CTLA4Ig-induced linked regulation of allogeneic T cell responses

The mechanisms by which CTLA4Ig exerts its powerful immunomodulatory effects are not clear. We show here that CTLA4Ig can induce linked regulation of allogeneic porcine T cell responses in vitro. Naive miniature swine SLA(dd) T cells were rendered hyporesponsive to specific allogeneic Ag after coculturing with MHC-mismatched SLA(cc) stimulators in the presence of CTLA4Ig. These Ag-specific hyporesponsive T cells were subsequently able to actively inhibit the allogeneic responses of naive syngeneic T cells in an MHC-linked fashion, as the responses of naive SLA(dd) responders against specific SLA(cc) and (SLA(ac))F(1) stimulators were inhibited, but allogeneic responses against a 1:1 mixture of SLA(aa) (I(a), II(a)) and SLA(cc) (I(c), II(c)) were maintained. This inhibition could be generated against either class I or class II Ags, was radiosensitive, and required cell-cell contact. Furthermore, the mechanism of inhibition was not dependent upon a deletional, apoptotic pathway, but it was reversed by anti-IL-10 mAb. These data suggest that CTLA4Ig-induced inhibition of naive allogeneic T cell responses can be mediated through the generation of regulatory T cells via an IL-10-dependent mechanism.

Schwann cell heparan sulfate proteoglycans play a critical role in glial growth factor/neuregulin signaling

Glial growth factors are proteins encoded by the neuregulin gene and are thought to signal via receptor tyrosine kinases. Many neuregulin gene products bind heparin, and we hypothesize that affinity for heparin may implicate cell surface heparan sulfate proteoglycans (HeSPGs) as co-receptors for the soluble neuregulin gene product, recombinant human glial growth factor 2 (rhGGF2). Using primary rat Schwann cell cultures, we show that exogenous heparin and heparan sulfate block rhGGF2-induced phosphorylation of putative neuregulin receptors, and block subsequent DNA synthesis; other glycosaminoglycans show no such effect. Inhibition of Schwann cell HeSPG biosynthesis by administration of beta-xyloside also blocks responsiveness to rhGGF2. In cell-free binding assays, rhGGF2 binds heparin and heparan sulfate with high affinity, while suramin and suramin-like molecules block this binding. These suramin-like molecules reversibly block Schwann cell responsiveness to rhGGF2 with a rank order of potency identical to that in the cell-free binding assay. Thus we demonstrate high affinity and specificity in the interaction of rhGGF2 with heparin-like molecules, and show that three distinct perturbations of this interaction on Schwann cells (exogenous heparin/ heparan sulfate treatment, inhibition of HeSPG biosynthesis, and treatment with suramin-like molecules) result in a loss of responsiveness to rhGGF2. These results support a model in which HeSPGs are critical components that modulate extracellular rhGGF2 signaling interactions with appropriate receptor tyrosine kinases.

The amino terminal domain of HIV-1 Rev is required for discrimination of the RRE from nonspecific RNA

The ability of HIV-1 Rev to successfully discriminate between specific Rev-responsive elements (RRE) and nonspecific binding sites in the presence of excess nonspecific RNA was examined using filter binding, gel shift, and gel filtration techniques, using purified M4 Rev mutant protein and endoproteinase Lys-C cleaved wild-type Rev. The M4 Rev displayed a slightly reduced binding affinity to the RRE, as well as a tenfold decrease in its ability to discriminate the RRE from non-specific RNA compared to the wild-type Rev. Gel shift and gel filtration chromotography data also showed decreased ability of the mutant to multimerize in the absence or presence of the RRE. The Lys-C cleaved Rev, which lacks the amino-terminal 20 amino acids of the protein, displayed less ability to discriminate the RRE from nonspecific RNA compared to either the wild-type or the M4 mutant Rev and appeared unable to form protein-protein interactions, yet still bound sense and antisense RNA species with high affinity (Kd was in the nanomolar concentration range). A 40 amino acid peptide containing the arginine-rich RRE binding domain of Rev was also observed to interact with both the RRE and antisense RNA fragments with a binding constant of about 1 x 10(-9) M. However, the peptide displayed almost no ability to discriminate between the RRE and a comparably sized antisense RRE. The loss in ability to discriminate correct from incorrect binding sites correlates with overall decreases in the alpha-helical character of the protein and perturbations within the amino terminus. The amino terminus of Rev is likely to maintain the conformational integrity of the arginine rich RRE binding domain which is required for specific RNA binding site discrimination or stabilization of specific Rev-RRE interactions.

