Bactericidal/permeability-increasing protein fold-containing family member A1 (BPIFA1) is abundantly secreted protein of the upper airways that plays a pleiotropic role in airway defence, including antimicrobial and bacteriostatic functions. The mechanisms by which BPIFA1 exhibits these diverse functions is not fully resolved.
I investigated the regions of BPIFA1 predicted to be functionally important by using BPIFA1-tagged proteins to visualise their ability to bind to bacteria. The ability of mouse and human BPIFA1 to bind to bacteria were compared. I also used in vitro differentiated mouse tracheal epithelial cells (mTECs) from wild-type (WT) and Bpifa1-/- mouse tracheas to investigate if BPIFA1 plays a role in host response to nontypeable Haemophilus influenzae (NTHi).
Human full-length and disulphide-bond mutant BPIFA1 proteins bound to both Gram-negative and Gram-positive bacteria. However, human BPIFA1 lacking residues 22-42 was not able to bind to bacteria, implying that this region may be important for BPIFA1’s binding. Mouse BPIFA1 also bound to both Gram-negative and Gram-positive bacteria. No apparent differences in the bacterial binding of human and mouse BPIFA1 proteins were observed. In vitro studies with WT and Bpifa1-/- mTECs showed the trend towards an increased NTHi colonisation in Bpifa1-/- mTEC cultures compared to WT. NTHi caused patchy infection of mTECs, suggesting that they exhibit resistance to NTHi. No clear differential infection between WT and Bpifa1-/- cells was detected. NTHi was capable of associating with the multiple cell types of tracheal epithelium, but not with BPIFA1-positive cells. NTHi infection disrupted cellular tight-junctions, suggesting that epithelial barrier function was impaired. This, in turn, allowed NTHi to cross through the mTECs. Differences in the BPIFA1 secretion by NTHi-exposed WT mTECs was observed compared to WT MOCK-exposed mTECs but no clear difference between WT and Bpifa1-/- mTECs inflammatory responses was detected.
Overall, the data from this study suggest that human and mouse BPIFA1 proteins play a role in airway defence by binding to Gram-negative and Gram-positive bacteria. The S18 region (G22-L42) of human BPIFA1, but not the disulphide bond, appears to be important for the binding activity of protein but further research is required to determine the true biological functions of this region. Mouse BPIFA1 binds to NTHi and appears to initially protect BPIFA1-positive cells from NTHi invasion, however BPIFA1 deficiency does not lead to significantly increased NTHi infection in mTECs. These findings imply that BPIFA1 is mucosal defence molecule that functions to modulate bacterial infection of the airways.
