Vaccines: around which corner?

Immunology Letters, 19 (1988)245-250

Elsevier IML 01113

Vaccines: around which corner? B r i d g e t M. Ogilvie The Wellcome Trust, 1, Park Square West, London NWI 4LJ, U.K.

(Received22 August 1988; accepted 22 August 1988)

1. Introduction During the past decade, the development and application of the techniques of molecular genetics has revolutionised our understanding of many aspects of biology. This has resulted in claims that the power of the techniques of this discipline will quickly result in new vaccines for even the most intractable infections. The purpose of this article is to address the question of what problems must be overcome in the development of vaccines and to suggest that many unsolved and frequently unacknowledged difficulties lie in the path which must be followed in vaccine production and delivery. The difficulties can be classified into three sets of problems which require attention: (i) the identification of key antigens and their production, (ii) the nature and specificity of the host's immune response, and (iii) the social, political and economic realities of vaccine production and delivery (Fig. 1). Whilst these three elements are interdependent they are also rather different in the problems they pose and the extent to which the problems have been solved or are capable of solution. They will therefore be discussed separately.

2. Antigen recognition and production Most antigens are predominantly protein in nature, so that the application of the techniques of molecular biology has resulted in rapid progress in the identification and production of antigens of this Key words: Vaccines Correspondence to: B. M. Ogilvie,The WellcomeTrust, 1, Park Square West, London NWI 4LJ, U.K.

type. In this situation the main question to answer is, how may we identify the appropriate molecules? With some viruses, where the complete structure is known it is possible to examine systematically the various component molecules in turn to assess their antigenicity in the induction of immunity [1-4]. Bacteria and parasites are very much larger, and here approaches to the identification of likely protective antigens are pragmatic, based on the obvious principle that the host's immune system is most likely to be exposed to secretions from or the surface of the organism concerned. Some of the most successful bacterial vaccines are based on secreted molecules, e.g., those used against tetanus and diphtheria. A major source of bias which may mislead in terms of antigen recognition is that most antigens have been selected for study by the use of antisera obtained from infected animals. The approach is well illustrated by the following quotation from Zanetti et al. [5]: "'Wherever possible thepeptide or site o f the antibody which actually dominates in a panel o f convalescent sera should influence the choice o f B-cell epitope f o r the vaccine immunogen".

Whilst at first sight this may seem the most logical approach to the problem, in the case of those infectious organisms which cause chronic disease, it has long been clear that the most prominent antibody responses induced by infection are not readily or directly related to protection. Nevertheless, this approach is a c o m m o n one with many infections, leading to the interesting recent discovery that certain of the antigens producing the dominant antibody responses post infection are at least in part identical with the so called "heat shock" or "stress" proteins [ 6 - 9]. The elucidation of why infectious organisms

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produce these proteins with such highly immunological epitopes is itself of considerable interest, though to this author at least it would seem improbable that the response to this set of molecules is likely to be important in the induction of protective immunity. For good practical reasons, the epitopes that might induce cell mediated immunity have been much less studied. Where they have, the results have been surprising. For example, James and her colleagues in studying the cell-mediated immune response (CMI) of mice to Schistosoma mansoni have demonstrated the central role of CMI in protection in this host to this parasite and shown that the antigen involved is paramyosin [10]. This molecule is found beneath the parasite surface in membrane bound bodies in the tegument [11]. Other epitopes that induce a degree o f protection to S. mansoni have been identified using antibodies directed against the surface of this parasite [12], It has become clear that many of the epitopes likely to be key in the protective immune response to bacteria and parasites are glycoproteins or even glycolipids [9]. Carbohydrates and lipids cannot be identified or produced by the direct application of DNA technology, though such methods are clearly valuable in identifying the synthetic machinery used to produce them. Moreover, sometimes key epitopes appear to be structural in nature, related to the 3-dimensional orientation of the antigenic molecule rather than to its peptide or non-peptide nature [13]. 246

