Limited Alleviation of Lysosomal Acid Lipase Deficiency by Deletion of Matrix Metalloproteinase 12

Lysosomal acid lipase (LAL) is the only known enzyme that degrades cholesteryl esters and triglycerides at an acidic pH. In LAL deficiency (LAL-D), dysregulated expression of matrix metalloproteinase 12 (MMP-12) has been described. The overexpression of MMP-12 in myeloid lineage cells causes an immune cell dysfunction resembling that of Lal knockout (Lal KO) mice. Both models develop progressive lymphocyte dysfunction and expansion of myeloid-derived suppressor (CD11b+ Gr-1+) cells. To study whether MMP-12 might be a detrimental contributor to the pathology of LAL-D, we have generated Lal/Mmp12 double knockout (DKO) mice. The phenotype of Lal/Mmp12 DKO mice closely resembled that of Lal KO mice, while the weight and morphology of the thymus were improved in Lal/Mmp12 DKO mice. Cytological examination of blood smears showed a mildly reversed lymphoid-to-myeloid shift in DKO mice. Despite significant decreases in CD11b+ Ly6G+ cells in the peripheral blood, bone marrow, and spleen of Lal/Mmp12 DKO mice, the hematopoietic bone marrow progenitor compartment and markers for neutrophil chemotaxis were unchanged. Since the overall severity of LAL-D remains unaffected by the deletion of Mmp12, we conclude that MMP-12 does not represent a viable target for treating the inflammatory pathology in LAL-D.