Sbds Deficient Neutrophils Exhibit Normal Numbers, Chemotaxis and Phagocytic Functions, but Impaired NADPH Oxidase Activity.

Shwachman-Diamond syndrome (SDS) is a recessive disease with multi-system involvement, and is characterized by hematological dysfunction, exocrine pancreatic insufficiency and skeletal abnormalities. Recurrent infections are a major cause of morbidity for SDS patients, due to neutropenia and proposed defects in neutrophil functions. SDS is caused by mutations in SBDS, a highly conserved gene that has been suggested to be involved in RNA metabolism and/or ribosome biogenesis. A conditional Cre-Lox Sbds allele was generated in the mouse using gene targeting technology, as complete ablation of Sbds has been shown to lead to early embryonic lethality. To investigate neutrophil functions and their specific sensitivity to loss of Sbds, we elected to delete the gene by breeding mice with the conditional allele to mice with Cre recombinase under the control of the lysozyme M promoter. While Sbds was confirmed to be ablated in mature neutrophils in adult offspring mice, their numbers and morphology were found to be normal in peripheral blood. Sbds-ablated neutrophils were also found to have normal chemotatic and chemokinetic abilities, and they demonstrated phagocytic function comparable to neutrophils of control mice. In contrast, NADPH oxidase activity was observed to be greatly reduced in response to phorbol-12-myristate 13-acetate stimulation, while the expression levels and phosphorylation of the NADPH oxidase complex components remained intact. To evaluate the net effect of the loss of Sbds in vivo, clearance of Pseudomonas aeruginosa from infected lungs was investigated, and found to be significantly reduced in mice with Sbds-abated neutrophils compared their wild type littermates. In summary, loss of Sbds does not to affect total peripheral neutrophil numbers directly, or chemotaxis and phagocytic abilities, but was essential for a robust NADPH oxidase response. These findings suggest that neutrophils are compromised with loss of Sbds, but that at least some of the neutrophil deficiencies of SDS patients are likely due to microenvironment deficiencies, or early hematopoiesis abnormalities.