Identification of non-immunological signals that can induce degranulation of mast cells during RSV infection

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Respiratory syncytial virus (RSV) is a major causative agent of respiratory tract infections in children worldwide. Previously, we reported that mast cell degranulation was induced by A549 cells infected with RSV and this was induced by some non-immunological signals expressed on RSV-infected A549 cells. Therefore, we had tried to identify the signals but we could not find it. However, in their instead, we found that cytokeratin 8 and 18 were the molecules that had some relation to the RSV replication. To elucidate the role of C8/18 in RSV replication, the replication kinetics of the virus were examined using cloned HMC-1-C8/18 cells, derived from the human mast cell line HMC-1 by the insertion of C8/18, and A549-C18KD cells, in which C18 was knocked down using a specific shRNA expression plasmid. By standard inoculation, the replication efficiency of RSV was slightly higher in HMC-1-C8/18 cells compared to HMC-1 cells. However, following spinoculation, RSV RNA replication was increased more than 10-fold compared with HMC-1 cells. In this situation, the intracellular titer of infectious virus in HMC-1-C8/18 cells matched that in A549 cells, while the release of virus into the culture fluid was much lower than with A549 cells. In addition, the RSV titer in A549-C18KD was log0.5 to log1 less than that in A549, not only in the cells, but also in the supernatant. This study demonstrates that C8/18 are directly associated with RSV replication, especially in the early stages.