Clearance of cervical human papillomavirus infections

It is intriguing that only a small percentage of young women infected with high-risk Human Papillomavirus (HR-HPV) in the cervix will develop a persistent infection, while the remaining young women clear the infection very rapidly.1, 2 Historically, this has been attributed to differences in the immune response among individuals.3 However, whereas some studies have found that cell mediated immune responses against HPV epitopes were associated with clearance of infection,4, 5 others studies have detected HPV specific T-cell responses predominantly in woman with high-grade lesions and not in women that cleared the HPV infection.6, 7, 8 Moreover, many studies have shown that HR-HPV has developed multiple strategies to efficiently evade detection and clearance by the immune system.9 Therefore, it remains possible that factors other than HPV-specific immune responses are important for the clearance of HR-HPV infections in women that possess no prior immunity. Owing to the continuous renewable nature of epithelial surfaces, cervical basal cells need to be infected in order to establish a long-term or a persistent HR-HPV infection. The basal cells consist of stem cells and transiently amplifying cells (TA-cells). Stem cells have unlimited capacity for self-renewal. In contrast, TA-cells are committed to differentiation and will divide a limited number of times before they undergo terminal differentiation and finally are pushed of the surface of the epithelium (Fig. 1). So far TA-cells have been considered irrelevant for the duration of HR-HPV infection due to their transient nature and short life span. Nonetheless, the fact is that nobody knows for how long TA-cells survive in human epithelia. Hierarchy of cells in the cervical epithelium. The cervical epithelium can be divided into clonal units. All cells in a clonal unit originate from 1 stem cell. The stem cell divides infrequently and gives rise the TA-cells that divide frequently to produce the amazing number of non-dividing differentiated cells necessary to maintain the epithelium. The stem cell has an unlimited capacity for self renewal. This means that the stem cell on average produces one stem cell and one TA-cell per cell division. The hierarchy of TA-cells has progressively less capacity for self-renewal and will therefore have shorter and shorter life span. The temporal dynamics of the epithelium is largely unknown. We believe that a subset of TA-cells in the cervical epithelium will have a large capacity for self renewal and therefore a sufficiently long life span to be host for productive HR-HPV infections. Based on this prediction, we suggest that a major contributor to the natural history of a HR-HPV infection in women without prior protective immunity is the life span of the infected cells. Thus, a stem cell infection will last until it is cleared by the immune system, whilst infections in TA-cells last for shorter periods of time that are determined by the life span of the infected cells or until cleared by the immune response (Fig. 2). This hypothesis provides a plausible molecular explanation for the observed large differences in the persistence of HR-HPV infections and clearance without any apparent activation of the immune system, although it remains a possibility that these observations are caused by inefficiency of current immunological assays. The TA-cell hypothesis: an alternative mechanism for clearance of HPV infections. Stem cell infection will last until it is cleared by the immune system, whilst infections in TA-cells last for shorter periods of time determined by the life span of the infected cells or until cleared by the immune response. The hypothesis presented above is based on the prediction of long-lived TA-cells in the human cervical epithelium. The life span of TA-cells is coupled to the frequency of stem cell division, which is unknown for the cervical epithelium. In general, stem cells divide infrequently to ensure that harmful mutations predominantly occurring during DNA replication or mitosis will accumulate almost exclusively in TA-cells. As TA-cells have limited life spans they will be less likely to develop into cancer cells.10 In higher primates, pluripotent hematopoietic stem cells (PHSCs) have been shown to divide very infrequently (less than one division per year in adults).11, 12 This is in contrast to mice PHSCs, which divide much more rapidly. A mouse lives for approximately 2 years and the mouse stem cells would therefore be expected to be able to divide much more frequently than human stem cells without causing cancer in the lifetime of the mouse.10 Epidermal stem cell research has also largely been based on experiments with mice and it is therefore possible that the frequency of stem cell division in human epidermis has been overestimated. An estimate of the frequency of human epidermal stem cell division comes from experiments with rafts of human foreskin keratinocytes transduced with a retroviral vector encoding LacZ that were grafted on nude mice. No distinctive arrangement of LacZ positive cells was found in the early postgrafting period. LacZ positive clusters of cells started to appear after 10 weeks and distinct columns representing clonal units could be seen after 40 weeks.13 These observations indicate that the life span of some TA-cells in human foreskin is between 10 and 40 weeks. The hypothesis presented here has an important implication, namely, that HR-HPV infections may frequently be cleared without immune intervention and acquisition of protective immunity. Thus, most women could experience several subclinical or transient infections during their lifetime before they develop protective immunity. This would help explain the rather high prevalence of HR-HPV infections that are observed in the general population and that reinfection with the same HR-HPV type occurs. We believe the results obtained in epidemiological studies of HR-HPV clearance are generated by a combination of immunological clearance and clearance by the mechanism described above. The HR-HPV types very efficiently evade the immune system and it is therefore possible that HR-HPV infections primarily will be detected by the immune system when the normal viral regulation somehow is abrogated. Most likely, such deregulation will in most cases require several separate events occurring in a cell. Since this is much more likely to occur in a stem cell (persistent) infection, we speculate that HR-HPV types with rather low infectiveness may have been positively selected during evolution to facilitate infections in TA-cells. Overall the spread of the virus may be facilitated by the ability to reinfect the same individual several times regardless of the reduced infectiveness of the virus. It will be difficult to directly verify that the mechanism described above plays a role in the clearance of HR-HPV infections, since no good model system for HR-HPV infection exist and no good markers for cervical stem cells and TA-cells have been found. Definitive verification of the presented hypothesis will therefore have to await technical advances. However, the current opinion that all HR-HPV infections are cleared by the immune response is also a hypothesis that never has been validated and cannot be validated with current experimental methods. When the current assays for T-cell responses have been properly validated, a longitudinal cohort study including young women with no previous exposure to HR-HPV could provide valuable information on the mechanism of clearance. This would enable simultaneous monitoring of HR-HPV infections and immune responses. If no immune responses were observed in the women with short transient infections, this would be an indication that some productive HR-HPV infections are cleared as a result of the limited life span of infected TA-cells. In summary, we suggest that the life span of the infected cells is important for the duration of HR-HPV infections in women without prior protective immunity. This mechanism of clearance is compatible with the existing experimental evidence and could explain some findings that are difficult to explain if the mechanism of HR-HPV infections is limited to immune dependent clearance. We believe that the mechanism described here is important for interpretation of all studies on the clearance of HR-HPV infections. Yours sincerely, Jeppe Vinther*, Bodil Norrild*, * The Protein Laboratory, Institute of Molecular Pathology, University of Copenhagen, Denmark.