Help for the induction of cytolytic T lymphocytes is mediated by dendritic cells (DC) that are conditioned by CD40 signaling. We identified tumor necrosis factor family member CD27L/CD70, which is expressed by cytolytic T lymphocytes on interaction with DC to control CD154 (CD40L) up-regulation on CD45RA(+) helper T cells for subsequent DC stimulation. The results show that the initiation of a cytolytic Immune response is determined by regulatory circuits, requiring simultaneous activation and differentiation of ail cells involved in T lymphocyte-DC cluster formation.