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The Ensembl project (http://www.ensembl.org) is a comprehensive genome information system featuring an integrated set of genome annotation, databases, and other information for chordate, selected model organism and disease vector genomes. As of release 51 (November 2008), Ensembl fully supports 45 species, and three additional species have preliminary support. New species in the past year include orangutan and six additional low coverage mammalian genomes. Major additions and improvements to Ensembl since our previous report include a major redesign of our website; generation of multiple genome alignments and ancestral sequences using the new Enredo-Pecan-Ortheus pipeline and development of our software infrastructure, particularly to support the Ensembl Genomes project (http://www.ensemblgenomes.org/).
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Bases de Dados Genéticas , Genômica , Animais , Variação Genética , Humanos , Internet , Alinhamento de SequênciaRESUMO
CCR5, a chemokine receptor expressed on T cells and macrophages, is the principal coreceptor for M-tropic HIV-1 strains. Recently, we described an NH2-terminal modification of the CCR5 ligand regulated on activation, normal T cell expressed and secreted (RANTES), aminooxypentane-RANTES (AOP-RANTES), that showed potent inhibition of macrophage infection by HIV-1 under conditions where RANTES was barely effective. To investigate the mechanism of AOP-RANTES inhibition of HIV infectivity we examined the surface expression of CCR5 using a monoclonal anti-CCR5 antibody, MC-1. We demonstrate that AOP-RANTES rapidly caused >90% decrease in cell surface expression of CCR5 on lymphocytes, monocytes/ macrophages, and CCR5 transfected Chinese hamster ovary (CHO) cells. RANTES also caused a loss of cell surface CCR5, although its effect was less than with AOP-RANTES. Significantly, AOP-RANTES inhibited recycling of internalized CCR5 to the cell surface, whereas RANTES did not. When peripheral blood mononuclear cells are cultured for prolonged periods of time in the presence of RANTES, CCR5 expression is comparable to that seen on cells treated with control medium, whereas there is no CCR5 surface expression on cells cultured in the presence of AOP-RANTES. Immunofluorescence indicated that both AOP-RANTES and RANTES induced downmodulation of cell surface CCR5, and that the receptor was redistributed into endocytic organelles containing the transferrin receptor. When RANTES was removed, the internalized receptor was recycled to the cell surface; however, the receptor internalized in the presence of AOP-RANTES was retained in endosomes. Using human osteosarcoma (GHOST) 34/CCR5 cells, the potency of AOP-RANTES and RANTES to inhibit infection by the M-tropic HIV-1 strain, SF 162, correlated with the degree of downregulation of CCR5 induced by the two chemokines. These differences between AOP-RANTES and RANTES in their effect on receptor downregulation and recycling suggest a mechanism for the potent inhibition of HIV infection by AOP-RANTES. Moreover, these results support the notion that receptor internalization and inhibition of receptor recycling present new targets for therapeutic agents to prevent HIV infection.
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Fármacos Anti-HIV/farmacologia , Quimiocina CCL5/análogos & derivados , HIV-1/efeitos dos fármacos , Receptores CCR5/metabolismo , Animais , Transporte Biológico , Células CHO , Quimiocina CCL5/farmacologia , Cricetinae , Regulação para Baixo , Endocitose , Endossomos/metabolismo , HumanosRESUMO
At the cell surface, chemokine receptors and CD4 act in concert to bind to human immunodeficiency virus (HIV) and trigger its entry into and infection of cells. Several different chemokine receptors can act as co-receptors for HIV entry, although either CCR5 or CXCR4 is used by all HIV-1 strains studied so far. The capacity of different HIV strains to exploit different chemokine receptors influences their cell tropism, cytopathicity and pathogenicity. Chemokines, the natural ligands for these receptors, can block HIV entry and are thus potential starting points for the design of novel therapeutic agents against HIV infection.
