Thymocyte development in major histocompatibility complex-deficient mice: evidence for stochastic commitment to the...

The mechanism resulting in commitment of precursor cells in the thymus to either the CD4 or CD8 lineage remains poorly understood. In principle, this may reflect a stochastic process or may reflect instructional signals from host major histocompatibility complex (MHC) molecules. We have examined the role of MHC products in subset commitment by using mice deficient in class I or class II MHC products. Normal numbers of committed CD4 intermediates (CD4+ CD8lo) develop in the thymus in the absence of class II molecules. Similarly, CD8 transitional cells (CD4loCD8+) are present in the thymus of mice lacking class I products. These findings suggest that commitment of CD4+8+ precursor cells to either lineage is a stochastic process that does not depend on instructive signals from MHC molecules (i.e., expression of alternative differentiative options by uncommitted precursor cells is independent of this environmental signal). These studies also suggest that an interaction between the T-cell antigen receptor (TCR) and MHC molecules that is independent of CD4/CD8 coreceptor engagement enhances stochastic coreceptor downregulation substantially and leads to upregulation of TCR expression as a prelude to selective events that require joint coreceptor/TCR engagement. We suggest that this initial interaction molds the TCR repertoire of stochastically generated T-cell subsets toward recognition of self-MHC products.