AIDS RESEARCH AND HUMAN RETROV1RUSES Volume 8, Number 8, 1992 Mary Ann Liebert, Inc., Publishers
The Next GORDON
Steps ADA,1
in HIV Vaccine
WAYNE
KOFF,2
INTRODUCTION decade, The quest (HIV). develop During
1990-1991 was a turning point in vaccines against human immunodefithose years, many examples of ciency virus immunization which protected against challenge with the homologous virus, either HIV or simian immunodeficiency virus (SIV), were described. These results led to a general concensus that it should be possible to develop several vaccine candidates which might either protect people against infection or delay the onset of disease in sympotomatic, infected people. Important questions to answer now include: start of this
Downloaded by Iowa State Univ from www.liebertpub.com at 01/25/19. For personal use only.
the
to
1. What are the epitopes in HIV antigens which are recognized by B cells or by T cells and which should be included in vaccines designed optimally to protect against infection or, when administered to HIV infected, asympotomatics, delay the progression to disease? 2. What types of vaccines will achieve responses to these epitopes most effectively? Do any current vaccine candidates
hold out particular promise? 3. How can such preparations be tested and made available to those in need,particularly in those countries where the incidence of infection is rising rapidly? 4. Once available for such trials, what assessments of efficacy can be applied to determine as soon as possible whether a vaccine is effective? This talk discusses mainly the first two aspects. It provides a background for later segments of this meeting which deal particularly with aspects of vaccine production and availability
for international
use.
and JOHN
Development PETRICCIANI3
tion against SIV infection by immunization with inactivated ' whole SIV. These basic findings recently have been extended further. Protection against heterologous strains of SIV differing genetically by 15% in the envelope glycoproteins has been achieved,2 and the time interval between the last immunization and the challenge extended to 8 months.3 Though the numbers of animals available has been and will remain a major limiting factor, chimpanzees immunized with combinations of different HIV antigens, including the envelope glycoproteins, have also been protected from HIV infection.4"6 In contrast, five independent studies using rDNA preparations of SIV glycoproteins applied under reasonably similar conditions, failed to protect. Passive transfer of immune sera specific for HIV or SIV has protected chimpanzees or monkeys against a homologous virus challenge,7X and more recently, a monoclonal antibody directed to the principal neutralizing domain (PND) of HIV gpl20 has
protected two chimpanzees.9 Clinical trails A number of preparations, including inactivated whole HIV but with a decreased complement of gpl20, different preparations of gp 120/160 produced by rDNA technology, and chimeric vaccinia virus constructs containing DNA coding for gpl60, have been given to seronegative, or in some cases, to seropositive, asymptomatic individiuals. So far all preparations have proved to be safe, but the numbers of volunteers involved are necessarily very small. The amount of antibody produced, particularly virus infectivity-neutralizing antibody, and the level of T-cell responses have varied considerably. One approach in particular, priming with a vaccinia virus-gpl60 construct and boosting with gpl60, has yielded high titers of antibody and
virus-neutralizing activity.
PROTECTION EXPERIMENTS
Important
A critical step in vaccine development is to see whether protection can be achieved in an animal model, using experimentally ideal conditions, namely to challenge the immunized animal with the homologous strain shortly after the last antigen administration. Several groups have
now
achieved such protec-
'"' ' '
immune responses and the
their enhancement
potential for
Though the method of protection elicited by whole inactivated SIV is unclear (and a recent finding12 has increased this uncertainty even more), the protection achieved following active immunization with HIV antigens, and the passive immuni-
'John Curtin School of Medical Research. Australian National University. Canberra, 2Division of AIDS. N.I.A.I.D., Washington. DC. 'Pharmaceutical Manufacturers Association. Washington. DC. 1317
1318
ADA ET AL.
zation with HIV antisera in chimpanzees is consistent with the following interpretation: the desirability for the vaccine to generate a high virus-neutralizing titer, and for much of this activity to be directed toward the PND, the intact V3 loop and the gpl20of HIV. Additional experiments suggest that antibodies directed to conformational determinants of gpl20 might be
broadly protective.
more
Downloaded by Iowa State Univ from www.liebertpub.com at 01/25/19. For personal use only.
Some of the protection experiments quoted above suggest that the most effective way to induce high neutralization titers is to immunize with preparations of gpl20 with the native conformation. Apart from our general knowledge of the role of different CMI responses in controlling viral infections,14 current information with these candidate vaccines gives little indication of their importance. However, one clinical trial in uninfected subjects suggests that poxviruses may be particularly effective vectors for priming for some useful responses, such as neutralizing for different CMI responses, activity; they also should prime '"'" such as cytotoxic T cell activity.
