Immunology Today, vol. 8, Nos 7and8, 1987
References 1 Dexter, T.M., Allen, T.D. and Lajtha, L.G. (1977)J. Cell. Physio191, 335-344 2 Dorshkind, K. and Phillips, R.A. (1983) J. Immunol. 131,2240-2245 3 Whitlock, C. and Witte, O. (1982) Proc Natl Acad. Sci. USA 79, 3608-3612 4 Denis, K.A., Treiman, L.J., St. Claire, J.I. and Witte, O.N. (1984) J. Exp. Med. 160, 1087-1101 5 Dorshkind, K., Denis, K.A. and Witte,
O.N. (1986)J. Immunol. 137, 3457-3463 6 Ham, A.W. and Cormack, D.H. (1979) Histology (Eighth Edition), J.B. Lippincott 7 Dick, J.E., Magli, MC., Huszar, D.E. et al. (1985) Ce1142,71-79 8 Keller, G., Paige, C., Gilboa, E. and Wagner, E.F.(1985) Nature 318, 149-154 9 Collins, L. and Dorshkind, K. (1987) J. ImmunoL 138. 1082-1087
Since the human immunodeficiency virus (HIV) started its spread through the human population, the AIDS epidemic has steadily increased on all con~inent~ unchecked by any therapeutic or preventive intervention, except for education. The search for an AIDS cure is facing great difficulties as HIV, a human retrovirus, is able to integrate and remain latent in the human genome for years. An effective vaccine thus remains the only foreseeable way to control and eventually eradicate AIDS. The unique pathogenicity and variability of HIV have raised new challenges in vaccine design, testing and evaluation. The status of the intense research efforts undertaken to solve these problems were assessedat a recent workshop on the AIDS vaccines*.
Towardsan AIDSvaccine:challenges and prospects
Different approaches to AIDS vaccines were presented but none has so far been shnwn tn c n n f p r t n r n plete protection in the only infectable animal, the chimpanzee. However, critical elements of a safe and effective vaccine development, evaluation and trial program were recognized and are summarized below. Candidate vaccines
10 Jyonouchi, H., Kimmel, M.D., Lee, G.
etal. (1985)J. Immunol. 135,
1891-1899 11 Landreth, K.S., Englehard, D., Beare, M. et al. (1985) J. Immunol. 134, 2304-2309 12 Song,Z.X., Shadduck, R.K., Innes, DJ. eta/. (1985)Blood66, 273-281 13 Palacios, R., Henson, G., Steinmetz, M. and McKearn, J. (1984)Nature 309, 126-134
fromJacquesHomsy,Kathe Steimerand RichardKaslow leagues at Genentech and G. Smith
et al. at Microgenesys, respectively. While rec gp130 glycosylation approaches that of native gp120, rec gp160 is glycosylated in a partial and different way. This latter structure may expose hidden neutralization epitopes of the envelope while conserving its conformational properties. Nonglycosylated envelope subunits may also uncover epitopes important for neutralization. Thus, PB1, a 180 amino-acid peptide of the C-terminal half of gp120 produced in E. coil by S. Putney et al. at Reoliaen as well as a synthetic n~ntide (pe pfi a e 73 5) -clerived f-r0mr-tl~e transmembrane portion of the HIV envelope gene (gp41) by G. Drees-
man and collaborators, (Southwest Foundation for Biomedical Research) are also being tested as potential vaccine products. Another approach, taken by A. Goldstein and colleagues at George Washington University, has been to synthesize and test a peptide derived from a 17kDa protein (p17) of HIV. This protein called HGP-30 was initially thought to be part of the viral core, but it has recently been located immediately underneath the viral envelope °,~---4,umay in fact protrude to the outside of the virion. The advantane nf thi~ ~nnrn~rh i~ that Hnlik~
env lop;:de; v;d-antigens'-(se;"
low), p17 is highly conserved among HIV variants.
Table 1. Portionof the HIVenvelopegene representedby variouscandidatevaccinesfor AIDS
5'
Purifiedviralantigens The external portion of the HIV envelope glycoprotein, gp120, is a major target for both antibodymediated neutralization and cytotoxic immunity; it is therefore considered a prime candidate for an AIDS vaccine (Tables 1 and 2). A recombinant form of gp120 (rec gp130) and the whole HIV envelope gene product (rec gp160) have been expressed in mammalian and insect cell systems by L. Lasky and col-
gp120 External
I
gp41
3'
HlVenvgene
Transmembrane gp 120 Recgp130 Recgp160 PB1 Pept735 ?
