Sendai virus infection up-regulates trypsin I and matrix metalloproteinase-9, triggering viral multiplication and matrix degradation in rat lungs and lung L2 cells.

To elucidate the virus-host cell interaction, we analyzed quantitatively the expression of various cellular proteases and tumor necrosis factor-alpha (TNF-alpha) after Sendai virus infection in rat lungs and lung L2 cells. After infection, TNF-alpha mRNA levels increased rapidly to a peak on day one, and then trypsin I and matrix metalloproteinase (MMP)-9, but not MMP-2, were significantly up-regulated with a peak on day 2 in vivo. These up-regulations were confirmed in L2 cells. Up-regulation of proMMP-9 and its active convertase trypsin I seems to synergistically enhance virus multiplication and the destruction of lung matrix, resulting in the progression of pneumonia.