Influence of macrophages on the insulin-synthesizing system under normal conditions and in alloxan diabetes

Abstract

Insulin-synthesizing cells (ISCs) of pancreatic gland are localized both in its islets, and in exocrine portion, as single cells or cellular agglomerates. ISCs differ in their morphological and functional characteristics, depending on characteristics of the microenvironment. Resident macrophages are also involved into formation of their microenvironment. Our purpose was to assess the effect of functional macrophages upon the insulinsynthesizing system (pancreatic islets, cell agglomerates, and separately lying insulin-synthesizing cells) under normal conditions and in alloxan diabetes.Alloxan diabetes was induced in mature male Wistar rats by intraperitoneal injection of alloxan (30 mg/100 g). Functional activity of macrophages was modeled with anti-inflammatory drug aminophthalhydrazide (AMP). Contents of insulin, glucose, and glycosylated hemoglobin were measured in blood of experimental animals. The levels of IL-1α, TNFα and IFNγ were determined in pancreatic homogenate. The number of macrophages was counted in histological preparations from the insular and exocrine parts of the organ, as well as the number of pancreatic islets, agglomerates, and single ISCs. The amounts of proliferating cells (insulin+Ki-67+), apoptotic forms (TUNEL+insulin+), and insulin content of ISCs at different sites (according to their fluorescence intensity) were determined. All pancreatic islets were divided into 3 types, according to intensity of insulin fluorescence, i.e., islets with high, median and low levels of fluorescence.In healthy rats, immunomodulation reduced total level of IL-1α in pancreatic parenchyma, without changing the overall parameters of carbohydrate metabolism. In the exocrine part of pancreas, the content of single ISCs in ductal epithelium was increased. Likewise, proliferation of the ISC agglomerates became higher. The intensity of β-cell apoptosis increased in pancreatic islets. The proportion of islets with high-level insulin fluorescence was decreased, along with lower density of macrophages and proliferation rates of β-cells, and higher apoptosis rates, than in intact animals. We have also revealed there an increased ratio of cells with average insulin levels. In the islets with low insulin content, immunomodulation did not cause morphological changes. Administration of AMP in alloxan diabetes contributes to a significantly decreased concentration of IFNγ in pancreatic tissues, stabilizes IL-1α content, along with reduced apoptosis of ISCs and macrophage infiltration in all parts of the gland. In the ductal epithelium, a large number of single ISCs with high synthetic activity was observed, with retained number of agglomerates and their increased cellularity. The number of dividing β-cells is increased in pancreatic islets.Modulation of the functional activity of pancreatic macrophages under physiological conditions provides a multidirectional effect on the insulin-synthesizing cells, depending on their localization. In exocrine part of the organ, where M2 macrophages are located, we have observed activated differentiation and proliferation of ISC precursors. Meanwhile, in the islets where M1 macrophages are present, apoptosis of β-cells was enhanced. In alloxan diabetes, immunomodulation was associated with reduced destruction of insulinocytes, along with high intensity of their proliferation. Heterogenous response of ISCs to the changes in the microenvironment depends on their synthetic activity. In healthy rats, the islets with high level of insulin fluorescence, the level of apoptosis is increased, and β-cell proliferation is reduced, while the morphological and functional characteristics of islets with low-level insulin fluorescence did not change. In alloxan diabetes, apoptosis prevailed in islets with high fluorescence values, whereas β-cell proliferation predominated in the islets with low insulin contents.