Interaction within clusters of dendritic cells and helper T cells during initial Th1/Th2 commitment

Cytokines are the main agents known to regulate Th1/Th2 commitment, where t hey may operate through paracrine activity within clusters of T cells gathe red around dendritic cells (DC). An in vitro system is used here to test th is possibility, using clusters around DC composed of naive TCR-transgenic o valbumin peptide 323-339-specific CD4(+) T cells as targets plus TCR-transg enic pigeon cytochrome C peptide 88-104-specific CD4(+) polarized Th1 or Th 2 cells as inducers. The polarized inducer cells exerted their maximum effe ct when the two T cell populations were activated within the same cluster, implemented by allowing a single DC to present both their epitopes. This fi nding thus supports the paracrine hypothesis. The system was then employed to explore the role of individual cytokines by means of inhibition by monoc lonal antibodies. Development of Th2 commitment proved strictly dependent o n the IL-4 produced by the Th2 inducers. For Th1 commitment, IFN-gamma and IL-12 were both needed, but with IFN-gamma required only during the initial period of culture. The rapid timing observed under these conditions places constraints on the molecular basis of commitment, and appears accurately t o reflect the physiological response in vivo.