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Human T-cell leukemia virus (HTLV), American PL isolate, was transmitted by cocultivation and by cell-free filtrates to a nonlymphoid human osteogenic sarcoma (HOS) cell line, designated HOS/PL, but not to nine other lines bearing receptors for HTLV. HOS and HOS/PL cells are not dependent on interleukin-2 and do not express interleukin-2 receptors that are recognized by anti-Tac monoclonal antibody. HTLV released by the Japanese MT2 cell line was also transmitted to HOS cells. The infected HOS cells release substantial titers of progeny HTLV which is antigenically indistinguishable from parental virus and is able to transform T cells.
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Deltaretrovirus/crescimento & desenvolvimento , Replicação Viral , Antígenos de Superfície/análise , Antígenos Virais/análise , Linhagem Celular , Transformação Celular Viral , Sistema Livre de Células , Deltaretrovirus/imunologia , Deltaretrovirus/ultraestrutura , Humanos , Interleucina-2/metabolismo , Microscopia Eletrônica , Linfócitos T/imunologiaRESUMO
Human T cell leukemia viruses (HTLV-I and HTLV-II) can infect many cell types in vitro. HTLV-I and HTLV-II use the same cell surface receptor, as shown by interference with syncytium formation and with infection by vesicular stomatitis virus (VSV) pseudotypes bearing the HTLV envelope glycoproteins. Human-mouse somatic cell hybrids were used to determine which human chromosome was required to confer susceptibility to VSV(HTLV) infection. The only human chromosome common to all susceptible cell hybrids was chromosome 17, and the receptor gene was localized to 17cen-qter. Antibodies to surface antigens known to be determined by genes on 17q did not block the HTLV receptor.
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Cromossomos Humanos Par 17 , Genes , Vírus Linfotrópico T Tipo 1 Humano/fisiologia , Vírus Linfotrópico T Tipo 2 Humano/fisiologia , Receptores Virais/genética , Animais , Bovinos , Linhagem Celular , Mapeamento Cromossômico , Cricetinae , Humanos , Células Híbridas/citologia , Células Híbridas/microbiologia , Camundongos , RatosRESUMO
The chemokine receptors CXCR4 and CCR5 have recently been shown to act as coreceptors, in concert with CD4, for human immunodeficiency virus-type 1 (HIV-1) infection. RANTES and other chemokines that interact with CCR5 and block infection of peripheral blood mononuclear cell cultures inhibit infection of primary macrophages inefficiently at best. If used to treat HIV-1-infected individuals, these chemokines could fail to influence HIV replication in nonlymphocyte compartments while promoting unwanted inflammatory side effects. A derivative of RANTES that was created by chemical modification of the amino terminus, aminooxypentane (AOP)-RANTES, did not induce chemotaxis and was a subnanomolar antagonist of CCR5 function in monocytes. It potently inhibited infection of diverse cell types (including macrophages and lymphocytes) by nonsyncytium-inducing, macrophage-tropic HIV-1 strains. Thus, activation of cells by chemokines is not a prerequisite for the inhibition of viral uptake and replication. Chemokine receptor antagonists like AOP-RANTES that achieve full receptor occupancy at nanomolar concentrations are strong candidates for the therapy of HIV-1-infected individuals.
