Defective thymocyte maturation in horses with severe combined immunodeficiency.

Six monoclonal antibodies, designated EqT2, EqT3, EqT6, EqT7, EqT12, and EqT13, which identify T lymphocyte antigens present at different stages of T cell maturation were used to examine T lymphocyte development in foals with severe combined immunodeficiency (SCID). Flow microfluorimetry demonstrated the presence of EqT12+ and EqT13+ prothymocytes and a few phenotypically mature EqT2+ and EqT3+ thymocytes within the thymic remnants of SCID foals. However, very few EqT6+ and EqT7+ resident cortical thymocytes were detected. The near absence of EqT6+ and EqT7+ cortical thymocytes was confirmed by immunofluorescence analysis of thymic tissue from SCID foals. Those cells present were larger than normal cortical thymocytes. Furthermore, their activities of adenosine deaminase, adenosine monophosphate-deaminase, and 5' nucleotidase differed from those of normal cortical thymocytes. The combined evidence of monoclonal antibody analysis, size parameters, and purine enzyme activities demonstrate the near absence of cortical thymocytes in horses with this genetically defined immunodeficiency disorder.