- Email: [email protected]
HIV types and testing
HIV types and testing Angus G. Dalgleish The London
Hospita1 Medical
College,
London,
UK
The past year has seen the discovery of new isolates which do not readily fit into the established categories HIV-1, HIV-2 and SIV, as well as the availability of new assays including the use of the polymerase chain reaction. New combination enzyme-linked immunosorbant assays can detect viruses from two or more groups where specific enzyme-linked immunosorbent assays have been developed using peptides and mono-
clonal antibodies.
Current Opinion in Immunology
Introduction In order to develop effective thempeutic and vaccine programs it is clearly imperative to dehne the target. This involves the careful characterization of different isolates that may possess markedly different functional properties with regards to their ability to evade existing therapies and immune responses which are mainly against the ‘wild’or prototype strains. Furthermore, it is important to know the enemy and its habits. This means that testsshould in theory be 100% speciflc and 100% sensiWe, and that negatlve results a certain time after potential infection ,should be meaningf~l. In this review of some of the papers published during the past year, it will be seen that considerable progress has been made in achieving many of these aims.
Types’ of HIV ‘Types’lnvolves the broad groupings of human immunodeficiency virus (HIV) isolates as well as those isolates that display subtle functional differences such as neutralization of escape mutants. Tivo broader groups are widely accepted i.e. HlV-1 and HIV-2. There are now several reports of isolates which are claimed to be signifïcantly divergent from wild-type HlV-1 and HN-2. It is of note that these isolates may be identilìed because they gW indeterminable results on ‘srandard’ HlV-1 and HIV-2 essays. Dietrich and colleagues [le] report a highly divergent HIV-2 isolate that appears to be closer to simian immuno-
deficiency (SIV) than previous isolates. It is of interest that these isolates have no cytopathic e6xr.s on lymphocytes and that the host from which tbe virus came is unaffected. Leys et UI![2*] have reported a distinct isolate obtained from two people from west central Africa. However, these
1991, 3:543-546
isolates are closer to HlV-1 than I-IIV-2 but differ at nearly every leve1 of analysis. Huet et al. [3] have reported a highly defective HIV-1 strain from a healthy Gabonese patlent that may be detected on an atypical western blot. This isolate is in fact a ‘string’ of HIV-1s that are closer to European than Afrlcan isolates. Sequencing of the tut gene showed that all the proviruses are defective and it is remarkable that no antibodies to the envelope were detected, virtually no cytopathic effect.5 were seen in vitm and the patient remained healthy throughout the study. Tbe authors conclude that HIV isolates represent a population of variable isolates. It is becoming increasingly clear that isolates with different cell tropisms and biological properties may exist concurrently in the patient as well as arise throughout the period of infection. Of particular interest, especlaUy in view of the disappointing results achieved in soluble CD4 clinical trials, is the report by Daar et al [4**] that higher concentrations of soluble CD4 are required to neutralize primary clinical HIV isolates compared with classic laboratory isolates. The demonstration of neutralization escape mutants in vitm [5], suggests that proviruses may be selected for thelr capacity to evade neutmlizing antibodies and that single amino acid changes may bring about markedly different neutmlizmg properties. However, demonstra’tion of an effect in vitro does not always mean that it is representative of a major interaction in vivo (as with soluble CD4). Berkower et al. [6], using a sensitive neutralizing plaque assay, have shown that most patients have neutralizing activity that is predominantly to a conserved epitope shared among three otherwise highly divergent isolates and rarely mutates. The authors optimistically argue that such a response bodes well for the development of a vaccine. The pressures for a virus to mutate or for a particular mutation to be selected for in vivo are still not clear. Tissue tropism certainly plays an important role.
Abbrewiations EUSA-enzyme-linked
immunosorbent
assay; HIV-human immunodeficiency virus; PCR-polymerase SlVsimian immunodeficiency virus.
