Multivalent vaccines may be subdivided into two groups: combination vaccines and poly-epitope vaccines. A combination vaccine usually refers to a mixture of several different components aiming at protecting an individual against several pathogens at once. Its main practical advantage is the reduction in the number of injections into a
Chapter 8: General discussion
single one. The use of combination vaccines requires extensive tests in order to ensure that the immunogenicity and the safety o f the mixture are not reduced compared with each component administered individually. A special issue of Clinical Infectious Diseases (Vol. 33, Sup. 4, 2001) is dedicated to different aspects o f combination vaccines from immunology to safety regulations. Poly-epitope vaccines are designed to elicit a polyclonal response against several determinants of a given antigen/pathogen. At present, most o f these vaccines are aimed at protecting against one pathogen at a time and are also being widely investigated as prospective therapeutic vaccines against cancer. Some examples o f positive and negative interactions between components o f combination and poly-epitope vaccines will be discussed in this section.
The relative immunogenicity o f each component of a combination vaccine may be reduced, enhanced or unchanged compared with monovalent vaccines. This variability may reflect the complexity o f the interactions between the different responding T and B cells within an individual and the genetic variation (polymorphism) between populations. It may also depend on the formulation and treatment schedule of the vaccines tested. This may be illustrated by the two following examples. First, in comparative studies between hepatitis A/B combination and monovalent vaccines, three groups reported a similar immunogenicity (Joines et
aL, 2001; Van Damme and Van der Wielen, 2001; Abraham et aL, 2002). One group
found that anti-hepatitis A titers were higher with the combined vaccine or simultaneous injection (i.e. at two separate sites) of the two monovalent vaccines than with the monovalent vaccine alone, but anti-hepatitis B titers were higher with the two simultaneous injections than with the combined formulation (Czeschinski et aL, 2000). Another group observed that the anti-hepatitis A response was at least as good in the combination vaccine as in the monovalent vaccine, but the anti-hepatitis B response was reduced (Frey et aL, 1999). The authors attributed this effect to “immunologic interference”, since HAV appears more immunogenic than HBV and thus its response may reduce the anti-HBV response by competition. This is consistent with observations by Czeschinski et al. (2000), where inoculation of a combined vaccine (favouring linkage and thus competition) resulted in lower anti- HBV response than simultaneous but separate inoculation (presumably limiting linkage and competition).
Chapter 8: General discussion
The second example concerns combination o f Haemophilus influenzae type b (Hib) vaccine with pertussis-diphteria-tetanus (± poliovirus) vaccine. All the responses to a five-component pertussis-diphteria-tetanus vaccine were enhanced when the vaccine was given combined with a tetanus toxoid (TT)-conjugated Hib vaccine compared with separate injections at different sites (Lee et aL, 1999). Good levels o f anti-Hib were also obtained. However, lower anti-Hib responses have been reported in other similar studies (Eskola et aL, 1999) with combined but not simultaneous inoculation. Different concentrations of active antigens or incompatible adjuvants (e.g. that have opposing effects) have been proposed as possible causes o f this interference. In another study, a TT-conjugated diphteria-tetanus-pertussis-poliovirus-Hib vaccine was used in combination with a tetravalent pneumococcal vaccine (Dagan et aL,
1998). No interference was observed, apart from a significantly lower anti-Hib response with the pneumococcal vaccine that was TT-conjugated. In contrast, no interference was detected if the latter was diphteria toxoid (DT)-conjugated. The authors thus attributed the interference to a common protein carrier. Finally, a pentavalent vaccine (Pentavac, diphteria-tetanus-pertussis-poliovirus-Hib, TT- conjugated) was combined with a hepatitis B vaccine (H-B-Vax II) to form Hexavac. The response to Hexavac was similar in most aspects to the response to simultaneously injected but not combined Pentavac and H-B-Vax II, except that with Hexavac, antibody titers to Hib and HBV were 2-fold lower and titers to poliovirus were enhanced (Mallet et al, 2000).
Despite the interference sometimes observed in these studies, it has been emphasised that the levels of antibody obtained with combination vaccines were still adequate for protection in most cases. The mechanisms underlying either enhancement or reduction of the response to one component within a combination vaccine remain unclear and may depend on factors such as the relative amount and immunogenicity o f each component. Furthermore, it is well established that the conjugation of an antigen with an immunogenic carrier protein such as TT or DT improves the response, an effect attributed to linkage (Mitchison, 1971). However, the number of carrier-specific T cells may be limiting and the overloading of antigens conjugated to the same carrier protein would then lead to competition between each antigen- specific population for help from a limited number of carrier-specific T cells, resulting in decreased responses (Dagan et aL, 1998; Fattom et aL, 1999).
Chapter 8: General discussion
In the case o f poly-epitope vaccines against a single pathogen, it is possible that less interference and more synergy occur, because determinants are related and may induce similar rather than antagonising responses. For example, in a tetravalent DNA vaccine against tuberculosis compared with monovalent immunisations, IgG titers were either unchanged for two components, enhanced 4-fold for one component or reduced 2-fold for the fourth component (Morris et aL, 2000). In a divalent vaccine against borreliosis, the level of protection achieved against B. burgdorferi challenge in mice was 100-fold higher than with only one o f the two components (Hanson et a l, 2000).
Several DNA vaccines have been designed for the delivery o f multiple epitopes, but most studies have so far been carried out only in animal models. Anti-malaria vaccines, for instance, have received considerable attention because protection against malaria requires immunity to the different stages o f the infection. In a mouse
model, immunisation with plasmids encoding four antigens o f Plasmodium
falciparum induces antibody levels similar or higher than single-plasmid injection (Grifantini et aL, 1998). The Thl/Th2 balance obtained (measured as IgG2a/IgGl ratio) was similar between the antigens and was not affected by the plasmid combination. In monkeys, a trivalent DNA vaccine induced, after four immunisations, 3 to 12-fold higher antibody titers against P. falciparum than single vaccines, suggesting absence o f interference and possible synergy between these antigens (Jones et aL, 2002). Some antigens of this parasite, such as CSP, contain polymorphic dominant epitopes, with limited cross-reactivity for them in humans and mice (Zevering et aL, 1998). In this context, immunisation with several epitope variants seems mandatory.
When epitopes are well characterised, it may be more practical to use “minigene” constructs, which express only the immunodominant epitopes from one or several antigens/pathogens (Gurunathan et aL, 2000). Efficient CTL induction may require both class I and class II MHC-restricted epitopes (Maecker et aL, 1998) or class I MHC-restricted epitopes only (Thomson et aL, 1998). Moreover, protection may be achieved with a B cell-specific epitope, rather than with the whole intact protein antigen using such construct (An and Whitton, 1997). In mice, detectable responses, sometimes conferring complete protection, were mounted after minigene