Antibody neutralization of HIV-1 and the potential for vaccine design

Neutralisation by antibody is, for a number of viruses, an in vitro correlate for protection in vivo. For HIV-1 this is controversial. However, the induction of a potent anti-HIV neutralising antibody response remains one of the principal goals in vaccine development. A greater knowledge of the fundamental mechanisms underlying the neutralisation process would help direct research towards suitable vaccine immunogens. The primary determinant of HIV neutralisation appears to be antibody affinity for the trimeric envelope glycoprotein spike on the virion, suggesting that epitope-specific effects are secondary and implying a single, dominant mechanism of neutralisation. Antibody interference with virion attachment to the target cell appears to be a major mechanism of neutralisation by gp120-specific antibodies. This is probably achieved both by antibody-induced dissociation of gp120 from gp41 and by direct inhibition of virus binding to receptor-coreceptor complexes. A gp41-specific antibody neutralises by interfering with post-attachment steps leading to virus membrane fusion. Recent advances in structural analyses of the HIV envelope glycoproteins coupled with data obtained from antibody mapping and neutralisation studies allow a greater understanding of Env function and its inhibition. This in turn should lead to a more rational basis for vaccine design aimed at stimulating highly effective neutralising antibodies.     

Antibody-dependent cellular cytotoxicity (ADCC) is directed against immunodominant epitopes of the envelope proteins of human immunodeficiency virus 1 (HIV-1).

In this study, epitopes of HIV envelope proteins that are involved in ADCC were identified. Peripheral blood mononuclear cells (PBMC) were obtained from adults with asymptomatic HIV infection or early symptoms of AIDS. These PBMC, which were reported to be "armed" in vivo with HIV-specific antibodies, were used as effector cells in 51Cr release assays. Target cells consisted of CD4 lymphocytes from healthy seronegative donors, coated with the IIIB strain of HIV-1 or with one of seven synthetic peptides. Cytotoxicity was detected against CD4 lymphocytes coated with HIV-1 IIIB or with the peptides env aa 507-518, corresponding to the carboxy-terminus of gp120, and env aa 597-611, corresponding to the region of the cysteine loop of gp41. The magnitude of target cell lysis was directly related to the quantity of peptide used. In contrast, target cells coated with the peptide gag aa 129-135, corresponding to the p17/p24 cleavage region of the gag precursor, were not killed. The same immunodominant regions which were involved in ADCC were recognized in enzyme-linked immunoabsorbent assays (ELISA) by the majority of 107 sera from HIV-infected adults. We conclude that the immunodominant epitopes located at the carboxy-terminus of gp120 and the cysteine loop of gp41 serve as recognition structure for antibodies, capable of mediating ADCC against HIV-infected cells.     

Utility of various commercially available human immunodeficiency virus (HIV) antibody diagnostic kits for use in conjunction with efficacy trials of HIV-1 vaccines.

There is a need for human immunodeficiency virus (HIV) screening assays which will distinguish uninfected HIV vaccine recipients from HIV-infected individuals. Commercial screening kits were used to test serum samples from low- and high-risk participants in clinical trials before and after immunization with various recombinant HIV type 1 (HIV-1) envelope glycoprotein 120 (gp120) candidate vaccines. All kits were 100% sensitive in detecting HIV infection. Both Murex Single Use Diagnostic System and United Biomedical, Inc., HIV type 1 or 2 (HIV-1/2) enzyme immunoassay (EIA) kits, which detect antibodies to HIV-1 gp41, were 98 to 100% specific when used to screen baseline or recombinant gp120-vaccinated populations as vaccine-induced antibodies to gp120 were nonreactive in these tests. The Abbott HIVAB HIV-1 EIA (lysate of whole infected cells, reactive with anti-gp120 antibodies) gave high levels of reactivity due to vaccine-induced antibodies and a high baseline rate of false positives (12 of 83) among nonvaccinated high-risk volunteers. Assays containing only gp41 and p24 solid-phase components are compatible with gp120-based vaccines but are unlikely to be useful in a similar role for vaccines containing gp160, gp41, or gp120 plus p24 antigens. Efficacy trials must be designed in concert with available diagnostic screening assays to avoid problems caused by vaccine-induced seroconversion in high-risk populations.     

