New cell antigens induced by viruses and their biological significance

Nearly all virus-infected cells are found to have new antigens not present in uninfected cells. Many such antigens are concerned in virus replication, either representing components of the virions themselves or, presumably, enzymes or other materials required for the replication process. Most antigens are short-term products: the host cells are killed or soon recover from the effects of the virus. However, in some cases the production of antigens can continue for a long time, with interesting consequences. In fact, the presence of such antigens is one of the main ways in which genetic modification of host cells by viruses—which is the subject of this meeting—can be detected. The term ‘antigen’ is here used in a broad sense for any component that can be demonstrated by immunological methods, not necessarily in the host species. Those that can elicit immune responses in the host are especially interesting because they are not laboratory artefacts and may be relevant to such phenomena as immunological surveillance against tumours and auto-immunity, as discussed below. The first point that requires consideration is the way in which viruses can bring about genetic modification of host cells. These can be classified into four main groups: (1) Expression of a persisting non-integrated virus genome. (2) Expression of an integrated virus genome. (3) Depression of host cell genes. (4) Alteration of the host cell genome. Examples of each of these will be considered, although in some cases the attribution of observed phenomena into the appropriate category is uncertain. Further studies should be directed towards resolving these uncertainties, because understanding of the mechanisms involved is of general importance in relation to understanding virus carcinogenesis and possibly some aspects of autoimmunity.