Optimal activation of T-lymphocytes requires at least two signals. A primary one is delivered by the T-cell receptor (TCR) complex after antigen recognition and additional costimulatory signals are delivered by the engagement of costimulatory receptors such as CD28. The best-characterized costimulatory pathways are mediated by a set of cosignaling molecules belonging to the CD28 superfamily, including CD28, CTLA4, ICOS, PD1 and BTLA receptors. These proteins deliver both positive and negative second signals to T-cells by interacting with B7 family ligands expressed on antigen presenting cells. Different subsets of T-cells have very different requirements for costimulation. CD28 family mediated costimulation is not required for all T-cell responses in vivo, and alternative costimulatory pathways also exist. Different receptors of the CD28 family and their ligands have different regulation of expression. CD28 is constitutively expressed on naive T cells whereas CTLA4 expression is dependent on CD28/B7 engagement and the other receptor members ICOS, PD1 and BTLA are induced after initial T-cell stimulation. The positive signals induced by CD28 and ICOS molecules are counterbalanced by other members of the CD28 family, including cytotoxic T-lymphocyte associated antigen (CTLA)4, programmed cell death (PD)1, and B and T lymphocyte attenuator (BTLA), which dampen immune responses. The balance of stimulatory and inhibitory signals is crucial to maximize protective immune responses while maintaining immunological tolerance and preventing autoimmunity. The costimulatory receptors CD28, CTLA4, ICOS and PD1 are composed of single extracellular IgV-like domains, whereas BTLA has one IgC-like domain. Receptors CTLA4, CD28 and ICOS are covalent homodimers, due to an interchain disulphide linkage. The costimulatory ligands B71, B72, B7H2, B7H1 and B7DC, have a membrane proximal IgC-like domain and a membrane distal IgV-like domain that is responsible for receptor binding and dimerization. CD28 and CTLA4 have no known intrinsic enzymatic activity. Instead, engagement by their physiologic ligands B71 and B72 leads to the physical recruitment and activation of downstream T-cell effector molecules.
Streuli M, Hall LR, Saga Y, Schlossman SF, Saito H.; ''Differential usage of three exons generates at least five different mRNAs encoding human leukocyte common antigens.''; PubMedEurope PMCScholia
Szczepanowska J.; ''Involvement of Rac/Cdc42/PAK pathway in cytoskeletal rearrangements.''; PubMedEurope PMCScholia
Rudd CE, Schneider H.; ''Unifying concepts in CD28, ICOS and CTLA4 co-receptor signalling.''; PubMedEurope PMCScholia
Ellis JH, Burden MN, Vinogradov DV, Linge C, Crowe JS.; ''Interactions of CD80 and CD86 with CD28 and CTLA4.''; PubMedEurope PMCScholia
Schneider H, Rudd CE.; ''Tyrosine phosphatase SHP-2 binding to CTLA-4: absence of direct YVKM/YFIP motif recognition.''; PubMedEurope PMCScholia
Migden MR, Rischin D, Schmults CD, Guminski A, Hauschild A, Lewis KD, Chung CH, Hernandez-Aya L, Lim AM, Chang ALS, Rabinowits G, Thai AA, Dunn LA, Hughes BGM, Khushalani NI, Modi B, Schadendorf D, Gao B, Seebach F, Li S, Li J, Mathias M, Booth J, Mohan K, Stankevich E, Babiker HM, Brana I, Gil-Martin M, Homsi J, Johnson ML, Moreno V, Niu J, Owonikoko TK, Papadopoulos KP, Yancopoulos GD, Lowy I, Fury MG.; ''PD-1 Blockade with Cemiplimab in Advanced Cutaneous Squamous-Cell Carcinoma.''; PubMedEurope PMCScholia
Woo SY, Kim DH, Jun CB, Kim YM, Haar EV, Lee SI, Hegg JW, Bandhakavi S, Griffin TJ, Kim DH.; ''PRR5, a novel component of mTOR complex 2, regulates platelet-derived growth factor receptor beta expression and signaling.''; PubMedEurope PMCScholia
