saari, J.M. Hoffman, J.S. Wai, T.E. Fisher, C.S. Rooney, A.M. Smith, C.M. Thomas, M. E. Goldman, J. A. O'Brien, J. H. Nunberg, J. C. Quintero, W. A. Schleif, E. A. Emini, and P. S. Anderson, J. Med. Chem. 34:2922-2925, 1991). In the present study, the RT-inhibitory activity and antiviral properties were characterized in detail. The inhibition of RT activity was template-primer dependent with 50% inhibitory concentrations of 0.018 to 0.50 microM and was noncompetitive with respect to deoxynucleoside triphosphates. L-696,229 inhibited RT activity in a mutually exclusive manner with respect to either phosphonoformate or azidothymidine triphosphate and was a weak partial inhibitor of the RNase H activity associated with HIV-1 RT. The compound did not significantly inhibit other retroviral or cellular polymerases at 300 microM.L-696,229 inhibited the spread of HIV-1 infection in cell cultures with all cell types and viral isolates tested, including human peripheral blood mononuclear cells and a virus isolate resistant to azidothymidine.

Prevention of HIV-1 infection in chimpanzees by gp120 V3 domain-specific monoclonal antibody.

The acquired immunodeficiency syndrome (AIDS) is the late-stage clinical manifestation of long-term persistent infection with the human immunodeficiency virus type 1 (HIV-1). Immune responses directed against the virus and against virus-infected cells during the persistent infection fail to mediate resolution of the infection. As a result, a successful AIDS vaccine must elicit an immune state that will prevent the establishment of the persistent infection following introduction of the virus into the host. The third hypervariable (V3) domain of the HIV-1 gp120 envelope glycoprotein is a disulphide-linked closed loop of about 30 amino acids which binds and elicits anti-HIV-1 type-specific virus-neutralizing antibodies. The in vitro characteristics of anti-V3 domain antibody suggest that this antibody could by itself prevent HIV-1 infection in vivo, an idea supported by chimpanzee challenge studies in which protection against the HIV-1 persistent infection seemed to correlate with the presence of anti-V3 domain antibody. Here we directly demonstrate the protective efficacy of anti-V3 domain antibody in vivo and propose that this antibody is potentially useful as both a pre- and post-exposure prophylactic agent.

Viral resistance to human immunodeficiency virus type 1-specific pyridinone reverse transcriptase inhibitors.

Human immunodeficiency virus type 1 (HIV-1)-specific pyridinone reverse transcriptase (RT) inhibitors prevent HIV-1 replication in cell culture (M. E. Goldman, J. H. Nunberg, J. A. O'Brien, J.C. Quintero, W. A. Schleif, K. F. Freund, S. L. Gaul, W. S. Saari, J. S. Wai, J. M. Hoffman, P. S. Anderson, D. J. Hupe, E. A. Emini, and A. M. Stern, Proc. Natl. Acad. Sci. USA 88:6863-6867, 1991). In contrast to nucleoside analog inhibitors, such as AZT, which need to be converted to triphosphates by host cells, these compounds act directly to inhibit RT via a mechanism which is noncompetitive with respect to deoxynucleoside triphosphates. As one approach to define the mechanism of action of pyridinone inhibitors, we isolated resistant mutants of HIV-1 in cell culture. Serial passage in the presence of inhibitor yielded virus which was 1,000-fold resistant to compounds of this class. Bacterially expressed RTs molecularly cloned from resistant viruses were also resistant. The resistant RT genes encoded two amino acid changes, K-103 to N and Y-181 to C, each of which contributed partial resistance. The mutation at amino acid 181 lies adjacent to the conserved YG/MDD motif found in most DNA and RNA polymerases. The mutation at amino acid 103 lies within a region of RT which may be involved in PPi binding. The resistant viruses, although sensitive to nucleoside analogs, were cross-resistant to the structurally unrelated RT inhibitors TIBO R82150 (R. Pauwels, K. Andries, J. Desmyter, D. Schols, M. J. Kukla, H. J. Breslin, A. Raeymaeckers, J. Van Gelder, R. Woestenborghs, J. Heykanti, K. Schellekens, M. A. C. Janssen, E. De Clercq, and P. A. J. Janssen, Nature [London] 343:470-474, 1990) and BI-RG-587 (V. J. Merluzzi, K. D. Hargrave, M. Labadia, K. Grozinger, M. Skoog, J. C. Wu, C.-K. Shih, K. Eckner, S. Hattox, J. Adams, A. S. Rosenthal, R. Faanes, R. J. Eckner, R. A. Koup, and J. L. Sullivan, Science 250:1411-1413, 1990). Thus, these nonnucleoside analog inhibitors may share a common binding site on RT and may all make up a single pharmacologic class of RT inhibitor. This observation may have important implications for the clinical development of these compounds.