HGP-30 Recvaccinia
*Sponsoredby the US PublicHealthServiceAIDS Task Forceand heldat the NationalInstitutesof Health,Bethesda,Maryland,25-27March,1987. (~ 1987,ElsevierPublications,Cambridge 0167 49191871502.00
Anti-id 193
ImmunologyToday,voL 8, NOS7andS, 1987
;C
L/revacones The viral envelope has been inserted into live virus vectors in the hope of improving its immunogenicity. Live vectors can mimic the presentation of HIV antigens through replication in the host, while avoiding the risk inherent to a live attenuated HIV vaccine. Both recombinant vaccinia virus, expressing either the envelope (gp160) or the gag (p25) gene products of HIV (B. Moss, NIH; G. Todaro, Oncogen; and M. Girard, Pasteur Vaccins), and recombinant adenovirus (A. Davis, Wyeth Labs) a~'e being studied in various animals, induding chimpanzees. A similar vaccinia vaccine expressing gp160 is being tested in humans in Africa by D. Zagury et aL from the Pierre et Marie Curie University in Paris. Ani/4d/oiyp/crace/he A vaccine based on molecular mimicry is being attempted by R. Kennedy et al. at the Southwest Foundation, using anti-idiotypic antibodies to either the HIV envelope or
to its natural receptor on T helper munogen. Therefore, any vaccine cells, the CD4 molecule. An anti- based on the HIV envelope will more idiotypic vaccine would be able to than likely have to include multiple present conformationally correct epi- subunits representative of the HIV topes with neither the risks inherent variants necessary to confer crosswith killed virus vaccines nor the loss reactive immunity. To identify and of immunogenicity that may be reduce the number of isolates that associated with synthetic or recom- will have to be used, the serotyping of the different HIV isolates must binant products. become an essential and urgent task and is in progress in several laborEvaluation All the candidate vaccines de- atories. Cell-mediated immunity (CMI) is a scribed are immunogenic and most have elicited the formation of neut- well-documented first line response ralizing antibodies in small animals, to many viral infections, but it is a monkeys or chimpanzees (Table 2~. complex process tostudy. As a reThe neutralization titers obtained sult, few CMI data on candidate vary from 1 : 10 to 1 : 100 except vaccines are complete to date. P. for PB1 that was reported to have Fishinger (NIH) reported preliminary elicited titers as high as 1 : 1000 in evidence that antibodies raised goats. It is possible that given ad- against native gp120 in chimpanzees ditional boosts and appropriate ad- failed to mediate antibody-depenjuvants the neutralizing titers in- dent cell-mediated cytotoxicity. In duced by other test vaccines will contrast, recombinant vaccinia virus increase as well. However, all neutra- expressing gp160 induces a strong lizing responses obtained thus far polyspecific CMI in rhesus monkeys appear type-specific for the HIV as well as in chimpanzees, suggeststrain used to produce the ira- ing that antigen presentation may
Table2.. Predinicalevaluationof candidatevaccines
194
CMI
Immunogen
Expression system/yield
Glycosylation
Advantages
Questions/ disadvantages
Neutralization titers/antibody source
gp 120
HIV-produdng celllines;low yield
Full
Native
Pooryield;toxic effecton CD4 cells?
1:100 Smallanimals Chimpanzee
Recgp130
Mammalian cells; highyield
Full,some differences
Native-like
Toxiceffecton CD4cells?
1:50 Smallanimals Chimpanzee
ND*
Recgp 160
Insectcells;high yield
Partial,different
Wholeenvelope, mayexpose hiddenepitopes
Different glycosylation
ND
ND
PB1
E co~i; highyield
None
Doesnot block fusion
1"1000 Goat
ND
Restricted specificity?
1:32 Chimpanzee
Mayexpose hiddenepitopes, do not bindCD4
ND
Pept735
Synthetic,gp41 region;highyield
None
HGP-30
Synthetic;high yield
None
Highlyconserved antigen
Doesnot block fusion?
1100 Smallanimals
Vacdniagp 160
Vacciniavirus; highyield
Full
Improvedantigen Liveviruspresentation associatedsideeffects
1:165 Rhesus Chimpanzee
Anti-idCD4
Immunized rabbit Molecular mimicry
Correct conformation
1:10 Smallanimals Baboon
* ND: notdone.