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HIV-1/efeitos dos fármacos , Macrófagos/virologia , Receptores de Quimiocinas , Receptores de Citocinas/antagonistas & inibidores , Receptores de HIV/antagonistas & inibidores , Linfócitos T/virologia , Animais , Ligação Competitiva , Antígenos CD4/metabolismo , Gatos , Linhagem Celular , Células Cultivadas , Quimiocina CCL4 , Quimiocina CCL5/metabolismo , Quimiocina CCL5/farmacologia , Quimiotaxia de Leucócito , HIV-1/fisiologia , Células HeLa , Humanos , Proteínas Inflamatórias de Macrófagos/metabolismo , Macrófagos/efeitos dos fármacos , Receptores CCR5 , Receptores de Citocinas/metabolismo , Receptores de HIV/metabolismo , Linfócitos T/efeitos dos fármacos , Replicação Viral/efeitos dos fármacosRESUMO
Kaposi's sarcoma-associated herpesvirus encodes a chemokine called vMIP-II. This protein displayed a broader spectrum of receptor activities than any mammalian chemokine as it bound with high affinity to a number of both CC and CXC chemokine receptors. Binding of vMIP-II, however, was not associated with the normal, rapid mobilization of calcium from intracellular stores; instead, it blocked calcium mobilization induced by endogenous chemokines. In freshly isolated human monocytes the virally encoded vMIP-II acted as a potent and efficient antagonist of chemotaxis induced by chemokines. Because vMIP-II could inhibit cell entry of human immunodeficiency virus (HIV) mediated through CCR3 and CCR5 as well as CXCR4, this protein may serve as a lead for development of broad-spectrum anti-HIV agents.
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Quimiocinas/antagonistas & inibidores , Quimiocinas/metabolismo , Herpesvirus Humano 8/genética , Receptores de Citocinas/metabolismo , Receptores de HIV/metabolismo , Sequência de Aminoácidos , Animais , Cálcio/metabolismo , Linhagem Celular , Quimiocina CCL5/antagonistas & inibidores , Quimiocinas/química , Quimiocinas/genética , Quimiocinas/farmacologia , Quimiotaxia de Leucócito , HIV-1/fisiologia , Humanos , Dados de Sequência Molecular , Monócitos/citologia , Receptores de Citocinas/antagonistas & inibidores , Receptores de HIV/antagonistas & inibidores , Proteínas Recombinantes/farmacologia , Transdução de SinaisRESUMO
HIV-1 cell tropisms are partly determined by the hypervariable loops V1/V2 and V3 in gp120, which also contain epitopes for neutralizing antibodies. Mutations conferring tropism changes can result in escape from neutralization and vice versa. We examine whether variant viruses that can colonize new cell types and simultaneously escape neutralizing antibodies have an enhanced advantage in vivo.
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Proteína gp120 do Envelope de HIV/química , Infecções por HIV/virologia , HIV-1/fisiologia , Imunidade Celular/imunologia , Tropismo/fisiologia , Sequência de Aminoácidos , Anticorpos Monoclonais/metabolismo , Proteína gp120 do Envelope de HIV/metabolismo , Infecções por HIV/imunologia , HIV-1/genética , Humanos , Imunidade Inata/genética , Região Variável de Imunoglobulina/metabolismo , Dados de Sequência Molecular , Mutação , Testes de Neutralização , Tropismo/genéticaRESUMO
We present a new method for paternity analysis in natural populations that is based on genotypic data that can take the sampling fraction of putative parents into account. The method allows paternity assignment to be performed in a decision theoretic framework. Simulations are performed to evaluate the utility and robustness of the method and to assess how many loci are necessary for reliable paternity inference. In addition we present a method for testing hypotheses regarding relative reproductive success of different ecologically or behaviorally defined groups as well as a new method for estimating the current population size of males from genotypic data. This method is an extension of the fractional paternity method to the case where only a proportion of all putative fathers have been sampled. It can also be applied to provide abundance estimates of the number of breeding males from genetic data. Throughout, the methods were applied to genotypic data collected from North Atlantic humpback whales (Megaptera novaeangliae) to test if the males that appear dominant during the mating season have a higher reproductive success than the subdominant males.
