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Opportunities to improve immune-based prevention of HPV-associated cancers

Immunization of adolescent girls with VLP vaccines, made of L1 proteins from the most medically significant high risk HPV types, is a major strategy for prevention of cervical cancer plus other HPV-associated cancers. Maximal population impact, including through herd immunity, requires high vaccination coverage. However, protection of unvaccinated women requires secondary prevention through cytology screening. Unfortunately in countries with the highest incidence/mortality due to cervical cancer HPV vaccination (or cytology screening) is not sufficiently available. Vaccination programme costs and a lack of accessibility of the populations for immunization remain significant hurdles. Several approaches could increase effective implementation of HPV vaccination. 1) Use of a single immunization of the current VLP vaccines. 2) Vaccination bundled with other paediatric vaccines with lower dosage to facilitate delivery, improve coverage and reduce costs through established logistics. 3) Local manufacture with lower cost systems (e.g. bacteria) for VLP or capsomer based vaccine production and utilization of additional protective epitopes (e.g L2) for increasing breadth of protection. However, all the latter need appropriate clinical validation. Gender neutral vaccination and extending routine vaccination strategies to women up to age 30 years in combination with at least one HPV screening test can also hasten impact on cancer incidence.


Introduction
Typical preventative vaccines act to limit rather than stop initial natural infection. Such contained infection elicits early and efficacious immune memory responses in the exposed vaccinated individual to prevent the development of clinical disease [1]. Because of the slow infectious process and spatially-separated productive virus life cycle of HPV, the VLP vaccines may differ from this paradigm in that L1-specific antibody can fully neutralize the inoculum to confer sterilizing immunity against the vaccine related types.
The available 2v-and 4v-vaccines both target the HPV 16 and 18 types, which account for about 70% of cervical cancers and 90% of HPV-related cancers at other sites, and the 4v-vaccine also contains VLPs for HPV 6/11, which cause benign genital warts. However, cross protection against other 16/18 related oncogenic HPV types is most evident with the 2v-vaccine, and the levels of protection against high grade cervical intraepithelial neoplasia in clinical trials was about 93%.
Trial data on a 9v-vaccine (4v-vaccine plus VLPs for HPV types 31, 33, 45, 52, 58) documented a similar level of protection against high grade disease. Time will tell if this robust efficacy mediated by type-specific and/or via cross protection show similar longevity. None of these VLP vaccines provide comprehensive protection against all oncogenic types, but they target the most medically impactful in cancer worldwide. Prevention of cervical cancer (∼0.5 million cases per annum), accounting 90% of HPV associated cancers in females, is a principle goal although there will undoubtedly be an impact on other HPV-associated anogenital and oropharyngeal cancers. With the focus on cervical cancer, the general vaccination strategy has been the immunization of adolescent girls twice 6 months apart. The available evidence suggests that the levels of neutralizing antibodies may provide life long protection [2]. Older individuals may also benefit from VLP vaccination as the majority are not infected, and for those already exposed there are obviously multiple additional HPV types where protective immunity would be valuable.

Limitations of current HPV vaccination
A key aspect of maximizing population protection is herd immunity for which vaccination coverage needs to be at least 70-80%. A genderneutral strategy can enhance the effectiveness of HPV vaccination for cross-protected HPV types with low to moderate coverage. Indeed, high coverage in males might be the key to a substantial public health benefit against anogential and oropharygeal cancers of men, and indirectly in unvaccinated females [3]. However, there is a considerable lag time of > 20 years for approaching the full impact on cervical cancer rates even with very efficient national HPV vaccination programs. Such programs are usually school based although this not be suitable in the context of the social structures of all countries [4]. HPV-FASTER aims to extend routine vaccination in women up to the age of 30 which in conjunction with a least HPV screening test at older ages has been modelled and predicts an accelerated cervical cancer reduction [5]. Secondary prevention through organized cervical screening will remain central to reduction in cervical cancer rates even in developed countries during this phase because the vaccines do not impact prevalent infection and disease [6]. While many women from high-income and upper-middle-income countries have or are being vaccinated against HPV, women from countries afflicted by the highest incidence and mortality of disease remain largely unreached [7]. Unfortunately, sufficiently organized screening/treatment programmes are also not widely available where there are the highest HPV associated cancer rates. While routine vaccination can provide for comprehensive protection against HPV associated disease, accessibility of populations for vaccination (school, gynaecologist, other), the lack of infrastructure in non-developed countries and the prioritisation of funding for this purpose (and competition for resources for cervical screening) remain imposing barriers. Bundling of HPV vaccine with other childhood vaccines might help address some of these challenges, given evidence of durable immunity.