Restricted specificity?No experiencein humans
ND
ND
Immunology Today, vol. 8, Nos 7 and 8, 1987 .
......
dfe
t
res l |
influence this type of response. Besides immunogenicity, toxicity is an important variable to consider before clinical use. There is evidence that the binding of gp120 to CD4 may in itself damage lymphocytes. In addition, live vaccinia virus bearing the HIV envelope may induce fusion of lymphocytes in vivo. However, unlike glycosylated antigens, PB1 peptide 735 and HGP-30 do not bind the CD4 molecule on helper cells. Whether these effects will be of significance for vaccine testing will have to be assessed in the early phases of clinical trials. Central to the question of vaccine evaluation is the issue of imrnunoprotection. Should an AIDS vaccine protect against infection or only prevent disease? Although opinions vary on this matter, there is a strong tendency to aim at the former goal due to high mortality caused by HIV. Two candidate vaccines, gp160vaccinia and peptide 735, have been tested in chimpanzees for their ability to block HIV infection in vivo; they both failed to protect the animals upon challenge. However, as many as 300000 HIV infectious doses (TCID-50) were used in the vaccinia challenge, a dose hardly representative of the clinical situation, and the experiment needs to be repeated. Yet, the number of chimpanzees available for AIDS vaccine testing is rapidly dwindling. Several investigators argued at the meeting for more extensive use of the rhesus model to study simian immunodeficiency virus vaccines in order to reserve the more scarce chimpanzees for the final pre-dinicai tests of HIV vaccines. Meanwhile, other researchers are increasingly questioning the validity of the chimpanzee model for human AIDS research. Some, in fact, deem animal testing altogether unlikely to reflect the human model and have already applied for phase I clinical trials. In any case, when all the available pre-clinical data are put together, it will be necessary to evaluate the different vaccines for further testing in humans. Currently, methods to assess neutralization titers, CMI, cytopathology and so on are almost as diverse as the laboratories using them and no consensus about standardizing these assays was reached during the workshop. Standardization is particularly important if chimpanzee trials are to be bypassed as the urgency of the epidemic may require.
Table 3. Social-epidemiologicalfactorsin the choiceof targetgroupsfor AIDSvaccinetrialsin the USAthat can be favourable,neutralor unfavourableto the evaluationof vaccineefficiency
Gay IVDU Prostitutes Std P-sp/std P-sp/hemo Prisoners Military
Avail.
Risk
F/up
F F F F F U N F
F F F U F F U? N
F U N U N F F (F)?
Re:r/ Compl. F U N N U? F F F
Educ.
Uab.
U F N N N F F N
N U? U? N N N U* F
f
I
Abbreviationsandsymbols:
Avail.: Risk: F/up: Recr/Compl.: Educ.: Liab.:
availabilityinnumbers exposureto HN accessibilityforfoll0w-upstudies recruitment/compliance influenceofeducationoninfectionrisk potentialliabl!ityproblemsinvolved
IVDU: Std: P-splstd: P-sp/hemo:
intravenousdrugusers sexuallytransmitteddiseasepatients partnersorspousesof stdpatients partnersorspousesof hemophiliacs
F: U: N:
favorablefactorforvaccinetrial unfavorable neutral
*Exceptionsto proscribedresearchcanbemade.
Design of dinkal trials In the USA, three clinical phases are part of any vaccine trial. For the AIDS vaccine tria!s, six vaccine evaluation units have already been selected by the National Institute of Allergy and Infectious Diseases. They are: Baylor College of Medicine in Houston, Vanderbilt University pediatric unit in Tennessee, Rochester University in New York, Marshall University, John Hopkins University and the University of Maryland. In phase I of a clinical trial, the safety and immunogenicity of ~andidate vaccines are tested on 10-20 individuals. Phase II has the same objectives but is carried out on a larger sample (40-60 individuals). The vaccine efficacy is tested in phase III on a representative population sample whose size will depend on the prevailing incidence of the disease and the confidence levels sought for significant protection from infection. In the case of AIDS, all phases are likely to include control groups in a double-blind fashion to avoid 'false hope' ~-~fects on the participants. Rather than placebo, it was suggested that other known vaccines be used as controls in order to account for unexpected side effects that may be encountered in the particular
population tested but not specifically due to the AIDS vaccine tested. In evaluating human trials, the distinction between seroconversions due to vaccine inoculation and due to actual infection will have to be made. Apparently, screening for antibodies to HIV antigens not included in a vaccine should answer this question. However, infected individuals do not always make antibodies to all vi~al proteins. It may therefore take =everal additional criteria such as early antigenemia or nucleic acid probing, if available, to make such distinction. This difficulty underlies the necessity of using low risk individuals for phase I trials, which in turn will require an extended follow-up period. The information gained from these trials should then permit the enrollment of high risk individuals for phase II and III trials where immunogenicity and efficacy will be the main variables under study.
