The potential of vaccines for t h e control of Al DS

Potential of vaccines for the control of AIDS. Can J Infect Dis 1994;5(SupplA):36A-41A.111c goal ofa prophylactic human immunodcncicncy (111v) vaccine is lo elicit immune response(s) that will, upon subsequent exposure lo 111v. prevent lnfeclion and/or disease. On lhc other hand. therapeutic administration ofa vaccine lo an individual in whom infection is already established might bcncfH lhe individual by augmenting existing funclional immune responses or inducing new ones. Development of vaccines for the prevention of AJOS offers unique challenges. Concerns regarding the safely of attenualcd and whole-killed products have led lo the pursuil of allernalivc designs. including recombi nanl proteins. vectors and parti.clcs. synU1eli(· peptides and naked DNA. Seven recombinant envelope. two reC'ombinanl vector and four other candidate vaccines that have entered into phase I trials in nonlnfccled iudivlduals have proven sale lo elate. and have differed In their ablllly lo induce:- funC'lional antibody and C'ylolox:lc T lymphocytes. 1wo recombinant cnvelopc products have recenUy progressed to phase 2 lcsling. F'iv<.' cnvclop<.'·bas<.'d and six other produC'ts hav<.' e:-nlered trial in 111v-infcctcd Individuals and have appeared to be safe. Evidence of new antibody. increased T cell proliferaUon and lncr<.'ased cytoloXiC' T lymphocyte activity have been reported. Addilional plaC'cbo controlled trials will be required to evaluate U1e impact of therapcullc vaccination on cu4 cell count. viral burdrn and clinical end-points. The status of 111v I AIDS vaccine development is reviewed. with emphasis on Lhc challenging task of nnding an efficacious. safe. prophylaclic vaccine.

personnc ravant.age de rc ha u sser sa proprc rcponse immun itairc ou d'en dcclcncllcr de nouvelles. Le dcvcloppcmc nl de vaccins pour la prcvcnlicm du SIDA offrc un polcnlic l unique. Les crainlcs qu·in s pircnl lcs produ ils altcnucs ou lues a regard de l'innoC'uile onl sli mule la recherchc en vuc de trouvcr des mocleles diffcrents. notammenl des prolcincs. des vcclcurs c l des pa,·li<.'ulcs rccombina ntes. des peplldes syntheliques cl de l'AoN denude. Pam1i les vaccins qui sont e nlrcs dans des essais de phase l <.'h<.'z des s uJC'ls non infcctcs ct qui se sont revcles sans danger jusqu·a present, notons-en sept <' t base d'cnvelo ppc rccombinanlc. clcux a base de vectcur recombinant el quatre d'autrcs types. Lcur capacitc a ind uirc la produ<.'tion cr anllcorps ct de lymph ocytes T cyto tox.iqucs foncllonnels est va,;able. Deux des produ ils ,1 base cl'<.'nveloppc rccombinantc onl rcccmmenl progrcsse vcrs des cssals de phase 11. Cinq produ its a base cl'cnveloppc et s ix prodults d'a utrcs lyp<.'s font robjcl d"cssais au pres de sujcls infcctcs au v111 ct semblcn l cgalcmcnl elrc san s danger. On a pu observer la presence de nouveaux a n tieo1v::.. la prolilcralion accrue de ccllulcs T cl une aclivit t' plus grandc des lymphocytes T cyloloxiqucs. o·aulrcs essais conlr6lcs avcc placebo seront neC'essaires pour cvalue r les repercussion::. d\me vaccination lhcrapcutique su r la numeration d<.'s cellulcs CD4. sur la charge viralc c l ks paramctres cliniqu es. L'cvolutton des vaccins eontrc le v111/s1DA esl passcc e n revu e c l ro n insistc sur le den quc rcprcscnlc la dccouvcrtc cl\111 vac<.'in prophylaC'tique s(1r cl c fficace. S INCE Tl IE DISCOVERY OF AIDS AND ITS ETIOLOGICAL AGENT, the human immunodeficiency virus (HIV). researchers from all sectors of the scientific community have been challenged to find effective therapies to treat HIV I AIDS and a vaccine to curb its spread ( 1-5). HIV/ AIDS has grown to be one of the major causes of deaU1 in the United Stales. Approximately one million individuals in the United States are infected (6,7). almost one quarter million have been diagnosed with AIDS. and approximately 65% of those individuals diagnosed with AIDS have died (8). The impact of IIIV/AIDS on other parts of the world. particularly parts of Africa and Asia, will likely be more devastating than the impact on the United States if current trends continue (9) . Since behavioural inlervention approaches have not proven entirely successful and access to antiretroviral therapies is generally limited to developed countries, the major hope for curbing the HIV/ AIDS epidemic worldwide is the development of an efficacious, inexpensive, easy to administer vaccine.