Hall RD, Gray JE, Chiappori AA.; ''Beyond the standard of care: a review of novel immunotherapy trials for the treatment of lung cancer.''; PubMedEurope PMCScholia
Alegre ML, Frauwirth KA, Thompson CB.; ''T-cell regulation by CD28 and CTLA-4.''; PubMedEurope PMCScholia
Sheppard KA, Fitz LJ, Lee JM, Benander C, George JA, Wooters J, Qiu Y, Jussif JM, Carter LL, Wood CR, Chaudhary D.; ''PD-1 inhibits T-cell receptor induced phosphorylation of the ZAP70/CD3zeta signalosome and downstream signaling to PKCtheta.''; PubMedEurope PMCScholia
Sarbassov DD, Guertin DA, Ali SM, Sabatini DM.; ''Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex.''; PubMedEurope PMCScholia
Crespo P, Schuebel KE, Ostrom AA, Gutkind JS, Bustelo XR.; ''Phosphotyrosine-dependent activation of Rac-1 GDP/GTP exchange by the vav proto-oncogene product.''; PubMedEurope PMCScholia
Manser E, Leung T, Salihuddin H, Zhao ZS, Lim L.; ''A brain serine/threonine protein kinase activated by Cdc42 and Rac1.''; PubMedEurope PMCScholia
Scheid MP, Marignani PA, Woodgett JR.; ''Multiple phosphoinositide 3-kinase-dependent steps in activation of protein kinase B.''; PubMedEurope PMCScholia
Schneider H, Cai YC, Prasad KV, Shoelson SE, Rudd CE.; ''T cell antigen CD28 binds to the GRB-2/SOS complex, regulators of p21ras.''; PubMedEurope PMCScholia
Teft WA, Chau TA, Madrenas J.; ''Structure-Function analysis of the CTLA-4 interaction with PP2A.''; PubMedEurope PMCScholia
Meier R, Alessi DR, Cron P, Andjelković M, Hemmings BA.; ''Mitogenic activation, phosphorylation, and nuclear translocation of protein kinase Bbeta.''; PubMedEurope PMCScholia
Parrini MC, Lei M, Harrison SC, Mayer BJ.; ''Pak1 kinase homodimers are autoinhibited in trans and dissociated upon activation by Cdc42 and Rac1.''; PubMedEurope PMCScholia
Murphy KM, Nelson CA, Sedý JR.; ''Balancing co-stimulation and inhibition with BTLA and HVEM.''; PubMedEurope PMCScholia
Sedy JR, Gavrieli M, Potter KG, Hurchla MA, Lindsley RC, Hildner K, Scheu S, Pfeffer K, Ware CF, Murphy TL, Murphy KM.; ''B and T lymphocyte attenuator regulates T cell activation through interaction with herpesvirus entry mediator.''; PubMedEurope PMCScholia
Watanabe R, Harada Y, Takeda K, Takahashi J, Ohnuki K, Ogawa S, Ohgai D, Kaibara N, Koiwai O, Tanabe K, Toma H, Sugamura K, Abe R.; ''Grb2 and Gads exhibit different interactions with CD28 and play distinct roles in CD28-mediated costimulation.''; PubMedEurope PMCScholia
Sadra A, Cinek T, Arellano JL, Shi J, Truitt KE, Imboden JB.; ''Identification of tyrosine phosphorylation sites in the CD28 cytoplasmic domain and their role in the costimulation of Jurkat T cells.''; PubMedEurope PMCScholia
Currie RA, Walker KS, Gray A, Deak M, Casamayor A, Downes CP, Cohen P, Alessi DR, Lucocq J.; ''Role of phosphatidylinositol 3,4,5-trisphosphate in regulating the activity and localization of 3-phosphoinositide-dependent protein kinase-1.''; PubMedEurope PMCScholia
Han J, Das B, Wei W, Van Aelst L, Mosteller RD, Khosravi-Far R, Westwick JK, Der CJ, Broek D.; ''Lck regulates Vav activation of members of the Rho family of GTPases.''; PubMedEurope PMCScholia
Tan S, Zhang H, Chai Y, Song H, Tong Z, Wang Q, Qi J, Wong G, Zhu X, Liu WJ, Gao S, Wang Z, Shi Y, Yang F, Gao GF, Yan J.; ''An unexpected N-terminal loop in PD-1 dominates binding by nivolumab.''; PubMedEurope PMCScholia
Slavik JM, Hutchcroft JE, Bierer BE.; ''CD28/CTLA-4 and CD80/CD86 families: signaling and function.''; PubMedEurope PMCScholia