Target groupsand ethics In phase Ill clinical trials, candidate vaccines will be tested on a population sample large enough to allow a valid assessment of the vaccine efficacy. The sample size will depend on the level of significance sought for the results, and on the popula-
195
Immunoloqy Today,voL 8, Nos 7and8, 1987
t
F
tion availability, reliability and rate of infection by HIV. These parameters will in turn determine the choice of the target population among the various groups at risk for AIDS (Table
3). The homosexua; risk group has proven extremely cooperative in other AIDS studies, but may have an incidence of infection too low for vaccine testing as a result of its high response to prevention and education efforts. In contrast, partners or spouses of hemophiliacs, who appear to have followed safe sex practice advice poorly, continue to be at high risk of infection by HIV and could be a very favorable target group. However, they may be too few in numbers to constitute a valid group for phase III trials, and wider publicity on prevention may rapidly change their behavior. None of these objections applies to intravenous ~.v.) drug abusers who by now are spreading HIV faster than any other group and respond very slowly to education. Yet i.v. drug abusers may bf. relatively less compliant and tell-
able for follow-up, a significant problem in a trial that may last several years. Alternatively, the military could be a suitable target population, since AIDS has affected thus far primarily young adult males. However, HIV seropositive individuals are no longer enrolled in the US military and the Defense Department is undertaking an intense education campaign. These factors are likely to reduce greatly the incidence of HIV infection in this group. J. Curran (Centers for Disease Control, Atlanta) suggested that the military in some African countries may constitute an appropriate target group since it would be composed of career professionals living in confined quarters, probably with a high HIV incidence rate. This choice, however, is complicated by the fact that the different HIV strains occurring in Africa may confuse the interpretation of results. The technical difficulties in choosing an adequate target group for vaccine trials are compounded by ethical and legal issues. For example,
INDEX TO ADVERTISERS Name of Advertiser Page Number A m g e n Biologicals ................................... Back Cover Applied
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intravenous drug abusers and prostitutes, two potential high-risk groups, are engaged in illegal activities, which could interfere with follow-up. There is a moral conflict in enrolling preferentially individuals unlikely to respond to education in order to ensure a constant rate of infection throughout the study. And it would be difficult to enroll prisoners in vaccine trials unless the stringent proscriptions on medical research involving this group are waived. Solutions to these problems will have to be found soon for the urgency of the AIDS pandemic will not allow any delay when a vaccine is ready for testing in humans. The Food and Drug Administration is already reviewing applications for phase I trials for a number of candidate vaccines. In the absence of any other means of controlling HIV spread, these requests are likely to be granted. However, the biggest task ahead remains overcoming the extensive genomic variation in HIV. Several variants have been identified already and many more may exist. Even if a finite number of HIV serotypes are eventually identified, their worldwide distribution, the potential for continuing emergence of new HIV variants (e.g. HIV-2 in West Africa), the specific population groups affected by the disease, the differences in disease manifestations as well as the ethical and liability problems discussed above imply that AIDS vaccine trials will require massive resources and extensive coordination at both national and international levels. One can only hope for a limited number of virulent serotypes or an effective vaccine using a conserved epitope. In either case, in the words of Assistant Secretary of Health Robert E. Windora, it will be several years until an AIDS vaccine reaches the market place. we wish to thank C. Cheng-Mayer,G. Thompson-Honnebier and J. A. Levy for their helpful comments and critical review of the manuscript.
Jacques Horr;sy is at the Cancer Research Institute, S-1280, Schoolof Medicine, University of California, San Francisco, California 94143, USA; Kathelyn Steimeris with Chiron Corporation, Emeryville, California 94608, USA; and Richard Kaslow is at the National Insitutesof Health, WestwoodBuilding, Room 739, Bethesda,Maryland 20892, USA.