Differences in the binding of blocking anti-CD11b monoclonal antibodies to the A-domain of CD11b

CD11b/CD18 (Mac-1) is a leukocyte integrin that plays a critical role in neutrophil adhesion and the initiation of acute inflammatory responses. Several Mac-1 blocking mAbs bind to the A-domain of CD11b, a approximately 200 amino acid region in the N-terminal portion of the protein that is involved in ligand binding and Mac-1 functional activity. We examined several CD11b blocking mAbs for different patterns of binding to A-domain. We used human/murine chimeric CD11b expression constructs and deletions of the A-domain to examine binding. We describe the binding characteristics of mAbs 60.1, LM2/1, LPM19C, M170, 44, and 904. All of these mAbs, except for 60.1, bind to the C-terminal half of the human A-domain (CD11b181-316). mAb 60.1 was unique in that it required regions of the N- and C-terminal ends of the A-domain for binding. mAbs 60.1, LPM19C, 904, and 44 all required the A-domain to be intact for binding. This suggests that these CD11b mAbs recognize a conformational epitope. LM2/1 was capable of binding to a fragment of the A-domain, CD11b285-300. Inasmuch as this system has been used to define different mAb binding sites, it may be used to analyze specific ligand binding sites in the A-domain of CD11b.

Differences in the binding of blocking anti-CD11b monoclonal antibodies to the A-domain of CD11b

CD11b/CD18 (Mac-1) is a leukocyte integrin that plays a critical role in neutrophil adhesion and the initiation of acute inflammatory responses. Several Mac-1 blocking mAbs bind to the A-domain of CD11b, a approximately 200 amino acid region in the N-terminal portion of the protein that is involved in ligand binding and Mac-1 functional activity. We examined several CD11b blocking mAbs for different patterns of binding to A-domain. We used human/murine chimeric CD11b expression constructs and deletions of the A-domain to examine binding. We describe the binding characteristics of mAbs 60.1, LM2/1, LPM19C, M170, 44, and 904. All of these mAbs, except for 60.1, bind to the C-terminal half of the human A-domain (CD11b181-316). mAb 60.1 was unique in that it required regions of the N- and C-terminal ends of the A-domain for binding. mAbs 60.1, LPM19C, 904, and 44 all required the A-domain to be intact for binding. This suggests that these CD11b mAbs recognize a conformational epitope. LM2/1 was capable of binding to a fragment of the A-domain, CD11b285-300. Inasmuch as this system has been used to define different mAb binding sites, it may be used to analyze specific ligand binding sites in the A-domain of CD11b.

Anti-CD11b monoclonal antibody reduces ischemic cell damage after transient focal cerebral ischemia in rat

We investigated the effect of an anti-CD11b monoclonal antibody (1B6c) on ischemic cell damage after transient middle cerebral artery occlusion. We divided animals into three groups: MAb 1 group (n = 5)--rats were subjected to 2 hours of transient occlusion and 1B6c (1 mg/kg) was administered intravenously at 0 and 22 hours of reperfusion; MAb 2 group (n = 5)--same experimental protocol as MAb 1 group, except that the initial dose of 1B6c was increased to 2 mg/kg; and control group (n = 5)--same experimental protocol as MAb 2 group, except that an isotype-matched control antibody was administered. Animals were weighed and tested for neurological function before and after occlusion of the middle cerebral artery. Forty-six hours after reperfusion, brain sections were stained with hematoxylin and eosin for histology evaluation. We observed a significant reduction of weight loss and improvement in neurological function after ischemia in the MAb 2 animals compared to MAb 1 and vehicle-treated animals (p < 0.05). The lesion volume was significantly smaller in the MAb 2 group (19.5 +/- 1.9%) compared to MAb 1 (29.9 +/- 2.6%) and vehicle-treated (34.2 +/- 5.4%) groups (p < 0.01). Tissue polymorphonuclear cell numbers were reduced in both 1B6c-administered groups. Our data demonstrate that administration of anti-CD11b antibody results in a dose-dependent, significant functional improvement and reduction of ischemic cell damage after transient focal cerebral ischemia in the rat.

