Intracellular co-localization of SPLUNC1 protein with nanobacteria in nasopharyngeal carcinoma epithelia HNE1 cells depended on the bactericidal permeability increasing protein domain.

Epithelial surfaces constitute natural immunobarriers against environmental threats. These barriers are brimming with fluids that bind, transport, cleave or degrade bacterial cells and their endotoxic by-products. Saliva and the airway surface-lining fluid (ASL) comprise the important fluid constituents. Short palate, lung and nasal epithelium clone 1 (SPLUNC1) is a potential host defensive protein that is secreted from the submucosal gland to the saliva and nasal lavage fluid. However, its antimicrobial spectrum and antimicrobial mechanism is not clear. Through green fluorescence protein (GFP) mediated subcellular localization experiments in nasopharyngeal carcinoma (NPC) HNE1 cell line, we determined that the intracellular GFP-tagged SPLUNC1 protein binds to a miniscule microorganisms, approximately 50-400nm in size, after the bactericidal permeability increasing protein (BPI) domain was deleted, GFP-tagged truncated SPLUNC1 protein lost its function of binding to the miniscule microorganisms. We verified that these microorganisms are nanobacteria (NB) with a negative staining using transmitted electronic microscope (TEM) and immunofluorescent analysis using an NB-specific antibody. We isolated and cultured the NB from the cultured nasopharyngeal carcinoma epithelia HNE1 cell supernatant. We found that the NB did not absorb the Hoechst stain, even when we extended the staining time to 35min. However, with the time extension the larger sized NB (larger than 300nm) did stain positively. From the biopsy specimen of NPC, we also detected the NB, which can lead to the swelling of mitochondria in the infected host cells. We hypothesize that SPLUNC1 and NB co-localization is due to the GFP-tagged SPLUNC1 protein binding to the lipopolysaccharide (LPS) of the Gram-negative NB, which can play an important role in the host defense of nasopharyngeal epithelium. This research sheds new light on the mechanism of SPLUNC1 involvement in the host upper respiratory tract defense system.