References 1 Dexter, T.M., Allen, T.D. and Lajtha, L.G. (1977)J. Cell. Physio191, 335-344 2 Dorshkind, K. and Phillips, R.A. (1983) J. Immunol. 131,2240-2245 3 Whitlock, C. and Witte, O. (1982) Proc Natl Acad. Sci. USA 79, 3608-3612 4 Denis, K.A., Treiman, L.J., St. Claire, J.I. and Witte, O.N. (1984) J. Exp. Med. 160, 1087-1101 5 Dorshkind, K., Denis, K.A. and Witte,
O.N. (1986)J. Immunol. 137, 3457-3463 6 Ham, A.W. and Cormack, D.H. (1979) Histology (Eighth Edition), J.B. Lippincott 7 Dick, J.E., Magli, MC., Huszar, D.E. et al. (1985) Ce1142,71-79 8 Keller, G., Paige, C., Gilboa, E. and Wagner, E.F.(1985) Nature 318, 149-154 9 Collins, L. and Dorshkind, K. (1987) J. ImmunoL 138. 1082-1087
Since the human immunodeficiency virus (HIV) started its spread through the human population, the AIDS epidemic has steadily increased on all con~inent~ unchecked by any therapeutic or preventive intervention, except for education. The search for an AIDS cure is facing great difficulties as HIV, a human retrovirus, is able to integrate and remain latent in the human genome for years. An effective vaccine thus remains the only foreseeable way to control and eventually eradicate AIDS. The unique pathogenicity and variability of HIV have raised new challenges in vaccine design, testing and evaluation. The status of the intense research efforts undertaken to solve these problems were assessedat a recent workshop on the AIDS vaccines*.
Towardsan AIDSvaccine:challenges and prospects
Different approaches to AIDS vaccines were presented but none has so far been shnwn tn c n n f p r t n r n plete protection in the only infectable animal, the chimpanzee. However, critical elements of a safe and effective vaccine development, evaluation and trial program were recognized and are summarized below. Candidate vaccines
10 Jyonouchi, H., Kimmel, M.D., Lee, G.
etal. (1985)J. Immunol. 135,
1891-1899 11 Landreth, K.S., Englehard, D., Beare, M. et al. (1985) J. Immunol. 134, 2304-2309 12 Song,Z.X., Shadduck, R.K., Innes, DJ. eta/. (1985)Blood66, 273-281 13 Palacios, R., Henson, G., Steinmetz, M. and McKearn, J. (1984)Nature 309, 126-134
fromJacquesHomsy,Kathe Steimerand RichardKaslow leagues at Genentech and G. Smith
et al. at Microgenesys, respectively. While rec gp130 glycosylation approaches that of native gp120, rec gp160 is glycosylated in a partial and different way. This latter structure may expose hidden neutralization epitopes of the envelope while conserving its conformational properties. Nonglycosylated envelope subunits may also uncover epitopes important for neutralization. Thus, PB1, a 180 amino-acid peptide of the C-terminal half of gp120 produced in E. coil by S. Putney et al. at Reoliaen as well as a synthetic n~ntide (pe pfi a e 73 5) -clerived f-r0mr-tl~e transmembrane portion of the HIV envelope gene (gp41) by G. Drees-
man and collaborators, (Southwest Foundation for Biomedical Research) are also being tested as potential vaccine products. Another approach, taken by A. Goldstein and colleagues at George Washington University, has been to synthesize and test a peptide derived from a 17kDa protein (p17) of HIV. This protein called HGP-30 was initially thought to be part of the viral core, but it has recently been located immediately underneath the viral envelope °,~---4,umay in fact protrude to the outside of the virion. The advantane nf thi~ ~nnrn~rh i~ that Hnlik~
env lop;:de; v;d-antigens'-(se;"
low), p17 is highly conserved among HIV variants.
Table 1. Portionof the HIVenvelopegene representedby variouscandidatevaccinesfor AIDS
5'
Purifiedviralantigens The external portion of the HIV envelope glycoprotein, gp120, is a major target for both antibodymediated neutralization and cytotoxic immunity; it is therefore considered a prime candidate for an AIDS vaccine (Tables 1 and 2). A recombinant form of gp120 (rec gp130) and the whole HIV envelope gene product (rec gp160) have been expressed in mammalian and insect cell systems by L. Lasky and col-
gp120 External
I
gp41
3'
HlVenvgene
Transmembrane gp 120 Recgp130 Recgp160 PB1 Pept735 ?
