Rev. Med. Virol. 2008; 18: 135–138.Published online in Wiley InterScience

(www.interscience.wiley.com)Reviews in Medical Virology DOI: 10.1002/rmv.579

Can we make vaccines that protect better thannatural immunity does?

Vaccines have delivered tremendous benefits toindividuals and populations at amazingly lowcosts, yet protection is mostly available againsta restricted set of viruses which cause acuteinfections. What are the prospects of developingvaccines able to protect against viruses whichestablish long-term persistent infections or whichcause recurrent infections despite inducing naturalimmunity?

Pessimists declare that we will never be able tomake vaccines to protect against viruses that per-sist in the host; after all these viruses have immuneevasion genes which allow them to bypass naturalimmune responses so they should be expected todo the same for vaccine-induced immunity. Theoptimists among us respond that all persistingviruses pass initially through an acute phase andso may be susceptible to intervention at thattime, before they have had the chance to expressmany of their immune evasion genes. Further-more, viruses which persist can be divided intothose which hide from the immune response andthose which run from it [1]. While the latter, suchas HIV and HCV, present formidable challenges tothe development of vaccines, the former may beeasier targets precisely because they are forced tohide from natural immunity. Thus, just as thevalue of free speech can be seen by the lengths towhich totalitarian regimes will go in order tosuppress it, so the potency of natural immunityis illustrated by the resources committed bysome viruses in order to hide from it. The hypoth-esis that viruses with the potential to become per-sistent can be prevented from establishing initialinfection and hence persistent infection has beentested three times now in humans and has comeup trumps on each occasion. Hepatitis B vaccine(our first vaccine against human cancer) readilyprevents establishment of the chronic phase ofHBV replication which is responsible for cirrhosisand hepatoma. Human HPV vaccine (our secondvaccine against human cancer) likewise prevents

establishment of the chronic phase of the onco-genic HPV16 and HPV18 infections which areresponsible for most (75%) cases of cervical cancer.Chickenpox vaccine (our first vaccine against ahuman herpesvirus), presumably protects againstprimary infection of VZV by neutralising the inputvirus, so also decreasing the chance of it establish-ing latency and reactivating later to cause zoster.

Of course, even better results might be obtainedif the viral vaccine could be engineered to removethese immune evasion genes in the first place. Thispossibility is being tested in mouse models ofHSV-2 or murine CMV. Wild-type HSV-2 inter-feres with type-I interferon production [2], impairsthe maturation of dendritic cells [3] and decreasestheir ability to migrate to lymphoid tissues [4]. Ithas been known for some time that these effectsare increased by virus replication [5], so thatstrains with deletions in essential genes no longerdemonstrate such evasion of innate and adaptiveimmune responses. Specifically, viruses with dele-tions in both UL5 (a component of the helicase-primase complex) and UL29 (a single strandedDNA-binding protein) can induce protectiveimmunity despite being unable to replicate in thehost or establish latency [6,7]. These vaccine candi-dates have to be propagated on complementingcell lines which provide the missing viral functionsin trans. Note that, although mechanistically simi-lar, such production differs from that of disabledinfectious single cycle vaccine strains because thelatter produce virions (for one round only) in thehost [8]. This emphasises that the qualities ofthe initial encounter with virus can alter the trajec-tory of future immune responses, perhaps reflect-ing antigen processing by antigen-presenting cellsor modulation of the immunological synapse.Regardless of the details, empirical studies showclearly that these double-deleted HSV strains pro-duce an immune response in animal modelswhose magnitude and durability are excellent,with neutralising antibody titres greater than those

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elicited in mice by recombinant soluble glycopro-tein-D (which has been studied in humans asa vaccine candidate [9]). Furthermore, deletionof a third gene (UL41 which encodes the virionhost shut off function) increased immuneresponses even further. Compared to the double-deleted HSV, the triple-deleted strain inducedincreased neutralising antibody titres and cellmediated immune responses, measured by classI restricted pentamers for CD8þ T-lymphocytesand intracellular cytokine staining for interferon-� in CD8þ and CD4þ T-lymphocytes afterstimulation in vitro with the appropriate peptides.These enhanced immune responses correlatedwith improved protection against intravaginalchallenge of mice with a high dose of wild-typeHSV-2 even when challenge was delayed until7 months after primary immunisation [10].

