Early detection of antibody to human immunodeficiency virus type 1 by using an antigen conjugate immunoassay correlates with the presence of immunoglobulin M antibody

Sensitivity of a rapid point of care assay for early HIV antibody detection is enhanced by its ability to detect HIV gp41 IgM antibodies

BACKGROUND

Anti-HIV-1 IgM antibody is an important immunoassay target for early HIV antibody detection.

OBJECTIVES

The objective of this study is to determine if the early HIV antibody sensitivity of the 60s INSTI test is due to detection of anti-HIV-1 IgM in addition to IgG.

STUDY DESIGN

To demonstrate HIV gp41 IgM antibody capture by the INSTI HIV-1 gp41 recombinant antigen, an HIV-IgM ELISA was conducted with commercial HIV-1 seroconversion samples. To demonstrate that the INSTI dye-labelled Protein A-based colour developer (CD) has affinity to human IgM, commercial preparations of purified human immunoglobulins (IgM, IgD, IgA, IgE, and IgG) were blotted onto nitrocellulose (NC) and probed with the CD to observe spot development. To determine that INSTI is able to detect anti-HIV-1 IgM antibody, early seroconversion samples, were tested for reduced INSTI test spot intensity following IgM removal.

RESULTS

The gp41-based HIV-IgM ELISA results for 6 early seroconversion samples that were INSTI positive determined that the assay signal was due to anti-HIV-1 IgM antibody capture by the immobilised gp41 antigen. The dye-labelled Protein-A used in the INSTI CD produced distinct spots for purified IgM, IgA, and IgG blotted on the NC membrane. Following IgM removal from 21HIV-1 positive seroconversion samples with known or undetermined anti-HIV-1 IgM levels that were western blot negative or indeterminate, all samples had significantly reduced INSTI test spot intensity.

CONCLUSIONS

The INSTI HIV-1/HIV-2 Antibody Test is shown to detect anti-HIV-1 IgM antibodies in early HIV infection which enhances its utility in early HIV diagnosis.

Development of an integrase-based ELISA for specific diagnosis of individuals infected with HIV

Currently, enzyme immunoassays (EIAs) are the most common immunological diagnostic methods that are used as the screening tool in HIV detection. Among all three major genes of HIV, the products of gag and env are usually used in EIAs (ELISAs and rapid tests). Hence, the presence of cross reacting antibodies against these antigens leads to the appearance of repetitive false positive results in screening tests. Re-testing the primary reactive samples with EIAs using other HIV antigens can considerably reduce the rate of false positive results. The products of pol gene may act as an appropriate candidate in this context. Integrase is a conserved and immunogenic product of HIV, encoded by the pol gene. The aim of this research was to determine the sensitivity and specificity of an ELISA detecting integrase antibodies. Recombinant integrase was produced in Escherichia coli to develop the integrase-based ELISA. Assay performance was evaluated by HIV positive and negative sera and an HIV panel of BBI (PRB-601). The sensitivity and specificity of assay was determined as 96.7 [95% confidence interval: 91.3-98.9%] and 100% [95% CI: 96.1-100%], respectively. High specificity of this assay may suggest its possible use in the detection of HIV.

Use of rapid HIV antibody testing for controlling the HIV pandemic

The HIV pandemic continues to expand throughout Africa and southern Asia. Despite recent advances in therapy, the primary means of prevention continues to be the identification of infected patients through diagnostic testing, and the provision of counseling services to reduce HIV transmission. In order to facilitate the identification of infected patients, great strides have been made during the past 10 years towards the development of simple, rapid HIV antibody assays that require no specialized equipment, are relatively stable at ambient temperatures and can be easily performed by people who do not have a laboratory background. Evaluations of these assays have shown that when used in multiple assay algorithm strategies, they perform comparably with current laboratory-based methods. Effective global implementation of these tests will require a concerted effort from public and private health agencies, however, expanding the use of these assays can provide a significant opportunity to slow the devastating spread of HIV.

Diagnosing acute HIV infection

Individuals with acute HIV infection (AHI) pose a greater transmission risk than most chronically HIV-infected patients and prevention efforts targeting these individuals are important for reducing the spread of HIV infection. Rapid and accurate diagnosis of AHI is crucial. Since symptoms of AHI are nonspecific, its diagnosis requires a high index of suspicion and appropriate HIV laboratory tests. However, even 30 years after the start of the HIV epidemic, laboratory tools remain imperfect and only a few individuals with AHI are identified. We review the clinical presentation of the acute retroviral syndrome, the laboratory markers and their detection methods, and propose an algorithm for the laboratory diagnosis of AHI.

Development and characterization of a bead-based, multiplex assay for estimation of recent HIV type 1 infection

Estimation of HIV-1 incidence is an important public health tool for understanding the status of the epidemic, identifying high-risk populations, and assessing various intervention strategies. Several laboratory-based methods have been developed for distinguishing recent from long-term HIV-1 infection; however, each exhibits some degree of misclassification, particularly among AIDS patients and those taking antiretroviral therapy (ART). To improve upon the limitations associated with measuring responses to a single analyte, we have developed a bead-based, multiplex assay for determination of HIV recent infection based on total antibody binding and antibody avidity to multiple analytes. An HIV-specific, multiplex panel was created by coupling the recombinant HIV-1 proteins p66, gp120, gp160, and gp41 to Bio-Plex COOH microspheres. Longitudinal plasma specimens from recent seroconverters were tested for reactivity to the coupled microspheres using the Bio-Plex 200 System. For each analyte, HIV-specific antibody binding and avidity increased for 1-2 years post-seroconversion, leading to a significant difference in reactivity between recent and long-term specimens. While the potential for misclassification of individuals diagnosed with AIDS or receiving ART appears to be minimal with avidity measures, the impact on total antibody binding was variable, depending on the individual analyte. This bead-based, HIV-specific multiplex assay measures several distinct immune responses in a single assay plate, allowing for sampling of multiple analytes in the determination of recent infection, which could aid in the development of improved statistical methods or algorithms that will more accurately estimate HIV incidence.

