Selection of thymocyte MHC restriction specificity in vitro.

The thymus is critical for the generation of mature T cells with restricted specificity for major histocompatibility complex (MHC) molecules of thymic genotype. The mechanism by which this T cell commitment to self-MHC gene products occurs in vivo prior to the exposure to foreign antigens is unknown. When thymocytes are cocultured with irradiated syngeneic accessory cells, a proliferation of cells specific for self-Class II MHC molecules is observed. This in vitro response is of interest in the context of the physiologic events that occur in the thymus. However, the functional significance of these in vitro proliferating cells, as well as their relationship to antigen-specific, MHC-restricted T cells, has not been defined. Further, whether this in vitro response is capable of influencing the MHC specificity of the thymocytes is unknown. In the work reported here, cells obtained from these cultures were tested for their ability to help a primed B cell antibody response. This approach allowed an analysis of function, antigen and MHC-restriction specificity, and the behavior of these cells upon transfer in vivo. Potent T-helper function was obtained from such cultures for an antigen-dependent, hapten-specific B cell response. When F1 thymocytes were cultured with F1 accessory cells, the T-helper cells recovered could help B cells of both parental MHC haplotypes. However, if the same F1 thymocytes were first cultured with parental accessory cells, they subsequently cooperated only with the corresponding parental or F1 B cells. This MHC restriction was determined by the MHC genotype of the accessory cell in primary culture. Monoclonal antibody blocking studies demonstrated that Ia molecules were critical to this process. Suppression did not account for the observed restrictions. The effector specificity of the T-helper cells could be mapped to the I-A subregion. On adoptive transfer to lethally irradiated hosts, the helper cells could localize to peripheral lymphoid organs. These cells retain their in vitro-imposed restriction specificity even when transferred back to and primed in an F1 environment. Using this experimental protocol, carrier antigen-specific, MHC-restricted priming of these thymic T-helper cells could be detected. These results demonstrate that one can select and amplify functional T-helper cells from cultures of thymocytes whose precise MHC-restriction specificity is determined in vitro.