HIV 外膜糖蛋白 gp120 和 gp41 在昆虫细胞中表达及其免疫学特性的评价

将编码完整ｇｐ１２０和完整ｇｐ４１的基因分别克隆到杆状病毒转移质粒中。使用重组转移质粒与野生杆状病毒（ＡｃＮＰＶ）ＤＮＡ共转染Ｓｆ９昆虫细胞，经挑选获得分别带有编码ｇｐ１２０和ｇｐ４１基因的重组杆状病毒。重组杆状病毒感染Ｓｆ９细胞后在细胞中分别表达了ＨＩＶ外膜糖蛋白ｇｐ１２０和ｇｐ４１。其重组蛋白的分子量分别为１２０×１０３和４１×１０３。此重组糖蛋白在免疫荧光、免疫印染和酶联免疫实验中都能被ＨＩＶ阳性血清所识别。动物免疫实验表明此重组糖蛋白能诱导很强的特异性抗体产生。     

Development and evaluation of ELISA procedures to detect antibodies against the major envelope protein (G(L)) of equine arteritis virus

Enzyme-linked immunosorbant assays (ELISAs) were developed for the detection of antibodies against the major envelope glycoprotein (G_L) of equine arteritis virus (EAV). A 6-Histidine tagged recombinant protein expressing the complete G_L ectodomain (G_L-6His), a glutathione-S-transferase recombinant protein expressing amino acids 55–98 of G_L (G_L-GST) and an ovalbumin-conjugated synthetic peptide representing amino acids 81–106 of G_L (G_L-OVA) were used as diagnostic antigens. An ELISA procedure was developed and optimised for each antigen. The G_L-OVA and G_L-6His assays showed the greatest specificity while the G_L-GST assay was slightly more sensitive that the G_L-OVA and G_L-6His assays; results based on the analysis of 50 virus neutralisation positive and 50 virus neutralisation negative sera. The G_L-OVA ELISA was selected for further evaluation since it was simpler to use than ELISAs based on recombinant antigens and did not suffer from background reactivity. The final sensitivity and specificity of the G_L-OVA ELISA were 96.75 and 95.6%, respectively, results based on the analysis of 400 virus neutralisation positive and 400 virus neutralisation negative sera. It also detected EAV antibody (100% efficiency) in seropositive shedding stallions and, in ponies infected experimentally with the UK93 isolate of EAV, the appearance of virus neutralising antibodies and G_L-OVA ELISA-specific immunoglobulins coincided.     

An O-linked carbohydrate neutralization epitope of HIV-1 gp 120 is expressed by HIV-1env gene recombinant vaccinia virus

Summary Previous studies have disagreed about the presence of O-linked carbohydrate epitopes on gp 120 of HIV, although antibodies against short-chain O-linked glycans neutralize HIV infection and block syncytium formation in vitro. To settle this question, we analysed the O-linked glycans of gp 120 by chemical methods using purified HIV-1 gp 120 from cells infected with recombinant vaccinia virus solely expressing gp 160 or gp 120. Alkaline borohydride degradation of recombinant gp 120 released monosaccharides and also slightly larger structures (di/trisaccharides) by a -elimination, confirming the presence of simple O-linked oligosaccharides. The functional activity as neutralisation epitopes of the O-linked oligosaccharides expressed on recombinant gp 120 was preserved, since fusion between uninfected CD 4⁺ cells and cells infected with recombinant vaccinia was blocked by monoclonal antibodies to the O-linked oligosaccharides of gp 120. Although the mechanism for HIV induction of O-linked oligosaccharide neoantigens is unknown, these results indicate that the O-linked neutralization epitopes are inherent to the glycoprotein itself, and that the unusual appearance of simple O-linked oligosaccharides on gp 120 is independent of any interaction between the host cell and retroviral genes other thanenv.     

双抗原夹心法检测抗HIV-1/2总抗体方法的建立和评价

目的 进一步提高HIv感染诊断试剂的敏感性。方法 以人工合成的HIv—1 gp41．1(sP1)、gp41．2(sP2)、gp120(sP3)、P24(sP4)和HIV—2gp36(sP5)5条多肽，采用双抗原夹心酶链免疫吸附试验(ELISA)原理，以sP1、sP3、sP4和sP5混合包被酶标板作为因相抗原，辣根过氧化物酶标记sp1、sp2、sP4和sP5多肽为标记物，建立了检测抗HIV—1／2总抗体的双抗原夹心ELISA法。结果 检测卫生部药品生物制品检定所第2代40份质控参比血清，其特异性和灵敏度均为100％，高于间接ELISA法(特异性为90％，灵敏度为65％)。检测210份其他病种患者血清均为阴性，与雅培HIVAB试剂比较检测如份健康献血员血清和88份HIv感染者血清，符合率为100％。结论 本方法特异性强、敏感性高，操作简便，适用于献血员的筛选和临床HIv感染的检测。     