Teng JM, King PD, Sadra A, Liu X, Han A, Selvakumar A, August A, Dupont B.; ''Phosphorylation of each of the distal three tyrosines of the CD28 cytoplasmic tail is required for CD28-induced T cell IL-2 secretion.''; PubMedEurope PMCScholia
Richter G, Burdach S.; ''ICOS: a new costimulatory ligand/receptor pair and its role in T-cell activion.''; PubMedEurope PMCScholia
Delcommenne M, Tan C, Gray V, Rue L, Woodgett J, Dedhar S.; ''Phosphoinositide-3-OH kinase-dependent regulation of glycogen synthase kinase 3 and protein kinase B/AKT by the integrin-linked kinase.''; PubMedEurope PMCScholia
Acuto O, Michel F.; ''CD28-mediated co-stimulation: a quantitative support for TCR signalling.''; PubMedEurope PMCScholia
Daniels RH, Bokoch GM.; ''p21-activated protein kinase: a crucial component of morphological signaling?''; PubMedEurope PMCScholia
Hehner SP, Hofmann TG, Dienz O, Droge W, Schmitz ML.; ''Tyrosine-phosphorylated Vav1 as a point of integration for T-cell receptor- and CD28-mediated activation of JNK, p38, and interleukin-2 transcription.''; PubMedEurope PMCScholia
Masteller EL, Chuang E, Mullen AC, Reiner SL, Thompson CB.; ''Structural analysis of CTLA-4 function in vivo.''; PubMedEurope PMCScholia
Nurieva RI.; ''Regulation of immune and autoimmune responses by ICOS-B7h interaction.''; PubMedEurope PMCScholia
Zhang B, Chernoff J, Zheng Y.; ''Interaction of Rac1 with GTPase-activating proteins and putative effectors. A comparison with Cdc42 and RhoA.''; PubMedEurope PMCScholia
Lei M, Lu W, Meng W, Parrini MC, Eck MJ, Mayer BJ, Harrison SC.; ''Structure of PAK1 in an autoinhibited conformation reveals a multistage activation switch.''; PubMedEurope PMCScholia
Burgering BM, Coffer PJ.; ''Protein kinase B (c-Akt) in phosphatidylinositol-3-OH kinase signal transduction.''; PubMedEurope PMCScholia
Fife BT, Bluestone JA.; ''Control of peripheral T-cell tolerance and autoimmunity via the CTLA-4 and PD-1 pathways.''; PubMedEurope PMCScholia
Yaghoubi N, Soltani A, Ghazvini K, Hassanian SM, Hashemy SI.; ''PD-1/ PD-L1 blockade as a novel treatment for colorectal cancer.''; PubMedEurope PMCScholia
Miyatake S, Nakaseko C, Umemori H, Yamamoto T, Saito T.; ''Src family tyrosine kinases associate with and phosphorylate CTLA-4 (CD152).''; PubMedEurope PMCScholia
Youngnak P, Kozono Y, Kozono H, Iwai H, Otsuki N, Jin H, Omura K, Yagita H, Pardoll DM, Chen L, Azuma M.; ''Differential binding properties of B7-H1 and B7-DC to programmed death-1.''; PubMedEurope PMCScholia
Watanabe N, Gavrieli M, Sedy JR, Yang J, Fallarino F, Loftin SK, Hurchla MA, Zimmerman N, Sim J, Zang X, Murphy TL, Russell JH, Allison JP, Murphy KM.; ''BTLA is a lymphocyte inhibitory receptor with similarities to CTLA-4 and PD-1.''; PubMedEurope PMCScholia
Kane LP, Weiss A.; ''The PI-3 kinase/Akt pathway and T cell activation: pleiotropic pathways downstream of PIP3.''; PubMedEurope PMCScholia
Han J, Luby-Phelps K, Das B, Shu X, Xia Y, Mosteller RD, Krishna UM, Falck JR, White MA, Broek D.; ''Role of substrates and products of PI 3-kinase in regulating activation of Rac-related guanosine triphosphatases by Vav.''; PubMedEurope PMCScholia
Schneider H, Rudd CE.; ''CD28 and Grb-2, relative to Gads or Grap, preferentially co-operate with Vav1 in the activation of NFAT/AP-1 transcription.''; PubMedEurope PMCScholia
Kline J, Gajewski TF.; ''Clinical development of mAbs to block the PD1 pathway as an immunotherapy for cancer.''; PubMedEurope PMCScholia
Baroja ML, Vijayakrishnan L, Bettelli E, Darlington PJ, Chau TA, Ling V, Collins M, Carreno BM, Madrenas J, Kuchroo VK.; ''Inhibition of CTLA-4 function by the regulatory subunit of serine/threonine phosphatase 2A.''; PubMedEurope PMCScholia