@ Current Biology Ltd ISSN 0952-7915 D
chain reaction;
543
544
Immunodeficiency
McNeameyet al [7*] showed that isolates from a clustered isolate outbreak were all remarkablysimilarover a 3-year period ( > 0.01% differente at the amino acid level) and that isolates from the brain and lung were more than 30 times more likelyto grow in monoq& than they were in lymphocytes.However, no envelope sequence differences between lung, brain and blood isolates were noted, in contrast to previous reports that had noted single amino acid changes in envelope proteins that exhibit differentcell tropisms.The diiference between brain tropism isolates and lymphotropic isolates has been reported by Cheng-Mayer[8] who also noted differentcell tropismfor paired isolates from the same patient.The authors suggestthat their findingsargue for a different subgroup for central nervous system (CNS) viruses.The im portance of a relevantgold standard for testing purposes bas been suggested by Zwart et al. [9*] who noted that the MN/SCisolate is much more representativeof clinical isolates with regards to eliciting neutralizingantibodies, and should be used in vaccine and similar studies. These studies show the diversityof HIV isolates and HIv populationsand the lack of consensus amongstworkers with regards to the relevante of different isolates in the development of HIv infection and pathogenesis of disease. Clearly, further studies require carelùl and openminded interpretation.
Testing’ of HIV
for I-IIV-1and HIV-2 using one kit is now a possibility,if costs allow. This latter consideration prompted Frösner et al [ 14**] to test the potentlal loss of sensitivitywhen testing pooled sera for public health surveillancesystems in low presalence population groups. A savingof about ninefold in the costs of testing and up to 50% of technicians time is reported. A new cheap screen@ system using iinger prick blood from antenatal clinics has been reported by Peckham et al. [ 15*]. Discriminationbetween HIV-1 and HIV-2 remains an important task espe&llywhen up to 70% of HIV-2seen may be detectable on HIV-1 EIISAs. Hunt et al. [I6*] have used mouse monoclonal antibodies to transmembrane proteins instead of peptide based assaysused earlier.The western blot still remains a popular assayfor distinguishing the two types of I-IIV. In this regard Jackson et UI! [17] examined the relevante of indeterminate reactions that do not meet the criteria for HIV-1 or HIV-2 positivity and conclude that such blots rarely, if ever, indicate HIVinfection in low-riskpopulations.They can however, alert one to a new virus isolate (HIV-2+ > as noted earlier. There are severalother reports, and comments upon the reports, of indeterminate blots and type discrimination [ 18,19,20]. These clinicianswho have had problems interpreting noted HIV results are recommended to .the articles by Benenson [ 211 and Drotman [ 221. A major testing problem is to determine whether or not babies bom to HIV-infectedmothers are infected, as they will cany maternal antibodies to the virus. An IgM response is present at birth or appears within the third month. The presence of p24 antigen is of ominous significance [23] and PCR has obvious implicationsin this group of patients. In another studythe prevalente of maternal antibodies to I-IIV-1gp120 was found to correlate with the outcome of children bom to I-IIV-infectedmothers. Sera collected from children at risk, who showed high levels of matemal antibodies to certain epitopes of. gp120, were less likely to be HIV infected than children without rnaternalantibodies to these epitopes [ 241. ‘T’his has important implications for the management of HIVpositlve mothers, if coniïrmed in other .studies.
HIV testing is required to establish the prevalente of the virus within a population, the iûnctional characteristics of the virus as well as delining markers and parameters that have prognostic value. The latter is particularlyimportantwith regardsto therapeutic trials.Most new studies address at least one of these issues. Detection of HIV may be limited by delayed seroconversion, or the presence of indetenninate assays.Wolinsky et al [lO] report the use of polymerase chain reaction (PCR) to investigate delayedseroconversion,and note that PCRidentilìed HIV in 20 of 24 HIV-1positive men prior to evidente of serological seroconversion. There could be a time interval of as long as 42 months (with a median of 18 months) before the appearance of a diagnostic western blot. Previous reports by Ranki et al [ 111 noted a long seroconversion time, judged by the presence of nef antibodies that were detectable before standard enqme-linked immunosor~t assays (EIISAs) read positive. This study would appear to coniïrm the existente of long seroconversionand the usemlness of PCR in the detection of HIV infection in high risk individuals.Cumming et ai! [12] have tried to increase the sensitivityof the ELISAsystem by using a short peptide from gp41 which is detected slightlyearlier than the Abbot envacore assays.
