Cathepsin L (Ctsl) is a proposed therapeutic target to control inflammatory responses in a number of disease states. However, Ctsl is thought to support host defense via its involvement in antigen presentation pathways. Hypothesizing that Ctsl helps combat
bacterial infection, we investigated its role in Mycoplasma pulmonis-infected mice as a model of acute and chronic infectious airway
inflammation. Responses to the airway inoculation of mycoplasma were compared in Ctsl(-/-) and Ctsl(+/+) mice. After
infection, Ctsl(-/-) mice demonstrated more
body weight loss, greater mortality (22% versus 0%, respectively), and heavier lungs than Ctsl(+/+) mice, but had smaller bronchial lymph nodes. The burden of live mycoplasma in lungs was 247-fold greater in Ctsl(-/-) mice than in Ctsl(+/+) mice after
infection for 3 days. Ctsl(-/-) mice exhibited more severe
pneumonia and neutrophil-rich, airway-occlusive exudates, which developed more rapidly than in Ctsl(+/+) mice. Compared with the conspicuous remodeling of lymphatics after
infection in Ctsl(+/+) mice, little lymphangiogenesis occurred in Ctsl(-/-) mice, but blood vessel remodeling and tissue
inflammation were similarly severe. Titers of mycoplasma-reactive
IgM,
IgA, and
IgG in blood in response to live and heat-killed organisms were similar to those in Ctsl(+/+) mice. However,
enzyme-linked
immunosorbent spot assays revealed profound reductions in the cellular IFN-γ response to mycoplasma
antigen. These findings suggest that Ctsl helps contain
mycoplasma infection by supporting lymphangiogenesis and cellular immune responses to
infection, and our findings predict that the therapeutic inhibition of Ctsl could increase the severity of mycoplasmal
infections.