Effect of Prophylactic Insulin Treatment on the Number of ER-MP23+Macrophages in the Pancreas of NOD Mice. Is the Prevention of Diabetes Based on β-cell Rest?

Abstract

Prophylactic insulin treatment has been shown to have beneficial effects in type 1 diabetes, both in humans and in various animal models of the disease. In experimental models, the protective effect of prophylactic insulin treatment was observed in two parameters: (1) progression of insulitis and (2) diabetes incidence. The mechanism of protection still remains to be investigated. We therefore analysed by immunohistochemistry the effect of prophylactic insulin treatment vs placebo treatment (from 4 to 13 weeks of age) on ER-MP23 + macrophage infiltration in and around pancreatic islets in the non-obese diabetic (NOD) mouse, a spontaneous model for type 1 diabetes. BALB/c mice were used as diabetes-free controls. Using conventional haematoxylin-eosin staining, we detected a protective effect of prophylactic insulin treatment in NOD females on the lymphocytic insulitis, significant at 13 weeks, but not at 9 weeks of age. However, when assessed by immunostaining for early infiltration of ER-MP23 + macrophages around islets, the reduction in severity of insulitis could already be detected as early as 9 weeks of age. With regard to the early accumulation of ER-MP23 + cells, we observed that their numbers per mm 2 surface area of the exocrine pancreas and per m at the circumference of the islet were higher in placebo-treated NODs (197±13.8 and 14±0.9, respectively) as compared to age-matched BALB/c mice (123.1±7.1 and 3.5±0.9, respectively). Prophylactic insulin treatment of NODs lowered the attraction of ER-MP23 + macrophages to the exocrine pancreas and to the circumference of the islets (156.3±8.5 and 7.9±1, respectively). Interestingly also, the islet size was found to be larger in placebo-treated NODs (51% was larger than 10 m 2 ) than in age-matched BALB/c mice (9% larger than 10 m 2 ). Prophylactic insulin treatment of NODs reduced their islet size to sizes found in the control BALB/c strain. In conclusion, the decrease in islet size by early insulin administration, and the lower attraction of ER-MP23 + macrophages to the islets are morphological indications that prevention of diabetes development by prophylactic insulin treatment results from a downregulation of islet metabolism and growth, with a concomitant decline in the release of islet factors attracting macrophages.

Islet Abnormalities Associated with an Early Influx of Dendritic Cells and Macrophages in NOD and NODscid Mice

“…In the latter, lacking functional lymphocytes, we therefore avoided the tolerizing effects of insulin on lymphocytes (Karounos et al, 1997). Figure 4 shows that prophylactic insulin treatment, administered from 3 to 6 weeks of age, decreased significantly the percentage of megaislets in NOD mice ( p ϭ 0.003), confirming previous data (Jansen et al, 1996). Moreover, it also decreased this percentage in NODscid mice to a similar extent ( p Ͻ 0.001).…”

“…Because insulin is known to down-regulate its own secretion (Argoud et al, 1987;Boden et al, 1993;Strubbe and Steffes, 1993) and to decrease pancreatic insulin content in NOD mice (Sai et al, 1996), we used it to prevent the transient ␤-cell hyperactivity that we previously described from 4 to 8 weeks of age in mice with the NOD genetic background (Amrani et al, 1998;Homo-Delarche, 1997). Here, we confirmed that prophylactic insulin treatment is able to reduce megaislet formation in NOD mice (Jansen et al, 1996). However, because the treatment could act on lymphocytes, via the induction of tolerance to insulin (Karounos et al, 1997), we also prophylactically treated NODscid mice, which lack functional lymphocytes.…”

“…In a previous study, we observed a comparable difference using Balb/c mice as a control strain (Jansen, 1996) #646. Moreover, our results on ␤-cell hyperplasia in NOD and NODscid mice are in agreement with studies showing a large ␤-cell mass in prediabetic NOD mice or in insulitis-free NOD stocks (NODscid, NOD-NON H-2 nb1 and NOD B2 m null (Serreze and Leiter, 1995).…”

“…Because ␤-cell hyperactivity was increased before the appearance of mega-islets (Amrani et al, 1998;HomoDelarche and Boitard, 1996;Jansen et al, 1996), we wondered whether it could be a contributing factor in their formation, in addition to immune cell infiltration. We thus assessed the effect of prophylactic insulin treatment, known to down-regulate ␤-cell activity (Argoud et al, 1987;Boden et al, 1993;Strubbe and Steffes, 1993), in NOD and NODscid mice.…”

“…Glucose tolerance tests also showed a more rapid blood insulin peak and a faster drop in blood glucose levels in prediabetic NOD mice compared with controls. Evidence for an early islet abnormality also has a morphological basis, because we previously showed a higher frequency of hyperplastic islets ("mega-islets") in prediabetic NOD mice compared with non-diabetic control strains (Jansen et al, 1996).…”