B7-1 and B7-2 are generally thought to have comparable structures and affinities for their receptors, CD28 and CTLA-4, each of which is assumed to be bivalent. We show instead (1) that B7-2 binds the two receptors more weakly than B7-1, (2) that, relative to its CTLA-4 binding affinity, B7-2 binds CD28 2- to 3-fold more effectively than B7-1, (3) that, unlike B7-1, B7-2 does not self-associate, and (4) that, in contrast to CTLA-4 homodimers, which are bivalent, CD28 homodimers are monovalent. Our results indicate that B7-1 markedly favors CTLA-4 over CD28 engagement, whereas B7-2 exhibits much less bias. We propose that the distinct structures and binding properties of B7-1 and B7-2 account for their overlapping but distinct effects on T cell responses.

In mammals, the classical B7 molecules expressed on antigen-presenting cells, B7-1 (CD80) and B7-2 (CD86), bind the structurally related glycoproteins CD28 and CTLA-4 (CD152), generating costimulatory signals that regulate the activation state of T cells. A recently identified human CD28-like protein, ICOS, also induces costimulatory signals in T cells when crosslinked with antibodies, but it is unclear whether ICOS is part of a B7-mediated regulatory pathway of previously unsuspected complexity, or whether it functions independently and in parallel. Here, we report that, rather than binding B7-1 or B7-2, ICOS binds a new B7-related molecule of previously unknown function that we call LICOS (for ligand of ICOS). At 37 degrees C, LICOS binds only to ICOS but, at lower, non-physiological temperatures, it also binds weakly to CD28 and CTLA-4. Sequence comparisons suggest that LICOS is the homologue of a molecule expressed by avian macrophages and of a murine protein whose expression is induced in non-lymphoid organs by tumour necrosis factor alpha (TNFalpha). Our results define the components of a distinct and novel costimulatory pathway and raise the possibility that LICOS, rather than B7-1 or B7-2, is the contemporary homologue of a primordial vertebrate costimulatory ligand.