Is Ia antigen expression by thymic epithelial cells constitutive?

Thymic epithelial cells express class II MHC antigens (Ia) in the normal thymus. In a variety of special circumstances, generally associated with decreased or absent thymocyte populations, TEC fail to express Ia antigens. Although such observations might be interpreted based on the hypothesis that TEC Ia antigens have an effect on the differentiation of thymocytes, it is also possible that instead, the thymocytes have an effect on TEC Ia antigen expression.