Histochemical analysis of lymphatic endothelial cells in the pancreas of non‐obese diabetic mice

We studied the relationship between insulitic development and function–structural changes of pancreatic lymphatics in non‐obese diabetic (NOD) mice using combined 5′‐nucleotidase (5′‐Nase) enzyme histochemical and secondary lymphoid tissue chemokine (SLC/CCL21) immunohistochemical methods. Interlobular lymphatic vessels were positive for 5′‐Nase throughout the pancreas, and dependent on both blood vessels and pancreatic ducts. Intralobular initial lymphatics were rare and occasionally ran in the neighbourhood of islets. During the non‐insulitic stage, the 5′‐Nase‐reactive product was evenly distributed on the surface of lymphatic endothelial cells (LECs) with weak expression of CCL21. The activity of 5′‐Nase on lymphatic vessels became slightly reduced as insulitis developed. The increasing blood glucose values appeared to be consistent with an increasing CCL21 expression by the endothelial lining, especially on the surface of LECs adjacent to the infiltrated islets and tissues. Lymphocytes and dendritic cells (DCs) were frequently located in the connective tissue, surrounding the lymphatic wall with deposition of 5′‐Nase precipitates. As the infiltration became severe, lymphocytes and DCs accumulated within lymphatic vessels and expressed high levels of CCL21. The most significant finding was that many DCs adhered to lymphatic vessels, transmigrating via the thin and indented endothelial walls. The activity of 5′‐Nase was increased on the adhesion surface between DCs (or lymphocytes) and LECs. The latter were characterized by open intercellular junctions and obvious cytoplasmic protrusions. These results suggest that LECs closely interact with DCs and lymphocytes, and play a key role in the migration of DCs and lymphocytes via lymphatic vessels during the pathological processes of insulitis in NOD mice.