The human polyomaviruses, JCV and BKV, establish lifelong persistent asymptomatic infection. Only under severe impairment of the immune system activated states of infection may lead to disease. Most prominent are progressive muldfocal leukoencephalopathy (PML) under AIDS and to a lesser extent hemorrhagic cystitis after bone marrow transplantation. PML regularly has a fatal outcome, as no beneficial therapy is available. This is due to the limited information on the virus life cycle and its interaction with the host. It is known for quite a while that the urogenital and the central nervous system are persistently infected, but only recently the lymphotropic nature of the viruses was detected. The understanding of the mechanisms responsible for the control of the viral life cycle at different states of infection and virus target cells is improved by reports on the interaction of cellular transcription factors with viral promoters. It became clear, that the functions regulated by the transcripdonal control region include not only basic and cell specific factors, but also immunomodulatory interactions on promoter binding sites. In this context, hematopoeric cells as mobile virus carriers and continuously interacting targets of the immune system appear to be deeply involved in viral pathogenesis.
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