A further feature of HIVinfection is the suspicion that the genetic type of the individualmight be as important or more important than the type or isolate of HIV. In support of this is the increasing association of certain haplotypeswith rapid progression to AIDS,noted previously and which receivesfurther support from Kaslowand colleagues [31*].
The sensitivityand speciIí&y of testing for HIVl and HIV-2 was sought in a large multicentre trial which showed that a recombinantHIV-1and HIV-2ELISAwas as sensitiveas a single I-IIV-1ELISAfor I-IIV-1and detected 99% of the I-IIV-2samples [ 13.1. Thus routine screening
The immune response must surely be one of the most misunderstoodcomponents in AIDS reseamh. Major dlfferences between chimpanzees and man with regards to why the latter, but not the former, have circulatingcytotoxic T lymphocytesthat lyse uninfected CD4+ cells [ 321
The remaining need for testing is to delìne‘prognostic factors and there are numerous papers addressing thls issue. It is becoming increasinglyclear that susceptibility to AIDS infection appears early in disease, is closely associated with CD4 kevelsand & microglobulin levels k5-301.
HIV types and testing Dalgleish have yet to be explained. This and rnany other similar
obsemations not readiíy explained by our current understanding of the immune response to HIV suggests that in future our attention, types and testing should also focus on testing human immune types.
References and recommended
of 18 Months Before a D@nostic Westem Blotz Evidente bom a Cohort of Homnsexual Men. Ann In&m Med 1989, 111:%1-+72. 11.
RANKIA,VAUESL,RROHNM~AL:L~~@L.~~~~~~P~~~~~~~ Ovest Seracnnvetsion in SexuaRy Ttsnsmitted HIV Infection. Luncet 1987, 2:58%593.
12.
CUMMING SA, MCPHEZDA, M&XUL WJ, KEMPBE, DOHERIYRR, ImmunodominaatEpitojx of G~~~ID:Ut~ofaCoaFerved
1.
DIETIUCHU, AD~KI M, KREUTZ R, SEIF’PA, KCRINELH, RUE~%MEN-WAIGMANN H: A HighIy Divergent HIV-2 ReIated IsoIate. Nature 1989, 342948-949. New isoiate betweeu HIV2 and SN. .
2. .
DE LEYS R, VANDEI(BORG~ B, VANDEN HAESEVE~DEM, HEYNDRICKXL, VAN GEEL A, WAUTERS C, BEMAERT~R SAMAN E, NIJS P, WU.IEM.SB, ET AL: Isolation and Pattial Chatacterization of An Unusual Humau ImmunoDeficIehcy Retrovhus from tio Petsous of West-CentraI AfrIcau Ot@n. J vd 1990, 64:1207-1216. New isoiste neater HIV-1 thau HIV2 and SN. Neither this isolate or that
in [l*] warrant a new label such as HlV-3. HUET T, DA?% MC, BRIJN-VEZINET F, ROEIAN?%GE, WAINHOBX>NS: A H@hIy DefectIve HIV-1 Stram IsoIated tiom a Heahhy Gabonese h&viduaI Rescnting an AtypicaI Western Blot. AIDS 1989, 3:707-716.