Postischemic administration of an anti-Mac-1 antibody reduces ischemic cell damage after transient middle cerebral artery occlusion in rats

BACKGROUND AND PURPOSE

Postischemic cerebral inflammation may contribute to ischemic cell damage. The CD11b/18 (Mac-1) integrin mediates stimulated neutrophil binding to endothelia. We therefore investigated the effect of administration of an anti-Mac-1 monoclonal antibody on cerebral ischemic cell damage in the rat.

METHODS

Rats (n = 10) were subjected to 2 hours of middle cerebral artery occlusion; the anti-Mac-1 antibody was administered at a dose of 2 mg/kg i.v. at 1 hour of reperfusion and 1 mg/kg i.v. at 22 hours of reperfusion or an isotype-matched control antibody (n = 10) was administered using the same experimental protocol. Rats were killed at 46 hours of reperfusion, and brain sections were stained with hematoxylin and eosin for histological evaluation. In a separate population of rats given either vehicle (n = 8) or anti-Mac-1 antibodies (n = 9), intraparenchymal neutrophils were measured by means of a myeloperoxidase assay.

RESULTS

The lesion volume was significantly smaller (28%) in the anti-Mac-1 antibody group compared with the vehicle control group (P < .01). Numbers of intraparenchymal polymorphonuclear cells were significantly reduced (P < .05) in the cortex of the anti-Mac-1 antibody group compared with the vehicle control group.

CONCLUSIONS

Our data demonstrate that administration of anti-Mac-1 antibody 1 hour after onset of reperfusion results in significant reductions of ischemic cell damage and intraparenchymal neutrophils after transient (2-hour) focal cerebral ischemia in the rat.

Molecular determinants of lipid mediator-induced leukocyte adherence and emigration in rat mesenteric venules

The objective of this study was to identify the molecular determinants of leukocyte rolling, adherence, and emigration elicited in postcapillary venules by the lipid mediators leukotriene B4 (LTB4) or platelet-activating factor (PAF). Leukocyte-endothelial cell adhesion and shear rate were monitored in rat mesenteric venules during superfusion with either LTB4 or PAF in the presence or absence of monoclonal antibodies (MAbs) directed against either leukocyte (CD18, CD11b) or endothelial cell [intercellular adhesion molecule 1 (ICAM-1), E-selectin, P-selectin] adhesion glycoproteins. In untreated animals and in animals receiving a nonbinding control MAb, LTB4 and PAF increased the number of both adherent (8- and 4-fold, respectively) and emigrated (14- and 8-fold, respectively) leukocytes, while reducing leukocyte rolling velocity (36 and 33%, respectively). The LTB4- and PAF-induced leukocyte adherence and emigration were significantly attenuated by pretreatment with MAbs directed against CD18, CD11b, ICAM-1, and E-selectin, but not P-selectin. The reduction in leukocyte rolling velocity induced by LTB4 was not affected by any of the MAbs; however, both P- and E-selectin MAbs significantly attenuated the reduction in leukocyte rolling velocity elicited by PAF. The results of this study indicate that the leukocyte adherence and emigration induced by both LTB4 and PAF are mediated by CD11b/CD18 on leukocytes and by ICAM-1 and E-selectin on endothelial cells. The molecular determinant of leukocyte rolling appears to be mediator specific, with the selectins mediating the rolling elicited by PAF.