多种单抗联合检测HIV抗原

目的 建立多种单抗联合早期检测HIV抗原的夹心ELISA方法.方法 以SAS盐析沉淀法和亲和层析法纯化抗HIV-1 p24、gp41、gp120及抗HIV-2 gp36的腹水型单克隆抗体(McAb),用高碘酸钠法将纯化的McAb以HRP进行标记.建立针对单个抗原的双抗体夹心ELISA法,对其灵敏度及特异性进行检测.将筛选得到的4株捕获McAb按比例混合作为捕获抗体,4株酶标McAb按比例混合作为检测抗体,建立多种单抗联合检测HIV抗原的夹心ELISA方法,检测混合HIV抗原.结果 按确定的最优反应条件建立的多种McAb联合夹心ELISA方法,检测到的最高稀释度的HIV混合抗原中各抗原的终浓度分别为:重组HIV-1 p24:0.625 pg/ml,gp41:6.25 ng/ml,gp120:6.25 ng/ml;HIV-2 gp36:9.25 ng/ml.结论 建立了具有高度敏感性的鸡尾酒式多种单抗联合检测HIV抗原的夹心ELISA法,为早期榆测HIV抗原提供了新的思路,为后续的研究奠定了一定基础.     

Enzyme-linked immunoassay for human immunodeficiency virus type 1 envelope glycoprotein 120.

An enzyme-linked immunosorbent assay (ELISA) that can measure picogram quantities of human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein 120 (gp120) in cell culture medium or body fluids has been developed. Recombinant, soluble CD4 immobilized in microtiter trays was used to capture gp120, which was then detected with polyclonal sheep antibody to gp120 followed by biotinylated rabbit anti-sheep immunoglobulin G and an avidin-alkaline phosphatase indicator system. With a reference recombinant gp120, the assay showed a linear relationship between optical density and concentrations ranging from 60 to 6,000 pg/100-microliters well; precision of the assay varied with the concentrations and ranged from +/- 40% with amounts smaller than 200 pg to +/- 10% with amounts larger than 200 pg. In a group of coded samples containing 60 pg (approximately 10(7) molecules) of reference gp120, the assay correctly identified the samples as containing gp120 99% of the time, with no false-positive results recorded for blank samples. Recombinant gp120 prepared in another cell culture system demonstrated a binding coefficient 13-fold lower than that of reference gp120. Mixing standard amounts of reference gp120 with increasing concentrations of human sera reduced assay sensitivity, although the linear relationship between gp120 concentration and optical density remained. With this assay we were able to detect gp120 in HIV-1 suspensions prepared from cultured lymphoblastoid cells and in the sera of HIV-1-infected patients. This ELISA for gp120 should be useful for studying the biological role of gp120 in HIV infection.     

Antibodies to MHC Class II Peptides are Present in HIV-1-Positive Sera

Seventy-five per cent of sera from HIV-1-infected individuals bind to the human B-lymphoma cells bearing the major histocompatibility class II molecule in enzyme-linked immunosorbent assay (ELISA). The binding is caused by the antibodies against the class II molecule present in the serum samples which prevent the interaction of murine anti-HLA.DR monoclonal antibody with B lymphoma in FACS analysis. The three highly conserved amino acid sequences in alpha- and beta-chains of the class II molecule and three homologous fragments in HIV-1 gp120 and gp41 were identified by computer search and synthesized. Using these peptides it was demonstrated that 28-48% of HIV-positive sera contain antibodies that cross-react with the peptide of HIV-1 origin and with the peptide from the class II molecule as well.     