DAARES, Lr KL, MOUIIGILT, HO DD: High Concenttations of RecombInaut Soluble CD4 ate RequIced to Neuttahze Pthuaty Humau Immunodeficiency Virus Type 1 IsoIates. Proc Natl Acud .%i USA 1990, 8765746578. Higher concenttations of soluble CD4 ate requited to neutmI& HIV 1 in vivo as opposed to ceR-cuitute-adapted HIV-1 isoiates. ThIs is a plausible teason why soiuble CD4 does not appear to work in viva However, otber factors may aiso be important. 4. ..
5.
MCKEATING JA, GOW J, Goucshut J, PEARL UI, MULDER C, WEE% RA ChatactetIzatIon of HIV-1 Neuttahzation Escape Mutauts. AIDS 1989, 3777-784.
6.
BERKOWER 1, Shmx GE, Gnu C, MIJRPHY D: Human Immunod&cIency Virus 1. Pmdomînance of a Group-Specific Neu-
F, G~~RTIER I, AYRESL, Avuxz F, GARCIA-BENITO A, Dw DENISF, LEONARD G, RANGER S, GIIOBP, JOILER-JEMEIXA H, I-IESS G, SE~L S, PUICICE H, SIMONF, BRUI+VEZINET F, SONDAGD, ~A,HAMPLH,SCHOENR,S~RS,TROONENH:MU~~~centet &&ation of a New Recombinant Enzpne Immunoassay fof the Combhmd DetectIon of AntIbody to IUV-1 and HIV-2. AIDS 15’90, 4:131-138. An ELISAthat detects both HIV-1 and HIV2 is described. 13. .
14. ..
FRO~NER t%, DOBIERG, VON SONNENBURG FJF: Cost ReductIon of Unhnked Test@ fat Anti-HIV hy InvestI8atIon of Pooled Sera. AWS 1990, 4:73-76. An important method to teduce tbe costs of large-scaie scteeuin8 for HN-1 is desctibed which Invoives screening pooled sets 15. .
PECKHAM CS, TEDDERRS, BRIGGSM, ADESAR, HJE~MM, WII~OX AH, PARRO-MEJL~ N, O’CONNERC: PrevaIeuce of Matemal HIV
fnfection Based on Unlimited Anonymous Testing of Newbom Babies. Luncet 1990, 335:516-518. A cheap screening assay for dried and blotted blood obtained from a fïnger prick appears sensitive and specific. 16. .
HUNTJC, JOHNSON-PAEPKE J, BOARDWAY K, GUTIERREZ R, HAMEL H,ALLENR,HEYENC,DESAIS,CASEYJ,~BYIETAL:D~S-
ctiminationEetween HIV-1 and HlV-2-Sempositive Individuais IJsin Mouse Mnnoclonal Andbodies Dimcted to HlV 1990, Transmembrane F’toteins.AIDS Res Hum Retnwiruw 6:883+98. A specific ELISA usinp mouse monoclonal antibodies as opposed to peptides. 17.
JIICKSON JB, MACDONAIDKL, CAD~EU.J, SUWAN C, KUNEWE, HANGENM, SANNERUD KJ, STRAMER SI, FUDESNJ, KWOK SY ET AL: Absente of HlV Infection in Bbod Donors with
Indetetminate Western Blot Tests for Antibody to HlV-1. N Eng1 J Med 1990, 322~217-222. 18.
KAKAIyo R, Ducos J ET AL: Intetpteting Western Blot Indetetminate HIV Antibody Tests. Lancet 1990,335:236.
miEzing Epitope That Petsists Despite Genetic Variation.J EA$)Af&? 1989, 170:x381-1696.
19.
COLJROUCE A-M: FoRowinE up IndetetminateHlV-1 Western Blots. Lamet 1989, 2:1330-1331.
MCNEARNFT T, WESTERVELT P, THIEIANBJ, TROWBRIDGE DB, GARCIAJ, WHITIIERR, RATNERL Limited Sequence Heterogeneity Atnon [email protected] Distinct Hwnan Immunodeficiency Vii ‘fjpe 1 Isnlates Bom Individuals Involved in a Clustered Infectieus Outbteak. Proc Nat1 Acad Sci USA 1990, 87:1917-1921. Sequenced isolates from a clustered outbreak are more similar than previously suspected.