HGP-30 Recvaccinia
*Sponsoredby the US PublicHealthServiceAIDS Task Forceand heldat the NationalInstitutesof Health,Bethesda,Maryland,25-27March,1987. (~ 1987,ElsevierPublications,Cambridge 0167 49191871502.00
Anti-id 193
ImmunologyToday,voL 8, NOS7andS, 1987
;C
L/revacones The viral envelope has been inserted into live virus vectors in the hope of improving its immunogenicity. Live vectors can mimic the presentation of HIV antigens through replication in the host, while avoiding the risk inherent to a live attenuated HIV vaccine. Both recombinant vaccinia virus, expressing either the envelope (gp160) or the gag (p25) gene products of HIV (B. Moss, NIH; G. Todaro, Oncogen; and M. Girard, Pasteur Vaccins), and recombinant adenovirus (A. Davis, Wyeth Labs) a~'e being studied in various animals, induding chimpanzees. A similar vaccinia vaccine expressing gp160 is being tested in humans in Africa by D. Zagury et aL from the Pierre et Marie Curie University in Paris. Ani/4d/oiyp/crace/he A vaccine based on molecular mimicry is being attempted by R. Kennedy et al. at the Southwest Foundation, using anti-idiotypic antibodies to either the HIV envelope or
to its natural receptor on T helper munogen. Therefore, any vaccine cells, the CD4 molecule. An anti- based on the HIV envelope will more idiotypic vaccine would be able to than likely have to include multiple present conformationally correct epi- subunits representative of the HIV topes with neither the risks inherent variants necessary to confer crosswith killed virus vaccines nor the loss reactive immunity. To identify and of immunogenicity that may be reduce the number of isolates that associated with synthetic or recom- will have to be used, the serotyping of the different HIV isolates must binant products. become an essential and urgent task and is in progress in several laborEvaluation All the candidate vaccines de- atories. Cell-mediated immunity (CMI) is a scribed are immunogenic and most have elicited the formation of neut- well-documented first line response ralizing antibodies in small animals, to many viral infections, but it is a monkeys or chimpanzees (Table 2~. complex process tostudy. As a reThe neutralization titers obtained sult, few CMI data on candidate vary from 1 : 10 to 1 : 100 except vaccines are complete to date. P. for PB1 that was reported to have Fishinger (NIH) reported preliminary elicited titers as high as 1 : 1000 in evidence that antibodies raised goats. It is possible that given ad- against native gp120 in chimpanzees ditional boosts and appropriate ad- failed to mediate antibody-depenjuvants the neutralizing titers in- dent cell-mediated cytotoxicity. In duced by other test vaccines will contrast, recombinant vaccinia virus increase as well. However, all neutra- expressing gp160 induces a strong lizing responses obtained thus far polyspecific CMI in rhesus monkeys appear type-specific for the HIV as well as in chimpanzees, suggeststrain used to produce the ira- ing that antigen presentation may
Table2.. Predinicalevaluationof candidatevaccines
194
CMI
Immunogen
Expression system/yield
Glycosylation
Advantages
Questions/ disadvantages
Neutralization titers/antibody source
gp 120
HIV-produdng celllines;low yield
Full
Native
Pooryield;toxic effecton CD4 cells?
1:100 Smallanimals Chimpanzee
Recgp130
Mammalian cells; highyield
Full,some differences
Native-like
Toxiceffecton CD4cells?
1:50 Smallanimals Chimpanzee
ND*
Recgp 160
Insectcells;high yield
Partial,different
Wholeenvelope, mayexpose hiddenepitopes
Different glycosylation
ND
ND
PB1
E co~i; highyield
None
Doesnot block fusion
1"1000 Goat
ND
Restricted specificity?
1:32 Chimpanzee
Mayexpose hiddenepitopes, do not bindCD4
ND
Pept735
Synthetic,gp41 region;highyield
None
HGP-30
Synthetic;high yield
None
Highlyconserved antigen
Doesnot block fusion?