Evaluation of a new third-generation ARCHITECT rHTLV-I/II assay for blood screening and diagnosis

In comparison to current on-market assays, the ARCHITECT rHTLV-I/II assay is the first fully automated assay that simultaneously detects human T-cell lymphotropic virus type I (HTLV-I) and type II (HTLV-II) in human serum and plasma. Specificity was assessed on 5646 blood donors and 692 clinical specimens. For sensitivity determination, 301 HTLV-I-positive and 105 HTLV-II-positive specimens were tested. Precision was between 3.98% and 4.31% coefficient of variation (CV) for specimens with 1 to 6 sample to cutoff. Specificity was 99.95% and 99.86% on specimens from blood donors and hospitalized patients, respectively. Sensitivity evaluation showed 100% detection on 301 HTLV-I and 105 HTLV-II specimens. HTLV-I and HTLV-II viruses are still circulating among general populations even in the low prevalence areas. To control the further spread of these retroviruses, we need to know that it is important to continue screening of blood. The performance evaluation data from this study demonstrate that the high throughput and fully automated ARCHITECT rHTLV-I/II chemiluminescence immunoassay effectively serves this purpose.

Evaluation of a new, fully automated immunoassay for detection of HTLV-I and HTLV-II antibodies

Screening blood donations for human T-lymphotropic virus types I and II (HTLV-I/II) continues to be important in protecting the safety of blood products and controlling the global spread of these retroviruses. We have developed a fully automated, third generation chemiluminescent immunoassay, ARCHITECT rHTLV-I/II, for detection of antibodies to HTLV-I/II. The assay utilizes recombinant proteins and synthetic peptides and is configured in a double antigen sandwich assay format. Specificity of the assay was 99.98% (9,254/9,256, 95% CI = 99.92-100%) with the negative specimens from the general population including blood donors, hospital patients and pregnant women from the US, Japan and Nicaragua. The assay demonstrated 100% sensitivity by detecting 498 specimens from individuals infected with HTLV-I (n = 385) and HTLV-II (n = 113). ARCHITECT rHTLV-I/II results were in complete agreement with the Murex HTLV-I/II reference assay and 99.7% agreement with the Genelabs HTLV Blot 2.4 confirmatory assay. Analytical sensitivity of the assay was equivalent to Murex HTLV-I/II assay based on end point dilutions. Furthermore, using a panel of 397 specimens from Japan, the ARCHITECT rHTLV-I/II assay exhibited distinct discrimination between the antibody negative (Delta Value = -7.6) and positive (Delta Value = 7.6) populations. Based on the excellent specificity and sensitivity, the new ARCHITECT rHTLV-I/II assay should be an effective test for the diagnosis of HTLV-I/II infection and also for blood donor screening.

A novel separation-free assay technique for serum antibodies using antibody bridging assay principle and two-photon excitation fluorometry

A new technique for separation-free detection of antigen-specific antibodies is presented. The new technique employs antibody bridging assay principle and the recently developed ArcDia TPX fluorescence detection technology. According to the assay scheme, antibody molecules from the sample bind with one arm to an antigen on polymer microspheres and with the other arm to a fluorescently labeled secondary antigen reagent. Consequently, fluorescent immunocomplexes are formed on the surface of microspheres in proportion to the concentration of the analyte in the sample. The fluorescence signal from individual microspheres is measured by means of two-photon excited fluorescence detection. In order to demonstrate the applicability of the new assay technique, an assay for anti-adenovirus antibodies was constructed. The function of the assay method was tested both with monoclonal anti-adenovirus antibody preparation (standard analyte), and with positive serum samples. Standard class-specific ELISA was used as a reference method. The new assay method provides comparable sensitivity and precision, and wider dynamic range for IgG antibodies than the ELISA method. The standard curve showed linear response (R(2)=0.999) with a dynamic range of three orders of magnitude, detection limit (mean+3S.D.) of 8 pM, and intra-assay signal precision of 5%. Applicability of the new method for clinical serodiagnostics is discussed.

Seroconversion following nonoccupational postexposure prophylaxis against HIV

BACKGROUND

The efficacy of antiretroviral postexposure prophylaxis (PEP) against infection with human immunodeficiency virus (HIV) following occupational exposures has prompted the use of PEP after nonoccupational exposures. There are, however, important differences between occupational and nonoccupational exposures, and the effectiveness of PEP following nonoccupational exposure is unknown. We sought to describe the occurrence and circumstances of HIV seroconversion following nonoccupational PEP.

METHODS

HIV uninfected individuals reporting potential sexual or injection drug use exposures to HIV in the preceding 72 h received a 28-day regimen of antiretroviral therapy and counseling in a nonrandomized trial. The level of HIV antibody was measured 12 weeks after PEP initiation.

RESULTS

Of 877 exposed subjects, 702 were evaluable 12 weeks after exposure. Seroconversion was detected in 7 subjects (1%; 95% confidence interval, 0.4%-2%). Three seroconverters reported having no exposures after PEP initiation and, thus, probably represent evidence of chemoprophylactic failure. In the other 4 subjects, additional exposures to HIV after PEP initiation or detection of HIV RNA in plasma specimens obtained at baseline precluded determination of the source of seroconversion. No exposure source was available to assess genetic concordance with the seroconverter's virus.

CONCLUSIONS

As for occupational exposure, PEP is not completely effective in preventing HIV infection following nonoccupational exposure. Therefore, primary prevention remains essential. In contrast to the occupational setting, the potential source of exposure is rarely available for testing in the nonoccupational setting, and exposures are often not isolated. Thus, it is often impossible to determine whether seroconversion resulted from failure of PEP or from other exposures, posing difficulties for future comparative studies seeking to evaluate the effectiveness of PEP.

Assays for HIV with improved sensitivity and specificity

Increased knowledge of the human immunodeficiency virus (HIV) and the infection it causes in humans has resulted in an enormous expansion in the understanding of viral parameters and host changes. HIV is a virus which mutates readily and rapidly, presenting many challenges to assay developers, and monitors of therapy and drug-resistance. Prolific viral replication at all the stages of the disease means that an accurate assessment of viral burden, viral load and changes to immune system markers is essential for effective clinical management and treatment. In the present review we have summarised current opinion on the kinetics of HIV infection and the pathogenesis of the disease it causes, and have provided a background to the evolution of HIV assays. Sensitivities and specificities of assays used for anti-HIV and HIV detection have improved, and new assays have been developed employing novel molecular techniques, which are being applied to meet continually evolving demands for more sensitive measurement of an increasing number of parameters. The future of HIV testing is also considered in the light of new knowledge concerning virus dynamics in vivo, the likelihood of the emergence of new subtypes and the changing approach to therapy. Assays will be, on the whole, used to quantify virus and to measure the host reactions to infection, often in the presence of antivirals. Thus, extreme sensitivity and specificity will be required.