20.
CARUSOBMT, DORKZI RM, TAGUAROF, G~VARINAM, FERRO & Dusr S, Bu\sIou B, TIUDENTE G: Rapid Von Be-
7. .
8.
CHENG-MAYER C, WEISSC, SETO D, Lar JA Isofates of Human ImmunodeficiencyVii ‘fj~xz 1 ftom the Brain May Constitute a Special Grnup of the AIDS Vii. Pnx Nat1 Acad Sci USA 1989, 86:8575-8579.
9.
JONGJJ, WOIPS T, VAN DERHOEK L, Shn~ I, ZWARTG, DEZ
.
RONDE4 TERSMETIX M, NARAP, Gourxha
tween HIV-1 and HIV-2 Infection. Lancet 1989, 2:1156-1157. 21.
22.
DROTMAN DP, VALDISERRI RO: Are Human ImmunodeEciency Virus Test Repntts Clear to Clinicians?J4MA 1989,262:3465.
23.
A, NO~ATIR, MARCHISIO P, ZANC~A N, D’ARMINIO-MONFORTE UBERTI-FOPPA C, TORNAG~ R, M&zIRONIE, L&z%w~ A, PRINCIPI N: EarIy D@nctsIs of HlV Infection in Mams. AIDS 1989,
24.
Ro~~I P, MOSCHESEV, BROLIDENPA, FUNDAROC, Q~NII 1, PLEBANI A, GL%QUINTO C, Tovo PA, I$JNGGRENK, ROS~NJ, WIGZu. H, JONDALM, WAHRENB: Pfesence of Matemal Anti-
5391-396.
DE
J: Predommsnce
WOUN%X SM, Rmx~0 CR, KWOK S, SNINSKYJJ, GUPTA P, IMAGAWA D, FARZADEGAN H, JACOB~ONLP, GRO~TTKs, IEE MH, CHMJEL JS, GINZEZURG H, KASLOWRA, m JP: Human Im-
munndeficiencyVims Type 1 @BV-1) Infection a Median
BENENSONAS, PEDDECORDKM, HOPHERRIK, .AXHER MS, TA~IORRN, HEARNTL RepottinE the Results of Human Rn-
munodeficiency Vims Testing. JXMX 1989, 26234353438.
of HIV-1 Semtype Distinct from JAV-l/HTLV-IIIB. Lancet 1990, 335:474. The laboratoty MN/SC Mate is more representative of clinical isolates than ether weU known laboratory ‘brands’. 10.
’
HIV Suttke Glycoprotein gp41 in the Detection of Early Antibodies. AIDS 1990,4:8386.
reading
Papets of special interest, published within the annual period of review, havebeenhi&lightedaS: . of interest .. of outstanding interest
3.
545
bodies to Human ImmunadeficiencyVims 1 Envelope Glycoprotein gp120 Epitopes Cotrelates with the Uninfected Status of Chikken Bom to Sempositive Mothers. Proc Natl Acad sci USA 1990,8680554058. 25.
SCHECHTER MT, CRAIBKJP, IE TN, MONTANER JSG, DOUGIASB, SESTAKP, WILUXJGHEIy B, O’SHAUGHNESSY MV: hSce@bllltg
??
546
Immunodeficiency to AIDS FVogredon
Appears Early
in
IUV Infection. AIDS
1990, 4185190. 26.
HOFMANNB, Ww Y, CUMBEIUAND WG, D~LS R, BOZORGMEHRI M, FAHEYfi Serum ~z-Microglobulin L.evel Inin HIV Infectioa Rektion to Seraconkrsion, CD4 =FaU aad proenosis. AIDS 195’0, 4:207-234.
27.
G,BowM,TIhwsA, -A,mCqEmmJ,JKERNOPPPB: Predicdoa of Progwssion to AIDS by Anmis of CD4 Lymphocyte Courts in a Haemophilic Cohort. AIDS 1990 5737-741.
