The resident macrophages in murine pancreatic islets are constantly probing their local environment, capturing beta cell granules and blood particles

Aims/hypothesis: We studied here the interactions between the resident macrophages of pancreatic islets with beta cells and the blood vasculature. We also examined the immunological consequences of such interactions. Methods: Islets were isolated from C57BL/6 mice expressing CX3C motif chemokine receptor 1–green fluorescent protein (CX3CR-GFP) and examined live by two-photon microscopy. Islets were also examined by electron microscopy to study the relationship of the intra-islet macrophages with the beta cells. In NOD.Rag1^−/− mice and young (non-diabetic) male mice, the acquisition of beta cell granules was tested functionally by probing with CD4⁺ T cells directed against insulin epitopes. Results: Two-photon microscopy showed that the islet resident macrophages were in close contact with blood vessels and had extensive filopodial activity. Some filopodia had direct access to the vessel lumen and captured microparticles. Addition of glucose at high concentration reduced the degree of filopodia sampling of islets. This finding applied to in vivo injection of glucose or to in vitro cultures. Ultrastructural examination showed the close contacts of macrophages with beta cells. Such macrophages contained intact dense core granules. Functional studies in NOD mice indicated that the macrophages presented insulin peptides to insulin-reactive T cells. Presentation was increased after glucose challenge either ex vivo or after an in vivo pulse. In agreement with the morphological findings, presentation was not affected by insulin receptor blockade. Conclusions/interpretation: Islet resident macrophages are highly active, sampling large areas of the islets and blood contents and capturing beta cell granules. After such interactions, macrophages present immunogenic insulin to specific autoreactive T cells.