Reliability of rapid diagnostic tests for HIV variant infection

The sensitivity of one ELISA method, six HIV-1/HIV-2 rapid screening tests, and one confirmatory test was evaluated in comparison with a third-generation EIA method (taken as the 'gold standard') and Western blot on well-characterized panels of sera. HIV diversity was represented by 50 HIV-1 group M subtype A to H, nine HIV-1 group O, 12 HIV-2, two HIV1+2 positive and six indeterminate Western blot profiles. Sensitivity during HIV-1 seroconversion was studied on 39 serial samples collected from six patients during early primary infection. Serial samples obtained from two primates during experimental primary SIV infection were used to mimic HIV-2 seroconversion samples. The sensitivity ranged from 100 to 94.6% according to the test. During seroconversion, rapid tests became positive 2-8 days later than the third-generation EIA. This reveals a major limitation of rapid tests, which are being recommended for use in developing countries. The lack of sensitivity seen during early HIV-1 seroconversion and/or limited specificity in some of the evaluated tests present serious limitations to their use in countries with high HIV incidence and variability. It is suggested that, as soon as possible, less sensitive rapid tests for blood bank screening should be abandoned in favor of highly sensitive rapid tests and/or more robust, more sensitive and cheaper ELISAs. These results stress the need for better screening tools and specific local evaluations.

Seven human immunodeficiency virus (HIV) antigen-antibody combination assays: evaluation of HIV seroconversion sensitivity and subtype detection

In this study, we evaluated the performance of two prototype human immunodeficiency virus (HIV) antigen-antibody (Ag-Ab) combination assays, one from Abbott Laboratories (AxSYM HIV Ag-Ab) and the other from bioMerieux (VIDAS HIV Duo Ultra), versus five combination assays commercially available in Europe. The assays were Enzygnost HIV Integral, Genscreen Plus HIV Ag-Ab, Murex HIV Ag-Ab Combination, VIDAS HIV Duo, and Vironostika HIV Uniform II Ag-Ab. All assays were evaluated for the ability to detect p24 antigen from HIV-1 groups M and O, antibody-positive plasma samples from HIV-1 groups M and O, HIV-2, and 19 HIV seroconversion panels. Results indicate that although all combination assays can detect antibodies to HIV-1, group M, subtypes A to G, circulating recombinant form (CRF) A/E, and HIV-1 group O, their sensitivity varied considerably when tested using diluted HIV-1 group O and HIV-2 antibody-positive samples. Among combination assays, the AxSYM, Murex, and VIDAS HIV Duo Ultra assays exhibited the best antigen sensitivity (at approximately 25 pg of HIV Ag/ml) for detection of HIV-1 group M, subtypes A to G and CRF A/E, and HIV-1 group O isolates. However, the VIDAS HIV Duo Ultra assay had a lower sensitivity for HIV-1 group M and subtype C, and was unable to detect subtype C antigen even at 125 pg of HIV Ag/ml. The HIV antigen sensitivity of the VIDAS HIV Duo and Genscreen Plus combination assays was approximately 125 pg of HIV Ag/ml for detection of all HIV-1 group M isolates except HIV-1 group O while the sensitivity of Vironostika HIV Uniform II Ag-Ab and Enzygnost HIV Integral Ag-Ab assays for all the group M subtypes was >125 pg of HIV Ag/ml. Among the combination assays, the AxSYM assay had the best performance for detection of early seroconversion samples, followed by the Murex and VIDAS HIV Duo Ultra assays.

Immune complex transfer enzyme immunoassay for antibody IgM to HIV-1 p17 antigen

The immune complex transfer enzyme immunoassay for antibody IgM to HIV-1 p17 antigen is described. Serum samples containing antibody IgM to HIV-1 p17 antigen were incubated simultaneously with 2,4-dinitrophenyl-bovine serum albumin-recombinant p17 (rp17) conjugate and rp17-beta-D-galactosidase conjugate, and the immune complex formed comprising the three components was trapped onto colored polystyrene beads coated with affinity-purified (anti-2,4-dinitrophenyl group) IgG. Subsequently, the immune complex was transferred to white polystyrene beads coated with monoclonal mouse (antihuman IgM) IgG in the presence of excess of epsilonN-2,4-dinitrophenyl-L-lysine. The signal for antibody IgM to p17 antigen was the fluorescence intensity by fluorometric assay of beta-D-galactosidase activity bound to the white polystyrene beads. The periods of time required for the formation, trapping, and transferring of the immune complex comprising the three components were more than 4 hr, 2 hr, and 3 hr, respectively. The immunoassay developed was shown to be specific by inhibition of transferring the immune complex in the presence of excess of nonspecific IgM but not IgG. Signals for antibody IgM to p17 antigen with serum samples of HIV-1 seroconversion serum panels,--that is, with serum samples in early stages of the infection--tended to be higher than those with serum samples from HIV-1 asymptomatic carriers probably long after the infection and patients with ARC and AIDS. In contrast, signals for antibody IgG to p17 antigen with serum samples of HIV-1 seroconversion serum panels tended to be higher than signals for antibody IgM to p17 antigen but were much lower than signals for antibody IgG to p17 antigen with serum samples from HIV-1 asymptomatic carriers and patients with ARC and AIDS.

Blood screening by nucleic acid amplification technology: current issues, future challenges

BACKGROUND

Nucleic acid amplification technology (NAT) is presently being evaluated in US clinical trials to determine the safety and efficacy of mini-pool testing for human immunodeficiency virus (HIV) and hepatitis C virus (HCV) RNA in the blood-donor population. Although the risk for transfusion-transmitted HIV and HCV infection is extremely low, there is still a small chance that blood donated by infected individuals before seroconversion can escape detection by current antibody-based assays.

METHODS

This report describes the amplification technologies being used and reviews several issues surrounding NAT-based blood screening. The performance features of NAT and current enzyme immunoassay technologies are compared, and the benefits of NAT in reducing transfusion-transmitted infections are discussed.

CONCLUSIONS

The current US clinical trials of mini-pool NAT testing for HIV and HCV RNA have successfully identified preseroconversion infectious blood units. Although the current NAT-based screening systems are semiautomated, mini-pool testing represents an unprecedented innovation among government and nongovernment agencies in the highly regulated blood transfusion industry. Despite cost-effectiveness issues, based on the public perception of infectious diseases acquired through blood transfusion, NAT-based screening of the blood supply is expected to become a standard in transfusion medicine.

