Efficient isolation of human metapneumovirus using MNT-1 , a human malignant melanoma cell line exhibiting early and distinct cytopathic effect Running title : Apparent CPE in HMPV-infected MNT-1 cell

Isolation of human metapneumovirus (HMPV) from clinical specimens is currently inefficient due to the lack of a cell culture system exhibiting a distinct cytopathic effect (CPE). The cell lines LLC-MK2, Vero, and Vero E6 are used for isolation of HMPV; however, the CPE in these cell lines is subtle and usually requires a long observation period and sometimes blind passages. Thus, a cell line that exhibits an early and distinct CPE following HMPV inoculation is highly desired by clinical virology laboratories. We demonstrate that the human malignant melanoma cell line MNT-1 shows obvious syncytium formation shortly after inoculation with HMPV-positive clinical specimens. In addition, the growth and isolation efficiency of HMPV was higher using MNT-1 than any other conventional cell line. Addition of this cell line to our routine viral isolation system for clinical specimens markedly enhanced isolation frequency, allowing isolation-based surveillance. MNT-1 has the potential to facilitate clinical and epidemiological studies of HMPV.