In these cases methods additional to those of molecular biology are needed and considerable ingenuity has already been applied to address these problems [5, 14-16]. In summary, it does not seem that we have reached the limits of scientific ingenuity in the identification and production of epitopes of this type. There is, however, an aspect of antigenicity which in one case has already revealed an insurmountable barrier to vaccine production: the ability of some infectious organisms to undergo antigenic variation. The best studied infections in this context are those induced by African trypanosomes, and here all concerned appear to agree that the exceptional ability of this protozoan to vary its surface antigens makes the control of the infection by vaccination virtually out of the question [17]. Whether this will prove to be the case with the human malaria parasite Plasmodium faiciparum, which has a remarkable capacity to vary its antigens [18] remains to be shown. A similar situation exists with HIV [1.] possibly also bacteria such as Neisseria and Borrelia spps. [19], but whether antigenic variation will prove to be so highly developed as to be insurmountable in the case o f these infections is unclear. In conclusion, I would suggest that in principle at least, the remarkable advances that have been made in identifying and producing antigens that induce protection suggest that, at least in terms of this aspect of vaccine production, we will fail in the development of vaccines only if the interesting organism's capacity to undergo antigenic variation makes

this approach to control impossible. It is important, however, to note that the understandable bias towards the use o f antibodies to identify such antigens might be misleading, and that the production of epitopes that are not peptides or are related to the structure o f the molecule are serious but probably not lethal impediments in the path o f vaccine development. 3. The host: nature and specificity of the response

The major problem facing vaccine development is how to identify the right kind o f long-lasting immune response. This is the area of vaccine production and use in which the majority of the unidentified and least understood problems o f immunology remain. We are still far from knowing how to switch on and control the precise immune response we require to prevent infections [20] even if we knew the exact nature and specificity of the protective immunological response required. This last situation scarcely exists, though it may soon be the case with influenza. The elucidation of the complete structure o f certain viruses has enabled scientists to determine the nature and specificity of the protective immunological response to each part of its structure. With influenza, the surface haemagglutinins which undergo rapid antigenic change are largely recognised by T-helper and antibody responses, whereas the relatively invariant nucleoprotein seems much more likely to induce cytotoxic T cell responses [3, 21]. Such detailed knowledge of the precise relationship between antigen structure and the host's immune response is, however, rare. We almost always have to find out by trial and error whether the epitope(s) we have identified will induce the right immune response. This is particularly difficult when animal models are scarce or very expensive. For example HIV, falciparum malaria, and the filarial nematodes are all highly host-specific and can be studied only in chimpanzees and in primates other than man. The importance o f understanding the nature of the host's response is demonstrated by the knowledge that the wrong or an inadequate immune response may lead to pathology or immunosuppression, not protection. For example, whilst there is evidence that in the human and rat host, protective immunity to S. mansoni is antibody-dependent, in both rats and man it has been shown that the same

antigen may induce both protective and blocking antibodies [22, 23]. Again, in the case of chronic infections induced by intracellular organisms such as Mycobacteria spp. and Leishmania spp., a spectrum o f responses is known to occur, varying from disseminated disease in which antibodies frequently dominate the immune response to no or localised lesions when cell-mediated immunity is the major response. It appears at present that protective immunity and the severe, frequently debilitating or disfiguring pathology found in these infections are closely associated. Thus, the following questions require attention. How can we induce cell-mediated responses rather than antibodies? Are the epitopes(s) that induce protection and pathology identical? How can we ensure that the cell-mediated response once switched on does not lead to pathology? These questions illustrate the undeniable fact that our present knowledge of immunity does not allow us to generate with precision the immune response we want, and nor do we know how to regulate a response once it is switched on. The hazards of immunising in the absence of full knowledge of the nature of the host's response are .illustrated by the result of attempts during the 1960's to immunise children with killed vaccines against measles and respiratory syncytial virus (RSV). In some of those vaccinated this way subsequent exposure to infection resulted in exacerbated rather than reduced disease [24]. Our knowledge concerning what controls the balance between types of immune responses and what switches on one in preference to another is slight. We have known for years that the nature o f the antigen, its route o f presentation, and the amount and frequency of administration all have an influence, [25, 26] but we do not understand why. In this context, recent progress concerning the nature o f T-helper cells and the interleukins that regulate the type of immune response is a hopeful development [27, 28], but far more knowledge is required before such information can be put to practical use. Another aspect of the host's response which may inhibit vaccine development is that antigen recognition is often poor. Here again the nature of the antigen is a key factor, but what determines whether an antigen is a good or bad immunogen and what makes a good adjuvant are extremely poorly understood areas of immunology [29]. Generally it seems 247