Performance of a multiplex qualitative PCR LCx assay for detection of human immunodeficiency virus type 1 (HIV-1) group M subtypes, group O, and HIV-2

Early detection of human immunodeficiency virus (HIV) in blood and blood products can be achieved by a sensitive nucleic acid amplification-based assay. We report on the performance of a PCR-based qualitative assay that detects both HIV type 1 (HIV-1) and HIV-2 with a sensitivity of 20 to 50 copies/ml. The assay has a specificity of 99.6% and an inhibition rate of 1.7%. One milliliter of sample is processed with a manifold system and Qiagen columns, and one-third of the extracted sample is used for PCR amplification. An internal control sequence, which is processed and amplified with each sample, monitors for amplification inhibition. Samples are reverse transcribed and are then amplified by reverse transcription-coupled PCR, after which HIV-1- and HIV-2-specific probes are hybridized to the amplified products. Following hybridization, samples are detected in the LCx instrument by microparticle enzyme immunoassay techniques. The detection system has an automated inactivation step that controls for PCR contamination. The HIV-1/2 qualitative RNA assay detects HIV-1 group M subtypes A, B, C, D, E, F, and G and group O. Testing of several HIV-1 seroconversion panels has demonstrated that the HIV-1/2 qualitative RNA assay detects HIV infection on the average of 6 days before p24 antigen can be detected and 11 days before antibodies can be detected.

Diagnostic testing for HIV type 1 RNA in seronegative blood

We studied the feasibility of routine diagnostic testing for HIV-1 RNA at a publicly funded testing site. HIV-1 RNA was determined with a commercial polymerase chain reaction assay in pooled seronegative blood samples submitted for HIV testing to a public health laboratory. Recovery of HIV-1 RNA from the samples was estimated as at least 8% of viral RNA that was found in freshly prepared plasma. We estimated that screening for HIV-1 RNA in serum pools would result in the identification of blood specimens from more than 95% of acutely infected patients. The frequency of HIV-1 RNA in seronegative blood samples was estimated to be between 19 and 601 per 10(6) submitted specimens. The ratio of HIV-1 RNA positive and seronegative samples to specimens with HIV-1 antibodies confirmed by Western blot was estimated to be between 0.2% and 6.6%. The reagent costs for identifying 1 HIV-infected blood sample were 10-fold higher with the commercially available HIV-1 RNA assay compared with the HIV antibody enzyme-linked immunosorbent assay. Diagnostic testing for HIV-1 RNA may be warranted in high-risk populations since acutely infected patients may benefit most from anti-retroviral therapy and are thought to contribute disproportionately to the HIV epidemic.

Immune complex transfer enzyme immunoassay for antibody IgM to HIV-1 p17 antigen

The immune complex transfer enzyme immunoassay for antibody IgM to HIV-1 p17 antigen is described. Serum samples containing antibody IgM to HIV-1 p17 antigen were incubated simultaneously with 2,4-dinitrophenyl-bovine serum albumin-recombinant p17 (rp17) conjugate and rp17-beta-D-galactosidase conjugate, and the immune complex formed comprising the three components was trapped onto colored polystyrene beads coated with affinity-purified (anti-2,4-dinitrophenyl group) IgG. Subsequently, the immune complex was transferred to white polystyrene beads coated with monoclonal mouse (antihuman IgM) IgG in the presence of excess of epsilonN-2,4-dinitrophenyl-L-lysine. The signal for antibody IgM to p17 antigen was the fluorescence intensity by fluorometric assay of beta-D-galactosidase activity bound to the white polystyrene beads. The periods of time required for the formation, trapping, and transferring of the immune complex comprising the three components were more than 4 hr, 2 hr, and 3 hr, respectively. The immunoassay developed was shown to be specific by inhibition of transferring the immune complex in the presence of excess of nonspecific IgM but not IgG. Signals for antibody IgM to p17 antigen with serum samples of HIV-1 seroconversion serum panels,--that is, with serum samples in early stages of the infection--tended to be higher than those with serum samples from HIV-1 asymptomatic carriers probably long after the infection and patients with ARC and AIDS. In contrast, signals for antibody IgG to p17 antigen with serum samples of HIV-1 seroconversion serum panels tended to be higher than signals for antibody IgM to p17 antigen but were much lower than signals for antibody IgG to p17 antigen with serum samples from HIV-1 asymptomatic carriers and patients with ARC and AIDS.

CXCR4 and CD4 mediate a rapid CD95-independent cell death in CD4(+) T cells

AIDS is characterized by a progressive decrease of CD4(+) helper T lymphocytes. Destruction of these cells may involve programmed cell death, apoptosis. It has previously been reported that apoptosis can be induced even in noninfected cells by HIV-1 gp120 and anti-gp120 antibodies. HIV-1 gp120 binds to T cells via CD4 and the chemokine coreceptor CXCR4 (fusin/LESTR). Therefore, we investigated whether CD4 and CXCR4 mediate gp120-induced apoptosis. We used human peripheral blood lymphocytes, malignant T cells, and CD4/CXCR4 transfectants, and found cell death induced by both cell surface receptors, CD4 and CXCR4. The induced cell death was rapid, independent of known caspases, and lacking oligonucleosomal DNA fragmentation. In addition, the death signals were not propagated via p56(lck) and Gialpha. However, the cells showed chromatin condensation, morphological shrinkage, membrane inversion, and reduced mitochondrial transmembrane potential indicative of apoptosis. Significantly, apoptosis was exclusively observed in CD4(+) but not in CD8(+) T cells, and apoptosis triggered via CXCR4 was inhibited by stromal cell-derived factor-1, the natural CXCR4 ligand. Thus, this mechanism of apoptosis might contribute to T cell depletion in AIDS and might have major implications for therapeutic intervention.