The
spread of the AIDS pandemic
The WHO estimates that by the year 2000, 40 million individuals will be infected by HIV. Given that other sexually transmitted diseases (STDs) facilitate HIV transmission and the occurrence of about 250 million fresh cases of STDs each year, the potential number of HIV infections is far greater. Though all appropriate methods to control the pandemic should be pursued, an active vaccination program has the greatest potential to slow the spread of new infections and possibly alleviate the disease in those already infected.
The
next
There
steps in HIV vaccine development
are
several
specific needs.
1. To obtain the necessary information about the requirement for an HIV vaccine to induce those immune responses, in addition to neutralizing antibody, which may limit viral spread and replication, and destroy HIV-infected cells. 2. To proceed apace with those additional investigations needed to convert the more promising of the current candidate vaccines into highly effective vaccines. Two types of information are needed. The first of these concerns aspects such as:
What needs to be done to generate a broadly reactive neutralizing antibody response? Will a 'cocktail' of env antigens be required? b. Can these vaccines be administered in a form which will induce strong mucosal immunity in the female reproductive tract? What route of administration will achieve this a.
most
effectively?
What type of immune response is needed to destroy transmitted infected cells either before or shortly after the infection is passed to host cells? 2. The second type of information concerns aspects such as: a. How can the most promising of the current vaccines be formulated so as to give long-lasting immunity after one or two administrations? There is a need to decide between or to use combinations of formulations such as liposomes, iscoms, and the different biodegradable microsphere/ c.
b.
capsule preparations which currently seem promising as a means to induce long-lived immunity. The affordability and heat stability of effective formula-
tions in third world countries. 3. The time has now arrived to consider seriously the early initiation of Phase 3 efficacy trials in selected populations with the most promising of the current candidate vaccines. There are powerful reasons for such a decision. The first reason is an acceptance of the realization that vaccination is potentially the most efficient and cost-effective way to prevent infection and possibly also to slow down the progression toward disease manifestation of those already infected. Second, such an action would be seen publicly as a demonstration that there really was an ongoing major international effort to develop HIV vaccines and the most promising of these are now ready for initial testing for efficacy in humans. The third (and not the least) reason is that we are bound to learn something from such an action. The information gained must be very useful in the further development of HIV vaccines. The rationale is as follows. 1. The major form of transmission of HIV worldwide is sexual intercourse and in developing countries at least,
by by
heterosexual activity. 2. The efficacy of heterosexual transmission of HIV is low, considerably less than 1%.'5 There is great variation. Very occasionally, one donor has infected several others after a single sexual contact with each; on the other hand, there are also examples where transmission has not occurred between an infected and a noninfected partner, even though there has been active unprotected sexual intercourse over a very long period. In addition and for many reasons, it is unlikely that human sexual practices can be experimentally mimicked by animal models. 3. One interpretation of such information16 is that in most situations, an infectious dose of virus is transmitted only rarely (< 1 % of contacts). If this interpretation is correct, it is likely that in many of the contacts which result in infection, only one or a few infectious doses is transmitted. In fact, an analysis of the antigenic specificity of virus isolated during the viremia which may occur shortly after infection has shown a restricted heterogeneity compared with virus isolated later during the infection,17 a finding consistent with the above interpretation. In such situations, a less-than-ideal vaccine might still be effective in many people, even though factors such as the antigenic specificity and route of administration might not be optimum. The difference between substantial success (say >50% protection) or failure would indicate the importance or otherwise of factors such as need for the vaccine to induce a very broad antigenic specificity, the need for strong, local mucosal
immunity, etc. Potential candidates for
early
human
efficacy
trials
Based upon the earlier information, three approaches seem at present to be quite promising.
Downloaded by Iowa State Univ from www.liebertpub.com at 01/25/19. For personal use only.