that living infections or living engineered vectors produce a better response than the injection o f nonliving antigens with or without adjuvant [30]. It is also clear that the response to small epitopes, whether peptides or sugars, is greatly improved when they are coupled to carrier molecules [31, 32]. The success of the vaccine for hepatitis B is related in part to the fact that the antigen is produced in a particulate and therefore highly immunogenic form both in the infected host and in expression vectors [4]. Host factors are also crucial in terms o f antigen recognition. Genetic constitution is often the determining factor in epitope recognition and there are now many examples which show that both M H C and non-MHC genes are involved in this aspect of immunity [15, 16, 18, 20]. The age of the host is frequently crucial. Young children do not respond well to the bacteria involved in respiratory infections (Haemophilus influenzae,

Neisseria meningitidis, Streptococcus pneumoniae) because their ability to recognize the crucial carbohydrate epitopes is not mature, although it can be enhanced by coupling the epitope to a carrier [31]. A similar situation exists in sheep infected with the important nematode of the gut, Haemonchus contortus, although the reason for the prolonged period of neo-natal immaturity in sheep to this organism is not known [13]. Another important aspect of neonatal immunology is that the presence of maternal antibodies in the young will impair their ability to respond to antigens. Finally, many important pathogens infect mucosal surfaces, and the oral route is in practical terms the best way to administer vaccines against any infection. But the immunology of mucosal surfaces is one of the least understood aspects of immunity. Here an interesting and promising advance is the development of strains o f Salmonella spps. which lack virulence genes for use as carriers o f genes that produce antigens from a variety of organisms [30, 33]. An ideal vaccine will induce a long-lasting immunological response without the need for constant restimulation. What is immunological memory? This is another ill-defined area. It takes time to develop, requires the activation of T cells, is hard to achieve with a single shot o f a non-living vaccine and is generally only achieved with living infections or living engineered vectors. Why it should be necessary to have antigen presented over a period of time 248

and exactly what the cellular and interleukin requirements are for the induction of immunological memory represent areas o f immunological ignorance which are of great importance to the development of vaccines. A key question which is well understood by veterinarians but may not be practical in human medicine is whether it is sufficient for a vaccine to control the disease associated with an infection or must the infection itself be completely prevented? In the case of AIDS, it would seem that any vaccine must give complete protection against the viruses involved if it is to be acceptable. But with parasites, successful vaccines have been developed for veterinary use which protect against disease whilst reducing but not preventing infection. A vaccine against dog hookworm, Ancylostoma caninum, consisting of irradiated infective larvae, gave reliable immunity with little disease although parasites were still present. This vaccine was a scientific and veterinary success which failed to convince dog owners [15, 16]. A vaccine for use in fowls against the protozoan parasites causing coccidiosis is in the late stages o f development. It consists o f attenuated strains of all the 7 species concerned, and gives excellent protection against disease although quite a high level of infection occurs [34]. In this context it is worth noting that Eimeria spps. are sporozoan parasites, as are the Plasmodia spps. which cause malaria. In areas o f endemic infection with malaria, disease is associated mainly with young children. The parasite persists in the rest of the human population, mostly in the absence of illness. Might it be possible to stimulate an immune response to malaria which prevents disease and transcends antigenic variation? Finally, in considering the nature of the host's immune response it is important to note that vaccines discovered and long in use in the developed world sometimes fail to work well when given to people in the third world [35]. Explanations for such vaccine failures may be related to many factors, but environmental effects on the host's immune system may sometimes be the determining factor. It has long been known that immune responses are impaired in all animals that are malnourished and subject to frequent intercurrent infections, sadly the fate of a high percentage of the human population of the world.