Detection of human immunodeficiency virus type 1 (HIV-1) RNA in pools of sera negative for antibodies to HIV-1 and HIV-2

A total of 234 pools were prepared from 10,692 consecutive serum samples negative for antibodies to human immunodeficiency virus type 1 (HIV-1) and HIV-2 collected at five virological laboratories (average pool size, 45 serum samples). Pools were screened for the presence of HIV-1 RNA by a modified commercial assay (Amplicor HIV-1 Monitor test) which included an additional polyethylene glycol (PEG) precipitation step prior to purification of viral RNA (PEG Amplicor assay). The sensitivity of this assay for HIV-1 RNA detection in individual serum samples within pools matches that of standard commercial assays for individual serum samples, i.e., 500 HIV-1 RNA copies per ml. Five pools were identified as positive, and each one contained one antibody-negative, HIV-1 RNA-positive serum sample, corresponding to an average of 1 infected sample per 2,138 serum samples. Retrospective analysis revealed that the five HIV-1 RNA-positive specimens originated from individuals who had symptomatic primary HIV-1 infection at the time of sample collection and who were also positive for p24 antigenemia. We next assessed the possibility of performing the prepurification step by high-speed centrifugation (50,000 x g for 80 min) of 1.5-ml pools containing 25 microl of 60 individual serum samples, of which only 1 contained HIV-1 RNA (centrifugation Amplicor assay). The sensitivity of this assay also matches the sensitivities of standard commercial assays for HIV-1 RNA detection in individual serum samples. The results demonstrate that both assays with pooled sera can be applied to the screening of large numbers of serum samples in a time- and cost-efficient manner.

Evaluation of 14 commercial HIV-1/HIV-2 antibody assays using serum panels of different geographical origin and clinical stage including a unique seroconversion panel

The performance of 14 commercially available HIV-1/2 antibody assays were compared using well-characterized serum panels containing in total 1500 1800 sera. The panels included consecutive HIV-negative blood donor sera from Sweden, unselected blood donor and patient sera from Tanzania and unselected sera from outpatient clinics in Guinea-Bissau. Furthermore selected HIV-1 antibody positive sera from Sweden and Tanzania and HIV-2 antibody positive sera from Guinea-Bissau were included in the panels. The HIV-1 antibody positive sera were from individuals at various stages of HIV infection, from primary infection, to asymptomatic phase and late stage disease. 12 of the 14 assays identified correctly all HIV-1 and HIV-2 antibody positive sera. One Tanzanian HIV-1 antibody positive sample with complete banding pattern on Western blot was not detected by two of the ELISAs employing synthetic peptides. There were small differences in sensitivity between the assays when used for analysis of seroconversion panels. The most sensitive assay, Abbott IMx HIV-1/HIV-2 III Plus detected antibodies in all nine samples collected from four individuals during the first week after onset of symptoms of primary HIV-1 infection. Most of the assays became reactive during the second week after onset of symptoms and the least sensitive assays were reactive from the third week. The assays showed a high specificity ranging from 99.2 to 100% when used for analysis of Swedish blood donor sera, while most of the assays showed a significantly lower specificity, 91.9-99.6%, when used for testing African specimens.

Reactivity of a new HIV-1 group O third generation A-HIV-1/-2 assay with an unusual HIV-1 seroconversion panel and HIV-1 group O/group M subtyped samples

It was shown previously that about 97% of the anti-HIV-1 group O strain-positive samples were detected by crossreaction with native HIV-1 gp160 (Van Binsbergen et al., Evaluation of a new third generation anti-HIV-1/anti-HIV-2 assay with increased sensitivity for HIV-1 group O, J. Virol. Methods 60 (1996) 131-137). Fourteen out of 17 new anti-HIV-1 group O positive samples, selected with the Enzygnost HIV-1/2 plus assay, were already reactive when tested with HIV-1 gp160. When tested by the Vironostika HIV Uni-Form II plus O microELISA all 17 samples were reactive, demonstrating the necessity to implement an HIV-1 group O-specific antigen in the assay. On the other hand, it was surprisingly found that 40 out of 43 (93%) of anti-HIV-1 group M-positive samples, belonging to strain A, B, C, D, E or F, were detected by crossreaction with the HIV-1 group O (strain ANT70) synthetic peptide incorporated in the Vironostika HIV Uni-Form II plus O. Only HIV-1 subtype D-positive samples did not react with this peptide, presumably because of the presence of a histidine residue in the immunodominant region of HIV-1 subtype D gp41. Both crossreactions make the Vironostika HIV Uni-Form II plus O microELISA also sensitive for anti-HIV-1-positive samples originating from different geographical regions and resulting from different HIV-1 subtype infections. With an unusual seroconversion panel in which p24 Ag was present persistently, many anti-HIV-1/-2 assays produce alternating positive/negative results in anti-HIV antibody-positive bleeds. It was shown that the use of viral p24 and gp160 in a direct sandwich, allowing detection of anti-HIV IgG and IgM, explains the identification of all anti-HIV-positive bleeds by the Vironostika HIV Uni-Form II plus O. The high sensitivity of the plus O assay was confirmed with clinical samples of a so-called anti-HIV-1 low titer panel. The specificity of the Vironostika HIV Uni-Form II plus O determined in five blood transfusion centers, based on 135070 tests, was 99.97%.

Sensitivity and specificity of standard and rapid HIV-antibody tests evaluated by seroconversion and non-seroconversion low-titre panels

BACKGROUND AND OBJECTIVES

The aim of this study is to compare the relative sensitivity and specificity of commercial HIV-antibody assays using seroconversion, non-seroconversion panels, and negative blood donor samples.

MATERIALS AND METHODS

We evaluated the sensitivity of five standard ELISA HIV-antibody assays: Vironostika HIV Uni-Form II, Abbott recombinant HIV-1/HIV-2 third-generation EIA, Biotest Anti-HIV-1/-2 recombinant, Recombigen HIV-1/ HIV-2 EIA and Wellcozyme HIV 1 + 2 (VK54/55), and three rapid screening tests, Capillus HIV-1/HIV-2, Abott Test Pack HIV-1/HIV-2 third-generation EIA, and Sensy-Test HIV 1/2. All tests were assessed using four panels of plasma samples obtained from individuals who were seroconverting and a low-titre HIV-antibody panel of samples. Specificity of the standard screening tests was determined on 3.500 HIV-antibody-negative blood donor samples.

RESULTS

There was no statistically significant difference in sensitivity between the five standard ELISA tests. One of these tests was significantly less specific than the others. The standard ELISA tests detected all the low-titre HIV-antibody-positive samples. Two of the rapid screening tests were significantly less sensitive on the seroconversion panels and all three tests failed to detect at least one of the positive samples in the low-titre panel.

CONCLUSIONS

The additional risk of using one or other of the standard ELISA tests under review of not detecting all HIV-positive units of blood is not statistically significant. Using some of the rapid screening tests will, however, add a significant additional risk. A rapid screening test should therefore be adopted only after careful consideration of the effect of a possible lack of sensitivity on the safety of the blood supply.

