Clearance of apoptotic beta-cells is reduced in neonatal autoimmune diabetes-prone rats.

The kinetics of beta-cell death in neonatal diabetes-prone (BBdp) and diabetes-resistant (BBdr) BioBreeding rats was investigated using both direct (histochemical) and indirect (mathematical modelling) techniques. In both BBdp and BBdr rats, the incidence of TUNEL positive beta-cells increased until 10 days of age before declining. The number of apoptotic beta-cells was significantly higher in BBdp as compared to BBdr neonates from birth until 20 days of age (P<0.05). Using a mathematical model applied to the time course of beta-cell mass and replication rate, a wave of net beta-cell loss was detected between 10 and 20 days of age in both strains. In contrast to the observed difference in the incidence of TUNEL positive beta-cells, with the model-based approach we found no difference in the rate of beta-cell apoptosis between BBdp and BBdr rats prior to weaning. As the number of apoptotic cells present in a tissue depends on the rate at which cells die and the rate at which the apoptotic cell debris is cleared, we compared in vitro phagocytosis of apoptotic thymocytes by peritoneal macrophages from 2-week-old BBdp and BBdr rats. Macrophages from BBdp neonates engulfed significantly less apoptotic cells as compared to BBdr neonates (P<0.0005). Taken together, these findings suggest that there is impaired clearance of apoptotic beta-cells in diabetes-prone BB rats during the neonatal period.

Neonatal beta-cell apoptosis: a trigger for autoimmune diabetes?

In neonatal rodents, the beta-cell mass undergoes a phase of remodeling that includes a wave of apoptosis. Using both mathematical modeling and histochemical detection methods, we have demonstrated that beta-cell apoptosis is significantly increased in neonates as compared with adult rats, peaking at approximately 2 weeks of age. Other tissues, including the kidney and nervous system, also exhibit neonatal waves of apoptosis, suggesting that this is a normal developmental phenomenon. We have demonstrated that increased neonatal beta-cell apoptosis is also present in animal models of autoimmune diabetes, including both the BB rat and NOD mouse. Traditionally, apoptosis has been considered a process that does not induce an immune response. However, recent studies indicate that apoptotic cells can do the following: 1) display autoreactive antigen in their surface blebs; 2) preferentially activate dendritic cells capable of priming tissue-specific cytotoxic T-cells; and 3) induce the formation of autoantibodies. These findings suggest that in some circumstances physiological apoptosis may, in fact, initiate autoimmunity. Initiation of beta-cell-directed autoimmunity in murine models appears to be fixed at approximately 15 days of age, even when diabetes onset is dramatically accelerated. Taken together, these observations have led us to hypothesize that the neonatal wave of beta-cell apoptosis is a trigger for beta-cell-directed autoimmunity.