THE NEXT STEPS VACCINE DEVELOPMENT 1. In view of the success with inactivated whole SIV, a similar preparation of HIV with a full complement of gpl20 should be tried. The polio, rabies, and Japanese encephalitis vaccines are examples of successful inactivated whole virus vaccines. One concern about the use of intact HIV is the presence of the viral DNA which might persist; however, it is possible to inactivate the nucleic acid component. 2. Priming with live recombinant poxviruses that express HIV antigens, followed by boosting with rDNA-derivedgpl20or gpl60. If the recombinant poxvirus is replication competent, it should only be administered to HIV seronegative people. However, there are recent encouraging reports of the use of avipox viruses as vectors for use in mammals (E. Paoletti, personal communication). These undergo only abortive infections in mammals and should be very safe to use. 3. Envelope glycoproteins derived from mammalian cells and having the correct native conformation. Such preparations could be formulated with adjuvants that promote persistence of antigen and generate long term immunological memory. At any one particular trial site, there might be only a few 'dominant' strains. Small amounts of vaccines sufficient for such efficacy trials and of one or a few of these specificities could be prepared within the next year or so. It is of the utmost importance that any preparation used in efficacy trials is shown to be safe and immunogenic in Phase I/I I trials, initially in the country of origin.
CONCLUSION It is critical now to establish as early as possible the ease or difficulty of preventing infection in humans by the sexual route. Vaccine efficacy trials, if carried out with maximum concern for the rights and welfare of participants, and with the most promising of the possible candidate preparations, could yield this information. It is hoped this presentation will stimulate a productive discussion on the merits and practicality of initiating such trials in the near future.
REFERENCES Murphey-Corb M: Whole inactivated SIV vaccines. In: AIDS Research Reviews. W.C. Koff, F. Wong-Staal, and R. Kennedy (Eds). Marcel Dekker, New York, 1991, pp 359-368. 2. Gardner M: SIV update. Ibid. 2, 1992. 1.
1319 3. Schulz AM, and Warren J: Surveillance of AIDS vaccine studies in non-human primates. VII IntConf AIDS, 1991; Abstract MA 1337. 4. Girard MP. and Eichberg JW: Progress in the development of HIV vaccines. AIDS 1990;4 (Suppl):SI43-S150. 5. Berman PW, Gregory TJ, RiddleLetal: Protection of chimpanzees from infection by HIV-1 after vaccination wilh recombinant gpl20 but notgpl60. Nature 1990;45:622-625. 6. Mannhalter JW, Barrett N. and Kupcu Z: Immunological parameters associated with protection against challenge with HIV-1 in rgp 160 immunized chimpanzees. VII Int. Conf. AIDS. 1991; Abstract Th A. 13. 7. Eichbert JW: Prevention of HI V infection by passive immunization with HIV IgorCD4 IgG. Int. Conf. Adv. in AIDS Vaccine Devel., Florida, October 1991; (Abstract) 8. Biberfeld G. Putkonen P. Thorstensson R. and Norrby E: Prevention of HIV-2 and SIVsm infection in cynomolgous monkeys by active or passive immunization. Ibid. Abst. 9. Emini EA, Schleif WA, Conley AJ et al. Pre- or post-challenge administration of HIV-1 V3 loop-specific monoclonal antibody prevents the establishment of HIV-1 infection in chimpanzees. 1992; Ibid. Abst. 10. Cooney E, Corey L, Hu SL et al: Enhanced immunogenicity in humans of an HIV-1 subunit vaccine regimen employing priming with a vaccinia gp 160 recombinant virus followed by boosting with recombinant HIV envelope glycoprotein (rpg 160). VI Int. Conf. AIDS. 1990; Abstract Th. A. 333. 11. Graham BS. Belshe R. Clements M-L et al: HIV-gp 160 recombinant vaccinia vaccination of vaccinia-naive adults followed by rpg 160 booster immunization. VII Int. Cong. AIDS 1991; Abstract F.A.I. 12. Stott J: ( 1991 ). Anti-cell antibody in macaques. Nature 353; 393. 13. Steimer KS. Haigwood N: Immunization of primates with native, recombinant HIV-SF2 gpl20 generates broadly effective neutralizing antibodies directed to conformational epitopes. Nati Cancer Inst. Lab. Tumor Cell Biol Washington, September 1991; Abst. 14. AdaGL: The immunological principles of vaccination. In: Modern Vaccines. Current Practice and New Approaches. Arnold, London, 1991. pp. 8-15. 15. Padian N. Marquis L, Francis DP et al: Male to female transmission of human immunodeficiency virus. JAMA 1987:258:788. 16. Wigzell H: Prospects for an HIV vaccine. FASEB J 1991;5:2406. 17. Goutsmit J, Zwart G, and Wolfs T: Assessment of antigenic diversification using serum as sample source. Int. Conf. Adv AIDS Vaccine Dev. October 1991: Florida. Abst.
