he study of T-cell activation re-mCCns an exciting endeavor, even though some of the molecular details of T-cell activation have become apparent. A number of molecules (receptor/counter-receptor pairs) thought to be crucial for the function of T celis have been identified. These include costimulatory molecules such as CD24, CD28, CD54 and CD58 which are widely distributed on many tissues, and CD80 (87-l) and CD86 (B7-2) molecules which are restricted to antigen-presenting cells (AI’Cs). However, levels of CD80 and CD86 on resting APCs are generally low, becoming upregulated following AK activation.

Many of the cosfimuiafovy molecules thought to be essential for T-cell activation have now been identified. The CD4U-CD40 ligand interacfion is one such receptor/counter-recepfor pair that hns been shown to be importanf in B-and T-cell cognate interactions. Here Iqbal Grewal and Richard Flavell examine recent data from studies using gene knockout techniques that have helped provide a better understanding of CD4OL’s role in vivo, in regulation of the immune response.