多种单抗联合检测HIV抗原

目的 建立多种单抗联合早期检测HIV抗原的夹心ELISA方法.方法 以SAS盐析沉淀法和亲和层析法纯化抗HIV-1 p24、gp41、gp120及抗HIV-2 gp36的腹水型单克隆抗体(McAb),用高碘酸钠法将纯化的McAb以HRP进行标记.建立针对单个抗原的双抗体夹心ELISA法,对其灵敏度及特异性进行检测.将筛选得到的4株捕获McAb按比例混合作为捕获抗体,4株酶标McAb按比例混合作为检测抗体,建立多种单抗联合检测HIV抗原的夹心ELISA方法,检测混合HIV抗原.结果 按确定的最优反应条件建立的多种McAb联合夹心ELISA方法,检测到的最高稀释度的HIV混合抗原中各抗原的终浓度分别为:重组HIV-1 p24:0.625 pg/ml,gp41:6.25 ng/ml,gp120:6.25 ng/ml;HIV-2 gp36:9.25 ng/ml.结论 建立了具有高度敏感性的鸡尾酒式多种单抗联合检测HIV抗原的夹心ELISA法,为早期榆测HIV抗原提供了新的思路,为后续的研究奠定了一定基础.     

Differential Patterns of T-Cell Receptor BV-Specific Activation of T Cells by gp120 from Different HIV Strains

Studies by several groups have suggested that HIV infection in vivo results in a BV-specific alteration of the TCR repertoire and that this might play a role in the pathogenesis of AIDS. Our earlier studies demonstrated tbat both a crude extract of HIV₄₅₁ as well as purified gp160 from HIV₄₅₁ could specifically activate, in vitro , T cells expressing a common set of TCRBV segments (TCRBV3, 12, 14, 15, and sometimes BV17 and 20) in individuals of disparate HLA type. Furthermore, purified gp120 from HIV₄₅₁ was shown to have a similar ability to activate T cells, although with a slightly diiferent TCRBV-specific pattern. In order to determine whether gp120 from other HIV strains could similarly activate T cells in a TCRBV-specific pattern, PBMC from HIV seronegative individuals of disparate HLA type were stimulated with gp120 from three strains of HIV (451, IIIB, and MN). The authors found that gp120 from all three strains activate T cells bearing TCRBV2 and BV3 in nearly every individual. T cells expressing other BV segments are also activated, but this is more variable and appears to be unique to each individual. Furthermore, gp120₄₅₁ and gp120 from HIV_IIIB and HIV_MNdiffer in their ability to activate T cells expressing these other TCRBV segments. These observations suggest that variation in the structure of gp120 and in the genetic and/or environmental background of the individual play an important role in determining which TCRBV segments are'triggered' by gp120. Furthermore, these observations may have important implications for the rate of disease progression in HIV-infected individuals.     

合成肽抗原抗人免疫缺陷病毒1／2型抗体酶联试剂盒…

根据人免疫缺陷病毒的基因结构和氨基酸序列，采用因相法合成了ＨＩＶ－１ｇｐ４１、ｂｐ１２０、ｐ２４和ＨＩＶ－２ｇｐ３６的４条多肽，混合包被酶标板做为固相抗原，采用间接酶联免疫吸附试验，建立了检测抗－ＨＩＶ－１／２ＩｇＧ抗体的酶联诊断试剂盒。检测卫生部药品和生物制品检定所提供的４１份质控参比血清，其特异性、敏感性均为１００％，变异系数小于１０％。检测１８６份其它病种病人血清均为阴性，与华怡、巴斯德、金     

HIV-1 膜蛋白基因片段杆状病毒转移载体克隆及重组体的构建

目的：构建编码HIV－1外膜糖蛋白的gp120和gp41基因杆状病毒转移载体，并在昆虫细胞中表达gp120和gp41重组蛋白。方法：从HIV－1基因克隆PNL4－3（NY5／LAV）中应用聚合链反应扩增目的基因gp120和gp41，克隆到PGEM－T载体中，限制性内切酶酶切、DNA序列分析鉴定目的基因，再经EcoRI和BamHI双酶切后定向克隆到杆状病毒转移载体PAC－SecG2T中，再次测序鉴定。通过在sf9昆虫细胞中同源重组、空斑筛选、病毒鉴定、SDS－PAGE、Western-blot对重组病毒和重组蛋白进行分析。结果：限制性内切酶酶切和DNA序列分析表明，gp120和gp41正确克隆到杆状病毒转移载体PACSecG2T中；SDS－PAGE和Eestern-blot结果表明在昆虫细胞中成功表达了HIV外膜糖蛋白gp120和gp41。结论：成功构建了PACSecG2T-gp120和PACSecG25-gp41杆状病毒转移载体，并在杆状病毒－昆虫细胞表达系统中表达了HIV－1 gp120和gp41重组蛋白，Western-blot证明具有较好的免疫原性。     

