Characterization of ζ-associated protein, 70 kd (ZAP70)-deficient human lymphocytes

Background: ζ-associated protein, 70 kd (ZAP70), deficiency in human subjects results in a combined immunodeficiency characterized by normal numbers of circulating CD4 T cells and CD8 lymphocytopenia. Patients who live beyond infancy can also experience autoimmune manifestations. Objectives: We sought to further characterize the nature of the T-cell populations found in ZAP70-deficient patients and explored the mechanisms that might predispose them to autoimmunity. Methods: T-cell development was assessed by examining T-cell receptor (TCR) gene rearrangements and thymopoiesis by measuring TCR exclusion circle levels. TCR repertoire on CD4 and CD8 T-cell populations was assessed by means of flow cytometry. T-cell gene expression patterns were examined by means of exonic microarray analysis and apoptotic responses by means of Annexin V binding and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling. Results: Cells displaying recombination events from all stages of TCR gene rearrangement were present in the peripheral blood of ZAP70-deficient patients; however, the late TCRD-deleting rearrangement was significantly reduced. TCR exclusion circle levels were also found to be low. Surprisingly, all Vβ families were detected in both CD4⁺ and CD8⁺ circulating T cells. Several Vβ families were significantly overrepresented, which is reminiscent of autoimmune disorders. Levels of mRNA for cytotoxic T lymphocyte-associated antigen 4, TGF-β, and IL-10 were found to be low, a signature of autoimmunity. Finally, Fas-mediated CD4 T-cell apoptosis was found to be reduced in vitro, and staining of thymus biopsy specimens revealed reduced thymocyte apoptosis. Conclusion: We show that in the absence of ZAP70, thymopoiesis is altered and differentiation to double-positive cells is hampered. Circulating T cells appear poorly regulated, do not differentiate into T_H2 T cells, lack a number of inhibitory growth controls, and display reduced apoptosis, all predisposing patients to exaggerated inflammation and autoimmunity.