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Teorema de Bayes , Impressão Genômica , Paternidade , Baleias/genética , Animais , Oceano Atlântico , MasculinoRESUMO
Bystander effects from ionizing radiation have been detailed for a number of cell systems and a number of end points. We wished to use a cell culture/ex vivo rat model of respiratory tissue to determine whether a bystander effect detected in culture could also be shown in a tissue. Examination by immunofluorescence techniques of tracheal cell cultures after exposure to very low doses of alpha particles revealed a large proportion of cells with proliferating cell nuclear antigen (PCNA) bound in their nuclei. PCNA was selected as an end point because it is involved in both DNA repair and the changes in cell cycle that are typical of many reported bystander effects. Maximum response can be detected in up to 28% of the cells in sub-confluent cultures with a dose of only 2 mGy. At this dose less than 2% of the cell nuclei have experienced a particle traversal and less than 6% of the cells have experienced an alpha-particle traversal through either their nucleus or some part of their cytoplasm. The hypothesis that this bystander response in nontargeted cells is mediated through secreted factor(s) is presented, and supporting evidence was found using partial irradiation and co-culture experiments. Examination of the effect with excised pieces of trachea demonstrated a response similar to that seen in culture.
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Efeito Espectador/fisiologia , Efeito Espectador/efeitos da radiação , Plutônio/efeitos adversos , Antígeno Nuclear de Célula em Proliferação/metabolismo , Mucosa Respiratória/metabolismo , Mucosa Respiratória/efeitos da radiação , Traqueia/metabolismo , Traqueia/efeitos da radiação , Partículas alfa , Animais , Núcleo Celular/metabolismo , Células Cultivadas , Relação Dose-Resposta à Radiação , Masculino , Ligação Proteica , Doses de Radiação , Ratos , Ratos Endogâmicos F344RESUMO
Human RANTES (CCL5) and MIP-1alpha (CCL3) bind and activate several CC chemokine receptors. RANTES is a high-affinity ligand for CCR1 and CCR5, and it binds CCR3 with moderate affinity and CCR4 with low affinity. MIP-1alpha has similar binding characteristics to RANTES except that it does not bind to CCR3. Here we have generated a chimera of human MIP-1alpha and RANTES, called MIP/RANTES, consisting of the eight amino terminal residues of MIP-1alpha preceding the CC motif, and the remainder of the sequence is RANTES. The chimera is able to induce chemotaxis of human monocytes. MIP/RANTES has >100-fold reduction in binding to CCR1 and does not bind to CCR3 but retains full, functional binding to CCR5. It has equivalent affinity for CCR5 to MIP-1alpha and RANTES, binding with an IC(50) of 1.12 nM, and is able to mobilize calcium and induce endocytosis of CCR5 in PBMC in a manner equi-potent to RANTES. It also retains the ability to inhibit R5 using HIV-1 strains. Therefore, we conclude that the amino terminus of RANTES is not involved in CCR5 binding, but it is essential for CCR1 and CCR3.
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Quimiocina CCL5/metabolismo , Proteínas Inflamatórias de Macrófagos/metabolismo , Receptores CCR5/metabolismo , Receptores de Quimiocinas/metabolismo , Sequência de Aminoácidos , Ligação Competitiva , Sinalização do Cálcio/efeitos dos fármacos , Quimiocina CCL3 , Quimiocina CCL4 , Quimiocina CCL5/análogos & derivados , Quimiocina CCL5/química , Quimiotaxia/efeitos dos fármacos , Regulação para Baixo/efeitos dos fármacos , HIV-1/fisiologia , Proteínas Inflamatórias de Macrófagos/química , Dados de Sequência Molecular , Monócitos/efeitos dos fármacos , Ligação Proteica , Estrutura Terciária de Proteína , Receptores CCR1 , Receptores CCR4 , Proteínas Recombinantes de Fusão/metabolismo , Proteínas Recombinantes de Fusão/fisiologia , Alinhamento de Sequência , Homologia de Sequência de AminoácidosRESUMO
OBJECTIVE: To study the induction of group-specific (gs) neutralizing antibodies to HIV-1 after seroconversion. DESIGN AND METHODS: Serum samples taken sequentially from seven Dutch homosexual men and four British haemophiliacs (anonymous sample, therefore sex not known) before and after seroconversion were tested for neutralizing antibodies effective against five diverse HIV-1 strains. Strains of HIV-1 tested included isolates from the United States, Europe and Africa. RESULTS: The gs neutralizing antibody response varied between individuals. Only five of the 11 individuals studied produced detectable neutralizing antibodies to laboratory-adapted HIV-1 strains (for example, IIIB) within 32 weeks of seroconversion. Most individuals initially produced antibodies effective against US/European isolates; the response then generally broadened to include the more diverse strains, i.e., African. CONCLUSIONS: These results suggest that the gs neutralizing target for HIV-1 is poorly immunogenic in vivo and is probably not highly conserved among diverse HIV-1 strains.