The combination of scientific challenges in HIV vaccine development is unparalleled in the field of vaccine development. First, the great degree of genetic diversity of HIV suggests that broadly cross-reactive immune responses will be needed to protect. an individual from all subtypes of virns to which the individual might be exposed. Five major clades or subtypes of HIV-1 have been identified (10)(11)(12). The virus in each clade differs from virus in other clades by 30 to 35% in the genetic sequence of env and gag genes. Antigenic variation. which has hindered the development of the influenza vaccine, may prove to be an even more significant obstacle in the case of HIV-I. Second, the virus exists in both free virion and cell-associated forms. Vaccines that induce both neutralizing antibodies. which block infection by free virions, and cytotoxic T lymphocytes (CTL), which may eliminate infected cells, may be required. However, the nature and amount of HIV antigen expression on the surface of cells that are not actively producing virus remains unknown; latently infected cells might not be vulnerable to attack by either antibody or CTLS. Third, the major mode of transmission of HIV-I is through sexual contact; an efficacious vaccine may require induction of HIV-I-specific mucosal immune responses through novel antigen presentation methods. Fourth, there is no documentation that individuals can completely clear 111v-1 from the body once infection is established. Thus. the immune response(s) that correlates with protection from HIV-induced disease is not known. Fifth, although monkeys infected with the simian immunodeficiency virus (SIV) have proven useful in evaluating vaccine concepts, there is as yet no animal model that can be employed to determine lhe effectiveness of an HIV-I candidate vaccine in preventing IIIV-I-induced disease (13). HIV-I can establish a chronic infection in chimpanzees, but does not cause immunodeficiency or disease. Thus, lhis expen- HIV /AIDS vaccine research sive model can be used to evaluate the ability of candidate HIV-I vaccines to block primary infection, but it will not provide information on the impact of the vaccine on disease induction or progression. Recent results wiU1 Macaca nemestrina suggesl that this species can be acutely infected with HIV-I (14): data are being accumulated to document ilie degree and reproducibility of this infection. Infection of monkeys wilh an 111v-s1v chimera, known as SIIIV, may also prove valuable in lhe evaluation of certain candidate HIV vaccines (15).
Most viral vaccines licensed in the United States are whole-killed (inactivated) or live-altenuated products (16). Because of safety concerns associated with administration of a live-attenuated retrovirns vaccine or killed 111v virions to noninfected, healthy individuals. most vaccine manufacturers have elected eiilier not to pursue 111v vaccine development or to pursue alternative approaches to antigen delive1y. These approaches have included: recombinant viral and bacterial vectors engineered to express one or more 111v antigens; purified recombinant HIV proteins; pseudovirions and other noninfectious particles that lack 111v genomic RNA: synthetic HIV peptides; and other approaches ( Table 1). Many of these candidate I IIV vaccines have entered early stages of clinical trial in noninfected and/or infected individuals.
Phase 1 testing in noninfected volunteers conducted by the National Institute of Allergy and Infectious Diseases (NIAID) AIDS Vaccine Evalution Group (AVEG) has yielded valuable information concerning the safety and immunogenicity of many of these firsl generation candidate vaccines (48-56). All 111v-1 vaccine candidates evaluated by the NLAID AVEG in noninfected individuals have been safe and well tolerated. Observed side effects such as local pain and tenderness. low grade fever and minor systemic symptoms, some of which also occur in placebo recipients, are typical of those observed with other vaccines. (When the immune modulator muramyl tripeptide-phosphatidyl ethanolamene is added, fever and malaise have been more prominent.) No major side effects such as renal, hepatic or neurological toxicity have been reported. With respect to immunogenicity. almost. all products evaluated induce binding antibody lasting from weeks to months (48,49,51 ,55,56). Almost all induce some level of functional antibody, usually against the strain of virus on which the vaccine is based. Functional antibody is defmed here as antibody that can neutralize virus in an in vitro acute HIV infection assay and/or inhibit fusion (syncytia formation) between noninfected and infected cells in culture. Recent preliminary results suggest. that gpl20 may induce more functional antibody than gpl60.
Recombinant (r) gpl60 has also been evaluated in combination with recombinant live vaccinia virus genetically engineered to express the gp 160 envelope protein (vac-env) (54,56). After two priming doses ofvac-env, little antibody was produced . Following subsequent immunization with gpl60. binding and functional antibody was elicited. Indeed, a higher percentage of individuals produced functional antibody to the vac-env plus gpl60 combination than to gpl60 alone in comparable immunization protocols (56).

