The formation of lung granulomas results in an extensive extracellular matrix remodeling. Collagen and elastin are the major components of the pulmonary matrix and it is believed that a cooperative action of multiple enzymes is required for its turnover. Cathepsin K has a unique ability to cleave highly efficiently triple helical collagen at multiple sites and to act as a potent elastase as well . Its strong expression in lung granulomas suggested an important role in granuloma formation and resolution, and indeed, lungs of Apoe-/- Ctsk-/- mice showed significant differences in granuloma appearance, size, and composition. The increased incidence of lung granulomas in cathepsin K-deficient mice (88% vs. 41%) correlates with the increased collagen fibers deposition (10% vs 20%) that may consequently result in a delayed granuloma resolution. Lung granuloma formation and their spontaneous resolution as commonly observed in patients with sarcoidosis  require remodeling of the extracellular matrix. It is tempting to speculate that the increased amount of collagen in granulomas (Figure 4H) and the presence of abundant collagen fibers between epithelioid cells in areas without well-formed granulomas (Figure 3F, G) in Ctsk-/- mice is due to the lack of the collagenase activity of cathepsin K. This accumulation of collagen fibers might also be responsible for the smaller number and size of granulomas. In our measurements, accumulations of epithelioid cells were considered as granulomas only when they were present as well-formed and ball-shaped structures that are characteristic for sarcoidosis. In Apoe-/- Ctsk-/- mice, epithelioid cells were often observed forming ring-like structures with dispersed collagen fibers, or were randomly distributed in fibrotic areas (Figure 3D, H).
The cell composition of granulomas in cathepsin K-deficient mice was also changed. They contained a significantly higher number of MGCs. This type of cells is formed by the fusion of cells from the monocyte-macrophage lineage . In atherosclerotic plaques from Apoe-/- Ctsk-/- mice, macrophages have an increased size  and microarray analyses have revealed the upregulation of several macrophage genes, including CD36 . Since CD36 participates in macrophage fusion , we suggest that it might be responsible for the increased number of MGCs in cathepsin K-deficient mice. The disruption of cathepsins L and S activities resulted in a more significant effect on granuloma formation. Cathepsin L deficiency reduced the size and the number of lung granulomas whereas mice treated with the cathepsin S inhibitor did not develop well-formed granulomas.
Analysis of available literature data suggests that the preventive effect of cathepsin L deficiency or cathepsin S inhibition on lung granuloma development may not only depend on their extracellular matrix-degrading activities, but on their involvement in antigen presentation as well [17, 38]. Cathepsin S is expressed in B cells, macrophages, and dendritic cells and is required for invariant chain (li) degradation and antigen processing . Cathepsin S-deficient mice have decreased MHC class II presentation in B and dendritic cells, and a reduced number of CD4+ T-cells . On the other hand, cathepsin L is expressed in cortical thymic epithelial cells and macrophages and is responsible for li degradation in CD4+ T cell selection in the thymus . Cathepsin L-deficient mice have a reduction in their numbers of CD4+ cells in thymus and peripheral organs . The roles of cathepsins S and L on the functions of CD4+ may have a direct influence on sarcoidosis development. T cell activation is mandatory for the development of granulomatous reactions and CD4+ T cells of the Th1-type are essential for the formation and the maintenance of granulomas [18–21]. The decreased amount of CD4+ cells in lungs of mice lacking cathepsin L or treated with cathepsin S inhibitor (Figure 7B, C) supports the suggestion that the effect of these two cathepsins on the granuloma formation is related to their role in antigen presentation and T-cell selection.
In addition to its involvement in antigen presentation, cathepsin L has been described to play a role in T-cell actin polymerization, shape polarization, chemotaxis. Cathepsin L deficiency significantly decreases the expression of laminin, fibronectin, and collagens I and II in thymus of cathepsin L-deficient mice . This might partially explain why Apoe-/- Ctsl-/- mice had significantly smaller thymi compared to Apoe-/- mice (Figure 7D, E).
In conclusion, our results show that the disruption of cathepsin L and S activities prevents the development of lung granulomas, whereas cathepsin K deficiency results in altered granuloma composition in mice. These results suggest that interventions in cathepsin S and L activities may yield a therapeutic benefit for patients with sarcoidosis, i.e., a potential prevention of disease progression. Such an approach, however, would require extreme caution as both cathepsins have been described as critical participants in antigen presentation and their inhibition may result in increased infection and cancer rates.