28
CIERICI M, STocKs NI, @Ac Q RN, Imm DR, Vw CS, SHWRERGM: Detection of 3 Distinct Patterns of T-Helper Cell Dysfunction in Asymptomatic, Human Immunode6ciency Virus-Seropositive Patients Independence of CDk+ Cell Numkrs and Clinical Staging. J Clin Invest 1939, 84X492-1899.
29.
DE Wow F, Gou~swr J, AD: Impairment of In V&o Immune Responses Occurs Witbin 3 Months Afte.r HlV-1 Seroconvetsion. AIDS 190 47741.
TEJ~~EN VJ, SIEBE~ UYIQEHMG FG, OsIEaus
30.
!?+XElIEKENs PT, Roos MT, DE WOIP F, IANGEJM, -Ehu F: LowT-CcllResponsivenesstoActivadonviaCD3/TCRis a -lastic Marker for Acquired Immun&&cY %Qdrome (AIDS) in Human Immunode6ciency Virus 1 (HIVl)-Infkcted Men. J Clfn Immurwl1!990, 10:121-127.
K~LOW RA, DUQVESN~U R, VANRADENM, Krísss~~ L m M, FRIEDMAN H, Su S, SAAHAJ, D~IS R, PHAIRJ ET AL: Al, Cw7, B8, DR3 HLA Antigen Combination Associated With Rapid Decliae of T-He@r Lymphocytea in HlV-1 InfectionA Report kom the Multicenter AIDS Cohort Study. Lancet 1990, 335927-930. that the rate of progression to AJDS is dependent A fwthur obsedon on MHC haplotypes. 31. .
32.
ZARUNGJM, IEDBETIERJA, Svu J, Fu~rz P, EICHBERGJ, GJEF%T G, MOW PA: HIV-Infected Humans, But Not ChimpnZees, Have Ckuhting Cytotoxic T Lymphocytes That Lyse Uninfected CD4+ Ce&. J Immund 1990, 144:2992-2998.
KH,
AG Dalgieish, Royl London Hospital, Department of Virology, Turner Streef, Iondon El 2AB, UK.
Hospita1 Medical
College,
London,
UK
The past year has seen the discovery of new isolates which do not readily fit into the established categories HIV-1, HIV-2 and SIV, as well as the availability of new assays including the use of the polymerase chain reaction. New combination enzyme-linked immunosorbant assays can detect viruses from two or more groups where specific enzyme-linked immunosorbent assays have been developed using peptides and mono-
clonal antibodies.
Current Opinion in Immunology
Introduction In order to develop effective thempeutic and vaccine programs it is clearly imperative to dehne the target. This involves the careful characterization of different isolates that may possess markedly different functional properties with regards to their ability to evade existing therapies and immune responses which are mainly against the ‘wild’or prototype strains. Furthermore, it is important to know the enemy and its habits. This means that testsshould in theory be 100% speciflc and 100% sensiWe, and that negatlve results a certain time after potential infection ,should be meaningf~l. In this review of some of the papers published during the past year, it will be seen that considerable progress has been made in achieving many of these aims.