Address
reprint requests
to:
Gordon Ada John Curtain School of Medical Research Australian National University Canberra, Australia
The Next GORDON
Steps ADA,1
in HIV Vaccine
WAYNE
KOFF,2
INTRODUCTION decade, The quest (HIV). develop During
1990-1991 was a turning point in vaccines against human immunodefithose years, many examples of ciency virus immunization which protected against challenge with the homologous virus, either HIV or simian immunodeficiency virus (SIV), were described. These results led to a general concensus that it should be possible to develop several vaccine candidates which might either protect people against infection or delay the onset of disease in sympotomatic, infected people. Important questions to answer now include: start of this
Downloaded by Iowa State Univ from www.liebertpub.com at 01/25/19. For personal use only.
the
to
1. What are the epitopes in HIV antigens which are recognized by B cells or by T cells and which should be included in vaccines designed optimally to protect against infection or, when administered to HIV infected, asympotomatics, delay the progression to disease? 2. What types of vaccines will achieve responses to these epitopes most effectively? Do any current vaccine candidates
hold out particular promise? 3. How can such preparations be tested and made available to those in need,particularly in those countries where the incidence of infection is rising rapidly? 4. Once available for such trials, what assessments of efficacy can be applied to determine as soon as possible whether a vaccine is effective? This talk discusses mainly the first two aspects. It provides a background for later segments of this meeting which deal particularly with aspects of vaccine production and availability
for international
use.
and JOHN
Development PETRICCIANI3
tion against SIV infection by immunization with inactivated ' whole SIV. These basic findings recently have been extended further. Protection against heterologous strains of SIV differing genetically by 15% in the envelope glycoproteins has been achieved,2 and the time interval between the last immunization and the challenge extended to 8 months.3 Though the numbers of animals available has been and will remain a major limiting factor, chimpanzees immunized with combinations of different HIV antigens, including the envelope glycoproteins, have also been protected from HIV infection.4"6 In contrast, five independent studies using rDNA preparations of SIV glycoproteins applied under reasonably similar conditions, failed to protect. Passive transfer of immune sera specific for HIV or SIV has protected chimpanzees or monkeys against a homologous virus challenge,7X and more recently, a monoclonal antibody directed to the principal neutralizing domain (PND) of HIV gpl20 has
protected two chimpanzees.9 Clinical trails A number of preparations, including inactivated whole HIV but with a decreased complement of gpl20, different preparations of gp 120/160 produced by rDNA technology, and chimeric vaccinia virus constructs containing DNA coding for gpl60, have been given to seronegative, or in some cases, to seropositive, asymptomatic individiuals. So far all preparations have proved to be safe, but the numbers of volunteers involved are necessarily very small. The amount of antibody produced, particularly virus infectivity-neutralizing antibody, and the level of T-cell responses have varied considerably. One approach in particular, priming with a vaccinia virus-gpl60 construct and boosting with gpl60, has yielded high titers of antibody and
virus-neutralizing activity.
PROTECTION EXPERIMENTS
Important
A critical step in vaccine development is to see whether protection can be achieved in an animal model, using experimentally ideal conditions, namely to challenge the immunized animal with the homologous strain shortly after the last antigen administration. Several groups have
now
achieved such protec-
'"' ' '
immune responses and the
their enhancement
potential for
Though the method of protection elicited by whole inactivated SIV is unclear (and a recent finding12 has increased this uncertainty even more), the protection achieved following active immunization with HIV antigens, and the passive immuni-
'John Curtin School of Medical Research. Australian National University. Canberra, 2Division of AIDS. N.I.A.I.D., Washington. DC. 'Pharmaceutical Manufacturers Association. Washington. DC. 1317
1318
ADA ET AL.
zation with HIV antisera in chimpanzees is consistent with the following interpretation: the desirability for the vaccine to generate a high virus-neutralizing titer, and for much of this activity to be directed toward the PND, the intact V3 loop and the gpl20of HIV. Additional experiments suggest that antibodies directed to conformational determinants of gpl20 might be
broadly protective.
more
Downloaded by Iowa State Univ from www.liebertpub.com at 01/25/19. For personal use only.
Some of the protection experiments quoted above suggest that the most effective way to induce high neutralization titers is to immunize with preparations of gpl20 with the native conformation. Apart from our general knowledge of the role of different CMI responses in controlling viral infections,14 current information with these candidate vaccines gives little indication of their importance. However, one clinical trial in uninfected subjects suggests that poxviruses may be particularly effective vectors for priming for some useful responses, such as neutralizing for different CMI responses, activity; they also should prime '"'" such as cytotoxic T cell activity.