Molecular determinants of reperfusion-induced leukocyte adhesion and vascular protein leakage

The adherence and emigration of leukocytes have been implicated as a rate-limiting step in the microvascular dysfunction associated with reperfusion of ischemic tissues. The objective of the present study was to define the relation between leukocyte-endothelial cell adhesion and albumin leakage in rat mesenteric venules exposed to ischemia and reperfusion (I/R). Leukocyte adherence and emigration as well as albumin extravasation were monitored in single post-capillary venules using intravital fluorescence microscopy. Ischemia (0, 10, 15, or 20 minutes) was induced by complete occlusion of the superior mesenteric artery, and all parameters were monitored for 30 minutes after reperfusion. The magnitude of the leukocyte adherence and emigration and albumin leakage elicited by I/R was positively correlated with the duration of ischemia. The albumin leakage response was also highly correlated with the number of adherent and emigrated leukocytes. Monoclonal antibodies against the adhesion glycoproteins CD18, CD11b, intercellular adhesion molecule-1 (ICAM-1) (at 10 and 30 minutes), and L-selectin (at 10 minutes), but not P- or E-selectin, reduced I/R-induced leukocyte adherence and emigration as well as albumin leakage. Platelet-leukocyte aggregates were formed in postischemic venules; the number of aggregates was reduced by antibodies against P-selectin, CD11b, CD18, and ICAM-1, but not E- or L-selectin. These results indicate that reperfusion-induced albumin leakage is tightly coupled to the adherence and emigration of leukocytes in postcapillary venules. This adhesion-dependent injury response is primarily mediated by CD11b/CD18 on activated neutrophils and ICAM-1 on venular endothelium and appears to require L-selectin-dependent leukocyte rolling.

Biochemical characterization of binding of multiple HIV-1 Rev monomeric proteins to the Rev responsive element

Recombinant HIV-1 Rev protein was overexpressed in Escherichia coli using translational coupling to the beta-glucuronidase gene and demonstrated to interact with high affinity and specificity with the Rev responsive element (RRE). A complex Rev-dependent binding pattern was observed using the gel shift assay which could be simplified to one or two primary bands in the presence of stoichiometric concentrations of RRE. Competition of these bands with a series of homopolymer RNA species demonstrated that Rev is essentially a poly-G binding protein, although poly-I was also shown to compete for specific RRE binding. The stoichiometry of the Rev-dependent gel shift complexes was determined using 125I-labeled Rev. The stable, lowest mobility complex was determined to possess a ratio of between 7 and 8 Rev molecules per RRE containing RNA fragment while the two fastest migrating complexes contained ratios of one and two Rev molecules per RRE, respectively. Using the Hill equation as a model for cooperative interactions, a Hill coefficient of n(app) = 2 was obtained from fitting of direct nitrocellulose filter binding assays, reflecting cooperatively bound Rev molecules on the RRE under equilibrium binding conditions. An increase in ionic strength from 0.0 to 0.3 M NaCl reduced cooperative Rev binding to the RRE, but specificity of Rev for the RRE relative to antisense RNA was increased 100,000-fold. At molar ratios of Rev to RRE above 2, Rev dissociated from the RRE with a T1/2 of approximately 20-25 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Perturbation of the carboxy terminus of HIV-1 Rev affects multimerization on the Rev responsive element