同时检测HIV抗体及p24抗原快速诊断试剂的研制

目的：研制可同时检测HIV－1、HIV－2抗体及p24抗原的胶体金快速诊断试剂。方法：利用重组杆状病毒－昆虫细胞系统进行HIV－1 gp41及HIV－2 gp36抗原的高效表达，以免疫亲和层析法纯化抗原。抗－HIV p24单克隆抗体杂交瘤细胞株，并制备抗－HIV p24单克隆抗体。以硝酸基纤维膜为载体，以纯化的HIV－1 gp41、HIV－2 gp36抗原及抗p24抗体点膜，20nm胶体金颗粒／抗人IgG和抗－HIV p24单克隆抗体进行标记，对33份已知HIV感染者阳性血清及6份阴性血清进行检测。结果：通过重组杆状病毒－昆虫细胞系统进行HIV－1 gp41及HIV－2 gp36抗原的表达，可获取浓度为2．0mg/L的纯化抗原。从抗p24单克隆抗体杂交瘤细胞株中培养上清液，通过葡萄球菌蛋白A免疫亲和层析柱可得到1．5mg/L的纯化抗体。利用纯化的抗原抗体进行标记，对39份已知血清进行检测，与荷兰Organon公司HIV1＋2抗体、p24抗原、ELISA诊断试剂同时检测结果进行比较，证实有较强的特异性及敏感性。结论：同时检测HIV－1、HIV－2抗体及p24抗原快速诊断试剂的问世，可为HIV感染的诊断提供一个简便、可靠、敏感的方法。     

B and T cell responses elicited by monoclonal anti-idiotypic antibody (Ab2beta) mimicking gp43 from Paracoccidioides brasiliensis

Paracoccidioidomycosis is a systemic mycosis endemic in Latin America, with a high prevalence in Brazil, Argentina, Colombia and Venezuela. The aetiological agent of disease is the thermal dimorphic fungus, Paracoccidioides brasiliensis. A glycoprotein of 43 kD (gp43) is the major antigen of P. brasiliensis. Antibodies directed to this antigen are detected in the sera of all patients with paracoccidioidomycosis (PCM). Recently, it has been shown that mice immunized with anti-gp43 monoclonal antibodies (MAbs) (Ab1), induce the idiotypic cascade in the gp43 system, which produced both, anti-Id antibodies (Ab2) and anti-anti-Id antibodies (Ab3). To further characterize the idiotypic cascade modulation in mice immunized with anti-gp43 MAb 17c, hybridomas were produced. Ab2 MAbs named 7.B12 inhibited (>95%) the binding of gp43 to MAb 17c (Ab1), suggesting that this anti-Id MAb bind to the idiotope, thus fulfilling the internal image criteria. To elucidate whether Ab2 MAb could act as antigen in serological assays, instead of gp43, sera from PCM patients were tested. Using an ELISA test, it was observed that antibodies from patients and not normal serum bound to Ab2. However, the ELISA test using Ab2 bound to the solid phase made possible to serologically monitor the patients after antifungal therapy, showing an equivalent curve when compared with ELISA test employing purified gp43. Our results also showed that, when mice were immunized with Ab2beta and their cells were exposed to gp43 in vitro, a T cell proliferation response was observed.     

Inhibition of DC-SIGN-mediated trans infection of T cells by mannose-binding lectin

Some dendritic cells (DC) express a cell-surface lectin called 'dendritic cell-specific intracellular adhesion molecule 3 (ICAM-3)-grabbing non-integrin' (DC-SIGN). DC-SIGN has been shown to mediate a type of infection called 'trans' infection, where DC bind human immunodeficiency virus (HIV) and efficiently transfer the virus to T cells. We investigated the possibility that mannose-binding lectin (MBL), a soluble lectin that functions as a recognition molecule in innate immunity and that binds to HIV, could block trans infection mediated by DC-SIGN. Binding studies with glycoprotein (gp)120/gp41-positive and -negative virus preparations suggested that DC-SIGN and MBL bind primarily to glycans on gp120/gp41, as opposed to glycans on host-cell-derived proteins, indicating a close overlap in the binding site of the two lectins and supporting the notion that MBL could prevent binding of HIV to DC-SIGN. Preincubation of X4, R5 or dual-tropic HIV strains with MBL prevented DC-SIGN-mediated trans infection of T cells. The mechanism of MBL blocking trans infection of T cells was at least partly caused by blocking of virus binding to DC-SIGN positive cells. This study shows that MBL prevents DC-SIGN-mediated trans infection of T cells in vitro and suggests that in infected persons, MBL may inhibit DC-SIGN-mediated uptake and spread of HIV.     