Strategies for increasing HPV vaccination impact
There are inherent limitations to the current VLP type vaccines as they do not protect against all hr HPV types, require a cold chain and are relatively expensive to manufacture. Extending the type specific coverage by adding more VLP types may not be sufficiently more efficacious or cost effective; for example, in terms of cancer prevention the 2v Cervarix vaccine adjuvanted with AS04 could possibly provide a similar impact on rates of cervical cancer as the 9v Gardasil, although the latter also targets benign genital warts. Only time will tell whether the cross protection has sufficient longevity. While VLP HPV vaccines are not designed to be therapeutic 9v Gardasil is being examined in a phase 3 study (NCT03848039) as adjuvant treatment after lesion excision to prevent recurrence or guard against another HPV type infection [8].
There are observations that indicate that a single immunization with an HPV VLP vaccine can provide efficacious protection [9]. This may result from the nature of the repetitive VLP protein structure, an optimal size for trafficking to the lymph nodes and subsequent acquisition by follicular dendritic cells with an optional spacing of the epitopes to engage the B cell receptors. These features provide for the generation of a pool of long-lived plasma cells able to provide long-term protection [10]. If single dose protection is validated in on-going clinical trials this could improve vaccination coverage, although the longevity of protection, especially if delivered in a paediatric setting, and the impact on cross-protection will also need careful assessment.
While sufficient herd immunity can impact HPV-associated cancers in males there are around 34,000 cases per annum including those involving the oropharynx, which are 2.5 fold more prevalent than in females. Increasing the vaccination of boys before first exposure can therefore also impact disease rates and promote herd immunity [11].
In the history of successful implementation of vaccination, successive developments have provided for increased range of specificity, improved coverage and logistics underwritten by improved cost effectiveness. Such potential opportunities are being investigated through different VLP production (e.g. in bacteria or the use of capsomere subunits) with selective adjuvantation and local manufacture. Extending the protection to more hr HPV types and beyond may also be achieved by increasing valency, or inclusion of the broadly cross-reactive and protective epitopes (e.g. RG-1) of the amino-terminal of L2 [2]. Table 1 summarizes the current VLP based vaccines that are in advanced clinical trials.
Sufficiency of population coverage is central to vaccine impact, and this remains the most formable obstacle to worldwide elimination of cervical cancer. While approaching this goal may eventually be achievable in countries with both high coverage cervical screening and vaccination (usually school based) programmes [Brotherton & Bloem; Canfell both this issue], this remains unfeasible in the poorest countries with inadequate infrastructure for repeat vaccination of young adults. By contrast, paediatric immunization against a variety of pathogens has achieved high levels in global routine vaccination coverage [12]. If HPV vaccination could be given as a single dose in infants, or bundled with other standard multi-dose childhood immunizations, the prospects for delivering substantial worldwide protection would be significantly improved (see Table 2A, B). This will require the appropriate bridging, dose-ranging and safety studies in infants and very long term follow up. Lower paediatric dosages may provide cost savings in addition to improved logistics for administration. Current generations would continue to rely on adolescent vaccination and screening in the meantime. Resource poor countries with no HPV vaccination or screening are those with the greatest need and the most to gain, and GAVI is one tool for helping to extend the benefits of HPV vaccination but this demands commitment from governments to sufficiently prioritize cervical cancer prevention.

Therapeutic vaccines
HPV therapeutic vaccine strategies have mostly focused on generating specific effector T-cells against the constitutive and functionally obligate expression of E6 and/or E7 oncogenes. Recently, several clinical trials of therapeutic HPV oncogene vaccines tested in patients with HPV-associated high-grade anogenital lesions have shown encouraging  HPV vaccination of adolescent girls will directly impact on established cervical screening programs. This is likely to result in an increased screening interval & the use of primary HPV testing. In developed countries it will take a long time before screening can be abandoned, if ever, but the overall costs will diminish in the long term.
Those countries with effective national cervical screening programs will continue to provide this service because of the prevalence of HPV infections in unvaccinated women. Current algorithms plan to exploit the negative predictive value of hr HPV testing to reduce the number of lifetime screens. HPV + individuals will be referred for cytology & treated surgically if necessary. There will be a significant number of individuals who present as HPV + but cytology negative & if this is sustained then they will be referred for colposcopy but that may not identify any lesion. clinical impact [13]. In the natural history of hr HPV infection, E6/E7 oncogene expression drives immune escape pathways that alter innate responses of immune and other lesion-associated cells which combine to blunt adaptive immunity, enabling the persistence and subsequent neoplastic transformation of HPV lesions [14], (Smola this issue). Thus further optimization will be required to improve efficacy. This is in progress through vaccine immunogen design (e.g. addition of E1/E2), co-administration with adjuvants/cytokines/chemokines (to maximize antigen presentation/T cell activation/migration) and optimization of the delivery vehicles [10]. However, the influences of local and systemic immune suppressive factors remain significant hurdles to efficacy, especially in more advanced disease. Novel strategies for targeting such immunosuppressive components, including a reevaluation of the functions of standard of care treatments in this context, are being investigated to provide for the maximum impact in tandem with therapeutic HPV vaccination [15]. Such vaccines would be particularly impactful for cancer patients and also when combined with screening and HPV testing. A combination of the remarkably successful prophylactic HPV vaccines with a therapeutic component remains an attractive goal, especially for implementation in sexually experienced populations, as this could achieve reductions in HPV-associated cancers sooner.

Conflicts of interest
RBSR is a member of Papivax LLC, has Papivax Biotech Inc. stock options and is a member of Papivax Biotech Inc.'s Scientific Advisory Board. Under a license agreement between Bravovax and the Johns Hopkins University, RBSR is entitled to distributions of payments associated with an L2-based invention described in this review. Under a license agreement between PathoVax LLC and the Johns Hopkins University, RBSR is entitled to distributions of payments associated with an invention described in this review. RBSR also owns equity in PathoVax LLC and is a member of its scientific advisory board. These arrangements have been reviewed and approved by the Johns Hopkins University in accordance with its conflict of interest policies.
PLS has been a scientific advisor for GSK, Alligator Biosciences, NeoTx Therapeutics and Oxford BioMedica.