CD8+ cytotoxic T lymphocytes (CTLs) provide defense against intracellular pathogens and tumors by recognizing and subsequently destroying infected or transformed cells. Recognition is achieved by T cell receptors (TCRs) on the CTL binding to non-self-peptide fragments presented in the cleft of class I MHC molecules (peptide–MHC complexes) on the surface of the target cell. TCR ligation triggers target cell destruction by the CTL through several mechanisms, including the delivery to target cells of a pore-forming protein that penetrate the target cell membrane (perforin) and specialized proteases (granzymes). It has been known for some time that the density of peptide–MHC (pMHC) complexes on the surface of target cells required to stimulate cytotoxic functions in CTLs is dramatically lower than that required to stimulate cytokine production and clonal expansion: very few agonist pMHC complexes, perhaps even a single one per target cell, can elicit measurable cytotoxic function at the population level (1, 2). In contrast, production of cytokines such as IFN-γ (which trigger effector functions in immune cells) and clonal expansion of CTLs require the presence of 100- to 1,000-fold more complexes (3). Separate thresholds for these biological responses fit within the context of CTL function: effector CD8+ T cells need to kill infected cells in response to the lowest possible level of foreign peptides to control infections at their early stages, but the immune system must also carefully control this potent cell population to avoid destruction of healthy tissue. Thus, cytokine production and proliferation likely occur only under conditions of high pMHC complex density, typically with delivery of costimulatory signals that in vivo will be provided by professional antigen-presenting cells (APCs). Although the phenomenon of dual antigen-density thresholds for CTL responses is well established at the population level, the mechanism behind these dual programs of function at the …