Genetic heterogeneity in the major histocompatibility complex of various BB rat sublines

Diabetes segregates as a single locus in crosses between inbred BB rats prone or resistant to diabetes

Diabetes-prone (DP) BB rats spontaneously develop insulin-dependent diabetes resembling type 1 diabetes mellitus in man. They also exhibit lifelong T cell deficiency. The segregation of both diabetes and lymphopenia was studied in crosses between this inbred line of rats and the related but nondiabetic and nonlymphopenic inbred diabetes-resistant (DR) BB rat line. Diabetes segregated as a single, autosomal recessive trait and was always accompanied by lymphopenia. Among the limited number of differences in the genomic DNA sequences of the two lines, DP and DR BB, one may account for the development of diabetes and lymphopenia in the DP BB rats. It may be possible to screen the genomic DNA for such differences to detect a marker for the phenotypes.

The relationship between insulin autoantibodies and islet cell histology in the diabetes prone BB rat

The relationship between insulin autoantibodies (IAA) and pancreatic islet cell histology was examined in 71 diabetes prone BB rats from the Toronto colony. Twenty-seven of the 71 became diabetic and of these, 18 (67%) were IAA positive by ELISA. IAA were also detected in 39/44 (89%) which did not develop diabetes, but in none of six control animals at 50-140 days of age. All 27 which became diabetic showed some evidence of lymphocytic infiltration scored + to ++++ histometrically and 26/27 evidence of beta cell degranulation. The frequency of diabetes increased with both intensity of insulitis and degree of beta cell degranulation, but there was no correlation between either and IAA. IAA are a marker for the BB strain of Wistar rat, but do not correlate with islet cell histology and do not predict clinical diabetes.

The genetics of insulin-dependent diabetes in the BB rat

Very little is known about the genes involved in the pathogenesis of IDDM. One component is known to be linked to the major histocompatibility complex, but the other components are unknown. We know from the major animals models of IDDM, both the NOD mouse and the BB rat, that the disease is under multigenic control. However, due to the size and complexity of the mammalian genome as well as to the lack of useful clues, the location and identity of the other genes remains a mystery. This is compounded by the fact that well-characterized genetic markers are not available for all regions of the mammalian genome, and it is likely that at least some of the genes of interest are located in these regions. The testing of pedigrees for the linkage of RFLP with the genetic factors involved in IDDM promises to be the most effective means of mapping, and ultimately identifying, these genes. However, the number of genes which are theoretically necessary to test for linkage makes even this approach impractical. Here, we have described here how the amount of work and time can be significantly reduced by utilizing repetitive DNA sequences as probes for the linkage of random RFLPs to diabetes. With each screening, one can simultaneously test multiple unlinked loci in the genome. Preliminary results which show promising linkage to two of the genetic components have been presented, thereby supporting the usefulness of this approach.

The gene for the T lymphocyte alloantigen, RT6, is not linked to either diabetes or lymphopenia and is not defective in the BB rat

In an outcross between a diabetic BB/H rat and a healthy Long Evans Hooded rat, the segregation of the RT6 gene was studied in the 207 F2 animals to look for linkage with diabetes or lymphopenia. The recessive gene, albino (c), was used as a marker for the RT6 gene because of the close proximity of these two genes on chromosome 1. Though most of the albino F2 rats should have been homozygous for the BB RT6 gene, we found no increase in the incidence of diabetes or lymphopenia among them when compared to their hooded littermates. Therefore, the RT6 gene was not linked to diabetes or lymphopenia in the BB rat. Moreover, the non-lymphopenic albino rats displayed normal RT6 expression when compared to the normal hooded rats showing that the RT6 gene from the BB/H grandfather was not defective. Any alteration in lymphocyte composition which could be specifically related to diabetes was studied by measuring all F2 rats for the major lymphocyte subsets including the RT6+ subset. We found that the typical pattern of lymphopenia described in diabetic BB rats was displayed by both diabetic and non-diabetic lymphopenic rats in the F2 generation. Thus, all these lymphocyte abnormalities including the depletion in RT6+ T lymphocytes appeared as a consequence of lymphopenia alone and could not be specifically related to diabetes.

The time course of insulin autoantibodies (IAA) in the diabetes prone BB rat

The BB rat is a widely used animal model for the study of insulin dependent diabetes. An enzyme linked immunosorbent assay, using purified rat insulin, was used to measure serial insulin autoantibodies (IAA) levels in coded sera from the BB/W/D rat colony in order to establish the time course of IAA. The animals included 26 diabetes-prone BB rats, six diabetes-resistant BB rats and six Wistar controls. There was an increase in both IAA frequency and titre with time in the diabetes-prone group: none were positive at 45 days, 17/19 (89%) were positive by day 90 and all were positive thereafter. Similar results were observed in the diabetes-resistant BB group (0/6 positive at day 51, 6/6 positive at day 90). None of the Wistar controls were positive at 105 days, although occasional positive sera were observed at 120 days. IAA seem to be acquired early on in the majority of BB rats, both diabetes-prone and diabetes-resistant, and much later, if at all, in controls. A clear homology of the MHC genes exists in both BB rat sublines, thus IAA appear to be a strain related phenomenon rather than a marker for IDDM.

Derivation of non-lymphopenic BB rats with an intercross breeding

Previous studies have suggested that the development of diabetes in the BB rats does not require the expression of T lymphopenia. In order to derive non-lymphopenic diabetic rats and define the relationship between the T cell abnormalities, MHC genotype, and diabetes, we performed a cross between BB/H and diabetes resistant BB/control followed by an intercross of the F1. In the F2, the overall incidence of diabetes and lymphopenia was 30% and 27%, respectively. Lymphopenia was strongly associated with diabetes (p less than 0.001) and was seen in 76% of the diabetic F2's. However, 6 of the diabetic were non-lymphopenic (24%) and 3 of the non-diabetics were lymphopenic (5%). In the non-lymphopenic diabetic animals, all T cell levels were within the normal range, but diabetes occurred at an earlier age than their lymphopenic littermates (p less than 0.001). In contrast to the strong association between the inheritance of lymphopenia and diabetes, no relationship between diabetes and Class I MHC restriction fragment length polymorphisms was found. We conclude: 1) Diabetes and lymphopenia are strongly associated inherited abnormalities in the BB rat and are not associated with Class I RFLP defined genotypes within the RTIu haplotype, 2) Animals in whom diabetes occurs in the absence of lymphopenia can be derived using this breeding approach 3) In our non-lymphopenic rats, diabetes occurred at an earlier age possibly reflecting the restoration of quantitative or qualitative T cell defects found in lymphopenic BB rats.