0 INSTITLJT PASTEURELSEVIER Paris 1995 Res. Viral. 1995, 146, 389-395 Electron microscopic evidence for infection of splenic dendritic cells by adenovirus in psittacine birds J.C. Gbmez-Villamandos (*), M.J. Bautista, L, Carrasco, J. He&s and M.A. Sierra Dpt. Anutomia Patoldgica, Facultad de Veterinaria, Avda. Medina Azahara 7, 14005 Chdoba (Spain) SUMMARY The spleens of four psit&acine birds suffering from natural type II avian adenoviral infection were studied by histopathological, immunohistochemical and ultrastructural techniques to determine the target cells of this virus in the spleen. Typical adenoviral replication sites were found in lymphocytes and dendritic cells. Key-words: Adenovirus, Dendritic cell, Spleen, T lymphocyte; Psittacine birds, Type II avian adenovirus, Replication, Ultrastructure. INTRODUCTION Avian adenoviruses replicate in hepatocytes, respiratory tract epithelial cells, intestinal epithe- lial cells, mononuclear phagocyte system cells and lymphocytes, giving rise to intranuclear amphophilic and basophilic inclusion bodies. These inclusions correspond to viral replication sites, and are formed by hexagonal virions of 70- 75 nm diameter, which may be arranged in para- crystalline arrays or scattered throughout the nucleus, and which are associated with moder- ately electron-dense amorphous material and adeno-associated particles roughly 20-25 run in diameter (Dutta and Pomeroy, 1967; Cheville and Sato, 1977 ; Desmidt et al., 1991; G6mez- Villamandos et al., 1992 ; Saunders ef al., 1993). The avian immune system is currently under study. Unlike mammalian spleens, the avian spleen possesses secondary germinal centres only, referred to as secondary follicles or lymphoid fol- licles, and the diffuse lymphoid tissue is located around arteries (per&e&l lymphatic sheath) and sheathed capillaries (Schweigger-Seidel sheaths) ; the latter structure is highly developed (Ride& 1987). Secondary follicles and periarterial lym- phoid sheaths contain dendritic cells, termed fol- licular and interdigitating, respectively (Eikle- boom, 1978 ; Balfour ef al., 1981). These cells respond differently to different viral infections: some viruses alter the distribution and morphol- ogy of dendritic cells, undermining their capacity to respond, while others such as herpes simplex type 1 (Sprecher and Becker, 1991) and vaccinia virus (Becker and Sprecher, 1989) trigger an increase in dendritic cell numbers. Dendritic cells may also be a replication site of both DNA and RNA viruses (Sprecher and Becker, 1993). In this paper, we have carried out a structural and ultrastructural study of the spleen of psitta- Submitted November 2, 1994, accepted August 1, 1995 (*)Corresponding author.