preparation and characterization of human hiv type 1 neutralizing reference sera

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  • AIDS RESEARCH AND HUMAN RETROVIRUSESVolume 11, Number 7, 1995Mary Ann Liebert, Inc.

    Preparation and Characterization of Human HIVType 1 Neutralizing Reference Sera



    Reference neutralizing antibody (NA) reagents are needed for laboratories to be able to compare results ofneutralization assays that will be used to monitor HIV-1 vaccine recipients. In an effort to establish such ref-erence reagents two asymptomatic, seropositive patients were identified with medium to high amounts of cross-reactive NA activity against a number of HIV-1 strains. Sera obtained from each individual at three or foursequential phlebotomies were pooled, and the two pools were each distributed in 3000 aliquots into glassampoules and lyophilized, and the ampoules were flame sealed. An HIV-1 antibody-negative reference serumwas prepared in a similar fashion after pooling serum from four individuals. Ampoules were tested for uni-formity of nil, sterility, moisture content, residual oxygen, stability, infectivity, and presence of antibody. Aninternational collaborative study was conducted to determine the potency of the samples in six laboratories,each using their own neutralization assays and reagents. The results indicated reasonable consistency betweenlaboratories and that both sera have sufficient titers against a variety of strains for Use as reference reagents.These reference sera have been included in the World Health Organization (WHO) AIDS Reagent Projectand are available through the three AIDS reagent repositories.


    Neutralizing antibody (NA) determinations are an im-portant feature of HIV-1 vaccine clinical trials. BecauseNAs play a critical role in protection from viral infections theyhave typically been measured to assess if vaccine recipientsmight be protected sufficiently by immunization. ' Whether NAsare beneficial in HIV-1 infection has not been shown. However,they will be important immunological markers of responses tovaccines.

    Presently, there is no single, generally used neutralizing an-tibody assay and laboratories use assays that may vary in re-gard to target cells, method of virus preparation, virus strains,virus potency, incubation times, duration of assay, and end-point markers used to determine virus replication.2-8 Three pre-vious international collaborative studies have been conductedto compare neutralization assays using either polyclonal sera ormonoclonal antibodies to HIV-1.4,9'10

    A World Health Organization (WHO) InternationalWorkshop on Standardization of Neutralizing AntibodyMeasurements to HIV and Related Viruses was held in London

    on October 3-5, 1988.n The need for standardized referencereagents was emphasized at that time and as a follow-up to thatmeeting we have prepared the sera described here for use asreference reagents. Another workshop, entitled "Neutralizationof HIV-1" held on April 19-20, 1993 in Bethesda, Maryland,again stressed the need for standardized reagents to be used inneutralizing antibody assays.7 Monoclonal antibodies could beused for these purposes because they would have the advantageof being in an unlimited supply and lacking the synergistic phe-nomena potentially occurring in polyclonal sera, but they havethe potential disadvantage that they may be too specific.Polyclonal human sera are broadly reactive and may better suitthe need for reference reagents for human vaccine trials.

    We selected two HIV-1-infected patients to be bled repeti-tively for sera to be used to prepare the reference reagents. Therepeated serum collections from each patient were pooled andwere treated and tested according to the WHO guidelines to beprepared in aliquots as reference reagents, including testing inan international collaborative study.1216 These samples havebeen designated to be reference reagents for research purposesin the WHO AIDS Reagent Project. They are now available

    'Center for Biologies Evaluation and Research, Food and Drug Administration, 1401 Rockville Pike, Rockville, Maryland 20852-1448.2Present address: Department of Preventive Medicine, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Road,

    Bethesda, Maryland 20814.



    through the three WHO AIDS Reagent repositories (the NIHAIDS Research and Reference Reagent Program [Rockville,MD], the National Institute for Biological Standards andControl [Hertfordshire, England], and the Agence Nationale deRecherches sur le SIDA [Paris, France]). An HIV-1 antibody-negative human serum has been prepared in a similar fashion.



