4. CAV-induced apoptosis Electron microscopic observations of thymuses from chickens experimentally infected with CAV at 1 day of age showed the presence of apoptotic bodies in the thymocytes, and similar changes were also observed in CAV-infected lymphoblastoid cell lines [18]. Agarose gel electrophoresis of
DNA extracted from thymuses of CAV-infected birds and from infected cell lines showed the characteristic pattern of apoptosis-specific nucleo-somal laddering. It was concluded that CAV causes apoptosis of thymocytes in vivo, and of lymphoblastoid cells following in vitro infection [18]. The induction of apoptosis by CAV has been studied in some detail [19]. The CAV genome has been shown to incorporate three overlapping open reading frames (ORFs) which code for three viral proteins designated VP1, VP2 and VP3, all of which are expressed in CAV infected cells. Only VP1 is present in puri®ed virus particles and is considered to be the capsid protein of CAV [20]. Studies with monoclonal antibodies demonstrated a close relationship between the non-structural protein VP3 and apoptotic structures [21]. It was shown that soon after virus infection of transformed lymphoblastoid cells in vitro, VP3 can be detected in close association with nuclear chromatin. As the virus infection proceeds, and apoptosis of the infected cells progresses, the VP3 protein was demonstrated in
aggregated form within the apoptotic structures [21]. It was also shown that VP3 alone was capable of inducing apoptosis in T lymphoblastoid cells. Transfection of the cells with an expression vector containing only the VP3 gene resulted in transient expression of VP3 proteinand apoptosis of the cells [21]. However, in similar transfection studies with non-transformed avian cells (chicken embryo fibroblasts) or with non-transformed human cell lines, VP3 failed to induce apoptosis, suggesting that its activity was limited to transformed cells [21,22]. The mechanism by which the virus induces apoptosis in chicken thymocytes in vivo therefore remains to be explained. Because of its close association with the development of apoptosis in transformed cells, the
name apoptin was suggested for the VP3 protein[23]. The affinity which VP3 shows for nuclear chromatin has been attributed to its highly proline rich composition which resembles nuclear location signals in some areas [19]. It has been suggested that binding of VP3 (apoptin) to chromatin may disturb the supercoiled structure,
resulting in fragmentation and aggregation, or that the proline-rich character of apoptin may act as an activation signal for apoptosis endonucleases or may indirectly induce gene sequences which mediate apoptosis [19].