Coinhibitory Pathways in the B7-CD28 Ligand-Receptor Family

Abstract

Immune responses need to be controlled for optimal protective immunity and tolerance. Coinhibitory pathways in the B7-CD28 family provide critical inhibitory signals that regulate immune homeostasis and defense and protect tissue integrity. These coinhibitory signals limit the strength and duration of immune responses, thereby curbing immune-mediated tissue damage, regulating resolution of inflammation, and maintaining tolerance to prevent autoimmunity. Tumors and microbes that cause chronic infections can exploit these coinhibitory pathways to establish an immunosuppressive microenvironment, hindering their eradication. Advances in understanding T cell coinhibitory pathways have stimulated a new era of immunotherapy with effective drugs to treat cancer, autoimmune and infectious diseases, and transplant rejection. In this review we discuss the current knowledge of the mechanisms underlying the coinhibitory functions of pathways in the B7-CD28 family, the diverse functional consequences of these inhibitory signals on immune responses, and the overlapping and unique functions of these key immunoregulatory pathways.

Figure 1. Coinhibitory pathways in the B7-CD28 family

T cell activation is initiated by recognition of peptide antigens presented by APCs to the TCR CD3 complex and T cell costimulatory signals provided by CD28 interactions with CD80 and CD86. Upon T cell activation, many coinhibitory pathways are upregulated and can attenuate TCR and costimulatory signals. Coinhibitory pathways in the B7-CD28 family control responses of naive, effector, regulatory and exhausted T cells. These receptors are expressed on T cells and some are also expressed on other hematopoietic cells, as described in the text. Their ligands can be expressed on APCs, non-hematopoietic cells and in tumors; some molecules are expressed on both APCs and T cells (indicated by *). Binding partners for B7-H3, B7-H4, VISTA, and BTNL2 have not yet been identified.

Figure 2. Regulation of CTLA-4 expression and functional effects of CTLA-4

(a) Dynamics of CTLA-4 expression and membrane cycling. Following synthesis in the Trans Golgi Network (TGN), CTLA-4 binds to T cell receptor-interacting molecule (TRIM), promoting formation of CTLA-4-containing vesicles. TCR signaling-mediated calcium influx induces CTLA-4 release from the vesicles to the cell surface, and CTLA-4 and TRIM no longer associate. CTLA-4 externalization also depends on phospholipase D (PLD) and GTPase adenosine diphosphate ribosylation factor 1 (ARF-1). Unphosphorylated CTLA-4 cytoplasmic domain binds to the clathrin adapter protein 2 (AP-2) which promotes rapid internalization to endosomes and lysosomes. Tyrosine phosphorylation of the CTLA-4 cytoplasmic domain retards internationalization. Upon T cell activation, CTLA-4-containing endosomes are recycled to the cell surface; this is regulated by lipopolysaccharide-responsive and beige-like anchor protein (LRBA). Association of CTLA-4 with adapter protein 1 (AP-1) mediates shuttling from the TGN to lysosomal compartments for degradation, a mechanism that controls the overall abundance of CTLA-4 in the TGN. (b) CTLA-4 can exert T cell-intrinsic and T cell-extrinsic functions. Intrinsic control. (1) Inhibitory signaling. Signals through CTLA-4 can interfere with proximal signaling by the T cell receptor (TCR) and CD28. (2) Competition for ligands. CTLA-4 is the higher affinity receptor than CD28 for CD80/CD86 and can outcompete CD28 for CD80/CD86 binding. (3) Promote adhesion or reduced stop signal. CTLA-4 can increase T cell/APC adhesion through a pathway mediated by LFA1 and decrease duration of APC/T cell interactions by inhibiting the TCR-mediated stop signal, resulting in reduced T cell activation. (4) Ligand-independent inhibition. A CTLA-4 splice variant that cannot bind to ligands may inhibit T cell activation through a similar signaling pathway as full length CTLA-4. Extrinsic control(1) Reverse signaling through ligands into APCs. CTLA-4 may reverse signal through CD80 and CD86 into APCs, leading to IDO production and suppression of T cell effector responses. (2) Reduce ligand expression/availability. Secreted factors such as IL10, TGFp or soluble splice variants of CTLA-4 reduce ligand expression or availability. (3) CTLA-4 removes ligands from APCs. CTLA-4 binding to CD80 or CD86 can result in transendocytosis of the ligands from the APC, resulting in lower levels of ligands on the surface of APCs.

Figure 3. Comparison of intracellular signaling by CTLA-4 and PD-1

PD-1 and CTLA-4 both inhibit Akt activation, but they target different signaling molecules. CTLA-4 engagement by its ligands CD80 and CD86, activates the serine/threonine phosphatase PP2A, which directly inhibits the TCR/CD28-mediated activation of Akt, but preserves PI3K activity, and therefor expression of Bcl-xL. PD-1 ligation by PD-L1 or PD-L2 leads to phosphorylation of ITSM/ITIM motifs in the PD-1 cytoplasmic domain, which results in recruitment of the tyrosine phosphatases SHP-1 and SHP-2, and inhibition of PI3K activity and therefor reduced expression of Bcl-xL. PD-1 ligation also inhibits PLCγ1 and downstream Ras-MEK-ERK signaling and leads to upregulation of the pro-apoptotic molecule BIM. In contrast to PD-1, CTLA-4 does not inhibit Ras-MEK-ERK and PLCγ1 signaling.