Excerpts from the Full Text
Carmen Lía Murall, Chris T. Bauch, Troy Day
DOI: 10.1098/rspb.2014.1069Published 26 November 2014
The human papillomavirus (HPV) vaccines hold great promise for preventing several cancers caused by HPV infections. Yet little attention has been given to whether HPV could respond evolutionarily to the new selection pressures imposed on it by the novel immunity response created by the vaccine. Here, we present and theoretically validate a mechanism by which the vaccine alters the transmission–recovery trade-off that constrains HPV’s virulence such that higher oncogene expression is favoured. With a high oncogene expression strategy, the virus is able to increase its viral load and infected cell population before clearance by the vaccine, thus improving its chances of transmission. This new rapid cell-proliferation strategy is able to circulate between hosts with medium to high turnover rates of sexual partners. We also discuss the importance of better quantifying the duration of challenge infections and the degree to which a vaccinated host can shed virus. The generality of the models presented here suggests a wider applicability of this mechanism, and thus highlights the need to investigate viral oncogenicity from an evolutionary perspective.
The evolutionary responses of viruses to vaccines are of serious concern, and they may appear several years after the introduction of such control measures . In a review, Read & Mackinnon contrast successful vaccines that stimulate natural immunity with novel vaccines that stimulate new responses that differ considerably from natural immunity. They warn that imposing new effector mechanisms can create very different selection pressures, with potentially unwanted consequences . Our findings appear to coincide with this scenario, in that the novel vaccine immunity favours increased virulence in order to allow for transmission during the short window of time before vaccine-induced clearance.
The HPV vaccines change the within-host ecology encountered by the virus in three main ways. First, the vaccine-targeted types experience a strong antibody response that is unnaturally high , and which we find drives the oncogene expression necessary for persistent circulation up further. Second, the vaccine-induced effector cells invade faster, and invasion can no longer be delayed through strategies using slow viral replication and signalling interference. We show that this effect changes the transmission–recovery trade-off such that low oncogene expression strategies are no longer favoured.
Finally, the vaccine adaptive response now exclusively targets epitopes of the capsid protein L1 , which is distinct from natural responses that target the early proteins, E2, E6 and E7, for clearance [13,36]. As the L1 is a late gene whose epitopes are expressed in the upper layers of epithelium or are exposed on the capsids , the vaccine-induced effectors will mainly target free virions and these terminating cells. However, infected cells of the mid- and lower levels of the epithelium express the early proteins, and so should be targeted less readily by the vaccine response. Though this detail is not present in our models, we expect that it could augment the effect we found by selecting against the re-infection strategy and favouring the self-proliferation strategy. In this new environment, variants of the vaccine-targeted types exhibiting higher than average cell proliferation would have an advantage.
Discussions of HPV evolutionary responses have been scant and have focused on the potential of L1 neutralization escape . We believe that we are the first to suggest this kind of evolutionary response in HPV types targeted by (or cross-reactive with) the vaccine. The main form of vaccine ‘leakiness’ that has been addressed in the HPV literature is that of type specificity and whether it can result in type replacement [51,52]. A ‘leak’ that has not been considered, and what we find here to be important, is what happens when the vaccine does not block infection and viral shedding. Given that challenge infections by vaccine-targeted types were detectable in vaccinated women  during HPV vaccine trials, we argue that the vaccine does not always fully block viral shedding. Indeed, a humoral response may not always provide perfect protection from viral challenge . As HPV is transmitted mechanically through the shedding of both free virions and dead infected keratinocytes from the epithelial surface , it is possible that even if the antibody response lowers the free virion population significantly, a vaccinated host could still transmit the virus by shedding infected keratinocytes. For comparison, consider once again the oncogenic MDV example in which shedding of epithelial cells was also involved in transmission. Indeed, the MDV vaccines are leaky because they do not block infection and viral shedding (though this leak is more pronounced compared with the HPV vaccine’s stronger prophylactic effect), which has played an important role in the subsequent virulence evolution of MDV [6,7]. In light of this, we strongly encourage studies of challenge infections in vaccinated hosts, their frequency, their duration and to what degree they shed infected cells. Cross-sectional epidemiological studies or longitudinal time-points separated six months apart will often lack the resolution to address these questions, especially if the challenges are short-lived.
Our model assumes that the high antibody response is instantaneous (δvac is a constant), and thus it captures the prophylactic effect of high neutralizing antibody titres the vaccine is intended to create. Locally, however, there should be lower levels of neutralizing antibodies (e.g. in cervicovaginal secretions)  and there should be a lag from the time of first challenge until the memory B cells induce antibodies and the subsequent cellular response invades at full force. We have not seen empirical estimates of how many days this takes, though their timing could have considerable consequences on the evolution of the virus and its transmission.
To demonstrate the essential ingredients of the phenomenon, our conceptual model had to idealize the viral replication process by neglecting many of its known details. So, although we demonstrate that virulence evolution is possible, we cannot determine with this study whether it is probable. It has been argued that accelerated carcinogenesis is not adaptive because cells in higher-grade lesions do not produce fully assembled virions . However, given that animal models can be infected with DNA plasmids to produce robust, productive infections [55,56], how infectious are keratinocytes containing HPV DNA? Even if cancer cells themselves are not infectious, how infectious are the cells in the lesions leading up to cancer? Experiments are needed to assess to what degree oncogene expression can rise while maintaining viable viral production, infectiousness and transmission. Furthermore, following several challenges to the prevailing view of slow dsDNA virus evolution (where mechanisms such as recombination are possible [57–61]), there is a need for more direct investigations into the evolutionary potential of HPV variants.
In a recent study, Orlando et al.  found that HR types are best suited for transmission in long partnerships (because HR infections last longer), while shorter partnerships with higher turnover rates allow for the persistence of LR types (because LR types are cleared faster). We show here that by artificially shortening the infection duration, targeted HR types can more strongly adopt the strategy of cell proliferation (a strategy that was costly in natural conditions) in order to increase their chance of transmission, thus adopting a similar strategy to LR types. Yet oncogenes of HR types have stronger cell-transforming abilities, and expression at higher levels should more readily cause cellular genetic instabilities and lead to faster progression towards cancer.
Our study does not contain a full population model of interacting hosts, so we cannot investigate the conditions needed for a host population to maintain an emergent vaccine-adapted type. Heterogeneity of hosts plays an important role in the emergence of strains , and indeed we found variation in the optimal oncogene expression required of the virus to persist in different sexual activity groups. For instance, superspreaders required lower viral loads for persistent transmission, and in a highly sexually active core group this could favour the emergence of a variant with higher oncogene expression. Emergence happens in stuttering transmission chains, potentially in small groups of individuals, and certain host groups are more likely to be carriers and superspreaders [63–65]. Therefore, future studies should consider how pockets of core-group individuals (the causal and superspreader groups in this study) or of immunodeficient individuals may contribute to the emergence and circulation of new variants.
In conclusion, the uniqueness of the HPV vaccines lies in that they target a virus that is avirulent for the majority of hosts but has strong cell transformation properties. Other oncoviruses have similar features to HPV, making it likely that this vaccination programme may be emulated in the future. Given that virulence is not a fixed trait in any pathogen, it is in our best interest to understand how we are changing the ecological landscape and the selection pressures acting on the virus, in order to more confidently declare a vaccine’s evolutionary robustness.