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Anticorpos Anti-HIV/imunologia , Soropositividade para HIV/imunologia , HIV-1/imunologia , Anticorpos Anti-HIV/biossíntese , Humanos , Masculino , Testes de NeutralizaçãoRESUMO
The cellular receptor for HIV-1 is the leucocyte differentiation antigen, CD4. Blocking of HIV-1 infectivity can be achieved with monoclonal antibodies (MAbs) to some, but not all epitopes of this antigen. We demonstrate here, by inhibition of virus infection, blocking of syncytium formation and inhibition of pseudotype infection with a panel of CD4 MAbs, that HIV-1, HIV-2 and simian immunodeficiency virus (SIV) isolates share the same cellular receptor, the CD4 glycoprotein. It is also shown that very similar epitopes of this molecule are involved in virus binding. We infer from these data that the binding sites on these viruses are highly conserved regions, and may therefore make good targets for potential vaccines. In addition, we show that cell surface expression of CD4 is similarly modulated after infection of cell lines by all the viruses.
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Antígenos de Diferenciação/imunologia , Epitopos/imunologia , HIV/imunologia , Haplorrinos/imunologia , Receptores Virais/imunologia , Retroviridae/imunologia , Animais , Anticorpos Monoclonais/imunologia , Anticorpos Antivirais/imunologia , Reações Antígeno-Anticorpo , Sítios de Ligação de Anticorpos , Ligação Competitiva , Humanos , Receptores de HIVRESUMO
The production of neutralizing monoclonal antibodies (MAbs) will permit the exact localization of neutralizing epitopes on the AIDS virus, HIV-1. We describe the properties of seven MAbs to the envelope of the LAV-1 isolate. Five MAbs recognise the central portion of gp110, amino acids 279-472, and four of these are capable of high-titre neutralization of HIV-1, by infection inhibition, syncytial inhibition and vesicular stomatitis virus (VSV) pseudotype neutralization. One of the two MAbs to gp41 inhibits syncytium formation. Neutralization, live cell immunofluorescence and immunoprecipitation of gp110 are type-specific and restricted to HIV-1 isolates closely related to LAV-1.
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Anticorpos Monoclonais/imunologia , Anticorpos Antivirais/imunologia , HIV/imunologia , Especificidade de Anticorpos , Antígenos Virais/imunologia , Epitopos/imunologia , Anticorpos Anti-HIV , Antígenos HIV , Humanos , Testes de Neutralização , Proteínas dos Retroviridae/imunologia , Proteínas do Envelope Viral/imunologiaRESUMO
The neutralization properties of three independent HIV-2 isolates were examined in comparison with four diverse HIV-1 strains. Human sera containing antibodies specific to HIV-2 can cross-neutralize HIV-1. By contrast, HIV-1 sera are group-specific and have no neutralizing effect on HIV-2. Therefore, HIV-2 antigens may be important components for the development of broadly cross-protective AIDS vaccines.
