Immunolocalization of monocyte chemoattractant protein-1 in islets of NOD mice during cyclophosphamide administration

The molecular processes that initiate insulitis in type 1 diabetes remain unclear. Chemokines, such as monocyte chemoattractant protein-1 (MCP-1), mediate chemotaxis and leukocyte migration to inflammatory sites. Although MCP-1 mRNA has been shown in islets isolated from NOD mice at 2 weeks of age and at later stages, the cellular sources of this chemokine, at the protein level, and its role in insulitis are unclear. The aims of the present study were to employ immunohistochemical techniques to examine the expression of MCP-1 and quantify its cellular sources in islets of NOD mice both after cyclophosphamide (Cy) administration and in spontaneous diabetes. Tissues were examined at days 1 (=day 73, first day of Cy), 4, 7, 11, and 14 and in age-matched control NOD mice. Pancreatic sections from NOD mice without Cy administration were also studied between days 21-65 and at onset of diabetes and from adult CD-1 mice. In the Cy group, a small number of peri-islet macrophages were immunopositive for MCP-1 at day 1 whereas at day 4, the number declined but increased subsequently at day 7. In the same group, it increased markedly at days 11 and 14 compared with age-matched control NOD mice. In young NOD mice, MCP-1 was present in selective macrophages in islets with early insulitis (day 45) but was absent at diabetes onset. MCP-1 was undetectable in beta cells and in most T cells. Islets from adult CD-1 mice did not show immunostaining for MCP-1. We conclude that MCP-1 is expressed in a proportion of islet and exocrine macrophages. This expression increases during the later stages of Cy-induced diabetes. Thus, MCP-1 positive macrophages that migrate to the islet periphery during the early stages of Cy-induced diabetes and preceding spontaneous diabetes may augment insulitis by further attracting macrophages and T cells.