Koilocytosis is a descriptive term derived from the Greek adjective koilos, meaning hollow. Koilocytosis is pathognomonic, though not required, for the diagnosis of low grade squamous intraepithelial lesion (LSIL). Koilocytes are squamous epithelial cells with perinuclear cavitation and nuclear features of LSIL, to include nuclear enlargement, coarse chromatin, and irregular nuclear membranes. A rim of condensed cytoplasm often surrounds the perinuclear cavitation, giving the cell a "halo" or cleared-out appearance around the dysplastic nucleus. Researchers and pathologists have identified koilocytic cells in epithelium throughout the body, but the initial description came from cervical samples. Cervical koilocytosis has since formed the basis for much of the research surrounding koilocytes, including their etiology and significance.
Koilocytosis describes the culmination of specific cellular events caused by infection with human papillomavirus (HPV). Koilocytes are thus indicative of and specific for HPV infection. HPV induces transformation of cells and is associated with malignant entities, such as certain carcinomas of the cervix, oral cavity, and larynx. HPV is also associated with benign entities such as condylomata, commonly known as genital warts.
HPV is transmitted through direct contact of skin or mucosal membranes, and commonly, the virus is sexually transmitted. It infects the immature basal cells of the squamous epithelium. When immature basal cells do not have the protective cover of more mature cells, they are vulnerable to infection. Exposure of the immature basal cells can occur through metaplastic processes, i.e., the transition zone of the cervix, or through external insults resulting in mucosal or epithelial injury. Once a virus infects a basal cell, it then replicates in the maturing squamous cells and can cause the cleared-out cytoplasm and nuclear changes characteristic of koilocytes.
Koilocytes are not visible grossly, but they can be seen microscopically. The anogenital and oropharyngeal regions are the main areas of the body that harbor koilocytes. Cervical cytology specimens, also known as Pap smears, commonly show koilocytosis. Koilocytes are less commonly encountered in urine cytology specimens and infrequently in anal cytology. With the Papanicolaou stain, the condensed cytoplasm can be pink, blue, or both. The cytoplasmic perinuclear cavitation is nearly transparent, but may occasionally contain a few condensed cytoplasmic granules. The nucleus is hyperchromatic with an irregular border and enlarged to at least 3 times the size of an intermediate squamous cell nucleus. Some cells may exhibit binucleation.
With the hematoxylin and eosin stain of fixed tissue, the findings are very similar. The pink cytoplasm is condensed around conspicuous perinuclear cavitation, and the enlarged nucleus exhibits hyperchromasia and membrane irregularity. Maturing squamous cells, those found in the superficial or intermediate layers, exhibit the koilocytic changes. Basilar squamous cells may be infected, but they do not show the characteristics of koilocytes. Reactive squamous epithelium can exhibit cells that mimic koilocytes with a vacuolar appearance. Strict criteria for koilocytes coupled with the context of the surrounding tissue can aid in proper classification.
Koilocytosis is a description used in anatomic pathology, but it may involve clinical pathology by prompting HPV DNA testing. Molecular biology techniques including nucleic acid hybridization, nucleic acid amplification, and signal amplification assays are used to type HPV. Many laboratories use polymerase chain reaction techniques for this purpose.
Koilocytosis is part of a precancerous pathway in cervical specimens. Therefore, identification of koilocytosis may prompt additional testing to identify the HPV type involved. The types of HPV are categorized as low- or high-risk, referring to their oncogenicity. If an HPV type has been isolated from cancer, it is generally categorized as high-risk.
HPV is a non-enveloped DNA virus with a circular double-stranded DNA. After infection of a basal cell, HPV transcribes 1one of its 2 DNA strands thus leading to viral protein formation. Two proteins, E6 and E7, are responsible for the oncogenic effects of the virus. They interrupt normal cell death pathways and promote cellular proliferation by interfering with the tumor suppressor genes RB and p53. If the virus incorporates its DNA into the genome (integrated viral DNA), it replicates in the maturing squamous cells. The koilocytic morphology results from E4 protein which acts to disrupt the cytoskeleton of the squamous cell. Krawczyk et al. suggest that HPV proteins E5 and E6 work together to promote the formation and fusion of perinuclear cavitations. This fusion likely creates the large cavitation that distinguishes HPV-infected cells by giving them their cleared-out appearance.
Koilocytosis more likely occurs in certain demographic groups and anatomic locations due to the frequent sexual transmission of HPV. Although, other transmission modes are possible, such as a vertical transmission from mother to infant. Researchers have even identified HPV DNA in breast milk.
Koilocytosis is not visible on physical exam, but colposcopy may reveal lesions that exhibit koilocytosis microscopically on subsequent biopsy. The correlation between HPV type and colposcopic appearance is an area of research.
Koilocytosis and its significance vary between organ systems. Koilocytosis reveals HPV infection, which is often transient. The immune system clears most HPV infections within 18 months. Persistent infection, however, is associated with various cancers, particularly cervical cancer. Many professional organizations recommend HPV testing of cervical specimens since HPV is the causative agent of cervical cancer. The American Society for Colposcopy and Cervical Pathology incorporated HPV testing into its algorithms for screening tests stratified by age. In women over 30 years of age, HPV co-testing will be performed regardless of the diagnoses; many of these women will have a diagnosis of LSIL in a cervical specimen. About half of LSILs regress, but those lesions associated with high-risk HPV may progress to cancer more frequently. High-risk HPV types 16 and 18 are most commonly associated with cervical cancer. The carcinogenic effects of HPV, especially in relation to cervical cancer, led to the development of HPV vaccines.
In certain cancers of the head and neck, HPV testing has prognostic significance, but the diagnostic algorithm does not include identification of koilocytes. Survival is better in patients with HPV-associated squamous cell carcinoma of the head and neck compared to squamous cell carcinoma that is not HPV-associated.
Anal cancers are frequently associated with HPV, and anal cytology specimens are evaluated in much the same manner as cervical Pap smears. Koilocytes are not frequently seen, but they are diagnostic of LSIL in this anatomic location as well.
As research continues, the role of koilocytosis in different organ systems may change. Currently, koilocytosis acts as a marker for HPV infection and may help direct further diagnostic decision-making.