Olfactomedin 4 Down-Regulates Neutrophil Killing of Gram-Positive and Gram-Negative Bacteria.

Olfactomedin 4 (OLFM4) is a member of olfactomedin-related glycoprotein family, which is specifically expressed in neutrophils and gastrointestinal tract. OLFM4 expression is upregulated in gastrointestinal cancer and inflammatory diseases such as chronic inflammatory bowel disease and Helicobacter pylori infection. It has been shown that OLFM4 is a target gene of NF-kB and Notch pathways and that OLFM4 down-regulates innate immunity to H. pylori infection. However, its potential biological functions in neutrophils still remain to be defined. The goal of this study is to determine whether OLFM4 is involved in the bactericidal activity of neutrophils using an OLFM4 deficient mouse model.

1. OLFM4 expression in neutrophils is upregulated in response to Staphylocococus aureus (Gram positive) and Escherichia coli (Gram negative) bacteria. 2. We have shown that neutrophils from OLFM4 deficient mice have increased intracellular killing of S. aureus and E. coli bacteria in vitro. 3. The OLFM4 deficient mice displayed enhanced bacterial clearance in vivo when the mice were challenged with intra-peritoneal injection of S. aureus and E. coli. 4. To elucidate the molecular mechanisms that mediate these effects, we performed a yeast 2-hybrid screen and found that OLFM4 interacts with cathepsin C (dipeptidyl peptidase I or DPPI), a lysosomal cysteine protease that has a degraditive function as an exopeptidase and is essential for activation of neutrophil granule-associated serine proteases including neutrophil elastase, cathepsin G and proteinase 3. The direct association of OLFM4 with cathepsin C was confirmed in human primary neutrophils. 4. We have demonstrated that OLFM4 is a direct substrate of DPPI and inhibits DPPI activity in transfected 293T cells. 5. The cathepsin C activity in neutrophils from OLFM4 deficient mice was significantly higher than that in neutrophils from wild-type littermate mice in the absence or presence of bacterial infection, suggesting that OLFM4 is an endogenous inhibitor of cathepsin C in neutrophils. 6. We have also demonstrated increased activities of neutrophil elastase, cathepsin G and proteinase 3 (whose processing and maturity require cathepsin C activity) in OLFM4 deficient neutrophils compared with wild-type neutrophils. 7. Activation of NADPH oxidase, myeloperoxidase (MPO) activity, and neutrophil phagocytosis were not altered in OLFM4 deficient neutrophils compared with wild-type neutrophils.

These results suggest that OLFM4 is an important regulator of neutrophil bacterial killing activity via negative regulation of cathepsin C activity and its down stream granule-associated serine proteases.

No relevant conflicts of interest to declare.