Perturbations within the transactivation and carboxy-terminal domains of HIV-1 Rev were examined for effects on Rev responsive element (RRE) binding activities in vitro and biological activity in vivo. Binding affinities, specificities, and multimerization of the transactivation mutants M10 and Rev/Rex M10-16 on the RRE were equivalent to wild-type Rev. Substitution of the Rex transactivation domain within Rev resulted in the incorporation of an internal methionine residue which, when cleaved with CNBr and subsequently purified, produced a protein species (CNBr-Rev) unable to fully multimerize on the RRE. Instead, two discrete protein-dependent species were generated in the gel shift assay. Furthermore, CNBr-Rev was observed to bind to the RRE with high specificity and an equilibrium binding constant of 6 x 10(-10) M. A C-terminal Rev deletion mutant (Rev M9 delta 14) lacking amino acids 68-112 displayed identical RRE binding characteristics to the CNBr-Rev protein. This protein, which lacks both the activation and the C-terminal domains, was biologically inactive but maintained the ability to discriminate the RRE from nonspecific RNA. Deletion of amino acids 92-112 resulted in a Rev mutant (Rev M11 delta 14) which bound to the RRE with wild-type affinity and high specificity. This purified mutant was observed to be aberrant in multimerization activity on the RRE with reduced multimerization apparent in the gel shift assay. However, Rev M11 delta 14 possessed biological activity equivalent to wild-type Rev in a cell-based p24 ELISA assay. These results suggest that polymerization on the RRE is dispensable for Rev activity and that two monomeric Rev proteins bound to the RRE are sufficient for biological activity. Furthermore, in vivo experiments using the Rev/Rex chimeric mutant and the M10 transdominant mutant as well as in vitro dissociation rate studies with Rev M11 delta 14 and Rev M9 delta 14 suggest that the M9 through M11 domain of the protein may be involved in RRE-dependent specific Rev dimerization.

Broadly neutralizing monoclonal antibodies to the V3 region of HIV-1 can be elicited by peptide immunization

The third hypervariable region (V3) of the HIV-1 envelope gp120 protein contains the principal neutralizing domain. Most neutralizing antibodies directed toward this region are very type-specific. Conserved sequences do exist within this region, however, and may prove useful in developing vaccines and therapeutic monoclonal antibodies (MABs) capable of targeting diverse HIV-1 isolates. We have used synthetic peptides containing conserved V3 sequences as immunogens to produce a panel of neutralizing MABs. The characterization of these MABs is described here. In addition, a series of in vitro assays has been developed that may be useful in predicting the neutralization potential of individual antibodies.

In vitro inhibition of a variety of human immunodeficiency virus isolates by a broadly reactive, V3-directed heteroconjugate antibody

The cytotoxic efficacy of a heteroconjugate antibody composed of OKT3 cross-linked to a broadly reactive antibody directed against the GPGRAF sequence of the gp120 V3 region has been characterized. The heteroconjugate antibody could completely inhibit viral replication of both the IIIB and MN isolates of human immunodeficiency virus (HIV) at concentrations as low as 0.5 ng/ml. At an antibody concentration of 1 micrograms/ml, heteroconjugate-mediated cytotoxicity occurred at effector-to-target ratios as low as 0.006:1. Eight different HIV isolates were tested for in vitro inhibition by the anti-V3-OKT3 conjugate, and all but one were completely inhibited for at least 7 days. These results indicate that heteroconjugate antibodies are a potent, effective means by which HIV-infected cells can be killed and viral replication suppressed.

Definition of the human immunodeficiency virus type 1 Rev and human T-cell leukemia virus type I Rex protein activation domain by functional exchange

The human retroviruses human immunodeficiency virus type 1 (HIV-1) and human T-cell leukemia virus type I (HTLV-I) are characterized by complex regulation of gene expression. Each virus encodes a posttranscriptional regulator, the 19-kDa HIV-1 Rev protein and the 27-kDa HTLV-I Rex protein, which is required for viral replication. Expression of these trans activators results in the cytoplasmic accumulation of unspliced or singly spliced viral mRNA which encode the gag, pol, and env gene products. The finding that the HTLV-I Rex protein is able to functionally substitute for the Rev protein of HIV-1 indicates that HIV-1 Rev and HTLV-I Rex may interact with the same component of a cellular pathway involved in either mRNA splicing or transport. In this study, we have generated functional Rev/Rex hybrid proteins by domain exchange. We have defined, using in vivo and in vitro analyses, the activation domains of Rev and Rex which are the putative targets of a common host cell factor(s) required for Rev and Rex function.

