During evolution, HPVs have adapted to specific epithelial niches, with different

types having different disease associations and disease prevalence [13], [14] and [23]. Amongst cutaneous HPVs, the diversity within the Alpha (species 2, 3, 4 and 14; see Fig. 1), Beta and Gamma genera contrasts sharply to what is seen in the apparently less successful Mu and Nu genera. The most well studied HPV types are, however, the mucosal Alpha types that cause cervical cancer (see Fig. 2A) [24], and for these the biology of disease is relatively well understood [3]. This is certainly the case for HPV16 (Fig. 2B) infections of the ectocervix and the cervical transformation zone where the majority of HPV16-associated Olaparib concentration cervical cancers develop (Fig. 3). The life-cycle organisation of HPV16 (and Alpha types in general) at other important epithelial sites, such as the anus, the endocervix, the penis [25] and [26] and the oropharynx [27] is, however, still poorly understood [28]. The Alpha PVs are divided into cutaneous and mucosal types, and the mucosal types are further subdivided into high-risk

and low-risk groups [1]. The cutaneous learn more Alpha types are also ‘low-risk’, and include HPV2 and 57, which cause common warts, and HPV3 and 10, which cause flat warts [1] and [20]. The low-risk mucosal types (Fig. 2A), which despite their name can also cause cutaneous genital lesions, share a low-risk HPV life-cycle organisation and do not typically cause neoplasia [29] (Figs. 4B and 5). Cutaneous lesions caused by Alpha, Beta, Gamma and Mu types can become difficult to manage in patients with SCID (severe combined immunodeficiency) [30] and EV (epidermodysplasia verruciformis) and in organ transplant recipients and others who are pharmacologically immunosuppressed [31], with certain Beta

types being associated with the appearance of neoplastic precursors (Bowen’s disease, actinic keratosis) [32] and the development of non-melanoma skin cancer at sun-exposed sites in these CYTH4 individuals [6], [31], [33] and [34]. A predisposition to HPV-associated disease and cancer progression is also seen in WHIM syndrome (warts, hypogammaglobulinemia, infections, and myelokathexis) patients, which is associated with defective CXCR4 signalling [35]. The molecular defects that underlie these conditions are known [36], but it is not yet clear (in most cases) exactly how they predispose to disease and whether it is the infected keratinocyte [37] and [38] or the immune system that is primarily compromised [39] and [40]. Thus, the low-risk viruses are occasionally found to be associated with human cancers and can in some instances be associated with papillomatosis, especially in individuals with immune defects. Carcinomas associated with the high-risk HPV types are, however, a far more significant burden [4] and [24].