Upstream-Downstream: CD28 Cosignaling Pathways and T Cell Function
1996; Cell Press; Volume: 4; Issue: 6 Linguagem: Inglês
10.1016/s1074-7613(00)80479-3
ISSN1097-4180
Autores Tópico(s)CAR-T cell therapy research
ResumoWhether due to the chronology of discovery, or the complexity of the system, an understanding of CD28 cosignaling pathways is only beginning to catch up with our understanding of its biology. Functional studies by numerous laboratories have shown that binding of CD28 by its ligands CD80 (B7-1) or CD86 (B7-2/B70) decisively determines the destiny of the T cell response (reviewed recently by28Jenkins M.K The ups and downs of T cell costimulation.Immunity. 1994; 1: 443-448Abstract Full Text PDF PubMed Scopus (237) Google Scholar, 7Bluestone J New perspectives of CD28–B7-mediated T cell costimulation.Immunity. 1995; 2: 555-559Abstract Full Text PDF PubMed Scopus (505) Google Scholar, 56Thompson C.B Distinct roles for the costimulatory liagnds B7-1 and B7-2 in T helper cell differentiation?.Cell. 1995; 81: 979-982Abstract Full Text PDF PubMed Scopus (362) Google Scholar, 38Linsley P.S Distinct roles for CD28 and cyotoxic T lymphocyte-associated molecule-4 receptor during T-cell activation.J. Exp. Med. 1995; 182: 459-465Crossref PubMed Scopus (100) Google Scholar). T cell responses require a nonantigen-driven cosignal provided by CD28 (and possibly other antigens), in addition to ligation of the T cell receptor ζ (TCRζ)–CD3 complex with CD4–p56lck or CD8–p56lck. Indeed, TCR ligation without costimulation can result in functional inactivation, or anergy of T helper 1 (Th1) cells (28Jenkins M.K The ups and downs of T cell costimulation.Immunity. 1994; 1: 443-448Abstract Full Text PDF PubMed Scopus (237) Google Scholar). In most T cells, CD28 lowers the threshold needed for activation and increases response longevity, effects linked to increased transcription and stability of lymphokine mRNAs, in particular those encoding interleukin-2 (IL-2) and IL-4 (21Fraser J.D Irving B.A Crabtree G.R Weiss A Regulation of interleukin-2 enhancer activity by the T cell accessory molecule CD28.Science. 1991; 251: 313-316Crossref PubMed Scopus (572) Google Scholar, 56Thompson C.B Distinct roles for the costimulatory liagnds B7-1 and B7-2 in T helper cell differentiation?.Cell. 1995; 81: 979-982Abstract Full Text PDF PubMed Scopus (362) Google Scholar). Consequently, CD28-deficient mice have markedly diminished T cell responses to certain antigens (53Shahinian A Pfeffer K Lee K.P Kuendig T.M Kishihara K Wakeham A Kawai K Ohashi P.S Thompson C.B Mak T.W Differential T cell costimulatory requirements in CD28-deficient mice.Science. 1993; 261: 609-612Crossref PubMed Scopus (1143) Google Scholar). Cosignals also amplify cytolytic responses in tumor and autoimmune models (15Chen L Ashe S Brady W.A Hellstroem I Hellstroem K.E Ledbetter J.A McGowan P Linsley P.S Costimulation of antitumor immunity by the B7 counter-receptor for the T lymphocyte molecules CD28 and CTLA-4.Cell. 1992; 71: 1093-1102Abstract Full Text PDF PubMed Scopus (1062) Google Scholar, 58Townsend S.E Allison J.P Tumor rejection after direct costimulation of CD8+ T cells by B7-transfected melanoma cells.Science. 1993; 259: 368-370Crossref PubMed Scopus (1125) Google Scholar) and may control the differentiation of Th1 versus Th2 subsets (32Kuchroo V.K Das M.P Brown J.A Ranger A.M Zamvil S.S Sobel R.A Weiner H.L Nabavi N Glimcher L.H B7-1 and B7-2 costimulatory molecules activate differentially the Th1/Th2 developmental pathways application to autoimmune disease therapy.Cell. 1995; 80: 707-718Abstract Full Text PDF PubMed Scopus (1620) Google Scholar, 7Bluestone J New perspectives of CD28–B7-mediated T cell costimulation.Immunity. 1995; 2: 555-559Abstract Full Text PDF PubMed Scopus (505) Google Scholar, 22Freeman G.J Boussiotis V.A Anumanthan A Bernstein G.M Ke X.