Burova E, Hermann A, Waite J, Potocky T, Lai V, Hong S, Liu M, Allbritton O, Woodruff A, Wu Q, D'Orvilliers A, Garnova E, Rafique A, Poueymirou W, Martin J, Huang T, Skokos D, Kantrowitz J, Popke J, Mohrs M, MacDonald D, Ioffe E, Olson W, Lowy I, Murphy A, Thurston G.; ''Characterization of the Anti-PD-1 Antibody REGN2810 and Its Antitumor Activity in Human PD-1 Knock-In Mice.''; PubMedEurope PMCScholia
Wittwer T, Schmitz ML.; ''NIK and Cot cooperate to trigger NF-kappaB p65 phosphorylation.''; PubMedEurope PMCScholia
Michel F, Grimaud L, Tuosto L, Acuto O.; ''Fyn and ZAP-70 are required for Vav phosphorylation in T cells stimulated by antigen-presenting cells.''; PubMedEurope PMCScholia
Truneh A, Reddy M, Ryan P, Lyn SD, Eichman C, Couez D, Hurle MR, Sekaly RP, Olive D, Sweet R.; ''Differential recognition by CD28 of its cognate counter receptors CD80 (B7.1) and B70 (B7.2): analysis by site directed mutagenesis.''; PubMedEurope PMCScholia
Pagès F, Ragueneau M, Klasen S, Battifora M, Couez D, Sweet R, Truneh A, Ward SG, Olive D.; ''Two distinct intracytoplasmic regions of the T-cell adhesion molecule CD28 participate in phosphatidylinositol 3-kinase association.''; PubMedEurope PMCScholia
Carter LL, Carreno BM.; ''Cytotoxic T-lymphocyte antigen-4 and programmed death-1 function as negative regulators of lymphocyte activation.''; PubMedEurope PMCScholia
Harada Y, Ohgai D, Watanabe R, Okano K, Koiwai O, Tanabe K, Toma H, Altman A, Abe R.; ''A single amino acid alteration in cytoplasmic domain determines IL-2 promoter activation by ligation of CD28 but not inducible costimulator (ICOS).''; PubMedEurope PMCScholia
Tybulewicz VL, Ardouin L, Prisco A, Reynolds LF.; ''Vav1: a key signal transducer downstream of the TCR.''; PubMedEurope PMCScholia
Manser E, Chong C, Zhao ZS, Leung T, Michael G, Hall C, Lim L.; ''Molecular cloning of a new member of the p21-Cdc42/Rac-activated kinase (PAK) family.''; PubMedEurope PMCScholia
Keir ME, Butte MJ, Freeman GJ, Sharpe AH.; ''PD-1 and its ligands in tolerance and immunity.''; PubMedEurope PMCScholia
Pearce LR, Huang X, Boudeau J, Pawłowski R, Wullschleger S, Deak M, Ibrahim AF, Gourlay R, Magnuson MA, Alessi DR.; ''Identification of Protor as a novel Rictor-binding component of mTOR complex-2.''; PubMedEurope PMCScholia
Okkenhaug K, Rottapel R.; ''Grb2 forms an inducible protein complex with CD28 through a Src homology 3 domain-proline interaction.''; PubMedEurope PMCScholia
Under conditions of growth and mitogen stimulation S473 phosphorylation of AKT is carried out by mTOR (mammalian Target of Rapamycin). This kinase is found in two structurally and functionally distinct protein complexes, named TOR complex 1 (TORC1) and TOR complex 2 (TORC2). It is TORC2 complex, which is composed of mTOR, RICTOR, SIN1 (also named MAPKAP1) and LST8, that phosphorylates AKT at S473 (Sarbassov et al., 2005). This complex also regulates actin cytoskeletal reorganization (Jacinto et al., 2004; Sarbassov et al., 2004). TORC1, on the other hand, is a major regulator of ribosomal biogenesis and protein synthesis (Hay and Sonenberg, 2004). TORC1 regulates these processes largely by the phosphorylation/inactivation of the repressors of mRNA translation 4E binding proteins (4E BPs) and by the phosphorylation/activation of ribosomal S6 kinase (S6K1). TORC1 is also the principal regulator of autophagy. In other physiological conditions, other kinases may be responsible for AKT S473 phosphorylation.