Virologic and serologic markers of rapid progression to AIDS after HIV-1 seroconversion

The association between early virologic and immunologic events after human immunodeficiency virus type 1 (HIV-1) infection and progression of HIV-1 infection to acquired immunodeficiency syndrome (AIDS) was studied among 59 homosexual men with documented time of seroconversion. Epidemiologic factors, such as number of lifetime sexual partners, history of sexually transmitted diseases, and other factors, also were studied. All 17 seroconverters in the cohort who developed AIDS within 3 years (rapid progressors = RPs) were compared with 42 men without AIDS for at least 6 years seroconversion (nonrapid progressors = non-RPs). Plasma levels of HIV-1 RNA, p24 antigen, antibodies to HIV-1 structural genes, beta-2 microglobulin, neopterin, and interferon-alpha were measured at four time points: (a) the last seronegative visit, (b) the first seropositive visit, (c) the visit closest to AIDS (or the corresponding visit for the non-RPs) and (d) 6 years after seroconversion (for non-RPs). Up to seroconversion, the RPs had a significantly higher number of lifetime sexual partners than non-RPs (503 versus 171, respectively). At the first seropositive visit, RPs had significantly higher concentrations of plasma HIV-1 RNA (p < 0.01) and prevalence of p24 antigenemia (p < 0.001) and significantly lower levels of antibodies to the HIV-1 gag proteins p17 and p24 (p < 0.01-0.001) compared with non-RPs. These differences increased during follow-up visits. Antibodies to p66 and gp120 were significantly different only at the visit closet to AIDS (p < 0.001), as were beta-2 microglobulin and interferon alpha. These findings suggest that early virologic-immunologic events after HIV-1 infection may determine the rate of progression to AIDS. Anti-gag immune response may prevent rapid progression of HIV-1 disease and should be considered for future vaccine studies.

Overexpression and purification of a recombinant chimeric HIV type 2/HIV type 1 envelope peptide and application in an accelerated immunobased HIV type 1/2 antibody detection system (AIBS): a new rapid serological screening assay

A chimeric HIV-2/HIV-1 envelope sequence containing an immunodominant region of HIV-2 gp36 and the corresponding region of HIV-1 gp41 was constructed and overexpressed in Escherichia coli. The recombinant product (rp21/18) was purified and applied in a novel antibody-screening assay. Characteristics in the design of this new principle are as follows: (1) the overall assay time is about 30 min; (2) the assay procedure includes three manipulation steps; and (3) the test shows a reliable performance with respect to sensitivity and specificity. The diluted serum sample and the protein G-horseradish peroxidase conjugate are added simultaneously into a coated (hybrid antigen HIV-1/2) and blocked microtiter plate well. The in-batch incubation of serum sample with protein G-horseradish peroxidase saves two manipulation steps that are normally necessary in the five-step procedure of a classical ELISA. AIBS was evaluated with commercially available seroconversion panels and with random negative serum samples from a blood bank. Seroconversion results demonstrated that AIBS has equivalent sensitivity to ELISAs and the third generation assays. The specificity was determined on a total blood donor population of 5012 (Red Cross Vienna, Austria). The repeat reactive rates for donor population were 0.02%. AIBS represents a general immunometric system (not only HIV antibodies). The entire assay procedure of AIBS evaluated for HIV-1/2 screening, including result reporting, can be performed automatically by several commercially available systems. Depending on these systems AIBS is potentially useful in laboratories or blood banks that have both high- and low-volume testing.

A field test for the detection of antibodies to human immunodeficiency virus types 1 and 2 in serum or plasma

In response to the need for simple and rapid tests for infectious diseases, we have devised a test for antibodies to human immunodeficiency virus type 1 (HIV-1) and HIV-2 which resembles many contemporary strip-style pregnancy tests in format and ease of use. The test was evaluated with 2,928 serum specimens (1,541 reactive and 1,387 nonreactive) collected and tested at a Mexico City hospital clinic and was compared with a laboratory assay (Abbott) performed simultaneously. The sensitivity and specificity of the test using these serum specimens were 99.68 and 99.71%, respectively (before the code of the blinded study was broken). This compares with 100% sensitivity and 97.55% specificity with the laboratory assay (specificity upon reassay after the code was broken, 99.21%). In a survey of HIV-2 specimens, reactive (positive) specimens were detected in 51 of 51 cases. The test was examined with 21 commercially available (HIV-1) seroconversion panels. The performance of the test was comparable to that of a group of Food and Drug Administration-approved (antibody-based) HIV tests.

Flow cytometric immunofluorescence assay for detection of antibodies to human immunodeficiency virus type 1 using insoluble precursor forms of recombinant polyproteins as carriers and antigens

A new serological assay, the recombinant flow cytometric immunofluorescence assay (r-FIFA), was developed for the early detection of human immunodeficiency virus type 1 (HIV-1) antibodies by using recombinant insoluble forms of HIV-1 Gag-p45, Gag-gp41 chimeric protein, gp160, Po197 polyprotein as antigens and autologous carriers through flow cytometry. These recombinant proteins were expressed in insect cells by a baculovirus expression system. Eight anti-HIV-1 seroconversion panels, a low-titer anti-HIV-1 panel from Boston Biomedica Inc. (BBI), and three HIV-1 seroconversion specimens from the Provincial Health Laboratory of Ontario, Toronto, Ontario, Canada (PHL), were tested and analyzed by r-FIFA. In sensitivity comparisons between r-FIFA and tests licensed by the U.S. Food and Drug Administration, which were used to test all of the HIV-1 panels from BBI, detection of HIV-1 antibody by r-FIFA was on average greater than 20 days earlier than that by enzyme immunoassay. The sensitivity of r-FIFA has permitted the detection of HIV-1-specific immunoglobulin G (IgG), IgM, and IgA antibodies during seroconversion. A kinetic analysis of HIV-1 antibody production of r-FIFA has shown that either IgG or IgM, or both, can be detected, depending on the phase and type of the immune response in the HIV-1-infected individual. Both primary and secondary immune responses were observed during this period. The r-FIFA results suggest that implementation of r-FIFA may significantly reduce the "window" period from the time of infection to the time of seroconversion, with earlier detection of antibodies after initial infection. This would also make it possible for us to understand the immune response and the precise mechanisms of immunopathogenesis in the early period of HIV-1 infection.

