The CD28/CTLA-4 and B7-1/B7-2 pathway is by far the best-understood costimulatory pathway. Although it has been clear for a while that CD28 helps to initiate T cell responses and CTLA-4 is crucial in the downregulation of responses, our recent studies have focused more on the cell biological lifestyle of these molecules as well as their signaling properties. Much of our understanding of the function of CTLA-4 has been reviewed in detail recently [2
]. In brief, the temporal and spatial separation of these two receptors is important in their function. Whereas CTLA-4 has a much higher affinity than CD28 for their ligands, it is not expressed constitutively on naive T cells and is mostly localized intracellularly. After stimulation by the T-cell antigen receptor, CD28 migrates very rapidly into the immunological synapse from the plasma membrane, whereas the intracellular vesicles containing CTLA-4 need to be repositioned to the area of the cytoplasm that is close to the synapse. Once these vesicles have been polarized beneath the T cell–antigen-presenting cell (APC) interface, CTLA-4 can be translocated into the synapse to engage its ligands. We have recently found the preferential recruitment of CTLA-4 into the synapse by B7.1, whereas B7.2 preferentially recruits CD28 [3
]. This suggests a previously unrecognized mechanism for tuning the response depending on the relative levels of B7.1/B7.2 expressed on APCs.
Interestingly, the translocation of CTLA-4 into the synapse is proportional to TCR signal strength [4
]. Hence, CTLA-4 might differentially restrict the expansion of T cells on the basis of the strength of the TCR signal they receive. Instead of being a simple inhibitor that attenuates T cell responses, CTLA-4 could shape the composition and functional activity (for example T helper 1 [Th1] versus Th2) of the overall pool of T cells with different specificities and affinities, which are activated during the course of an immune response [2
]. Indeed, it has recently been reported that even in the absence of Stat6 (a key signal transducer for interleukin-4 [IL-4]), CTLA-4-deficient T cells can efficiently differentiate into Th2 cells [7
]. It was suggested that the increased signal strength of high-affinity T cells that are no longer restricted by CTLA-4 could result in an increased bias towards a Th2 phenotype [7
]. However, the issue of whether increased TCR signal leads to Th2 differentiation remains very controversial.
Although the inhibitory effects of CTLA-4 are clear, a variety of endogenous versus exogenous mechanisms have been proposed. Whereas we have focused on understanding the cell-endogenous mechanisms of inhibition [2
], others have suggested that CTLA-4 has a role in immunosuppression by CD4+
regulatory T cells (Treg
cells; discussed below). It has also been suggested that CTLA-4 has a role in the induction of anergic T cells [8
] that could in turn be suppressive [9
]. These mechanisms are not necessarily mutually exclusive and might act in concert.
More recently, a splice variant of mouse CTLA-4 was discovered that has a fully intact open reading frame encoding a transmembrane isoform that lacks the B7-1/B7-2-binding domain (liCTLA-4) as a result of skipping exon 2 [10
]. There is an association between the autoimmune susceptible strain of NOD mice with a fourfold decrease in the expression of liCTLA-4, which is in turn associated with a silent mutation in exon 2. A ligand-independent isoform for CD28 has also been reported [11
]. Future studies will have to reconcile the potential functions of these ligand-independent forms, with our recent findings that ligand binding is required for localizing CTLA-4 to the immunological synapse [3
]. Perhaps liCTLA-4 provides a 'tonic' inhibitory signal that decreases the T cell activation threshold during the transient non-specific interactions between T cells and dendritic cells (DCs) that occur continuously in the lymph nodes.