Characterization of HIV-1 REV protein: binding stoichiometry and minimal RNA substrate

The HIV-1 REV protein binds to the stem II region of the REV-responsive element (RNA). Studies to further define the RNA sequence and structure specifically bound by REV protein identify a minimal RNA element of 40 nucleotides. Analysis of RNA fragments by gel retardation and filter binding suggest that a core element composed of one particular stem with flanking sequences capable of forming a second double stranded region is essential for specific recognition by REV protein. Stable REV-RNA complexes are formed in a stoichiometry of 1 REV: 1 RNA. The minimal RNA element binds 1 REV molecule while the stem II saturates at 3 REV molecules per RNA. These results establish that REV recognizes a primary binding site within the RRE and support the notion that the initial viral transcript binding event involves a monomeric REV protein.

Circular dichroism studies of the HIV-1 Rev protein and its specific RNA binding site

The circular dichroism (CD) spectrum of the Rev protein from HIV-1 indicates that Rev contains about 50% alpha helix and 25% beta sheet at 5 degrees C in potassium phosphate buffer, pH 3, and 300 mM KF. The spectrum is independent of protein concentration over a 20-fold range. At neutral pH, Rev is relatively insoluble but can be brought into solution by binding to its specific RNA binding site, the Rev-responsive element (RRE), at a Rev:RNA ratio of about 3:1. Nonspecific binding to tRNA does not solubilize Rev. As judged by difference CD spectra, the conformation of Rev when bound to the RRE at neutral pH is similar to the conformation of unbound Rev at pH 3, although changes in the RNA may also contribute to the difference spectrum. Indeed, some difference is observed near 260 nm, consistent with a conformational change of the RRE upon Rev binding. Rev alone at pH 3 shows irreversible aggregation as the temperature is raised, while Rev bound to the RRE at neutral pH shows a reversible transition with a Tm of 68 degrees C.

C-terminal fragments of gp120 and synthetic peptides from five HTLV-III strains: prevalence of antibodies to the HTLV-III-MN isolate in infected individuals

The immunoreactivity of HTLV-III-infected individuals and virus-inoculated chimpanzees with gp120 synthetic peptides of the HTLV-III gp120 envelope principle neutralizing domain (amino acid 301-324 sequences), derived from the HTLV-III isolates 3B, RF, MN, WMJ2, and SC were determined. Sequential bleeds from an infected lab worker and chimpanzees, both infected with the HTLV-IIIB, were immunoreactive only with the 3B peptide. In contrast, 33 HTLV-III-infected individuals were immunoreactive with the HTLV-III(MN) peptide. Of these 33 individuals, 23 were also immunoreactive with the HTLV-III(SC) peptide, and 18 with the HTLV-III(WMJ2) peptide. The data suggest that HTLV-III strains related to MN are most prevalent among HTLV-III-infected individuals. The binding specificities of goat sera generated against either of these synthetic peptides or the C-terminal fragment of gp120 (PB-1, amino acid 287-467, derived from the HTLV-III isolates 3B, RF, MN, WMJ2, and SC) were also determined. Four different ELISA formats (peptide sera/peptide antigens, peptide sera/PB-1 antigens, PB-1 sera/PB-1 antigens, and PB-1 sera/peptide antigens) were utilized to determine the cross-reactivity patterns of goat sera with the antigens. Goat sera generated against MN and SC sequences (PB-1 proteins, as well as synthetic peptides) were highly cross reactive. Thus, patient sera cross reactivity to multiple strains of the principal neutralizing domain may reflect the antigenic relatedness of the virus isolates rather than multiple infection events or strains generated during disease progression.

HIV-1 structural gene expression requires binding of the Rev trans-activator to its RNA target sequence

Expression of human immunodeficiency virus type 1 structural proteins requires both the viral Rev trans-activator and its cis-acting RNA target sequence, the Rev response element (RRE). The RRE has been mapped to a conserved region of the HIV-1 env gene and is predicted to form a complex, highly stable RNA stem-loop structure. Site-directed mutagenesis was used to define a small subdomain of the RRE, termed stem-loop II, that is essential for biological activity. Gel retardation assays demonstrated that the Rev trans-activator is a sequence-specific RNA binding protein. The RRE stem-loop II subdomain was found to be both necessary and sufficient for the binding of Rev by the RRE. We propose that the HIV-1 Rev trans-activator belongs to a new class of sequence-specific RNA binding proteins characterized by the presence of an arginine-rich binding motif.

