Increased expression of CCL2 in insulin-producing cells of transgenic mice promotes mobilization of myeloid cells from the bone marrow, marked insulitis and diabetes

Abstract

Objective: To define the mechanisms underlying the accumulation of monocytes/macrophages in the islets of Langerhans.

Research Design and Methods: We tested the hypothesis that macrophage accumulation into the islets is caused by overexpression of the chemokine CCL2. To test this hypothesis we generated transgenic mice and evaluated the cellular composition of the islets by immunohistochemistry and flow cytometry. We determined serum levels of CCL2 by ELISA, determined numbers of circulating monocytes, and tested whether CCL2 could mobilize monocytes from the BM directly. We examined development of diabetes overtime and tested whether CCL2 effects could be eliminated by deletion of its receptor, CCR2.

Results: Expression of CCL2 by β-cells was associated with increased numbers of monocytes in circulation and accumulation of macrophages in the islets of transgenic mice. These changes were promoted by combined actions of CCL2 at the level of the BM and the islets and were not seen in animals in which the CCL2 receptor (CCR2) was inactivated. Mice expressing higher levels of CCL2 in the islets developed diabetes spontaneously. The development of diabetes was correlated with the accumulation of large numbers of monocytes in the islets, and did not depend on T and B cells. Diabetes could be induced in normoglycemic mice expressing low levels of CCL2 by increasing the number of circulating myeloid cells.

Conclusions: These results indicate that CCL2 promotes monocyte recruitment by acting both locally and remotely and that expression of CCL2 by insulin-producing cells can lead to insulitis and islet destruction.