The activation of dendritic cells is marked by changes both on their cell surfaces and in their functions. We define EWI-2/CD316 as an early activation marker of dendritic cells upregulated by Toll-like receptor ligands clearly before CD86 and CD83. By expression cloning, human heat shock protein A8 (HSPA8), a member of the hsp70 family, was identified as the ligand for EWI-2. Soluble EWI-2 bound both to cells expressing HSPA8 and also to immobilized HSPA8 protein. Although heat shock proteins are evolutionarily well conserved, other members of this class, including human hsp60 and mycobacterial hsp65, did not bind to EWI-2. The ligation of EWI-2 enhanced the CCL21/SLC-dependent migration of activated mature dendritic cells but attenuated their antigen-specific stimulatory capacities. Important functions of recently activated dendritic cells are thus critically modulated by the newly discovered HSPA8-EWI-2 interaction.Dendritic cells are the most crucial sentinels of the immune system. Although different DC sublineages have heterogeneous phenotypes and functions, they share similarities in their maturation processes (2, 3). First, precursors are generated continuously in the bone marrow. They circulate as immature DCs and then enter their destination compartment, where they mature. These resident DCs then are activated by danger signals derived primarily from pathogens and possibly also from endogenous metabolic processes. The activation step is very rapid and is characterized by cellular and morphological changes: the upregulation of surface markers, the expression of cytokines and chemokines, and the formation of dendrites. Several such activation/maturation markers have been described, including, for example, members of the B7 superfamily, like CD80 and CD86, which then later provide costimulatory signals during the priming of effector cells.However, the appearance of these already known molecules follows the activation event with a certain delay. We wanted to understand the very early changes on the dendritic cell surface after stimulation, because we reasoned that surface molecules appearing immediately early after stimulation might be involved in controlling the maturation and fate of the DC itself. For example, immediate early activation molecules could amplify the external activation signal, could modify the migratory behavior, or could serve as a stop signal by initiating a negative feedback loop. We searched for such first-line activation markers by comparing naïve and early activated DCs in a differential display analysis. Here we identify EWI-2 as an early induced transcript whose presence on dendritic cells has not been described before. EWI-2 (11) is also known as CD316, PGRL (PG regulatory-like protein), KAI/CD82-associated surface molecule (34), and CD81-binding partner 3 (CD81P3) (10). EWI-2 contains four immunoglobulin domains, a transmembrane region, and a short cytoplasmic tail that does not bear any signature motif for signal transduction. However, work of the Hemler and Rubinstein groups in...