Histologic characterization of the thymus in canine X-linked severe combined immunodeficiency.

This study describes the thymic morphology in 52 dogs (ranging in age from 1 to 79 days) with X-linked severe combined immunodeficiency disease (XSCID). The thymuses from the XSCID dogs and age-matched controls were evaluated histologically for the presence of Hassall's corpuscles and branchial duct remnants, the degree of corticomedullary differentiation, and lymphoid development and organization. Within this population of XSCID dogs with the same genetic defect, three histologic patterns of thymic dysplasia were recognized. Simple dysplasia, noted in 27 XSCID thymuses, was characterized by varying numbers of lymphocytes, no corticomedullary demarcation, and an absence of Hassall's corpuscles. Dysplasia with Hassall's corpuscles was noted in 21 dogs and consisted of varying numbers of lymphocytes, no corticomedullary demarcation, and varying numbers of Hassall's corpuscles. Dysplasia with partial corticomedullary demarcation, noted in 4 dogs, consisted of relatively normal-looking thymuses with well-defined corticomedullary demarcation and numerous Hassall's corpuscles; however, the lobules were extremely small and the subcapsular cortical region was devoid of lymphocytes. Cystic branchial duct remnants were present in 46 of the 52 XSCID thymuses and were more numerous in those thymuses with the pattern of simple dysplasia. The thymuses of XSCID pups less than 4 weeks of age were of the simple dysplastic type and thymuses of XSCID dogs greater than 4 weeks of age were more developed, as evidenced by increased numbers of Hassall's corpuscles and greater corticomedullary demarcation. In conclusion, the thymic dysplasia and lymphoid hypoplasia associated with X-linked severe combined immunodeficiency disease in the dog does not appear to be due to a developmental arrest but rather due to an active process dependent on factors probably related to the overall genetic defect.