1100 Smallanimals
Vacdniagp 160
Vacciniavirus; highyield
Full
Improvedantigen Liveviruspresentation associatedsideeffects
1:165 Rhesus Chimpanzee
Anti-idCD4
Immunized rabbit Molecular mimicry
Correct conformation
1:10 Smallanimals Baboon
* ND: notdone.
Restricted specificity?No experiencein humans
ND
ND
Immunology Today, vol. 8, Nos 7 and 8, 1987 .
......
dfe
t
res l |
influence this type of response. Besides immunogenicity, toxicity is an important variable to consider before clinical use. There is evidence that the binding of gp120 to CD4 may in itself damage lymphocytes. In addition, live vaccinia virus bearing the HIV envelope may induce fusion of lymphocytes in vivo. However, unlike glycosylated antigens, PB1 peptide 735 and HGP-30 do not bind the CD4 molecule on helper cells. Whether these effects will be of significance for vaccine testing will have to be assessed in the early phases of clinical trials. Central to the question of vaccine evaluation is the issue of imrnunoprotection. Should an AIDS vaccine protect against infection or only prevent disease? Although opinions vary on this matter, there is a strong tendency to aim at the former goal due to high mortality caused by HIV. Two candidate vaccines, gp160vaccinia and peptide 735, have been tested in chimpanzees for their ability to block HIV infection in vivo; they both failed to protect the animals upon challenge. However, as many as 300000 HIV infectious doses (TCID-50) were used in the vaccinia challenge, a dose hardly representative of the clinical situation, and the experiment needs to be repeated. Yet, the number of chimpanzees available for AIDS vaccine testing is rapidly dwindling. Several investigators argued at the meeting for more extensive use of the rhesus model to study simian immunodeficiency virus vaccines in order to reserve the more scarce chimpanzees for the final pre-dinicai tests of HIV vaccines. Meanwhile, other researchers are increasingly questioning the validity of the chimpanzee model for human AIDS research. Some, in fact, deem animal testing altogether unlikely to reflect the human model and have already applied for phase I clinical trials. In any case, when all the available pre-clinical data are put together, it will be necessary to evaluate the different vaccines for further testing in humans. Currently, methods to assess neutralization titers, CMI, cytopathology and so on are almost as diverse as the laboratories using them and no consensus about standardizing these assays was reached during the workshop. Standardization is particularly important if chimpanzee trials are to be bypassed as the urgency of the epidemic may require.
Table 3. Social-epidemiologicalfactorsin the choiceof targetgroupsfor AIDSvaccinetrialsin the USAthat can be favourable,neutralor unfavourableto the evaluationof vaccineefficiency
Gay IVDU Prostitutes Std P-sp/std P-sp/hemo Prisoners Military
Avail.
Risk
F/up
F F F F F U N F
F F F U F F U? N
F U N U N F F (F)?
Re:r/ Compl. F U N N U? F F F
Educ.
Uab.
U F N N N F F N
N U? U? N N N U* F
f
I
Abbreviationsandsymbols:
Avail.: Risk: F/up: Recr/Compl.: Educ.: Liab.:
availabilityinnumbers exposureto HN accessibilityforfoll0w-upstudies recruitment/compliance influenceofeducationoninfectionrisk potentialliabl!ityproblemsinvolved
IVDU: Std: P-splstd: P-sp/hemo:
intravenousdrugusers sexuallytransmitteddiseasepatients partnersorspousesof stdpatients partnersorspousesof hemophiliacs
F: U: N:
favorablefactorforvaccinetrial unfavorable neutral
*Exceptionsto proscribedresearchcanbemade.
Design of dinkal trials In the USA, three clinical phases are part of any vaccine trial. For the AIDS vaccine tria!s, six vaccine evaluation units have already been selected by the National Institute of Allergy and Infectious Diseases. They are: Baylor College of Medicine in Houston, Vanderbilt University pediatric unit in Tennessee, Rochester University in New York, Marshall University, John Hopkins University and the University of Maryland. In phase I of a clinical trial, the safety and immunogenicity of ~andidate vaccines are tested on 10-20 individuals. Phase II has the same objectives but is carried out on a larger sample (40-60 individuals). The vaccine efficacy is tested in phase III on a representative population sample whose size will depend on the prevailing incidence of the disease and the confidence levels sought for significant protection from infection. In the case of AIDS, all phases are likely to include control groups in a double-blind fashion to avoid 'false hope' ~-~fects on the participants. Rather than placebo, it was suggested that other known vaccines be used as controls in order to account for unexpected side effects that may be encountered in the particular
population tested but not specifically due to the AIDS vaccine tested. In evaluating human trials, the distinction between seroconversions due to vaccine inoculation and due to actual infection will have to be made. Apparently, screening for antibodies to HIV antigens not included in a vaccine should answer this question. However, infected individuals do not always make antibodies to all vi~al proteins. It may therefore take =everal additional criteria such as early antigenemia or nucleic acid probing, if available, to make such distinction. This difficulty underlies the necessity of using low risk individuals for phase I trials, which in turn will require an extended follow-up period. The information gained from these trials should then permit the enrollment of high risk individuals for phase II and III trials where immunogenicity and efficacy will be the main variables under study.