Phosphorylation of AKT on S473 by TORC2 complex is a prerequisite for AKT phosphorylation on T308 by PDPK1 (Scheid et al. 2002, Sarabassov et al. 2005).
Once AKT is localized at the plasma membrane, it is phosphorylated at two critical residues for its full activation. These residues are a threonine (T308 in AKT1) in the activation loop within the catalytic domain, and a serine (S473 in AKT1), in a hydrophobic motif (HM) within the carboxy terminal, non-catalytic region. PDPK1 (PDK1) is the activation loop kinase; this kinase can also directly phosphorylate p70S6K. The HM kinase, previously termed PDK2, has been identified as the mammalian TOR (Target Of Rapamycin; Sarbassov et al., 2005) but several other kinases are also able to phosphorylate AKT at S473. Phosphorylation of AKT at S473 by TORC2 complex is a prerequisite for PDPK1-mediated phosphorylation of AKT threonine T308 (Scheid et al. 2002, Sarabassov et al. 2005).
Once phosphorylated on serine residue S473, AKT bound to PIP3 forms a complex with PIP3-bound PDPK1 i.e. PDK1 (Scheid et al. 2002, Sarabassov et al. 2005)
PIP3 generated by PI3K recruits phosphatidylinositide-dependent protein kinase 1 (PDPK1 i.e. PDK1) to the membrane, through its PH (pleckstrin-homology) domain. PDPK1 binds PIP3 with high affinity, and also shows low affinity for PIP2 (Currie et al. 1999).
PIP3 generated by PI3K recruits AKT (also known as protein kinase B) to the membrane, through its PH (pleckstrin-homology) domains. The binding of PIP3 to the PH domain of AKT is the rate-limiting step in AKT activation (Scheid et al. 2002). In mammals there are three AKT isoforms (AKT1-3) encoded by three separate genes. The three isoforms share a high degree of amino acid identity and have indistinguishable substrate specificity in vitro. However, isoform-preferred substrates in vivo cannot be ruled out. The relative expression of the three isoforms differs in different mammalian tissues: AKT1 is the predominant isoform in the majority of tissues, AKT2 is the predominant isoform in insulin-responsive tissues, and AKT3 is the predominant isoform in brain and testes. All 3 isoforms are expressed in human and mouse platelets (Yin et al. 2008; O'Brien et al. 2008). Note: all data in the pathway refer to AKT1, which is the most studied.
CTLA4 binds with high affinity to the ligands B71 and B72. The interaction of B7 molecules with CTLA4 provides inhibitory signals required for downregulation of the TCR response. The interaction is mediated by the CDR3 analogous loop of CTLA4, composed of an MYPPY motif, with a concave surface on the B7 formed predominately by the G, F, C, C' and C" strands. Its also been demonstrated that B7 binding is not totally essential for CTLA-4 activity.
Cot functions upstream of NIK in the CD28-costimulation signaling pathway leading to activation of NF-kB. Cot binds avidly to NIK and induces NIK phosphorylation in vivo.
PD-1 delivers inhibitory signals and downregulates antigen receptor signaling through direct dephosphorylation of signaling intermediates. The phosphatases SHP-1 and SHP-2 dephosphorylate CD3 zeta and inhibit the phosphorylation of ZAP-70 and PKC theta.