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Memory T cells may be essential lo induct.ion of a rapid protective immune response upon subsequent exposure to virus. HIV-specific lymphoproliferative responses to HIV antigen, indicative of HIV-specific T cell memory, have been induced by the candidate vaccines in every trial from which data are available (52)(53)(54)(55). CTLS may be needed to clear virus-infected host cells and produce a 'sterilizing immunity'. Thus far , CDS+ major histocompatibility complex (MHC) class I -restricted cytotoxic T lymphocytes (CDS+ CTL) have been observed only in volunteers immunized with vac-env followed by boosts with rgpl60 (57,58; personal communication). CTLS cloned from selected volunteers demonstrated cytolytic activity specific for HIV-infected cells in vitro (57). The vac-env and gpl60 combination also elicited CD4+ MHC class 2-restricted CTLS (CD4+ CTL). In one study, CDS+ CTL clones from these volunteers were more potent than CD4+ CTL in lysing Epstein-Barr virus-transformed autologous B cells transfected with a eukaryotic expression vector carrying the lllV envelope gene (57) . One CDS+ clone lysed target cells at effector:target ratios as low as 0.03:1 (57). Furthermore. CD4+ CTL clones, but not CDS+ CTL clones, lysed gpl20-pulsed noninfected cells, suggesting that CD4+ CTL might have a detlimental effect in the presence of significant levels of gp 120, although the physiological significance of this observation remains unknown (57).
Pelipheral blood lymphocytes isolated from selected volunteers primed with vac-env and then boosted with rgpl60 were able to protect severe combined immuno-us deficiency mice from HIV infection (59). Protection correlated more closely with T cell proliferation responses than with the level of antibody in the donor. However. 100% protection was nol achieved, and lhe number of protective cells waned with time after the gpl60 boost.
A number of novel adjuvanls are being developed for use in human vaccine fon11ulations (60,61). Recent meeting reports of animal studies (62)(63)(64) suggest that. in addition lo increasing the breadth, magnitude and duration of the antibody response, some of these novel adjuvants show promise in stimulating cos+ CTL. Several of U1ese adjuvants are expected to be employed in prototype HIV vaccine formulations in clinical trials in the near future, in hopes thal both humoral and cellular HIV-specific immune responses can be elicited or augmented.
All products evaluated to date are based on HIV-1 LAI. HIV-t MN or HIV-I SF2, which are all laboratory strains of the virus that belong to the same clade or subtype of 1 nv-1. A high research priority is lo determine the degree lo which antibodies induced by these candidate vaccines recognize HIV isolated from infected individuals in the United Slates and other countries. Multiple products or cocktails of products may be required lo achieve sufficiently broad in1mune responses.
Most available information is from trials of a candidate HIV vaccine, rgpl60, which include the only completed, double-blind, placebo controlled trials of rgpl60 in infected individuals (65). Candidates for which safety information has been made available have not elicited any significant toxicities (65.66). Furthermore, rgpl20 and rgpl60 have been shown to elicit both new humoral and new cellular immune responses in HIV-1infected individuals. Specifically, rgpl60 has elicted new antibody responses directed against llie Cl, c2, C3 and V3 regions of the HIV envelope, augmented levels of HIV-specific CTLS, induced delayed-type hypersensitivity responses lo 111v envelope and increased gp 160-specific lymphocyte proliferative responses.
Ongoing and plarmed placebo controlled phase 2 and 3 trials will help determine if any of these inmrnne responses lead lo changes in viral load and/or stabilization of CD4 cell counts. However, it. is uncertain whellier these virological markers prognosticate clinical out.come. Furthermore. clinical end-points in HIVinfected ind ividuals with more than 500 CD4 cells/mm 3 would take an inordinate time to reach. These considerations have led lo a proposal that larger trials should begin in the absence of phase 2 data demonstrating thal immunization leads to positive changes in progres-

SUMMARY
Candidate HIV vaccines are being developed in hopes thal one or more will prolecl noninfecled individuals from infection upon subsequent exposure lo virus, and/or will elicit new responses of llierapeutic benefit in individuals in whom infection is already established. Safety concerns associated with the more lradilional approaches of vaccine design, combined with the opportunilies provided by advances in biotechnology, have resulted in the pursuit of numerous I IIV-1 vaccine designs. The techniques of molecular biology are being applied to an unprecedented extent in the design and delivery of candidate 1-1rv-1 vaccines. Mosl prototype vaccines are pu1;fied recombinant. I IIV envelope proteins. Results with these products have been encouraging. All products for which information is available have proven safe and immunogenic in noninfected and/or infected individuals. Additional information on the duration, breadth and functionality of the immune responses will be needed before efficacy trials in noninfect.ed individuals are likely lo begin. Import.ant information on the benefit of HIV vaccines in infected individuals will likely emerge int.he nexl few years.
Additional novel candidate vaccines are in earlier stages of development, entering human trials in 1993 and beyond. These include viral and bacterial vectors that express one or more HIV proteins, peptide-based approaches, new particle designs and others. In addition, llie success of live-altenualed s1v in prolecling monkeys from infection by SIV has resulted in renewed interest in live-attenuated designs as prophylaclic vaccines (68,69). Should problems or disappointing results with current prophylactic candidates arise, the risks and benefits of live-allenualed products will have to be reconsidered. In llie interim, additional preclinical and safety and efficacy studies of live-allenuated and other novel designs is of high priority.