IL‐12‐mediated STAT4 signaling and TCR signal strength cooperate in the induction of CD40L in human and mouse CD8+ T cells

CD40L is one of the key molecules bridging the activation of specific T cells and the maturation of professional and nonprofessional antigen‐presenting cells including B cells. CD4+ T cells have been regarded as the major T‐cell subset that expresses CD40L upon cognate activation; however, we demonstrate here that a putative CD8+ helper T‐cell subset expressing CD40L is induced in human and murine CD8+ T cells in vitro and in mice immunized with antigen‐pulsed dendritic cells. IL‐12 and STAT4‐mediated signaling was the major instructive cytokine signal boosting the ability of CD8+ T cells to express CD40L both in vitro and in vivo. Additionally, TCR signaling strength modulated CD40L expression in CD8+ T cells after primary differentiation in vitro as well as in vivo. The induction of CD40L in CD8+ T cells regulated by IL‐12 and TCR signaling may enable CD8+ T cells to respond autonomously of CD4+ T cells. Thus, we propose that under proinflammatory conditions, a self‐sustaining positive feedback loop could facilitate the efficient priming of T cells stimulated by high affinity peptide displaying APCs.