A costimulatory signal involved in T cell activation is transmitted by CD28. This signal is delivered by the interaction of the CD28 receptor on T cells with its ligand B7-1 on the antigen-presenting cell and modulates T cell antigen recognition. Despite the homodimeric structure of CD28, it interacts with B7-1 as though it has a single binding site. The V-like domains of CD28 contains a strictly conserved MYPPPY sequence motif that maps to the CDR3-analogous loop and is critical for recognition of its ligands. B7-1 and B7-2 bind to overlapping but not identical sites on CD28
Gads (Grb2-related adaptor protein 2) is essential for CD28 mediated NF-kB activation. This signal is mediated by the binding of Gads to the CD28 YMNM motif. The CD28 cytoplasmic PxxP motif is also required for this association.
The p85 unit of PI3K is the only signaling molecule identified so far that interacts with ICOS. ICOS contains several conserved motifs also found in CD28, including the YxxM motif in the cytoplasmic tail, which binds the lipid kinase phosphatidylinositol 3-kinase (PI3K) upon tyrosine phosphorylation after complex formation with ICOS. However, ICOS costimulation shows greater PI3K activity than CD28 in T cells.
The Programmed cell death protein 1 (PD-1) is functionally similar to CTLA4 and exerts an inhibitory signal on T cell activation. PD-1 binds the ligands B7H1 and B7DC but with different affinities. Interaction of PD-1/B7DC exhibited a 2-6-fold higher affinity and had different association/dissociation kinetics compared with the interaction of PD-1/B7H1.
Once phosphorylated, SH2-domain containing tyrosine phosphatases SHP-1 and SHP-2 bind to the ITIM and ITSM motifs of PD-1. The association between SHP-1 and PD-1 appears to be weaker than the interaction of PD-1 with SHP-2.
CTLA4 associates with the SH2 domain containing tyrosine phosphatase SHP2 and this interaction is mediated through the YVKM motif of CTLA-4. Phosphorylation of tyrosine in the YVKM motif recruits SHP-2. Still the association of SHP-2 with CTLA-4 is unclear and remains controversial. Several other studies have reported that CTLA-4 does not directly associate with SHP-2. The interaction between the phosphatase and CTLA-4 may be an indirect event, possibly mediated by PI3-kinase/SHP-2 binding.
When CTLA4 is engaged by B7 molecules, PP2A disassociates from CTLA4 in a phosphorylation dependent manner. Released PP2A acts downstream of early TCR and CD28 signaling, by inhibition of the PKB/Akt pathway.
PI3K inducibly associates with CD28: the SH2 domains of the PI3K p85 adaptor subunit interact with a cytoplasmic YMNM consensus motif at residues 173-176 of CD28.
Cot/Tpl 2 is a serine/threonine kinase of the mitogen activated protein kinase kinase kinase (MAP3K) family. Cot is observed as one of the downstream effectors of Akt. Based on in-vitro kinase assays and following overexpression in cell lines its been showed that AKT can phosphorylate Cot under non-physiological conditions. Akt and Cot physically interact through the amino terminus of Cot, and this interaction results in the phosphorylation of Cot on serine 400. Cot was shown to activate the IkB kinase (IKK) complex, possibly acting through NF kB inducing kinase (NIK).
CD28 delivers the costimulatory signal by interacting with its ligand B7-2 on the antigen-presenting cell and modulates T cell antigen recognition. The V-like domains of CD28 contains a strictly conserved MYPPPY sequence motif that maps to the CDR3-analogous loop and is critical for recognition of its ligand B7-2. Engagement of CD28 with B7-1 and with B7-2 have different bilogical functions in-vivo. Rapid dissociation of B7-2 from CD28 may not permit the robust tyrosine phosphorylation that the prolonged binding of B7-1 induces.
Vav1, once activated by PIP3 binding and phosphorylation by Fyn, stimulates the GDP/GTP exchange activity of Rac. Vav1 is selective for Rac and catalyses exchange of bound GDP for GTP.