Murine CMV possesses a series of immune eva-sion genes which are mostly sited towards the ter-mini of the genome. In a recent study, a total of32 genes were deleted from these termini, includ-ing the 3 known to modulate MHC Class I expres-sion, the 5 known to interfere with the ability ofNK cells to recognise infected cells and an addi-tional gene which impairs dendritic cell function[11]. The deleted virus replicated to wild-typelevels in cell culture but was attenuated in vivo,including in immunodeficient mice. Unlike thewild-type, no reactivated virus was seen in spleenscollected 11 days after immunosuppression bydepletion of CD8þ and CD4þ T-cells plus totalbody irradiation. Despite this attenuation andapparent inability to reactivate, the deleted virusacted as a good immunogen, inducing good levelsof CTLs (although lower levels of antibody) andprotecting mice when challenged with the wild-type virus 20 weeks post-vaccination [11].

Turning to viruses which do not persist, butcause recurrent infections, a rigid timetable coversthe selection of influenza strains to be included ineach year’s vaccine. Vaccine strains have to beupdated annually because the protective antigen,haemagglutinin, undergoes antigenic drift to par-tially escape from neutralising antibody inducedby natural infection or by prior vaccines of thesame haemagglutinin subtype. Why could wenot immunise against a conserved antigen whichdoes not change frequently and so remove theneed for this annual ritual? Immune responseswhich cross react with influenza strains of differ-

ent A subtypes are termed heterosubtypic andthere is evidence for involvement of both B-celland T-cell responses in conferring such cross pro-tection. The diversity of B-cells is importantbecause mice with knocked-out genes encodingcomponents of B-cell function do not show hetero-subtypic immunity, although they are protectedagainst challenge with a homologous virus [12].Potential conserved targets for neutralising antibo-dies include those expressed on the exterior of theinfluenza virion; for influenza subtype A, theseinclude M2 and haemagglutinin (at distinctsites from those targeted on seasonal strains ofvirus) (reviewed in Reference [13]). Interestingly,immune responses are not usually made againstthe relevant parts of these molecules duringnatural infection or vaccination; perhaps becauseof competition with the vast excess of immuno-genic distal regions of the haemagglutinin ectodo-main [14–16]. It is tempting, therefore, to speculatethat some viruses have been selected to expressimmunodominant epitopes to divert the immunesystem away from the most potent sites for neutra-lising antibodies. There has been much interest inM2 as an immunogen because it is highly con-served in influenza strains and because immu-nisation with M2 can protect mice againstheterosubtypic challenge [17–19] For example,recent results show that a DNA vaccine encodingM2 followed by an adenovirus-expressing M2(prime-boost strategy) protected mice againstlethal challenge with two divergent (3-aa differ-ences in M2 ectodomains) subtype A virusesand against one H5N1 strain [20]. Depletionand adoptive transfer studies implicated bothB-cells and T-cells in mediating this protection[20]. Likewise, mice were protected by immunisa-tion with synthetic peptides which span the con-served region of the HA0 precursor proteinwhich is cleaved to form the HA1 and HA2subunits [14,21]. It is interesting that HA0 peptidesprovided better protection against influenza Bthan against influenza A [14]; because the latteris already the target of M2 vaccines, a combinedimmunisation against M2 plus HA0 of influenzaB could potentially provide broad protectionagainst all circulating strains.

In summary, substantial progress has beenmade in several animal models of viruses forwhich licensed vaccines do not exist (HSV, CMV)or which do exist but are suboptimal (influenza).

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Copyright # 2008 John Wiley & Sons, Ltd. Rev. Med. Virol. 2008; 18: 135–138.DOI: 10.1002/rmv

Note that the design of the HSV vaccines was dif-ferent to that of the murine CMV vaccine because,in HSV, the aim was to prevent replication of thevaccine strain in vivo, whereas, for murine CMV,the aim was to allow replication but preventexpression of immune evasion genes. Likewise,while M2 is being considered as an immunogenagainst influenza virus, other studies are doingthe opposite; namely deleting M2 from the wild-type to make viruses with a growth defect inmice [22,23]. Perhaps the take-home message isthat the wild-type has been selected for optimumtransmission within a community, so that interfer-ence with components which modulate replicationor with those which mediate immune evasion bothrepresent rational approaches in producing vac-cines. While we must always be cautious aboutthe relevance of data from animal models, includ-ing the dose of virus used in challenge studies,the results so far are robust enough to stimulatetransfer into humans for definitive randomisedcontrolled trials to determine if they are superiorto the currently licensed vaccines.

P D Griffiths

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Copyright # 2008 John Wiley & Sons, Ltd. Rev. Med. Virol. 2008; 18: 135–138.DOI: 10.1002/rmv