PSP expression in murine lacrimal glands and function as a bacteria binding protein in exocrine secretions

Nonobese diabetic (NOD) mice, an animal model for type I autoimmune diabetes and autoimmune sialoadenitis, abnormally express parotid secretory protein (PSP) in the submandibular glands (Robinson, C. P., H. Yamamoto, A. B. Peck, and M. G. Humphreys-Beher. Clin. Immunol. Immunopathol. 79: 50-59, 1996). To evaluate possible PSP gene dysregulation in the NOD mouse, we have examined a number of organs and tissues for PSP mRNA transcripts and protein expression. Results indicate that PSP is produced in the lacrimal glands of NOD mice as well as most laboratory mouse strains. Although purified salivary PSP from C3H/HeJ or BALB/c mice fails to affect amylase enzyme activity in in vitro assays, PSP bound to whole bacteria in a Zn2+-dependent manner. Additionally, radiolabeled protein bound to specific bacterial membrane proteins using a ligand binding assay. PSP gene transcription, but not protein production, was observed in the heart and pancreas from NOD mice, indicating abnormal transcription of the PSP gene. Sequence analysis of PSP cDNA from NOD mice revealed numerous base differences (compared with the published PSP sequence) capable of leading to significant amino acid substitutions, suggestive of strain-specific differences for the protein in mice. Together these results suggest that there exists in the NOD mouse a dysregulation of PSP transcription in various tissues. However, except for C3H/HeJ mice, PSP appears as a normal product of the lacrimal glands where, as in saliva, it may function as a nonimmune antimicrobial agent in the protection of tissue surfaces exposed to the external environment.