Types’ of HIV ‘Types’lnvolves the broad groupings of human immunodeficiency virus (HIV) isolates as well as those isolates that display subtle functional differences such as neutralization of escape mutants. Tivo broader groups are widely accepted i.e. HlV-1 and HIV-2. There are now several reports of isolates which are claimed to be signifïcantly divergent from wild-type HlV-1 and HN-2. It is of note that these isolates may be identilìed because they gW indeterminable results on ‘srandard’ HlV-1 and HIV-2 essays. Dietrich and colleagues [le] report a highly divergent HIV-2 isolate that appears to be closer to simian immuno-
deficiency (SIV) than previous isolates. It is of interest that these isolates have no cytopathic e6xr.s on lymphocytes and that the host from which tbe virus came is unaffected. Leys et UI![2*] have reported a distinct isolate obtained from two people from west central Africa. However, these
1991, 3:543-546
isolates are closer to HlV-1 than I-IIV-2 but differ at nearly every leve1 of analysis. Huet et al. [3] have reported a highly defective HIV-1 strain from a healthy Gabonese patlent that may be detected on an atypical western blot. This isolate is in fact a ‘string’ of HIV-1s that are closer to European than Afrlcan isolates. Sequencing of the tut gene showed that all the proviruses are defective and it is remarkable that no antibodies to the envelope were detected, virtually no cytopathic effect.5 were seen in vitm and the patient remained healthy throughout the study. Tbe authors conclude that HIV isolates represent a population of variable isolates. It is becoming increasingly clear that isolates with different cell tropisms and biological properties may exist concurrently in the patient as well as arise throughout the period of infection. Of particular interest, especlaUy in view of the disappointing results achieved in soluble CD4 clinical trials, is the report by Daar et al [4**] that higher concentrations of soluble CD4 are required to neutralize primary clinical HIV isolates compared with classic laboratory isolates. The demonstration of neutralization escape mutants in vitm [5], suggests that proviruses may be selected for thelr capacity to evade neutmlizing antibodies and that single amino acid changes may bring about markedly different neutmlizmg properties. However, demonstra’tion of an effect in vitro does not always mean that it is representative of a major interaction in vivo (as with soluble CD4). Berkower et al. [6], using a sensitive neutralizing plaque assay, have shown that most patients have neutralizing activity that is predominantly to a conserved epitope shared among three otherwise highly divergent isolates and rarely mutates. The authors optimistically argue that such a response bodes well for the development of a vaccine. The pressures for a virus to mutate or for a particular mutation to be selected for in vivo are still not clear. Tissue tropism certainly plays an important role.
Abbrewiations EUSA-enzyme-linked
immunosorbent
assay; HIV-human immunodeficiency virus; PCR-polymerase SlVsimian immunodeficiency virus.
@ Current Biology Ltd ISSN 0952-7915 D
chain reaction;
543
544
Immunodeficiency
McNeameyet al [7*] showed that isolates from a clustered isolate outbreak were all remarkablysimilarover a 3-year period ( > 0.01% differente at the amino acid level) and that isolates from the brain and lung were more than 30 times more likelyto grow in monoq& than they were in lymphocytes.However, no envelope sequence differences between lung, brain and blood isolates were noted, in contrast to previous reports that had noted single amino acid changes in envelope proteins that exhibit differentcell tropisms.The diiference between brain tropism isolates and lymphotropic isolates has been reported by Cheng-Mayer[8] who also noted differentcell tropismfor paired isolates from the same patient.The authors suggestthat their findingsargue for a different subgroup for central nervous system (CNS) viruses.The im portance of a relevantgold standard for testing purposes bas been suggested by Zwart et al. [9*] who noted that the MN/SCisolate is much more representativeof clinical isolates with regards to eliciting neutralizingantibodies, and should be used in vaccine and similar studies. These studies show the diversityof HIV isolates and HIv populationsand the lack of consensus amongstworkers with regards to the relevante of different isolates in the development of HIv infection and pathogenesis of disease. Clearly, further studies require carelùl and openminded interpretation.