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Anticorpos Antivirais/fisiologia , HIV/imunologia , Soros Imunes/farmacologia , Testes de Neutralização , Especificidade de Anticorpos , Reações Cruzadas , Ensaio de Imunoadsorção Enzimática , Anticorpos Anti-HIV , Humanos , Testes de Neutralização/métodos , Testes de PrecipitinaRESUMO
BACKGROUND: HIV-1 strains R5 and X4 can infect CD4 memory T cells in vivo. Anti-CD3/28 stimulation induces beta-chemokines and CCR5 down-regulation and renders these cells resistant to R5 HIV-1 infection. Here we describe an additional cellular mechanism that blocks productive R5 HIV-1 infection of CD4 memory T cells. METHODS: Blood-derived CD4 memory T cells and CD4 T-cell clones were infected with primary R5 and X4 HIV-1 strains. Virus replication was correlated with CCR5 expression and beta-chemokine production. Virus entry and infectivity were measured by PCR for early and late products of HIV reverse transcription respectively. RESULTS: R5 strains were up to 1000-fold less infectious than X4 viruses for CD4 memory T cells. This resistance was independent of CCR5 levels and of the Delta-32 mutation and the CCR2-V64I/CCR5-59653T linked mutations. Blocking endogenous beta-chemokines relieved minimally this restriction. At the single cell level, CD4 memory cells were either permissive or non-permissive for R5 HIV-1 infection. R5 HIV titre was up to 10-fold lower than X4 virus titre even in a permissive clone. However, R5 viruses replicated as efficiently as X4 viruses in the permissive clone when neutralizing anti-beta chemokine antibodies were added. Non-permissive cells blocked a post-entry step of the virus life-cycle and expressed early but not late HIV transcripts. Neutralizing anti-beta chemokine antibodies promoted R5 virus replication marginally in the non-permissive clone. CONCLUSION: Some blood memory CD4 T cells retard R5 HIV-1 replication via endogenous beta-chemokines whereas others block productive R5 HIV-1 infection by an additional mechanism that interferes with a post-entry step of the virus life cycle. These natural barriers might contribute to lower pathogenicity of R5 HIV-1 strains for CD4 memory T cells than X4 viruses that emerge late in disease.
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Linfócitos T CD4-Positivos/virologia , Quimiocinas CC/farmacologia , HIV-1/patogenicidade , Memória Imunológica , Replicação Viral , Linhagem Celular , Células Cultivadas , Quimiocinas CC/biossíntese , Infecções por HIV/prevenção & controle , Infecções por HIV/virologia , HIV-1/efeitos dos fármacos , HIV-1/genética , HIV-1/fisiologia , Humanos , Polimorfismo Genético , Receptores CCR5/genética , Receptores CCR5/metabolismo , Replicação Viral/efeitos dos fármacosRESUMO
The envelope glycoproteins of HIV, gp120 and gp41, contain epitopes recognized by neutralizing antibodies. Studies of human sera from infected individuals indicate that group-specific neutralization antigens common to most isolates of HIV-1 exist, and that some HIV-2 antisera cross-neutralize HIV-1. Neutralization epitopes for HIV-1 have been identified and mapped, including a group-specific antigen on gp41, and a type-specific antigen on gp120. Neutralization "escape" mutants have been selected in vitro with a neutralizing mab to the type-specific antigenic loop. The CD4 antigen binds HIV-1 gp120 with high affinity and acts as the receptor on human and simian T-lymphocytes and monocytes for all strains of HIV-1, HIV-2, and SIV tested. Following binding to the CD4 receptor, HIV becomes internalized by a pH-independent process. The principle binding domain for gp120 is located in the N-terminal V domain of CD4. Anti-idiotypic sera to CD4 mabs recognizing the same site weakly neutralize HIVs of many strains, and soluble, recombinant forms of CD4 strongly neutralize HIV. Neither anti-CD4 mabs nor sCD4 inhibit the low level of plating of HIV observed on tumour cells in culture of glial (brain) and muscle origin, indicating that CD4 is not essential for infection of these cell types.
