Positive selection of CD8⁺ T cells induced by major histocompatibility complex binding peptides in fetal thymic organ culture

We have used an in vitro system to study the effects of major histocompatibility complex class I binding peptides on thymic development. Fetal thymus lobes from mice deficient in the class I light chain (β₂ microglobulin or β₂M-/-) were cultured for 10 d in vitro, during which time T cell precursors develop into mature T cells. In these organ cultures, as in the adult or neonatal β₂M-/- thymus, CD8⁺ mature T cells did not develop, demonstrating that the mature T cells seen during early murine thymic development are the result of the positive selection process. To these cultures we added various class I binding peptides with or without a source of exogenous β₂M. CD8⁺ T cells developed to various degrees only in the presence of β₂M and peptides. Using peptide mixtures of differing complexity, we snowed that the efficiency of this process is dependent more on peptide complexity than on peptide concentration. These data argue for a specific role for peptides in the process of positive selection. Furthermore, this culture system should be useful in identifying peptides that can promote positive selection of cells expressing a specific T cell receptor (TCR) in TCR transgenic mice.