Diabetes development in non-obese diabetic mice is under control of the toll-like receptor 4

Objectives: Animal models of human type 1 diabetes have shown that cells of the innate immune system, such as macrophages, play a central role in the initiation and progression of immune reactivity directed against autologous insulin producing pancreatic beta cells. Recent studies identified the family of toll-like receptors (tlrs) as important regulators of innate immune cell activities and implicated a role of tlrs also in the induction of autoimmune disorders. In our present study we investigated the effect of tlr4, a prominent member of the tlr family, on the development of autoimmune diabetes in mice.

Methods: For our investigations the tlr4 defect of the C57BL10/ScCr mouse (tlr4-/-) was backcrossed onto the background of the non-obese diabetic (NOD) mouse and tlr4+/+, tlr4+/- and tlr4-/- mice were generated.

Results: Until 210d of age 71% of female tlr4+/+ mice developed diabetes (mean age of manifestation 177±22d). Interestingly, tlr4-/- and tlr4+/- mice showed a significant acceleration of diabetes development. In tlr4-/- mice the diabetes incidence reached its maximum (80%) already at 146d (mean age of manifestation 118±21d, p<0.01 vs. tlr4+/+). Tlr4+/- mice showed a diabetes incidence of 67% (mean age of manifestation 129±40d, p<0.05 vs. tlr4+/+). Pancreata of 100d old tlr4-/- and tlr4+/- mice showed advanced stages of severe inflammation of the islets (intra-insulitis) (p<0.05 vs. tlr4+/+) and of the islet-surrounding exocrine tissue. As assessed by trypan blue exclusion exposure of isolated islet cells to a mixture of the cytokines TNFα (50 U/ml), IL-1β (5 U/ml) and IFNγ (10 U/ml) for 6d resulted in the lysis of 45.9±4.7% of tlr4+/+ and of 42.5±6.8% of tlr4-/- islet cells, indicating that islet cell susceptibility to inflammatory mediators is independent of tlr4.

Conclusions: Our data show that tlr4 controls the development of autoimmune diabetes in NOD mice. Tlr4 does not affect beta cell susceptibility to inflammatory damage but seems to act on immunoregulatory processes that may involve the activity of regulatory T-cells.