Testing’ of HIV
for I-IIV-1and HIV-2 using one kit is now a possibility,if costs allow. This latter consideration prompted Frösner et al [ 14**] to test the potentlal loss of sensitivitywhen testing pooled sera for public health surveillancesystems in low presalence population groups. A savingof about ninefold in the costs of testing and up to 50% of technicians time is reported. A new cheap screen@ system using iinger prick blood from antenatal clinics has been reported by Peckham et al. [ 15*]. Discriminationbetween HIV-1 and HIV-2 remains an important task espe&llywhen up to 70% of HIV-2seen may be detectable on HIV-1 EIISAs. Hunt et al. [I6*] have used mouse monoclonal antibodies to transmembrane proteins instead of peptide based assaysused earlier.The western blot still remains a popular assayfor distinguishing the two types of I-IIV. In this regard Jackson et UI! [17] examined the relevante of indeterminate reactions that do not meet the criteria for HIV-1 or HIV-2 positivity and conclude that such blots rarely, if ever, indicate HIVinfection in low-riskpopulations.They can however, alert one to a new virus isolate (HIV-2+ > as noted earlier. There are severalother reports, and comments upon the reports, of indeterminate blots and type discrimination [ 18,19,20]. These clinicianswho have had problems interpreting noted HIV results are recommended to .the articles by Benenson [ 211 and Drotman [ 221. A major testing problem is to determine whether or not babies bom to HIV-infectedmothers are infected, as they will cany maternal antibodies to the virus. An IgM response is present at birth or appears within the third month. The presence of p24 antigen is of ominous significance [23] and PCR has obvious implicationsin this group of patients. In another studythe prevalente of maternal antibodies to I-IIV-1gp120 was found to correlate with the outcome of children bom to I-IIV-infectedmothers. Sera collected from children at risk, who showed high levels of matemal antibodies to certain epitopes of. gp120, were less likely to be HIV infected than children without rnaternalantibodies to these epitopes [ 241. ‘T’his has important implications for the management of HIVpositlve mothers, if coniïrmed in other .studies.
HIV testing is required to establish the prevalente of the virus within a population, the iûnctional characteristics of the virus as well as delining markers and parameters that have prognostic value. The latter is particularlyimportantwith regardsto therapeutic trials.Most new studies address at least one of these issues. Detection of HIV may be limited by delayed seroconversion, or the presence of indetenninate assays.Wolinsky et al [lO] report the use of polymerase chain reaction (PCR) to investigate delayedseroconversion,and note that PCRidentilìed HIV in 20 of 24 HIV-1positive men prior to evidente of serological seroconversion. There could be a time interval of as long as 42 months (with a median of 18 months) before the appearance of a diagnostic western blot. Previous reports by Ranki et al [ 111 noted a long seroconversion time, judged by the presence of nef antibodies that were detectable before standard enqme-linked immunosor~t assays (EIISAs) read positive. This study would appear to coniïrm the existente of long seroconversionand the usemlness of PCR in the detection of HIV infection in high risk individuals.Cumming et ai! [12] have tried to increase the sensitivityof the ELISAsystem by using a short peptide from gp41 which is detected slightlyearlier than the Abbot envacore assays.
A further feature of HIVinfection is the suspicion that the genetic type of the individualmight be as important or more important than the type or isolate of HIV. In support of this is the increasing association of certain haplotypeswith rapid progression to AIDS,noted previously and which receivesfurther support from Kaslowand colleagues [31*].
The sensitivityand speciIí&y of testing for HIVl and HIV-2 was sought in a large multicentre trial which showed that a recombinantHIV-1and HIV-2ELISAwas as sensitiveas a single I-IIV-1ELISAfor I-IIV-1and detected 99% of the I-IIV-2samples [ 13.1. Thus routine screening
The immune response must surely be one of the most misunderstoodcomponents in AIDS reseamh. Major dlfferences between chimpanzees and man with regards to why the latter, but not the former, have circulatingcytotoxic T lymphocytesthat lyse uninfected CD4+ cells [ 321
The remaining need for testing is to delìne‘prognostic factors and there are numerous papers addressing thls issue. It is becoming increasinglyclear that susceptibility to AIDS infection appears early in disease, is closely associated with CD4 kevelsand & microglobulin levels k5-301.
HIV types and testing Dalgleish have yet to be explained. This and rnany other similar
obsemations not readiíy explained by our current understanding of the immune response to HIV suggests that in future our attention, types and testing should also focus on testing human immune types.
References and recommended
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