The greatest risk factor for the development of cervical and other cancers that have been linked to the human papillomavirus (HPV) family is the persistence of the virus. To persist for the decades required to develop HPV-related cancers, the virus must escape host immunity. HPV is a simple DNA virus that has evolved to escape immune attack by a combination of stealth and interference. This review focuses on the mechanisms by which HPV can evade recognition by the host immune system.

Mechanisms used by human papillomaviruses to escape the host immune response

Cervical cancer is a leading cause of death by cancer among women worldwide. High-risk human papillomaviruses (HPVs) are the major etiological agents for cervical cancer, but other factors likely contribute to cervical cancer, because these cancers commonly arise decades after initial exposure to HPV. Estrogen is thought to be one such cofactor; however, its temporal requirements in human cervical cancer are not known. Here we evaluate the temporal requirements of estrogen in cervical carcinogenesis in a mouse model for HPV-associated cervical cancer. Tumors arising in HPV16 transgenic mice treated with estrogen for 9 months were greatly increased in their size compared with tumors developing after 6 months of estrogen treatment. HPV16 transgenic mice treated 6 months with estrogen followed by 3 months without exogenous estrogen had significantly fewer tumors and the tumors were smaller and less aggressive than those arising in mice treated the full 9 months. Importantly, cervical cancers that arose in the mice treated the first 6 of 9 months with estrogen must have regressed, based upon the reduced incidence of cancers in these mice compared with those treated for 6 months with estrogen, then immediately analyzed. We conclude that estrogen plays a critical role not only in the genesis of cervical cancer but also in its persistence and continued development in this mouse model. These findings raise the clinically relevant possibility that, if human cervical cancer has a similar dependence on estrogen for continued tumor growth, then antiestrogen therapy may be effective in the treatment of cervical cancer.

https://www.pnas.org/content/pnas/102/7/2490.full.pdf

Estrogen contributes to the onset, persistence, and malignant progression of cervical cancer in a human papillomavirus-transgenic mouse model

Evasion of host immune defenses by human papillomavirus.

We have created a transgenic mouse with tissue-specific expression of the human papilloma virus (HPV) 16 E6 and E7 oncoproteins in the thyroid as a model of HPV transformed cancer. The expression of the transgenes results in the formation of palpable thyroid tumors. E7 is not expressed in other tissues but is expressed in medullary thymic epithelial cells, which have been implicated in the control of negative selection. We show that Listeria-based vaccines against E7 can induce the regression of solid implanted tumors in the transgenic mice, although at a lower frequency than in wild type (WT) mice. E7-specific CD8+ T cells induced in transgenic mice are of both lower avidity and lower frequency when compared to the WT mice. In this model, Listeria-based vaccines against E7 appear to be overcoming central tolerance by expanding low avidity CD8+ T cells specific for E7 that are not deleted during thymopoesis and can eliminate solid tumors.

https://cancerimmunolres.aacrjournals.org/content/canimmarch/7/1/2.full.pdf

Listeria-based vaccines can overcome tolerance by expanding low avidity CD8+ T cells capable of eradicating a solid tumor in a transgenic mouse model of cancer.

Screening for colorectal cancer is commanding increasing attention. Other cancer screening programmes have been a part of public consciousness for some time, but, until recently, colorectal cancer screening has remained in the background. Fuelled by new research, market opportunities and increased recognition of individual risk, screening for colorectal cancer is becoming a recommended procedure, but controversy about how best to implement widespread screening remains.

The case for population-based screening for colorectal cancer.

The E7 oncoprotein of human papillomavirus 16 (HPV16) transforms basal and suprabasal cervical epithelial cells and is a tumor-specific antigen in cervical carcinoma, to which immunotherapeutic strategies aimed at cytotoxic T-lymphocyte (CTL) induction are currently directed. By quantifying major histocompatibility complex class I tetramer-binding T cells and CTL in mice expressing an HPV16 E7 transgene from the keratin-14 (K14) promoter in basal and suprabasal keratinocytes and in thymic cortical epithelium, we show that antigen responsiveness of both E7- and non-E7-specific CD8+ cells is down-regulated compared to non-E7 transgenic control mice. We show that the effect is specific for E7, and not another transgene, expressed from the K14 promoter. Down-regulation did not involve deletion of CD8+ T cells of high affinity or high avidity, and T-cell receptor (TCR) Vbeta-chain usage and TCR receptor density were similar in antigen-responsive cells from E7 transgenic and non-E7 transgenic mice. These data indicate that E7 expressed chronically from the K14 promoter nonspecifically down-regulates CD8+ T-cell responses. The in vitro data correlated with the failure of immunized E7 transgenic mice to control the growth of an E7-expressing tumor challenge. We have previously shown that E7-directed CTL down-regulation correlates with E7 expression in peripheral but not thymic epithelium (T. Doan et al., J. Virol. 73:6166-6170, 1999). The findings have implications for the immunological consequences of E7-expressing tumor development and E7-directed immunization strategies. Generically, the findings illustrate a T-cell immunomodulatory function for a virally encoded human oncoprotein.

Nonspecific Down-Regulation of CD8+T-Cell Responses in Mice Expressing Human Papillomavirus Type 16 E7 Oncoprotein from the Keratin-14 Promoter

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Nonspecific Down-Regulation of CD8+T-Cell Responses in Mice Expressing Human Papillomavirus Type 16 E7 Oncoprotein from the Keratin-14 Promoter