Virus-induced immunosuppression is linked to rapidly fatal disease in infant rhesus macaques infected with simian immunodeficiency virus

Six newborn rhesus macaques were experimentally infected with pathogenic Simian immunodeficiency virus of macaques (SIVmac251), and three newborn macaques were infected with avirulent SIVmac1A11. The former developed rapidly fatal simian AIDS and died within 26 wk of age, whereas the latter remained clinically normal. Infant monkeys that developed rapidly progressive disease had rapid declines in CD4+ cells and were unable to mount IgG and IgA antibody responses to SIV or to an unrelated antigen, tetanus toxoid. IgM antibody responses were near normal to both SIV-specific and nonspecific antigens. Cytotoxic T lymphocyte (CTL) responses to SIV envelope were observed in animals infected with either virulent or avirulent SIV. These studies demonstrated that virulent SIVmac infection induced a rapid immunosuppression that was both SIV-specific and nonspecific in nature. The observation that virulent strains of SIV can rapidly induce a global immunosuppression provides one explanation for the rapid disease course in some HIV-infected children and supports the strategy of early and vigorous antiviral drug therapy to alter the disease course even if this does not prevent infection.

Performance characteristics of recombinant enzyme immunoassay to detect antibodies to human immunodeficiency virus type 1 (HIV-1) and HIV-2 and to measure early antibody responses in seroconverting patients

We investigated the performance of a double-antigen sandwich recombinant enzyme immunoassay (EIA; Abbott Laboratories, North Chicago, Ill.) and compared it with that of a synthetic-peptide-based EIA (Biochem Immunosystems, Montreal, Quebec, Canada) for the detection of human immunodeficiency type 1 (HIV-1) and HIV-2 antibodies in 2,321 clinical serum samples. The results of both EIA methods and Western blot (immunoblot) were in agreement for 1,046 HIV-1 and 10 HIV-2 specimens from a panel of known positives. From a prospective panel of 1,085 specimens, 38 proved to be positive by both EIAs and Western blot, 3 were positive by the recombinant EIA only, and 9 were positive by the peptide EIA only, for calculated specificities of 99.71 and 99.04%, respectively. Of 180 specimens from a seroconversion panel collected from 77 patients, the results for 170 were in agreement by all antibody testing methods and 10 were found to be repeat reactive for HIV antibodies by the recombinant EIA only. All 10 were initial specimens of seroconverting patients; 7 were also reactive for HIV p24 antigen. An examination of four of these sera by radioimmunoprecipitation assay showed gp120 and gp160 bands in each. Analysis of the anti-Env antibody class in three of these samples showed that one consisted of immunoglobulin M (IgM) only and two contained both IgG and IgM antibodies. Although both EIA procedures were sensitive and specific in the detection of antibodies to HIV-1 and HIV-2 and both were capable of detecting early antibodies, the recombinant assay was more sensitive for antibody detection during early seroconversion.

Multicenter evaluation of new double-antigen sandwich enzyme immunoassay for measurement of anti-human immunodeficiency virus type 1 and type 2 antibodies

A new enzyme immunoassay (EIA), the Cobas Core Anti-HIV-1/HIV-2 EIA DAGS (also referred to as Roche DAGS), for the detection of antibodies to human immunodeficiency virus type 1 (HIV-1) and HIV-2 was evaluated in four centers. The assay is based on the double-antigen sandwich (DAGS) format, which enables the detection of all classes of antibodies. The antigens consist of recombinant proteins in their native conformation and of synthetic peptides. Of a total of 5,836 negative serum samples, including 95 samples likely to produce false reactivities, 6 were false positive, resulting in a specificity of 99.9%. None of 35 sera that were from noninfected individuals but contained p24-cross-reacting antibodies as revealed by Western blot (immunoblot) analysis were reactive by the Roche DAGS assay. In samples from individuals infected with HIV-1 group M (n = 499) and HIV-2 (n = 200), the sensitivity of the assay was 100%. Although containing antigens with sequences from subtype B only, the assay was also able to correctly identify with high optical density/cutoff ratios samples from subjects infected with HIV-1 subtype O (n = 10). In 17 of 19 seroconversion panels tested, the assay detected the presence of HIV-1 antibodies as early as another sandwich EIA. Eight of these panels were also analyzed by an indirect second-generation assay, which detected antibodies 2 to 10 days later than did the DAGS assay under evaluation. The excellent specificity and sensitivity of the new Cobas Core Anti-HIV-1/HIV-2 EIA DAGS are the result of the DAGS format as well as of the native, naturally folded form of the recombinant protein used as the gag antigen.

Evaluation of the reliability of 6 current anti-HIV-1/HIV-2 enzyme immunoassays

The sensitivity for early detection of HIV antibodies and specificity of 6 anti-HIV-1/HIV-2 screening enzyme immunoassays (ELISAs) currently on the market were investigated by testing a panel of 249 well-characterized serum samples. The panel included sera from AIDS patients or children with congenital HIV infection, high-risk individuals and patients with conditions unrelated to AIDS. 'Tricky' sera (repeatedly positive results by ELISA and negative or indeterminate results by Western blot; n = 69) were also used in this evaluation along with 6 seroconversion panels. One second-generation assay (Biotest) and two third-generation assays (Abbott and Murex) showed the highest sensitivity for early detection of HIV-1 antibodies in seroconversion panels. A high specificity was achieved with the Cambridge Biotech (100%) and Ortho ELISA (99.4%). A relatively high rate of false-positive results was obtained with the Biotest (n = 10) and the Pasteur assays (n = 8) by testing 'tricky' sera and samples from high-risk individuals and from patients with other acute viral infections. In conclusion, it remains difficult to combine high specificity with an accurate detection of early seroconversion for anti-HIV-1/HIV-2 screening enzyme immunoassays.

Multicentre study for diagnostic evaluation of an assay for simultaneous detection of antibodies to HIV-1, HIV-2 and HIV-1 subtype 0 (HIV-0)

The aim of the study was to evaluate a new ELISA for detection of HIV-1, HIV-2 and HIV-1 subtype 0 (HIV-0) antibodies. The assay format is based on the antigen sandwich principle. To enable specific detection of HIV-0 antibodies, in addition to HIV-1 and HIV-2 antigens HIV-0 antigen is used for coating the solid phase and for the conjugate. The results show that all 12 HIV-0 samples tested were detected with a high degree of reactivity, as were all the 1,144 anti-HIV-1 and 424 anti-HIV-2 positive samples. The capacity of the test to enable early detection of seroconversions is equivalent to that of other sandwich ELISAs. The specificity of the assay was determined to be 99.89/99.94% (initial/after retest) using 58,366 samples, which is superior to the other ELISAs used for comparison. Even with difficult samples (i.e. samples of African origin, samples known to cause false-positive reactivity in different ELISAs, or samples containing potential interference factors) there were very few false-positive reactions. Therefore, the new assay is well suited for screening blood donations as well as for evaluating samples from patients of different geographic origin.

