Beta cells activate islet resident macrophages in a culture bioassay

Abstract

Abstract The pancreatic islets of all species contain a small number of resident macrophages. Transcriptional profiles of such islet macrophages indicate that they are in an activated state in steady state conditions expressing high MHC-II, TNFα, and IL-1β. However the reason for this activation is still unknown. To understand the molecular mechanism how the islet environment affects the biology of its resident macrophages we have developed a culture system that reproduces the beta cell: macrophage interaction taking place in islets. Our protocol involves culturing beta cells or insulinomas with bone marrow precursors in CSF-1 conditioned media for seven days. During the period of co-culture the insulinoma cells begin to bud off the plate and cluster with the developing macrophages forming a pseudo-islet structure. Two photon microscopic analysis of the organoids showed the presence of macrophages in the cluster closely attached to beta cells. Macrophages in the organoid capture insulin granules and express high level of MHC-II and PDL1. Microarray analysis shows upregulation of inflammatory genes of the NFκB pathway and a tight correlation with gene signature of bone marrow macrophages stimulated with LPS. The macrophage differentiation and activation is specific for insulinomas and does not take place with other immortalized cell lines; it requires cell contact between β cells and macrophage precursors: it does not take place with already differentiated macrophages. In sum, the beta cell:macrophage symbiosis can be generated in vitro by co-culturing beta cells and bone marrow precursors. We are currently investigating the mechanism how beta cells modulate the activation profile of macrophages using this in vitro co-culture system.