Salivary gland changes in the NOD mouse model for Sjögren's syndrome: is there a non-immune genetic trigger?

Sjögren's syndrome is a systemic autoimmune disease characterized by patient complaints of oral and ocular dryness accompanied by clinical observations of a progressive loss of salivary and lacrimal function, related to the presence of a focal, periductal leukocyte infiltrate. Progress in understanding the mechanisms involved in the development of autoimmune diseases in general, and Sjögren's syndrome specifically, has been generated as a result of renewed interest in animal models, such as the NOD mouse, which mimics autoimmune sialoadenitis. Biochemical analyses have indicated that the salivary glands have reduced beta-adrenergic, muscarinic, and neuropeptide signal transduction responses that correlate to reduced receptor density and the appearance of autoantibodies directed against these and other cell surface proteins. Using the NOD-scid mouse (lacking functional B- and T-lymphocytes) it has been determined that salivary flow rates are normal; however, these animals show abnormal changes in protein biosynthesis with increasing age. Histological evaluation of the submandibular gland from older NOD-scid mice revealed a loss of acinar cells accompanied by a potential increase in the ductal cell component of the tissue. Consistent with this finding, we recently have observed increased levels of cell death-associated cysteine proteases in the submandibular glands of 20 week NOD and NOD-scid mice but not in BALB/c and young NOD controls. Other novel protease activity was detected in the parotid and submandibular glands from NOD mice, which were able to generate the aberrantly processed PSP from purified BALB/c protein. Taken together, these data paint a complex picture of the development of Sjögren's syndrome-like disease in the NOD mouse model. The presence of activated lymphocytes appears to be necessary for the ultimate loss of exocrine gland function, potentially through the loss of tolerance to glandular proteins. However, the findings of high levels of apoptosis and aberrant protein expression in the submandibular gland in the absence of an immune response (NOD-scid) suggests that genetic alterations in glandular homeostasis involving the death program contribute to disease progression or even the initial trigger of autoimmunity.