-Y Rennert P.D Gray G.S Gribben J.G Nadler L.M B7-1 and B7-2 do not deliver identical costimulatory signals, since B7-2 but not B7-1 preferentially costimulates the initial production of IL-4.Immunity. 1995; 2: 523-532Abstract Full Text PDF PubMed Scopus (572) Google Scholar). As if this were not enough, CD28 also regulates expression of Bcl-xL (8Boise L.H Minn A.J Noel P.J June C.H Accavitti M.A Lindsten T Thompson C.B CD28 costimulation can promote T cell survivial by enhancing the expression of Bcl-xL.Immunity. 1995; 3: 87-98Abstract Full Text PDF PubMed Scopus (1058) Google Scholar), and a variety of surface antigens such as CTLA-4, the high affinity IL-2 receptor, and the CD40 ligand (CD40L), each of which is needed for successful progression of T cell responses. By contrast, the structurally related molecule CTLA-4 negatively regulates T cell proliferation. Anti-CTLA-4 antibodies inhibit TCR and TCR–CD28 triggered proliferation (62Walunas T.L Lenschow D.J Bakker C.Y Linsley P.S Freeman G.J Green J.M Thompson C.B Bluestone J.A CTLA-4 can function as a negative regulator of T cell activation.Immunity. 1994; 1: 405-413Abstract Full Text PDF PubMed Scopus (1674) Google Scholar), while T cells from CTLA-4 negative mice are hyperresponsive to antigen and undergo spontaneous proliferation that leads to extreme lymphadenopathy (66Waterhouse P Penninger J.M Timms E Wakeham A Shahinian A Lee K.P Thompson C.B Griesser H Mak T.W Lymphoproliferative disorders with early lethality in mice deficient in Ctla-4.Science. 1995; 270: 985-988Crossref PubMed Scopus (2242) Google Scholar, 57Tivol E.A Borriello F Schweitzer A.N Lynch W.P Bluestone J.A Sharpe A.H Loss of CTLA-4 leads to massive lymphoproliferation and fatal multiorgan tissue destruction, revealing a critical negative regulatory role of CTLA-4.Immunity. 1995; 3: 541-547Abstract Full Text PDF PubMed Scopus (2246) Google Scholar). The emerging picture points to an interplay of positive and negative signals mediated by TCR–CD3, CD4/CD8–p56lck, CD28, and CTLA-4. TCRζ–CD3 and CD4/CD8–p56lck initiate a complex array of intracellular signals that have been reviewed extensively elsewhere (67Weiss A Littman D.R Signal transduction by lymphocyte antigen receptors.Cell. 1994; 76: 263-274Abstract Full Text PDF PubMed Scopus (1921) Google Scholar). By contrast, as outlined in this review, the proximal and distal events of CD28 signaling are just beginning to be understood. Proximal events are initiated by phosphorylation of CD28 at the cytoplasmic pTyr–Met–Asn–Met (pYMNM) motif, followed by recruitment of the signaling proteins phosphatidylinositol 3-kinase (PI 3-kinase), growth factor receptor–bound protein-2 (GRB-2), and T cell–specific protein-tyrosine kinase (ITK). Distal events include activation of the serine kinase modules for mitogen-activated kinase (MAPK), stress-activated protein kinase/c-jun amino-terminal protein kinase (SAPK/JNK), and mitogen activation protein kinase p38 (p38/HOG1), followed by c-jun phosphorylation and initiation of IL-2 transcription. The pathways that bridge early and late events are unresolved, although studies increasingly implicate PI 3-kinase. PI 3-kinase binding also implicates this receptor in a diverse number of functions that have not previously been attributed to CD28. What is the nature of the intracellular cosignal(s) that accounts for CD28 function? Initial studies distinguished TCRζ–CD3 signaling from CD28 signaling based on the strict dependency of the former on calcium and sensitivity to cyclosporin A (CsA). Two parts of CD28 signaling were suggested, a calcium-dependent pathway with characteristics similar to TCRζ–CD3 signaling and a pathway that is CD28 specific, calcium independent, and CsA resistant (see29June C.H Bluestone J.A Nadler L.M Thompson C.B The B7 and CD28 receptor families.Immunol. Today. 