The
spread of the AIDS pandemic
The WHO estimates that by the year 2000, 40 million individuals will be infected by HIV. Given that other sexually transmitted diseases (STDs) facilitate HIV transmission and the occurrence of about 250 million fresh cases of STDs each year, the potential number of HIV infections is far greater. Though all appropriate methods to control the pandemic should be pursued, an active vaccination program has the greatest potential to slow the spread of new infections and possibly alleviate the disease in those already infected.
The
next
There
steps in HIV vaccine development
are
several
specific needs.
1. To obtain the necessary information about the requirement for an HIV vaccine to induce those immune responses, in addition to neutralizing antibody, which may limit viral spread and replication, and destroy HIV-infected cells. 2. To proceed apace with those additional investigations needed to convert the more promising of the current candidate vaccines into highly effective vaccines. Two types of information are needed. The first of these concerns aspects such as:
What needs to be done to generate a broadly reactive neutralizing antibody response? Will a 'cocktail' of env antigens be required? b. Can these vaccines be administered in a form which will induce strong mucosal immunity in the female reproductive tract? What route of administration will achieve this a.
most
effectively?
What type of immune response is needed to destroy transmitted infected cells either before or shortly after the infection is passed to host cells? 2. The second type of information concerns aspects such as: a. How can the most promising of the current vaccines be formulated so as to give long-lasting immunity after one or two administrations? There is a need to decide between or to use combinations of formulations such as liposomes, iscoms, and the different biodegradable microsphere/ c.
b.
capsule preparations which currently seem promising as a means to induce long-lived immunity. The affordability and heat stability of effective formula-
tions in third world countries. 3. The time has now arrived to consider seriously the early initiation of Phase 3 efficacy trials in selected populations with the most promising of the current candidate vaccines. There are powerful reasons for such a decision. The first reason is an acceptance of the realization that vaccination is potentially the most efficient and cost-effective way to prevent infection and possibly also to slow down the progression toward disease manifestation of those already infected. Second, such an action would be seen publicly as a demonstration that there really was an ongoing major international effort to develop HIV vaccines and the most promising of these are now ready for initial testing for efficacy in humans. The third (and not the least) reason is that we are bound to learn something from such an action. The information gained must be very useful in the further development of HIV vaccines. The rationale is as follows. 1. The major form of transmission of HIV worldwide is sexual intercourse and in developing countries at least,
by by
heterosexual activity. 2. The efficacy of heterosexual transmission of HIV is low, considerably less than 1%.'5 There is great variation. Very occasionally, one donor has infected several others after a single sexual contact with each; on the other hand, there are also examples where transmission has not occurred between an infected and a noninfected partner, even though there has been active unprotected sexual intercourse over a very long period. In addition and for many reasons, it is unlikely that human sexual practices can be experimentally mimicked by animal models. 3. One interpretation of such information16 is that in most situations, an infectious dose of virus is transmitted only rarely (< 1 % of contacts). If this interpretation is correct, it is likely that in many of the contacts which result in infection, only one or a few infectious doses is transmitted. In fact, an analysis of the antigenic specificity of virus isolated during the viremia which may occur shortly after infection has shown a restricted heterogeneity compared with virus isolated later during the infection,17 a finding consistent with the above interpretation. In such situations, a less-than-ideal vaccine might still be effective in many people, even though factors such as the antigenic specificity and route of administration might not be optimum. The difference between substantial success (say >50% protection) or failure would indicate the importance or otherwise of factors such as need for the vaccine to induce a very broad antigenic specificity, the need for strong, local mucosal
immunity, etc. Potential candidates for
early
human
efficacy
trials
Based upon the earlier information, three approaches seem at present to be quite promising.
Downloaded by Iowa State Univ from www.liebertpub.com at 01/25/19. For personal use only.