Specific binding of HIV-1 recombinant Rev protein to the Rev-responsive element in vitro

The human immunodeficiency virus type 1 (HIV-1) genome encodes the regulatory protein Rev, of relative molecular mass 13,000, which is synthesized from fully processed viral transcripts before synthesis of HIV-1 structural proteins. Rev has been postulated to exert control within the nucleus at the level of messenger RNA processing. The availability of Rev in the nucleus serves to increase the proportion of unspliced and singly spliced mRNA species relative to fully spliced mRNA molecules, resulting in an increased synthesis of viral structural proteins. A highly conserved cis-acting sequence termed the Rev-responsive element (RRE) has been identified in the envelope gene (env) of the viral transcript that seems to control mRNA processing in a Rev-dependent manner. Genetic studies have identified rev gene mutants with dominant phenotypes, supporting the hypothesis that Rev interacts directly with the RRE. Here we demonstrate that Rev protein, purified from Escherichia coli, binds in a sequence-specific manner to the RRE element in vitro.

Principal neutralizing domain of the human immunodeficiency virus type 1 envelope protein

The principal neutralizing determinant of human immunodeficiency virus type 1 (HIV-1) is located in the external envelope protein, gp120, and has previously been mapped to a 24-amino acid-long sequence (denoted RP135). We show here that deletion of this sequence renders the envelope unable to elicit neutralizing antibodies. In addition, using synthetic peptide fragments of RP135, we have mapped the neutralizing determinant to 8 amino acids and found that a peptide of this size elicits neutralizing antibodies. This sequence contains a central Gly-Pro-Gly that is generally conserved between different HIV-1 isolates and is flanked by amino acids that differ from isolate to isolate. Antibodies elicited by peptides from one isolate do not neutralize two different isolates, and a hybrid peptide, consisting of amino acid sequences from two isolates, elicits neutralizing antibodies to both isolates. By using a mixture of peptides of this domain or a mixture of such hybrid peptides the type-specificity of the neutralizing antibody response to this determinant can perhaps be overcome.

The human immunodeficiency virus type 2 (HIV-2) contains a novel gene encoding a 16 kD protein associated with mature virions

The HIV-2 genome contains an open reading frame (designated X-orf) that does not have a counterpart in HIV-1. To establish whether X-orf is a gene, we studied its expression in HIV-2-infected individuals and in infected cells in vitro. An HIV-2 proviral DNA fragment containing the X-orf was expressed in E. coli, and the recombinant protein was used in an immunoblot assay. The X-orf protein was recognized specifically by the sera of HIV-2-infected people but not by the sera of SIV-infected monkeys or HIV-1-infected humans. A rabbit antiserum raised against the recombinant X-orf protein recognized a 16 kD protein in HIV-2-infected cells. The native X-orf protein was not glycosylated or phosphorylated, was localized in the cytoplasm of HIV-2-infected cells, and appeared to be associated with mature virions.

Type-restricted neutralization of molecular clones of human immunodeficiency virus

In a study of the immunologic significance of the genetic diversity present within single isolates of human immunodeficiency virus type 1 (HIV-1), the neutralization of viruses derived from molecular clones of the HIV-1 strain HTLV-IIIB by an extensive panel of sera was compared. Sera from HIV-1-infected patients and from goats immunized with polyacrylamide gel-purified HIV-1 envelope glycoprotein (gp120), native gp120, or gp120-derived recombinant peptides, showed marked heterogeneity in neutralizing activity against these closely related viruses. The change of a single amino acid residue in gp120 may account for such "clonal restriction" of neutralizing activity.