Target groupsand ethics In phase Ill clinical trials, candidate vaccines will be tested on a population sample large enough to allow a valid assessment of the vaccine efficacy. The sample size will depend on the level of significance sought for the results, and on the popula-
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tion availability, reliability and rate of infection by HIV. These parameters will in turn determine the choice of the target population among the various groups at risk for AIDS (Table
3). The homosexua; risk group has proven extremely cooperative in other AIDS studies, but may have an incidence of infection too low for vaccine testing as a result of its high response to prevention and education efforts. In contrast, partners or spouses of hemophiliacs, who appear to have followed safe sex practice advice poorly, continue to be at high risk of infection by HIV and could be a very favorable target group. However, they may be too few in numbers to constitute a valid group for phase III trials, and wider publicity on prevention may rapidly change their behavior. None of these objections applies to intravenous ~.v.) drug abusers who by now are spreading HIV faster than any other group and respond very slowly to education. Yet i.v. drug abusers may bf. relatively less compliant and tell-
able for follow-up, a significant problem in a trial that may last several years. Alternatively, the military could be a suitable target population, since AIDS has affected thus far primarily young adult males. However, HIV seropositive individuals are no longer enrolled in the US military and the Defense Department is undertaking an intense education campaign. These factors are likely to reduce greatly the incidence of HIV infection in this group. J. Curran (Centers for Disease Control, Atlanta) suggested that the military in some African countries may constitute an appropriate target group since it would be composed of career professionals living in confined quarters, probably with a high HIV incidence rate. This choice, however, is complicated by the fact that the different HIV strains occurring in Africa may confuse the interpretation of results. The technical difficulties in choosing an adequate target group for vaccine trials are compounded by ethical and legal issues. For example,
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intravenous drug abusers and prostitutes, two potential high-risk groups, are engaged in illegal activities, which could interfere with follow-up. There is a moral conflict in enrolling preferentially individuals unlikely to respond to education in order to ensure a constant rate of infection throughout the study. And it would be difficult to enroll prisoners in vaccine trials unless the stringent proscriptions on medical research involving this group are waived. Solutions to these problems will have to be found soon for the urgency of the AIDS pandemic will not allow any delay when a vaccine is ready for testing in humans. The Food and Drug Administration is already reviewing applications for phase I trials for a number of candidate vaccines. In the absence of any other means of controlling HIV spread, these requests are likely to be granted. However, the biggest task ahead remains overcoming the extensive genomic variation in HIV. Several variants have been identified already and many more may exist. Even if a finite number of HIV serotypes are eventually identified, their worldwide distribution, the potential for continuing emergence of new HIV variants (e.g. HIV-2 in West Africa), the specific population groups affected by the disease, the differences in disease manifestations as well as the ethical and liability problems discussed above imply that AIDS vaccine trials will require massive resources and extensive coordination at both national and international levels. One can only hope for a limited number of virulent serotypes or an effective vaccine using a conserved epitope. In either case, in the words of Assistant Secretary of Health Robert E. Windora, it will be several years until an AIDS vaccine reaches the market place. we wish to thank C. Cheng-Mayer,G. Thompson-Honnebier and J. A. Levy for their helpful comments and critical review of the manuscript.
Jacques Horr;sy is at the Cancer Research Institute, S-1280, Schoolof Medicine, University of California, San Francisco, California 94143, USA; Kathelyn Steimeris with Chiron Corporation, Emeryville, California 94608, USA; and Richard Kaslow is at the National Insitutesof Health, WestwoodBuilding, Room 739, Bethesda,Maryland 20892, USA.