Vav1 protein is a cytoplasmic guanine nucleotide exchange factor (GEF) for Rho-family GTPases. CD28 co-stimulation resulted in a prolonged and sustained phosphorylation and membrane localization of Vav1 in comparison to T-cell receptor activation alone. Vav1 contains a unique arrangement of signaling motifs a calponin homology domain, an acidic domain, a DBL homology (DH) domain, a pleckstrin homology (PH) domain, a cysteine-rich domain (CR), and a SH2 domain flanked by two proline-binding SH3 domains. Vav-1 may be recruited to the membrane through its PH domain by binding PI(3,4,5)P3 produced by CD28-bound PI3K and also by binding to CD28:Grb2 complexes by the dimerized SH3 domains in both the molecules.
In its unbound state PAK is maintained in an inactive conformation through intra domain interactions. On stimulation PAK is translocated to the plasma membrane where it specifically interacts with Rho family GTP-binding proteins Rac1-GTP and Cdc42-GTP. All PAK isoforms are direct effectors of the Rac and Cdc42. Rac and Cdc42 bind to a highly conserved motif in the amino terminus of PAK known as p21-binding domain (PBD) or Cdc42/Rac interactive binding (CRIB) domain. This binding is thought to induce a conformational change in PAK that relieves autoinhibition of the catalytic carboxy terminal domain, thereby inducing autophosphorylation at several sites and enabling the phosphorylation of exogenous substrates.
CD28 is capable of binding the Src homology 3 (SH3) domains of several proteins, including Grb2. The phospho-YMNM motif in CD28's cytoplasmic domain facilitates tandem SH2–SH3 domain binding. Grb-2 has been shown to bind to the CD28 YMNM motif with additional SH3 domain binding to the diproline motif in the C-terminal portion of the cytoplasmic domain of CD28.
The interaction of CD28 with its ligands, B7-1 (CD80) and B7-2 (CD86) on antigen-presenting cells enhances a number of TCR-mediated responses like production of interleukins. CD28 mediated costimulation is dependent upon phosphorylation of tyrosine residue 191 of the CD28 cytoplasmic tail, present in a 'YMNM' motif. p56Lck and p59Fyn phosphorylate CD28 and the phosphorylated residue allows the recruitment of PI3K, growth factor receptor-bound protein 2 (GRB2) and Grb2-related adaptor downstream of SHC (GADS) via their src-homology region 2 (SH2) domains.
PI3K enzyme bound to co-stimulatory protein CD28 catalyzes the phosphorylation of phosphatidylinositol 4,5-bisphosphate (PIP2) to generate phosphatidylinositol 3,4,5-trisphosphate (PIP3). This PIP3 acts as a membrane anchor for the downstream proteins like PDK1 and PKB.
Vav1 serves as a GEF for Cdc42-GTPase. It interacts with and activates Cdc42. The activated Cdc42 in turn transmits its signals through the downstream target, the PAK1 kinase.
The cytoplasmic domain of PD-1 has two tyrosine motifs, ITIM and ITSM. On engagement with B7 ligands B7DC and B7H1, PD-1 is phosphorylated on tyrosine residues 223 and 248 within these motifs. Kinases Lck and Csk also bind to these motifs and these kinases may be involved in the phosphorylation of PD-1.
Vav1 exists in an auto-inhibitory state folded in such a way as to inhibit the GEF activity of its DH domain. This folding is mediated through binding of tyrosines in the acidic domain to the DH domain and through binding of the CH domain to the C1 region. Activation of Vav may involve at least three different events to relieve this auto-inhibition. Phosphorylation of the tyrosines causes them to be displaced from the DH domain, binding of a ligand to the CH domain may cause it to release the C1 domain and PIP3 may bind to the PH domain, altering its conformation. Vav1 is phosphorylated on at least three tyrosines (Y142, Y160 and Y174) in the acidic domain, and this phosphorylation results in an increase in GEF activity. Fyn tyrosine kinase phosphorylates Vav1 after CD28 ligation.
The cytoplasmic tail of BTLA contains three tyrosine residues that are conserved in most organisms. The tyrosine residues Y257 and Y282 are both present in ITIM motif sequences. These tyrosine residues are phosphorylated after BTLA cross-linking, and both ITIM motifs recruit the tyrosine phosphatases SHP1 and SHP2. The targets of SHP1 and SHP2 recruited to BTLA are not known, although it is possible that they also have a role in dephosphorylating signaling intermediates downstream of antigen receptors in lymphocytes or in specifically targeting the PI3K-AKT pathway.