THE NEXT STEPS VACCINE DEVELOPMENT 1. In view of the success with inactivated whole SIV, a similar preparation of HIV with a full complement of gpl20 should be tried. The polio, rabies, and Japanese encephalitis vaccines are examples of successful inactivated whole virus vaccines. One concern about the use of intact HIV is the presence of the viral DNA which might persist; however, it is possible to inactivate the nucleic acid component. 2. Priming with live recombinant poxviruses that express HIV antigens, followed by boosting with rDNA-derivedgpl20or gpl60. If the recombinant poxvirus is replication competent, it should only be administered to HIV seronegative people. However, there are recent encouraging reports of the use of avipox viruses as vectors for use in mammals (E. Paoletti, personal communication). These undergo only abortive infections in mammals and should be very safe to use. 3. Envelope glycoproteins derived from mammalian cells and having the correct native conformation. Such preparations could be formulated with adjuvants that promote persistence of antigen and generate long term immunological memory. At any one particular trial site, there might be only a few 'dominant' strains. Small amounts of vaccines sufficient for such efficacy trials and of one or a few of these specificities could be prepared within the next year or so. It is of the utmost importance that any preparation used in efficacy trials is shown to be safe and immunogenic in Phase I/I I trials, initially in the country of origin.
CONCLUSION It is critical now to establish as early as possible the ease or difficulty of preventing infection in humans by the sexual route. Vaccine efficacy trials, if carried out with maximum concern for the rights and welfare of participants, and with the most promising of the possible candidate preparations, could yield this information. It is hoped this presentation will stimulate a productive discussion on the merits and practicality of initiating such trials in the near future.
REFERENCES Murphey-Corb M: Whole inactivated SIV vaccines. In: AIDS Research Reviews. W.C. Koff, F. Wong-Staal, and R. Kennedy (Eds). Marcel Dekker, New York, 1991, pp 359-368. 2. Gardner M: SIV update. Ibid. 2, 1992. 1.
1319 3. Schulz AM, and Warren J: Surveillance of AIDS vaccine studies in non-human primates. VII IntConf AIDS, 1991; Abstract MA 1337. 4. Girard MP. and Eichberg JW: Progress in the development of HIV vaccines. AIDS 1990;4 (Suppl):SI43-S150. 5. Berman PW, Gregory TJ, RiddleLetal: Protection of chimpanzees from infection by HIV-1 after vaccination wilh recombinant gpl20 but notgpl60. Nature 1990;45:622-625. 6. Mannhalter JW, Barrett N. and Kupcu Z: Immunological parameters associated with protection against challenge with HIV-1 in rgp 160 immunized chimpanzees. VII Int. Conf. AIDS. 1991; Abstract Th A. 13. 7. Eichbert JW: Prevention of HI V infection by passive immunization with HIV IgorCD4 IgG. Int. Conf. Adv. in AIDS Vaccine Devel., Florida, October 1991; (Abstract) 8. Biberfeld G. Putkonen P. Thorstensson R. and Norrby E: Prevention of HIV-2 and SIVsm infection in cynomolgous monkeys by active or passive immunization. Ibid. Abst. 9. Emini EA, Schleif WA, Conley AJ et al. Pre- or post-challenge administration of HIV-1 V3 loop-specific monoclonal antibody prevents the establishment of HIV-1 infection in chimpanzees. 1992; Ibid. Abst. 10. Cooney E, Corey L, Hu SL et al: Enhanced immunogenicity in humans of an HIV-1 subunit vaccine regimen employing priming with a vaccinia gp 160 recombinant virus followed by boosting with recombinant HIV envelope glycoprotein (rpg 160). VI Int. Conf. AIDS. 1990; Abstract Th. A. 333. 11. Graham BS. Belshe R. Clements M-L et al: HIV-gp 160 recombinant vaccinia vaccination of vaccinia-naive adults followed by rpg 160 booster immunization. VII Int. Cong. AIDS 1991; Abstract F.A.I. 12. Stott J: ( 1991 ). Anti-cell antibody in macaques. Nature 353; 393. 13. Steimer KS. Haigwood N: Immunization of primates with native, recombinant HIV-SF2 gpl20 generates broadly effective neutralizing antibodies directed to conformational epitopes. Nati Cancer Inst. Lab. Tumor Cell Biol Washington, September 1991; Abst. 14. AdaGL: The immunological principles of vaccination. In: Modern Vaccines. Current Practice and New Approaches. Arnold, London, 1991. pp. 8-15. 15. Padian N. Marquis L, Francis DP et al: Male to female transmission of human immunodeficiency virus. JAMA 1987:258:788. 16. Wigzell H: Prospects for an HIV vaccine. FASEB J 1991;5:2406. 17. Goutsmit J, Zwart G, and Wolfs T: Assessment of antigenic diversification using serum as sample source. Int. Conf. Adv AIDS Vaccine Dev. October 1991: Florida. Abst.
Address
reprint requests
to:
Gordon Ada John Curtain School of Medical Research Australian National University Canberra, Australia