Pyridinone derivatives: specific human immunodeficiency virus type 1 reverse transcriptase inhibitors with antiviral activity.

Derivatives of pyridinones were found to inhibit human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) activity and prevent the spread of HIV-1 infection in cell culture without an appreciable effect on other retroviral or cellular polymerases. 3-[( (4,7-Dimethyl-1,3-benzoxazol-2-yl) methyl]amino ]-5-ethyl-6-methylpyridin-2(1H)-one (L-697,639) and 3-[[ (4,7-dichloro-1,3-benzoxazol-2-yl) methyl]amino]-5-ethyl-6-methylpyridin-2(1H)-one (L-697,661), two compounds within this series, had HIV-1 RT IC50 values in the range of 20-800 nM, depending upon the template-primer used. The most potent inhibition was obtained with rC.dG and dA.dT as template--primers. With rC.dG, reversible slow-binding non-competitive inhibition was observed. [3H]L-697,639 bound preferentially to enzyme-template-primer complexes. This binding was magnesium-dependent and saturable with a stoichiometry of 1 mol of [3H]L-697,639 per mol of RT heterodimer. Displacement of [3H]L-697,639 was seen with phosphonoformate. In human T-lymphoid-cell culture, L-697,639 and L-697,661 inhibited the spread of HIV-1 infection by at least 95% at concentrations of 12-200 nM. Synergism between 3'-azido-3'-deoxythymidine or dideoxyinosine and either of these compounds was also demonstrated in cell culture. Based upon their specificity for HIV-1 RT activity, template-primer dependence on potency and ability to displace [3H]L-697,639; a tetrahydroimidazo [4,5,1-jk] [1,4]-benzodiazepin-2(1H)-thione derivative R82150 and the dipyridodiazepinone BI-RG-587 appear to inhibit RT activity by the same mechanism as the pyridinones.

Release of human serum albumin from poly(lactide-co-glycolide) microspheres.

Human serum albumin (HSA) was encapsulated in a 50:50 copolymer of DL-lactide/glycolide in the form of microspheres. These microspheres were used as a model formulation to study the feasibility of controlling the release of large proteins over a 20- to 30-day period. We show that HSA can be successfully incorporated into microspheres and released intact from these microspheres into various buffer systems at 37 degrees C. A continuous release of the protein could be achieved in physiological buffers at 37 degrees C over a 20- to 30-day period from microspheres with high protein loadings (11.6%). These results demonstrate the potential of poly(DL-lactide-co-glycolide) microspheres for continuous delivery of large proteins.

Recombinant feline herpesviruses expressing feline leukemia virus envelope and gag proteins.

We constructed recombinant feline herpesviruses (FHVs) expressing the envelope (env) and gag genes of feline leukemia virus (FeLV). Expression cassettes, utilizing the human cytomegalovirus immediate-early promoter, were inserted within the thymidine kinase gene of FHV. The FeLV env glycoprotein expressed by recombinant FHV was processed and transported to the cell surface much as in FeLV infection, with the exception that proteolytic processing to yield the mature gp70 and p15E proteins was less efficient in the context of herpesvirus infection. Glycosylation of the env protein was not affected; modification continued in the absence of efficient proteolytic processing to generate terminally glycosylated gp85 and gp70 proteins. A recombinant FHV containing the FeLV gag and protease genes expressed both gag and gag-protease precursor proteins. Functional protease was produced which mediated the proteolytic maturation of the FeLV gag proteins as in authentic FeLV infection. Use of these recombinant FHVs as live-virus vaccines may provide insight as to the role of specific retroviral proteins in protective immunity. The current use of conventional attenuated FHV vaccines speaks to the wider potential of recombinant FHVs for vaccination in cats.

Controlled release of interleukin-2 from biodegradable microspheres.

We have evaluated the use of biodegradable poly(DL-lactide-co-glycolide) microspheres for the controlled release of interleukin-2 (IL-2) and its modified forms: succinyl IL-2 (SIL-2) and polyethylene glycol-modified IL-2 (PEG IL-2). We show that a microsphere formulation can be prepared from PEG IL-2 using HSA as an excipient which, after an initial burst, releases 2-3% PEG IL-2 per day in a bioactive form continuously over a 20- to 30-day period.