Detection of specific human immunodeficiency virus IgM antibodies

This study was done to demonstrate whether the use of the antigen-sandwich human immunodeficiency virus (HIV) antibody-screening assays (3rd generation assays), which detect all classes of anti-HIV immunoglobulins, leads to an earlier detection of HIV IgM compared to the 2nd generation HIV antibody-screening assays. We tested sequential bleeds of three donors obtained from commercially available seroconversion panels. Anti-HIV testing was done before and after high-performance liquid chromatography separation of IgG and IgM fractions. The positive result of the first bleedings from all three panels was linked to the IgM fraction, while at that time the IgG fraction was still negative. For subsequent samples drawn 5-9 days later, a positive signal was obtained with the IgG fraction in addition to a stronger positive signal obtained with the IgM fraction. We conclude that an assay capable of simultaneously detecting different immunoglobulin classes, including IgM, will help to narrow the "window period" for serological detection of seroconversion to HIV by detecting anti-HIV IgM-containing samples earlier than conventional assays using only anti-human IgG enzyme conjugates (indirect anti-HIV-screening assay, 2nd generation assays).

Anti-human immunodeficiency virus type 1 antibodies of noninfected subjects are not related to autoantibodies occurring in systemic diseases

Indeterminate Western blot (WB) (immunoblot) patterns for anti-human immunodeficiency virus type 1 (HIV-1) antibodies are often observed when testing serum samples from noninfected individuals. We investigated here the possible involvement of some frequently occurring autoantibodies (anti-SmB/B', U1snRNP [68 kDa, A, and C], Ro/SS-A [60 and 52 kDa], and Jo-1) in the generation of such indeterminate HIV-1 WB. In particular, the role of a reported sequence homology between p24 gag and the SmB/B' autoantigen was investigated. Serum samples were obtained from 50 healthy controls, 51 patients with systemic lupus erythematosus (SLE), 46 with systemic sclerosis, 6 with Sjögren's disease, 3 with mixed connective tissue disease, and 41 healthy subjects with persistent indeterminate HIV-1 WB. Reactivity to HIV-1 p24 gag was slightly but not significantly more frequent in patients with SLE than in controls (25.5% versus 14.0%; P > 0.1), whereas reactivity to HIV-1 p17 gag was significantly more frequent in the former subjects (23.5% versus 8.0%; P = 0.03). Simultaneous reactivity to p17 and p24 was observed in patients with SLE (11.8%; P = 0.014) or systemic sclerosis (8.7%; P = 0.049) but not in controls. There was no association found between the presence of any autoantibody and the occurrence of indeterminate HIV-1 WB nor between the presence of p24-reactive antibodies and anti-SmB/B'; this indicates that most p24-reactive antibodies are directed to epitopes other than the proline-rich sequences shared by p24 gag and SmB/B'.

Testing blood donors for HIV: current controversies

Despite an estimated risk of HIV infection from anti-HIV screened blood transfusions of less than one in 225,000 per unit, there continues to be strong pressure to implement additional donor screening and viral inactivation procedures. Decisions to implement such procedures must be based on analyses that incorporate accurate estimates of residual risk, and data-based projections for the reduction in risk that would result from each measure. Since the residual risk of HIV is primarily due to donations given in the infectious pre-seroconversion window, effort must be directed at: reducing donations by persons in this window; employing tests that narrow the window; and development and implementation of procedures that inactivate viral compartments that predominate during the window. Unfortunately, as the risk of HIV has declined to near-undetectable levels, the challenge of generating appropriate data to evaluate new measures, and thereby support rational policy decisions, has increased inversely. To meet this challenge, we must refine our understanding of the virological characteristics of early HIV seroconversion, and of the types of donors who present in the seroconversion window. Thoughtful application of a thorough understanding of the seroconversion window, in the context of accurate HIV incidence data in the donor settings, should enable us to assure the public of a safe blood supply while resisting inappropriate implementation of unnecessary and usually non-specific procedures.

Development of the anti-gp120 antibody response during seroconversion to human immunodeficiency virus type 1

We have studied the development of the antibody response to the surface glycoprotein gp120 of human immunodeficiency virus type 1 in three individuals who presented with primary human immunodeficiency virus type 1 infection syndrome. Serum anti-gp120 antibodies were first detected 4 to 23 days after presentation, after p24 antigen and infectious-virus titers in the peripheral blood had declined manyfold from their highest values. Whether anti-gp120 antibodies present at undetectable levels are involved in clearance of viremia remains unresolved. Among the earliest detectable anti-gp120 antibodies were those to conformationally sensitive epitopes; these antibodies were able to block the binding of gp120 monomers to soluble CD4 or to a human monoclonal antibody to a discontinuous epitope overlapping the CD4-binding site. Some of these antibodies were type specific to a degree, in that they were more effective at blocking ligand binding to autologous gp120 than to heterologous gp120. However, the appearance of these antibodies did not correlate with that of antibodies able to neutralize the autologous virus in vitro by a peripheral blood mononuclear cell-based assay. Antibodies to the V3 loop were detected at about the same time as, or slightly later than, those to the CD4-binding site. There was a weak correlation between the presence of antibodies to the V3 loop and autologous virus-neutralizing activity in two of three individuals studied. However, serum from the third individual contained V3 antibodies but lacked the ability to neutralize the autologous virus in vitro, even immediately after seroconversion. Thus, no simple, universal correlate of autologous virus-neutralizing activity in a peripheral blood mononuclear cell-based assay is apparent from in vitro assays that rely on detecting antibody interactions with monomeric gp120 or fragments thereof.

HIV and human complement: mechanisms of interaction and biological implication

Human complement, although not lytic for HIV-1, interacts with the virus and is closely involved in the infectious process. It enhances infection in the absence of antibody, and turns neutralizing antibodies into agents which increase viral infectivity. In this review M.P. Dierich et al. summarize available information and discuss possible biological implications.