Defects in the thymic epithelial stroma of diabetes prone BB rats

The thymus of diabetes prone BB rats (DP) was studied and compared with diabetes resistant (DR) BB rats and normal WAG rats. Thymuses were obtained from 4-5 week old animals, i.e. before the onset of disease. Analysis included specific immune histology using a panel of monoclonal antibodies directed against markers both on thymocytes and stromal cells. No striking differences were observed between the three groups with regard to the expression and distribution of the various T cell markers. There was however a marked difference for thymic class II MHC antigen expression between the various groups. Whereas WAG rats displayed a regular class II MHC pattern both in the cortex and the medulla, both DR and DP rats showed large areas in the cortex where there was no class II MHC staining. The lack of expression of class II MHC antigens in these 'holes' was associated with a complete absence of epithelial cells in that area. Also in the medulla of DP and DR thymuses 'holes' in the keratin-stroma were observed. The stromal aberrations in these auto-immune prone rats are discussed in the context of the deficient T cell system in these animals.