Iddm 1 in the BioBreeding (BB) rat designates the genomic region on rat Chromosome (Chr) 4 that harbors the gene causing peripheral T cell lymphopenia (Lyp) and diabetes (Jacob et al. 1992). We here report the generation of a novel congenic Lyp/Lyp Dr rat line, in which iddm 1 has been introgressed onto the diabetes-resistant BB/Dr background. This was accomplished by marker-assisted selection (MAS)(Lande and Thompson 1990) in nine cycles of cross-intercross breeding of diabetes-prone Dp with Dr BB rats. As a result, we have generated a congenic line in which diabetes and lymphopenia are controlled solely by iddm 1. The lymphopenia phenotype correlates completely with homozygosity for the Dp alleles of D4Mit6 and D4Mit24, which are flanking markers of iddm 1. D4Mit6 and D4Mit24 have a genetic distance of about 1 cM (Jacob et al. 1992; Pettersson et al. 1995; Hornum et al. 1995). During the generation of the congenic line, genomic DNA from every rat in the F2 generations was genotyped by PCR for Dp or Dr allele sequence polymorphisms of these two markers (Jacob et al. 1992). All rats were also analyzed for lymphopenia with the monoclonal anti CD5 T cell antibody OX19 (Pettersson et al. 1995). Rats that were typed Dp/Dp, DpIDr, or Dr/Dr for D4Mit6 and D4Mit24 were designated LyplLyp, Lypl+ and+/+, respectively. All lymphopenic rats in the F 2 generations were Lyp/Lyp, and all non-lymphopenic rats were either Lyp/+ or+/+(wildtype BB/Dr).

We have previously reported the establishment of inbred BB/Dp and BB/Dr lines that were born and kept under specific pathogen-free (SPF) conditions (Markholst et al. 1991; Eastman et al. 1991). The introgression of iddm 1 was initiated in 1989 by a cross between a male BB/Dp and a female BB/Dr rat. The resulting F, rats (all Lyp/+) were intercrossed to generate Lyp/Lyp, Lyp/+ and+/+ F2 rats. A male diabetic Lyp/Lyp F2 rat was selected and crossed with a+/+ BB/Dr rat to generate the next F 1 generation. In this approach of introgression, there is a theoretical reduction of the donor genome (BB/Dp) by 50% after each cycle of crossintercross breeding. The BB/Dr rat line has been kept in sisterbrother breeding in parallel to the cyclic cross-intercross breeding. The frequency of peripheral CD5+ cells in the Lyp/Lyp rats was< 15% compared with 50-80% in Lyp/+ or+/+ rats as determined by flow cytometry (Jacob et al. 1992; Eastman et al. 1991). The onset of insulin-dependent diabetes mellitus (IDDM) was indicated by a reduction of weight gain (Markholst et al. 1993) and by blood glucose levels equal to or exceeding 13 mmol/L with an Accu-Chek II glucose meter (Boehringer Mannheim, Indianapolis, IN). No Lyp/+ or+/+ F, or F2 rat ever developed lymphopenia or diabetes, nor has diabetes been detected in our inbred SPF BB/Dr line. The frequency of lymphopenic animals in the F2 generations