SUMMARY:Induction of apoptosis has been documented during infection with a number of different viruses. In this study, we used transmission electron microscopy (TEM) and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling to investigate the effects of Ebola and Marburg viruses on apoptosis of different cell populations during in vitro and in vivo infections. Tissues from 18 filovirus-infected nonhuman primates killed in extremis were evaluated. Apoptotic lymphocytes were seen in all tissues examined. Filoviral replication occurred in cells of the mononuclear phagocyte system and other well-documented cellular targets by TEM and immunohistochemistry, but there was no evidence of replication in lymphocytes. With the exception of intracytoplasmic viral inclusions, filovirus-infected cells were morphologically normal or necrotic, but did not exhibit ultrastructural changes characteristic of apoptosis. In lymph nodes, filoviral antigen was co-localized with apoptotic lymphocytes. Examination of cell populations in lymph nodes showed increased numbers of macrophages and concomitant depletion of CD8 ϩ T cells and plasma cells in filovirus-infected animals. This depletion was particularly striking in animals infected with the Zaire subtype of Ebola virus. In addition, apoptosis was demonstrated in vitro in lymphocytes of filovirus-infected human peripheral blood mononuclear cells by TEM. These findings suggest that lymphopenia and lymphoid depletion associated with filoviral infections result from lymphocyte apoptosis induced by a number of factors that may include release of various chemical mediators from filovirus-infected or activated cells, damage to the fibroblastic reticular cell conduit system, and possibly stimulation by a viral protein. (Lab Invest 2000, 80:171-186).I nfections with filoviruses cause severe and often fatal hemorrhagic disease in humans and nonhuman primates, killing up to 90% of infected individuals. Ebola (EBO) and Marburg (MBG) viruses comprise the family Filoviridae (Murphy et al, 1995). The EBO species consists of the Zaire and Sudan subtypes first isolated during separate outbreaks in 1976, the Reston subtype initially isolated from monkeys imported from the Philippines to the United States in 1989, and the Cô te d'Ivoire subtype discovered in the Tai Forest of the Ivory Coast in 1994. The recent re-emergence of EBO subtype Zaire (EBO-Z) in former Zaire and Gabon, and subtype Reston (EBO-R) in the United States, are but the last of a succession of filoviral outbreaks that have occurred sporadically since the initial zoonotic outbreak of MBG in 1967. Although outbreaks have been self-limiting, the lack of proven effective prophylactic or therapeutic regimens to treat filoviral infections has heightened concerns about the public health threat of these pathogens.The factors contributing to the development of severe lesions in EBO and MBG hemorrhagic fevers are unknown. Filoviral infections of humans and nonhuman primates are characterized by a failure o...