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Antígenos de Diferenciação de Linfócitos T/imunologia , Antígenos HIV/imunologia , HIV/imunologia , Animais , Variação Antigênica , Epitopos/imunologia , Humanos , Testes de Neutralização , Receptores Imunológicos/imunologia , Receptores Virais/imunologia , Linfócitos T/imunologiaRESUMO
It is widely assumed that the mating system of the humpback whale. Magaptera novaeangliae, is similar to that of most mammals in that it represents some form of polygyny or promiscuity, but this cannot be tested without observations of copulation or data on paternity of offspring. Microsatellite DNA markers were used to examine the paternity of calves born to individually identified mature female humpback whales from the Gulf of Maine. Skin biopsies were obtained from three females, and several (range: three to five) of their known offspring. Multiple paternity of offspring, indicated by the presence of at least three different paternal alleles, was evident in all three females at either three or four of the six microsatellite loci surveyed. Such promiscuous mating is expected given current knowledge of the social ecology of this species. It is also consistent with resightings of individually identified female humpbacks with different male associates during two or more breeding seasons.
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Repetições de Microssatélites , Baleias/genética , Animais , Feminino , Marcadores Genéticos , Masculino , Comportamento Sexual AnimalRESUMO
We have identified 6 sera containing autoantibodies to CD4 in 174 human immunodeficiency virus-type (HIV-1) positive sera tested in an antigen-capture enzyme-linked immunosorbent assay (ELISA) using sCD4, and none in 34 HIV type 2 sera. These autoantibodies do not bind to cellular CD4, but react with sCD4 to increase its binding in ELISA to monoclonal antibodies and the HIV surface glycoprotein gp120. The effect of CD4 autoantibodies is mimicked by monoclonal antibodies to the third and fourth domains of CD4. The enhanced sCD4 binding to gp120 in ELISA is reflected by a reduction in the concentration of sCD4 required to neutralize HIV-1 and HIV-2 infection in tissue culture when CD4 autoantibodies or the relevant monoclonal antibodies were present.
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Anticorpos Monoclonais/imunologia , Autoanticorpos/imunologia , Antígenos CD4/imunologia , Proteína gp120 do Envelope de HIV/imunologia , Infecções por HIV/imunologia , HIV-1/imunologia , Sequência de Aminoácidos , Animais , Linhagem Celular , Cricetinae , Ensaio de Imunoadsorção Enzimática , Humanos , Masculino , Dados de Sequência Molecular , Testes de Neutralização , SolubilidadeRESUMO
We have developed a series of murine monoclonal antibodies to a region of the 120 kD envelope glycoprotein (gp120) of human immunodeficiency virus type 1 (HIV-1). This region has previously been implicated as a site for virus neutralization by antisera raised to recombinant proteins and by antibodies made to full-length gp120 purified from virus. The antigen employed was a synthetic peptide containing 15 amino acids, representing amino acid residues 308-322, RIQRGPGRAFVTIGK, of env gp120 (HTLV-IIIB isolate). Five of the monoclonal antibodies raised to this antigen have reactivity with gp120 from divergent strains of HIV-1 in Western blot assays. The two of these five which were tested with live cells infected with the divergent HIV-1 isolates IIIB, MN, and RF were specifically reactive by fluorescence analyses with cells infected with the MN and IIIB isolates. Four of the five monoclonal antibodies blocked the fusion of IIIB-infected cells with uninfected MOLT-4 target cells. The monoclonal antibody most reactive with MN-infected cells by fluorescence, #5025A, blocked the fusion of MN-infected cells with uninfected MOLT-4 cells. Four of the five monoclonal antibodies neutralized the IIIB isolate of HIV-1 in vitro, but none neutralized the MN or RF isolates at the levels of antibody tested (less than or equal to 50 micrograms/ml). Taken together these data indicate that monoclonal antibodies to the immunodominant neutralizing domain of HIV-1 gp120 display different levels of group reactivity depending on the assay system being examined.