Infectivity of influenza B virus in cultured human muscle

Severe muscle symptoms, particularly in children, are frequently associated with influenza B virus infection. In this study we examined the effects of influenza B virus (Lee Strain) on cultured human muscle by light and electron microscopy (EM), immunofluorescence, hemadsorption and plaque assays. Muscle injury was also evaluated by the appearance of muscle-specific creatine kinase (CK) in the culture medium. By fluorescence immunocytochemistry viral antigen was demonstrated in muscle cell nuclei within 3 h postinoculation (p.i.) and in the cytoplasm at 6 h p.i. Membrane-associated viral antigen was seen at 16 h p.i., at which time budding influenza virus-like particles could be demonstrated by EM, both in myoblasts and multinucleated myotubes. At 16 h all cells were hemadsorption positive. Plaque assays showed peak virus production at 48 h (p.i.), at which time cytopathic effects (cell retraction, pycnosis and cytoplasmic vacuolization) were prominent and some cells detached from the substratum. Leakage of muscle-specific CK isozyme into the culture medium could be demonstrated as early as 6 h p.i. with peak enzyme activity around 40-48 h p.i. Cytopathic changes and virus production were observed both in myoblasts and myotubes indicating that both cell types are susceptible.