Expression of apoptosis-related antigens, Fas, bcl-2, p53, and Mib-1 proliferation index in the hypoplastic thymus of DiGeorge syndrome

Apoptosis, along with cellular proliferation, plays a major role in normal developmental processes and tissue homeostasis. We hypothesized that altered apoptosis-related pathways and/or reduced cell proliferation might play a role in the thymic hypoplasia or aplasia in DiGeorge syndrome (DG). We used immunohistochemistry to evaluate the apoptosis-related antigens Fas (CD95), bcl-2, and p53, as well as Mib-1 proliferation index in the thymuses from six patients with DG. The results were compared with those from the thymuses from six patients with non-DG congenital heart disease. All DG patients (age 32 weeks GA to 4 months) had hypoplastic thymuses ranging from microscopic foci to 2.7 g in weight (expected for age, 4.7 +/- 3.6 g to 10 +/- 6 g). The thymic weights from the patients with non-DG congenital heart disease (age 37 weeks GA to I month) ranged from 3 to 5.6 g and were at the lower range of expected weight by age (expected for age, 8.4 +/- 5.6 g to 12 +/- 7 g). All thymuses showed histologic features of stress-induced involution. In both groups, a greater than or equal to 50% Mib-1 proliferation index was found in the cortical thymocytes, whereas < 5% Mib-1 labeling was seen in the medullary thymocytes; Fas stained medullary epithelial cells (3 +) and cortical epithelial cells (1 +); bcl-2 stained medullary thymocytes (3 +) and cortical thymocytes (1 +); p53 stained less than 1% of nuclei in all sections. No significantly altered Mib-1 proliferation index or expression of Fas, bcl-2, and p53 was observed in the hypoplastic thymuses in DG, compared to these same measures in non-DG. These results suggest that thymic hypoplasia in DG may be mediated by mechanisms other than reduced cellular proliferation and/or altered Fas, bcl-2, and p53 apoptotic pathways.