    The donors that were selected for the reference sera wereparticipants in a longitudinal study of progression of HIV in-fection and underwent extensive medical evaluation at each pe-riodic visit, including testing of their immunological status. Thisstudy was conducted at the National Institutes for Health (NIH,Bethesda, MD) with informed consent and it has been reviewedyearly by the Institutional Review Board. A panel of 10 donorswas initially screened in order to select 2 patients with high NAtiters against HIV-lmn- The two asymptomatic, HIV-1-infectedindividuals that were selected were phlebotomized under med-ical supervision for approximately 500 ml of blood on three orfour occasions over the course of 10 months. The units werecollected without anticoagulant and serum was removed aftercentrifugation. Each individual donation was tested for neu-tralizing antibody titer and the final pooled samples for eachdonor were also tested. The final pooled individual serum col-lections consisted of 350 ml for patient 1 and 375 ml for pa-tient 2. An HIV-1 antibody-negative serum was prepared bypooling sera from four uninfected individuals with hemochro-

    matosis who were being maintained in normal iron balance byperiodic phlebotomy. The volume of this serum pool was 720ml. The pooled sera were heat inactivated at 56C for 30 min.

    Distribution of sera into vialsThe sera were diluted 1:4 with half-strength phosphate-

    buffered saline (PBS), pH 7.0, made with pyrogen-free doubleglass-distilled water prior to aliquoting. Samples were stirredthroughout the filling process and a Wheaton (Millville, NJ)peristaltic dispenser was used to place 0.5-ml amounts in 2-mlcapacity flat-bottom Wheaton Vacules made of type 1 borosil-icate glass, class A, that were washed and autoclaved prior tothe fill, and then were closed with Wheaton low-extractablegray butyl slotted stoppers. The fills were performed in a lam-inar flow cabinet in a class 100 clean room with a HEPA-fil-tered, positive-pressure air supply. Ampoules were stopperedby hand and placed in a precooled Hull (Hatsboro, PA) freezedryer. Three probes were distributed through the product loadto monitor the product temperature and the lots were subjectedto a 3-day freeze-dry cycle, stoppered under vacuum, and thenremoved from the freeze dryer. The ampoules were stored at4CC until flame-sealed under vacuum, reinforced with a coat ofparaffin, tested for sealing defects with a high-frequency gen-erator (model BD10A; Electro Technic Products, Chicago, IL),and stored upright in boxes at +4C.

    Quality control testsTo ensure uniformity of fill of the liquid samples before

    lyophilization at least 3 weight measures per each tray of 942

    Table 1. Summary of Neutralization Assay Methods in Collaborative Study




    Length ofassay (days)



    A (2)





    F (6)







    90 min/37 C

    60 min/37 C60 min/37 C

    90 min/room temperature

    90 min/room temperature

    120 min/4 C

    60 min/37 C


    p24 AgSyncytiaColorimetric


    SyncytiaAcid reduction

    p24 Ag


    aHuman lymphoblastoid cell line.bNo reference provided.cHuman umbilical cord blood lymphocyte (T lymphoid).dHuman male spleen cell subclone of AA derived from WIL-2 human splenic EBV+ lymphoblastoid line).Peripheral blood mononuclear cells.


    ampoules were taken. After lyophilization, the moisture con-tent of the aliquots was analyzed using the gravimetric or losson drying method.16 Samples were tested for sterility in fluidthioglycolate medium at 31C and soybean casein digestmedium at 22C for 14 days. Because the samples were sealedunder vacuum, the oxygen content of the ampoules atmospherewas not tested. However, the samples were subjected to thehigh-frequency vacuum testing coil previously described toconfirm that vacuum was present. A short-term stability studywas conducted with five samples each at 56, 35, 27, and 4C.Samples were reconstituted every 1-4 weeks depending on thetemperature and tested for neutralizing activity.17"19 The serawere also checked for infectivity by adding a vial of each tocultures of phytohemagglutinin (PHA)-stimulated peripheralblood lymphocytes (PBLs) and incubated at 37C in the pres-ence of a 5% C02-in-air atmosphere for a period of 4 weeks.20Samples were checked daily for indication of giant cell forma-tion and weekly samples were taken for testing on a Du Pont(Wilmington, DE) HIV-p24 core profile enzyme-linked im-munosorbent assay (ELISA) test kit.

    ELISAThe reference sera were tested by HIV-1 enzyme im-

    munoassay (EIA), HIV-2 EIA, and a combination HIV-1/HIV-2 EIA (all made by Genetic Systems Corporation, Seattle, WA).

    PotencyAn international collaborative study was designed with six

    laboratories participating from the United States and Europe.The investigators were as


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