Blind comparison of Abbott and Dupont HIV antigen ELISA tests for detecting antigenaemia in asymptomatic human immunodeficiency virus antibody positive homosexual men.

Three hundred and ninety eight serum samples from 270 human immunodeficiency virus (HIV) antibody positive asymptomatic homosexual men were tested in the Abbott and Dupont HIV antigen ELISA tests. In the Abbott test 62 (16%) of the sera were positive, according to the manufacturer's instructions, compared with 55 (14%) in the Dupont test. Twenty six sera were positive with the Abbott test but negative with the Dupont test and 19 sera were positive only by the Dupont test. Only 36 (9%) of the sera were positive in both tests. The Abbott confirmatory neutralisation test gave excellent agreement with the initial Abbott HIV antigen ELISA test; the Dupont confirmatory test was only in agreement with the initial positive Dupont antigen ELISA test in one third of the sera tested. Although the overall sensitivity of each of the two commercial assays tested was similar, the Abbott method may be preferable for clinical purposes if confirmation of an initial ELISA positive test result by neutralisation assay is required.     

Peptides from the principal neutralizing and CD4-binding domain: similar immunoreactive properties and structure pattern]

The human immunodeficiency virus (HIV) proteins gp120 and gp41 are the principal immune target in HIV infection. One of the most important trends in the study of AIDS is linked to the mapping of sites involving in the binding to the cell receptor CD4 and in the induction of virus-neutralizing antibodies (VNA). Recent studies have revealed that gp120 as the major domain contains inducing type-specific BNA (PND) and a binding region with CD4 (CD4-BR). PND is located in the hypervariable loop of gp120 (residues 301-336 for a BRU strain), and CD4-BR is in the conservation area (residues 410-450). By using the synthetic fragments from these areas (BRU and MN strains) and HIV-infected persons' sera, the authors established that the immune response to PND and CD4-BR is somewhat interrelated: there is a synchronized response of HIV antibodies to peptides from the two regions in ELISA (r = 0.82). For analysis of this phenomenon, experiments with cross-linked immunoreactivity of rabbit antisera to peptides from PND and CD4-BR with homologous and heterologous peptides were performed by applying three control peptides from HIV and hepatitis B virus. It has been found that there is a cross reactivity between rabbit anti-PND (MN, BRU) and anti-CD4-BR abs. Peptide homological analysis revealed common structural elements for PND and CD4-BR despite significant differences in their proposed functions. There is a large amount of positively charged aa within both PND and CD4-BR which may be involved in gp120-CD4 interaction. Acetylation of Lys residues resulted in complete loss of peptide reactivity.     

Confirmation of HIV seropositivity: comparison of a novel radioimmunoprecipitation assay to immunoblotting and virus culture

A recently developed radioimmunoprecipitation assay, using 125I-labeled human immunodeficiency virus (HIV) viral proteins enriched for glycoproteins gp120env, gp41env (GRIPA), was compared to the immunoblot assay with respect to sensitivity and specificity for the detection of antibodies to HIV. Longitudinal sets of serum samples of seroconverting homosexual men were studied, as were sera of six blood-bank donors likely to be false-positive in immunoblot. In addition, HIV isolation was attempted from white blood cells of these blood-bank donors and of seropositive and seronegative individuals. In sets of seroconversion samples, the GRIPA appeared at least as sensitive as the immunoblot. Some sera already were clearly positive in the GRIPA at a time when there was only weak reactivity in immunoblot. In contrast, sera from blood-bank donors that were regarded as false-positive in immunoblot were negative in GRIPA. Virus culture from these donors was also negative. It is concluded that reactivity in immunoblot to core proteins only may well be false-positive, whereas antibody reactivity in the radioimmunoprecipitation assay to p24gag solely suggests ongoing seroconversion. This feature, in addition to a sensitivity for anti-gp120env comparable to immunoblotting, makes the GRIPA a useful confirmatory assay in sera that yield conflicting results in other HIV-antibody assays.