Marek's disease virus (MDV) is classified as an oncogenic lymphotropic herpesvirus of chickens. MDV productively and cytolytically infects B, αβT and γδT lymphocytes and latently infects T-helper lymphocytes. The aims of this study were to identify whether MDV infects macrophages in vivo and, if so, whether quantitative differences in macrophage infection are associated with MDV strain virulence. Chickens were infected with either virulent MDV (HPRS-16) or 'hypervirulent' MDV (C12/130). Flow cytometry with monoclonal antibodies recognizing MDV pp38 antigen and leukocyte antigens was used to identify MDV lytically infected cells. Macrophages from HPRS-16- and C12/130-infected chickens were pp38+. It is demonstrated that macrophages are pp38+ because they are infected and not because they have phagocytosed MDV antigens, as assessed by confocal microscopy using antibodies recognizing MDV antigens of the three herpesvirus kinetic classes: infected cell protein 4 (ICP4, immediate early), pp38 (early) and glycoprotein B (gB, late). Spleen macrophages from MDV-infected chickens were ICP4+, pp38+ and gB+, and ICP4 had nuclear localization denoting infection. Finally, MDV pp38+ macrophages had high inherent death rates, confirming cytolytic MDV infection, although production of virus particles has not been detected yet. These results have two fundamental implications for understanding MDV pathogenesis: (i) MDV evolved to perturb innate, in addition to acquired, immunity and (ii) macrophages are excellent candidates for transporting MDV to primary lymphoid organs during the earliest stages of pathogenesis.
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