An increased MRP8/14 expression and adhesion, but a decreased migration towards proinflammatory chemokines of type 1 diabetes monocytes

Abstract

In the early development of type 1 diabetes macrophages and dendritic cells accumulate around the islets of Langerhans at sites of fibronectin expression. It is thought that these macrophages and dendritic cells are derived from blood monocytes. Previously, we showed an increased serum level of MRP8/14 in type 1 diabetes patients that induced healthy monocytes to adhere more strongly to fibronectin (FN). Here we show that MRP8/14 is expressed and produced at a higher level by type 1 diabetes monocytes, particularly after adhesion to FN, creating a positive feedback mechanism for a high fibronectin-adhesive capacity. Also adhesion to endothelial cells was increased in type 1 diabetes monocytes. Despite this increased adhesion the transendothelial migration of monocytes of type 1 diabetes patients was decreased towards the proinflammatory chemokines CCL2 and CCL3. Because non-obese diabetic (NOD) mouse monocytes show a similar defective proinflammatory migration, we argue that an impaired monocyte migration towards proinflammatory chemokines might be a hallmark of autoimmune diabetes. This hampered monocyte response to proinflammatory chemokines questions whether the early macrophage and dendritic cell accumulation in the diabetic pancreas originates from an inflammatory-driven influx of monocytes. We also show that the migration of type 1 diabetes monocytes towards the lymphoid tissue-related CCL19 was increased and correlated with an increased CCR7 surface expression on the monocytes. Because NOD mice show a high expression of these lymphoid tissue-related chemokines in the early pancreas it is more likely that the early macrophage and dendritic cell accumulation in the diabetic pancreas is related to an aberrant high expression of lymphoid tissue-related chemokines in the pancreas.

Fig. 1

Adhesion and MRP8/14 expression of monocytes in diabetes mellitus type 1 (DM1). (a) Monocytes of DM1 patients (n = 12) showed increased surface expression of MRP8/14 compared to healthy controls (n = 13) and DM2 patients (n = 3). The MRP8/14 expression is shown relative to that observed in healthy control subjects. The inlay shows representative histograms of monocytes of a DM1 patient and a healthy control; IgG controls and a DM2 patient are not shown to avoid overcrowding the figure. (b) The production of MRP8/14 by monocytes after 24 h was increased in DM1 patients (n = 9) in comparison to healthy controls (n = 9), both with and without fibronectin (FN) stimulation. (c) Monocytes of DM1 patients showed increased adhesion to FN in comparison to monocytes of healthy control subjects. Adhesion after 24 h is shown of nine DM1 patients and nine healthy control subjects. Data are presented as average ± s.e.m., *P < 0·05 as determined with unpaired two-tailed Students’t-test.

Fig. 2

Expression of proinflammatory cytokines by monocytes under fibronectin (FN)-adherent condition. (a) When stimulated with fibronectin, monocytes of diabetes mellitus type 1 (DM1) patients showed an increased production of CCL2 compared to monocytes of healthy control subjects. (b) The production of CCL3 was also increased by monocytes of DM1 patients after FN stimulation. Data are presented as average ± s.e.m., n = 9, *P < 0·05, **P < 0·01 as determined with unpaired two-tailed Students’t-test.

Fig. 3

Monocyte adhesion and transmigration across human umbilical vein endothelial cells (HUVEC). (a) Monocytes of type 1 diabetic patients (n = 7) showed increased adhesion to HUVEC compared to monocytes of healthy subjects (n = 7) and type 2 diabetes patients (n = 4). (b) Transmigration in response to CCL2 and CCL3 of type 1 diabetes monocytes (n = 12) was strongly decreased compared to healthy control monocytes (n = 12). Adhesion is shown relative to that observed with monocytes of healthy control subjects. Data are presented as average ± s.e.m., *P < 0·05, **P < 0·01 as determined with unpaired two-tailed Students’t-test.

Fig. 4

Disturbed chemotactic response of monocytes of type 1 diabetes patients. Monocytes of type 1 diabetes patients displayed a decreased chemotactic response to fMLP and CCL2, while in contrast the response towards CCL19 was increased. Data are presented as average ± s.e.m., n = 12, *P < 0·05, **P < 0·01 as determined with unpaired two-tailed Students’t-test.

Fig. 5

Chemokine receptor expression on monocytes. Representative histograms of CCR1, CCR2, CXCR4 and CCR7 expression of 16 type 1 diabetes patients, nine type 2 diabetes patients and 11 healthy control subjects show that the expression of CCR1, CXCR4 and CCR7 was increased in type 1 diabetes patients.