4. Social, political and economic realities These factors are frequently crucial in determining whether vaccines will be developed for, delivered to, or properly assessed in the population at risk. It is not widely appreciated that cloning the gene(s) for the protective immunogen(s) is the first, easiest and probably the cheapest step in the long process of vaccine development. In the case of the one successful engineered vaccine so far produced for human use, the hepatitis B surface antigen vaccine, it has been suggested that this step represents only about 10% of the eventual cost of its development [36]. Thus, development costs are high, and vaccine markets are usually small and highly targetted. Whereas many antibiotics can be used against a variety of infections, vaccines must be specifically developed for each infection, and even in the USA, returns for even very successful vaccines are small [15, 16]. Postmarketing surveillance is a continual high cost necessity, patent protection is difficult and the risk of litigation over presumed deleterious consequences of vaccination are ever increasing. The combination of these factors has resulted in many companies deciding not to continue the production and marketing of vaccines. Government action is required, not only in the centralised economies where these factors are not so determining, but increasingly in Western countries as a consequence o f the financial realities associated with vaccine development. Those companies that have continued to produce vaccines do so largely as a public service, but dependence on this kind of goodwill will not solve the problem. There are major difficulties in measuring the effectiveness of vaccines particularly for infections that are not acute. Diagnosis may not be easy, and in many countries not reliable. For example, where malaria is endemic the occurrence of fever is usually equated with the presence of malaria. Should the presence o f disease or the presence of the organism be the diagnostic criterion? This is not a trivial matter in many countries without well-organised public health systems, and especially in dealing with chronic infections that may be quite rare. If the immunogenicity of a vaccine is assessed immunologically, it is almost always by measuring the antibody rather that the cell-mediated immune response. The reason is largely practical, because antibody re-

sponses are relatively much easier to measure than T cell responses, but it is illogical to measure the antibody response when the infection is controlled by a cell-mediated reaction. The T cell response and cellular responses in general are tremendously understudied because the techniques required are complex and need good laboratories and trained personnel. There seems to be a real need and plenty of potential still for the development of simplified field kits for the more ready assessment of this arm of the immune response. The practical problems of handling vaccines under the conditions that exist in much o f the world should not be underestimated. The maintenance of a cold chain, storage under adverse conditions, the problems of temperamental communications all affect the viability o f vaccines even before they are delivered [16]. Organisation and commitment are essential in vaccine delivery: if multiple vaccinations are necessary before individuals are protected, the problems of ensuring recall can be overwhelming. Assessment of vaccine efficacy takes time and requires trained staff. It is therefore hardly surprising that many already available, tried and tested vaccines are underused although they are cost-effective. Galazka et al. (1984) [351 pointed out that in tropical countries one child in 100 dies from tetanus, 1 in 50 from pertussis and 1 in 200 from polio, all conditions for which vaccines have long been in use in the developed world. In conclusion, there are three interlinked but distinct aspects of vaccine production and delivery. Rapid advances and real success have occurred in antigen recognition and production. The fulfilment of these advances is being impeded by existing holes in our knowledge of the nature and specificity of the host response whether there is an overemphasis on antibodies with corresponding neglect of cellular responses. Our knowledge of the cellular side is gathering pace, but much more is required before many practical problems can be dealt with. On the host side too, genetic unresponsiveness and poor immunogenicity are serious problems preventing advance. Finally, the economic, social and political realities that may impede or prevent vaccine production and delivery are extremely important factors. The solution requires government action, both nationally and internationally. There seems little doubt that if the World Health Organisation did not al249

ready exist, it would need to be created to deal with this problem.

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