Upon TCR-CTLA-4 complex formation, CTLA4 is tyrosine phosphorylated. Src family tyrosine kinases Fyn, Lyn, and Lck associate with CTLA-4 and phosphorylate both Y-165 and Y-182 that are mainly responsible for interaction with Fyn through its SH2 domain. Once tyrosine 165 is phosphorylated, PP2A is activated and disassociates from CTLA4; this correlates with T cell inactivation.
The B and T lymphocyte attenuator, BTLA, is one of the co-inhibitory receptors of CD28 superfamily along with CTLA-4 and PD-1. BTLA differs from other CD28 members by having an extracellular Ig C-like domain, instead of a V-like one. Herpesvirus entry mediator (HVEM) is the external ligand for BTLA, providing the first example of a functional interaction between a TNFR and an Ig superfamily member. Binding of HVEM to BTLA delivers an inhibitory signal to T cells.
Newly synthesized CTLA4 expressed on the transmembrane associates with PP2A, and under these conditions the inhibitory function of CTLA4 is inactive. CTLA4 homodimer has a PP2A trimer bound to each tail. The A subunit of PP2A binds the lysine-rich motif located at lysine residues 152, 155, and 156 of the juxtamembrane region of the CTLA-4 tail, and the C subunit binds the Y165 residue.
The sequence around Y226 in the BTLA cytoplasmic domain is a predicted recruitment site for Grb2. Despite the prediction there is no direct evidence of protein recruitment to this tyrosine motif.
Inducible T cell co-stimulatory (ICOS) protein is the third member of the CD28 family that regulates T-cell activation and function. ICOS interacts with B7H2 (ICOSL, B7RP-1), a member of the B7 family expressed on the antigen-presenting cell.
The positive signals induced by CD28 and ICOS molecules are counterbalanced by other members of the CD28 family, including cytotoxic T-lymphocyte associated antigen (CTLA)4, programmed cell death (PD)1, and B and T lymphocyte attenuator (BTLA), which dampen immune responses. The balance of stimulatory and inhibitory signals is crucial to maximize protective immune responses while maintaining immunological tolerance and preventing autoimmunity.
The costimulatory receptors CD28, CTLA4, ICOS and PD1 are composed of single extracellular IgV-like domains, whereas BTLA has one IgC-like domain. Receptors CTLA4, CD28 and ICOS are covalent homodimers, due to an interchain disulphide linkage. The costimulatory ligands B71, B72, B7H2, B7H1 and B7DC, have a membrane proximal IgC-like domain and a membrane distal IgV-like domain that is responsible for receptor binding and dimerization. CD28 and CTLA4 have no known intrinsic enzymatic activity. Instead, engagement by their physiologic ligands B71 and B72 leads to the physical recruitment and activation of downstream T-cell effector molecules.
Original Pathway at Reactome: http://www.reactome.org/PathwayBrowser/#DB=gk_current&FOCUS_SPECIES_ID=48887&FOCUS_PATHWAY_ID=388841
Try the New WikiPathways
View approved pathways at the new wikipathways.org.Quality Tags
Ontology Terms
Bibliography
History
External references
DataNodes
PIP3
THEM4/TRIB3pBTLA
HVEMAKT
PIP3PDK1
active AKTCTLA4
B7-1/B7-2pBTLA
HVEMPDPK1
PIP3Src kinases
SHP2PIP3 Grb2
CD28Annotated Interactions
PIP3
THEM4/TRIB3AKT
PIP3CTLA4
B7-1/B7-2Phosphorylation of AKT on S473 by TORC2 complex is a prerequisite for AKT phosphorylation on T308 by PDPK1 (Scheid et al. 2002, Sarabassov et al. 2005).
Still the association of SHP-2 with CTLA-4 is unclear and remains controversial. Several other studies have reported that CTLA-4 does not directly associate with SHP-2. The interaction between the phosphatase and CTLA-4 may be an indirect event, possibly mediated by PI3-kinase/SHP-2 binding.
PDPK1
PIP3PIP3 Grb2
CD28PIP3 Grb2
CD28PIP3 Grb2
CD28