1. Coinhibitory Pathways in the B7-CD28 Ligand-Receptor Family
Frank A. Schildberg, Sarah R. Klein, Gordon J. Freeman, Arlene H. Sharpe 
Immunity 
Volume 44, Issue 5, Pages (May 2016)
DOI: /j.immuni
Copyright © 2016 Elsevier Inc. Terms and Conditions

2. 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, memory, 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 asterisk). Binding partners for B7-H3, B7-H4, VISTA, and BTNL2 have not yet been identified.
Immunity  , DOI: ( /j.immuni )
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3. Figure 2
Regulation of CTLA-4 Expression and Functional Effects of CTLA-4
(A) Dynamics of CTLA-4 expression and membrane cycling. After 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 adaptor 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 adaptor 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 is provided by the following. (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 can inhibit T cell activation through a similar signaling pathway as full-length CTLA-4. Extrinsic control is provided by the following. (1) Reverse signaling through ligands into APCs. CTLA-4 can 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 IL-10, TGF-β, 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.
Immunity  , DOI: ( /j.immuni )
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4. 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 therefore 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 therefore 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.
Immunity  , DOI: ( /j.immuni )
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