Developmental regulation of bcl-2 expression in the thymus

An important factor in shaping the T-cell receptor (TcR) repertoire during thymocyte development is the susceptibility of double-positive (CD4+ CD8+) thymocytes to induction of apoptosis (negative selection) when the TcR is engaged by 'self'-antigens. Recent evidence has suggested that this susceptibility to apoptosis may be influenced by the expression of bcl-2, a proto-oncogene known to increase the resistance to apoptosis in various cell systems. Using a semi-quantitative polymerase chain reaction (PCR) technique in conjunction with staged embryonic material and purified thymocyte subpopulations we have investigated patterns of bcl-2 expression during normal T-cell development. Our results show that while bcl-2 alpha gene expression is readily detectable in immature CD3-CD4-CD8- thymocytes and in mature single-positive TcRhi cells, it is drastically reduced in TcR negative double-positive (CD3- CD4+ CD8+) cortical thymocytes of intermediate maturity. Careful mapping of bcl-2 alpha re-expression in relation to the onset of TcR expression within the population of embryonic thymocytes indicates that bcl-2 alpha is up-regulated as soon as TcR molecules are expressed on the surface of CD4+ CD8+ thymocytes. Therefore, thymocytes susceptible to apoptosis on TcR ligation express bcl-2 alpha mRNA suggesting that changing levels of bcl-2 expression are unlikely to be the only determinant regulating susceptibility to apoptosis in the thymus. The possible implications of these changes in bcl-2 expression regarding other facets of thymocyte development will be discussed.