Activation and inhibition of lymphocytes by costimulation (original) (raw)

The requirement for costimulation allows lymphocyte activation to be strictly regulated by modulating the expression of either member (or both) of the costimulatory receptor/ligand pair. An additional level of regulation is achieved by the expression of inhibitory receptors that can initiate negative signals. The balance between such negative signals and positive signals from antigen receptor and costimulatory molecules apparently sets the threshold for lymphocyte activation. The interaction between costimulatory and inhibitory signals appears to be a critical aspect of the regulation of lymphocyte function.

Perhaps the best-studied inhibitory receptor is CTLA-4. CTLA-4 was the second member of the CD28 family to be identified. Like CD28, it binds to B7.1 and B7.2, although with significantly higher affinity. Resting T cells express little surface CTLA-4, but surface levels are increased upon activation, due to both redistribution of an intracellular pool and increased synthesis. CTLA-4 inhibits T cell proliferation and IL-2 synthesis in response to stimulation with anti-CD3 and anti-CD28 antibodies. Furthermore, preventing CTLA-4/B7 interactions enhances T cell responses in vitro and in vivo (37). These findings suggest a model in which the dynamic regulation of CTLA-4 and B7 levels is responsible for setting the activation threshold of T cells. Importantly, CD28 costimulation upregulates CTLA-4 expression, such that costimulation is a self-limiting process. CTLA-4 can also inhibit ICOS signaling (38).

CTLA-4 can interfere with T cell activation by both passive and active mechanisms. When CTLA-4 levels are high, and B7 levels limiting, the higher affinity of CTLA-4 for B7 allows it to outcompete CD28 for B7 binding, preventing CD28/B7 interactions (39). CTLA-4 can also engage inhibitory signal transduction machinery, including the phosphatases SHP-2 (40, 41) and PP2A (42), which may oppose the action of kinases downstream of CD3 and CD28. CTLA-4 ligation has been shown to inhibit activation of extracellular signal–regulated kinase (ERK) and Jun N-terminal kinase (43), both of which are at the ends of kinase cascades. CTLA-4–associated SHP-2 has been proposed to reduce activation-induced phosphorylation of LAT and TCRζ (41), although effects on TCR/CD3–proximal events are controversial (43). In addition, most evidence places SHP-2 in positive, rather than negative, signaling pathways (44). PP2A is able to dephosphorylate and deactivate Akt (45), suggesting that it may play a similar role in CTLA-4 inhibition of costimulatory signals. There is also growing evidence that CTLA-4 interacts with other inhibitory pathways, since mutation of the tyrosine residue critical for the SH2-mediated binding of SHP-2 and PP2A does not abolish CTLA-4 function (46–49). Intriguingly, CTLA-4 also interacts with PI3K, further complicating models of PI3K’s contribution to lymphocyte activation and inhibition (7).

PD-1 is another inhibitory member of the CD28 family. As with the other CD28 family members, its ligands, PD-L1 (50) and PD-L2 (51), are related to the B7 proteins. PD-1 is expressed on activated B and T cells (52), and engagement of PD-1 has been shown to inhibit T cell proliferation and cytokine production in response to anti-CD3 and anti-CD28 antibody stimulation (50, 51). Further, mice deficient in PD-1 expression develop autoimmune disorders characterized by production of high titers of autoantibodies (53), indicating an important role in the regulation of autoreactive B cells. Unlike CTLA-4, the cytoplasmic tail of PD-1 contains a sequence known as the immunoreceptor tyrosine-based inhibition motif (ITIM; see Billadeau and Leibson, this Perspective series, ref. 54). The ITIM sequence is found in several classes of inhibitory receptors, including the killer inhibitory receptors found on NK cells and FcγRIIB on B cells, and functions by recruiting SH2-containing phosphatases (55). There appear to be receptor and cell type specificities in the identity of the phosphatase (SHP-1, SHP-2, or SHIP) recruited by the ITIM motif, and it is unknown which phosphatase is involved in PD-1 function, although, as with CTLA-4, there is some evidence for preferential activation of SHP-2 (50, 51).