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microbiology today
Mechanisms of virus action Broadly speaking there are two mechanisms by which viruses cause tumours – direct and indirect. The direct mechanism involves the virus infecting a cell and expressing its own genes. These gene products then enhance the growth potential and/or survival of that cell. Next, over time, if other growth enhancing changes occur in the same cell it may grow into a cancer, for which the virus would be an essential element but insufficient on its own. The indirect mechanism of tumourigenesis involves the virus acting as a cofactor for the tumour but not actually being present in the tumour cells. Human immunodeficiency virus (HIV) is a good example here, since by causing severe immunosuppression it allows other viruses, such as EBV and Kaposi’s sarcoma-associated herpesvirus, to act opportunistically and cause uncontrolled cell growth in the absence of the normal immune control mechanisms. Cancer treatment and prevention The importance of the identification of an association between viruses and various types of cancer is that it opens up new possibilities for cancer prevention and treatment. Because virus-associated cancer cells express viral antigens, they can be recognized as ‘foreign’ by the immune system. So vaccines can be developed which induce an effective immune response to the virus and can thereby prevent infection and consequent tumour production. Vaccines for HBV and HPV are at present being tested in clinical trials and are giving encouraging results. Also, where tumours develop in the setting of immunosuppression, the key elements of the immune response controlling the virus infection, cytotoxic T cells, can be grown in the laboratory and given to patients to prevent or treat the tumour. With these various strategies, hopefully it will not be that long before the worldwide incidence of virus-associated cancers is dramatically reduced. Dorothy H. Crawford Professor of Medical Microbiology and Head of School of Biomedical & Clinical Laboratory Sciences, University of Edinburgh, Hugh Robson Building, George Square, Edinburgh EH8 9XD (t 0131 650 3142; f 0131 650 3711; e d.crawford@ed.ac.uk) Table 1. Viruses associated with cancer in humans Family Virus Benign disease Tumour Retroviridae Human T-lymphotropic virus 1 Tropical spastic paraparesis Adult T-cell leukaemia/lymphoma Papillomaviridae Human papillomaviruses Benign warts Cancer of cervix, skin, anus, penis Hepadnaviridae Hepatitis B virus Hepatitis, cirrhosis Hepatocellular cancer Flaviviridae Hepatitis C virus Hepatitis, cirrhosis Hepatocellular cancer, lymphoma Herpesviridae Kaposi’s sarcoma-associated herpesvirus Castleman’s disease Kaposi’s sarcoma, Body cavity lymphoma Epstein–Barr virus Infectious mononucleosis Burkitt’s lymphoma, Hodgkin’s lymphoma, B lymphoproliferative disease, Nasopharyngeal carcinoma |
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