Investigation of human papillomavirus (HPV) infection began in earnest in the 1980s after Harold zur Hausen postulated that infection with these viruses was associated with cervical cancer. It is now recognized that HPV infection of the human genital tract is extremely common and causes clinical states ranging from asymptomatic infection to genital warts (condylomata acuminata); dysplastic lesions or invasive cancers of the anus, penis, vulva, vagina, and cervix; and a subset of oropharyngeal cancers. This chapter describes the epidemiology of HPV in general and as a pathogen, the natural history of HPV infections and associated cancers, strategies to prevent HPV infection and HPV-associated disease, and treatment modalities.
HPV is an icosahedral, nonenveloped, 8000-base-pair, double-stranded DNA virus with a diameter of 55 nm. Like those of other papillomaviruses, HPV’s genome consists of an early (E) gene region, a late (L) gene region, and a noncoding region that contains regulatory elements. The E1, E2, E5, E6, and E7 proteins are expressed early in the growth cycle and are necessary for viral replication and cellular transformation. The E6 and E7 proteins cause malignant transformation by targeting the human cell cycle–regulatory molecules p53 and Rb (retinoblastoma protein), respectively, for degradation. Translation of the L1 and L2 transcripts and splicing of an E1^E4 transcript occur later. The L1 gene encodes the 54-kDa major capsid protein that makes up the majority of the virus shell; the 77-kDa L2 minor protein constitutes a smaller percentage of the capsid mass.
More than 125 HPV types have been identified and are numerically designated according to a unique L1 gene sequence. Approximately 40 HPV types are regularly found in the anogenital tract and are subdivided into high-risk and low-risk categories on the basis of the associated risk of cervical cancer. For example, HPV-6 and HPV-11 cause genital warts and ~10% of low-grade cervical lesions and are thus designated low risk. HPV-16 and HPV-18 cause dysplastic lesions and invasive cancers of the cervix and are considered high risk.
HPV targets basal keratinocytes after microtrauma has exposed these cells to the virus. The HPV replication cycle is completed as keratinocytes undergo differentiation. Virions are assembled in the nuclei of differentiated keratinocytes and can be detected by electron microscopy. Infection is transmitted by contact with virus contained in these desquamated keratinocytes (or with free virus) from an infected individual.
Immune Response to HPV Infection
A cell-mediated immune response plays an important role in controlling the progression of natural HPV infection. Histologic examination of lesions in individuals who experience regression of genital warts demonstrates infiltration of T cells and macrophages. CD4+ T cell regulation is particularly important in controlling HPV infections, as evidenced by the higher rates of infection and disease among immunosuppressed individuals, particularly those who are infected with HIV. Specific T-cell responses may be measured ...