Cultured chicken bone-marrow-derived macrophages have been assayed for their susceptibility to infection with various avian viruses. Three criteria of infection were employed: (1) Virus-induced alterations in cell morphology ; (2) presence of intracellular viral antigens detectable by immunofluorescence; (3) kinetics of virus release by infected macrophages. Macrophages proved to be resistant to Marek's disease virus (MDV), herpesvirus of turkeys (HVT-FC126), infectious bronchitis virus (IBV) and reticuloendotheliosis virus (REV). MDV included the pathogenic HPRS-16 strain prepared from feather follicles, and the apathogenic HPRS-24 strain adapted to growth in chick embryo fibroblast cultures. IBV included both embryo-propagated and tissue culture-adapted variants of the apathogenic Beaudette strain and a pathogenic Massachusetts-type strain. REV comprised the strains REV-C, CSV and oncogenic virus of the REV-F strain. Adenovirus, infectious laryngotracheitis (ILT) virus, reovirus, infectious bursal disease virus (IBDV) and Newcastle disease virus (NDV) replicated in macrophages causing different but characteristic cytopathic effects, or alterations in cell morphology associated with macrophage activation. The most prominent effect of IBDV and lentogenic NDV infection were morphological signs of macrophage activation, i.e. enlargement or 'transformation' of cells which tended to survive in infected cultures and were usually free of detectable amounts of immunofluorescent viral antigens. Macrophage cultures were less susceptible to infection with adenovirus (OTE strain), pathogenic ILT virus and lentogenic NDV (B1 strain) than permissive chicken kidney cell (CKC) cultures. In contrast, macrophage cultures were significantly more susceptible to infection with reovirus than CKC cultures, indicating that bone-marrow-derived macrophages might be the major target cells of this virus species. Virus restriction by cultured bone-marrow-derived macrophages was expressed to various degrees among the different avian virus species and among different strains of the same virus species, however, it was not generally correlated with the pathogenicity of these viruses in chickens.

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