Lack of evidence for a role of islet autoimmunity in the aetiology of canine diabetes mellitus

Abstract

Aims/hypothesis: Diabetes mellitus is one of the most common endocrine disorders in dogs and is commonly proposed to be of autoimmune origin. Although the clinical presentation of human type 1 diabetes (T1D) and canine diabetes are similar, the aetiologies may differ. The aim of this study was to investigate if autoimmune aetiology resembling human T1D is as prevalent in dogs as previously reported.

Methods: Sera from 121 diabetic dogs representing 40 different breeds were tested for islet cell antibodies (ICA) and GAD65 autoantibodies (GADA) and compared with sera from 133 healthy dogs. ICA was detected by indirect immunofluorescence using both canine and human frozen sections. GADA was detected by in vitro transcription and translation (ITT) of human and canine GAD65, followed by immune precipitation. Sections of pancreata from five diabetic dogs and two control dogs were examined histopathologically including immunostaining for insulin, glucagon, somatostatin and pancreas polypeptide.

Results: None of the canine sera analysed tested positive for ICA on sections of frozen canine or human ICA pancreas. However, serum from one diabetic dog was weakly positive in the canine GADA assay and serum from one healthy dog was weakly positive in the human GADA assay. Histopathology showed marked degenerative changes in endocrine islets, including vacuolisation and variable loss of immune-staining for insulin. No sign of inflammation was noted.

Conclusions/interpretations: Contrary to previous observations, based on results from tests for humoral autoreactivity towards islet proteins using four different assays, and histopathological examinations, we do not find any support for an islet autoimmune aetiology in canine diabetes mellitus.

Figure 1. Pancreatic specimens stained with hematoxylin-eosin (A, C, E) and immunostained for insulin (B, D, F).

The samples shown in A and B come from a non-diabetic male Swedish Elkhound, C and D come from a diabetic male Polish Owczarek Nizinny dog and E and F from a diabetic female English Setter. The islet in Fig D contains few insulin-immunoreactive cells except the vacuolated cells, while these normal-appearing insulin-stained cells are numerous in Fig F.

Figure 2. Canine pancreas stained by immunofluorescence.

A) Stained with a mouse monoclonal Ab to human GAD65, B) stained using a GADA positive human serum and C) stained using a GADA positive serum from a patient with stiff person syndrome and D) canine serum from a dog with diabetes. Nuclear stain in blue (DAPI). Green is secondary anti rat Ig-FITC and anti-canine Ig-FITC used in A) and D). Red is anti-human Ig -Cy5 used in B) and D).

Figure 3. Immunoreactivity to A) Canine GAD65 and B) Human GAD65.

DM =  diabetes mellitus, n = 121, Ctrl  =  healthy control, n = 133, LT  =  lymphocytic thyroiditis, n = 13 and AI  = adrenal insufficiency n = 5, Cut off index for positive vs. negative to GADA indicated by dotted line.