1994; 15: 321-331Abstract Full Text PDF PubMed Scopus (181) Google Scholar). However, a direct link between CD28 and an intracellular signaling pathway was first provided with the finding that CD28 could be phosphorylated and then bind to the lipid kinase, PI 3-kinase (59Truitt K.E Hicks C.M Imboden J.B Stimulation of CD28 triggers an association between CD28 and phosphatidylinositol-3-kinase in Jurkat T cells.J. Exp. Med. 1994; 179: 1071-1076Crossref PubMed Scopus (186) Google Scholar, 45Prasad K.V.S Cai Y.-C Raab M Duckworth B Cantley L Shoelson S.E Rudd C.E T cell antigen CD28 interacts with the lipid kinase phosphatidylinositol 3-kinase by a cytoplasmic Tyr(P)–Met–Xaa–Met motif.Proc. Natl. Acad. Sci. USA. 1994; 91: 2834-2838Crossref PubMed Scopus (264) Google Scholar, 1August A DuPont B CD28 of T lymphocytes associates with phosphatidylinositol 3-kinase.Int. Immunol. 1994; 6: 769-774Crossref PubMed Scopus (97) Google Scholar). A phosphotyrosine-based motif pYMNM is conserved between species (human, mouse, rat, and chicken) and serves as a binding motif for the Src homology domain 2 (SH2 domain) of PI 3-kinase (Figure 1). PI 3-kinase is a heterodimer, composed of a p85 adaptor subunit linked to a p110 catalytic domain that phosphorylates the D-3 position of the inositol ring of phosphatidylinositol, phosphatidylinositol 4-phosphate, and phosphatidylinositol 4,5 bisphosphate generating PI 3-P, PI 3,4-P2, and PI 3,4,5-P3, respectively. The p85 adaptor chain has two SH2 domains, each of which is formed by two α helices and intervening β-pleated sheets that form grooves that bind to a core phosphotyrosine residue and adjacent residues within the pYMNM motif (Figure 2). Methionine residues determine the specificity of p85 SH2 domain binding (54Songyang Z Shoelson S.E Chandhuri M Gish G Pawson T Haser W.G King F Roberts T Ratnofsky S Lechleider R.J Neel B.G Birge R.B Fajardo J.E Chou M.M Hanafusa H Schaffhausen B Cantley L.C SH2 domains recognize specific phosphopeptide sequences.Cell. 1993; 72: 767-778Abstract Full Text PDF PubMed Scopus (2351) Google Scholar). In this regard, the avidity of p85 SH2 binding to CD28 is comparable to that of growth factor receptors such as the platelet-derived growth factor receptor (PDGF-R) (45Prasad K.V.S Cai Y.-C Raab M Duckworth B Cantley L Shoelson S.E Rudd C.E T cell antigen CD28 interacts with the lipid kinase phosphatidylinositol 3-kinase by a cytoplasmic Tyr(P)–Met–Xaa–Met motif.Proc. Natl. Acad. Sci. USA. 1994; 91: 2834-2838Crossref PubMed Scopus (264) Google Scholar). The C-terminal domain has 10-fold greater avidity than the N-terminal SH2 domain, and is thus likely to dominate in binding. CD28 ligation recruits PI 3-kinase (45Prasad K.V.S Cai Y.-C Raab M Duckworth B Cantley L Shoelson S.E Rudd C.E T cell antigen CD28 interacts with the lipid kinase phosphatidylinositol 3-kinase by a cytoplasmic Tyr(P)–Met–Xaa–Met motif.Proc. Natl. Acad. Sci. USA. 1994; 91: 2834-2838Crossref PubMed Scopus (264) Google Scholar) and, as shown in earlier work, can generate D-3 lipids (64Ward S.G Westwick J Hall N.D Sansom D.M Ligation of CD28 receptor by B7 induces formation of D-3 phosphoinositides in T lymphocytes independently of T cell receptor/CD3 activation.Eur. J. Immunol. 1993; 23: 2572-2577Crossref PubMed Scopus (116) Google Scholar).Figure 2CD28 Binds to Multiple Intracellular Signaling ProteinsShow full caption(Top) The 41 aa cytoplasmic tail of CD28 binds to three intracellular signaling molecules: PI 3-kinase, ITK, and the signaling complex GRB-2–SOS. The SH2 domains of PI 3-kinase and GRB-2 bind to the pYMNM motif, while the mechanism of ITK binding has yet to be established. Based on peptide binding, GRB-2 binds with 10- to 100-fold lower avidity than PI 3-kinase.(Bottom) The p85 subunit of PI 3-kinase consists of an SH3 domain, two proline motifs, a Bcr homology region, two SH2 domains, and an SH2 domain intervening region (SH2i). The p85 C-terminal domain of PI 3-kinase binds with 10-fold greater avidity than its N-terminal SH2 domain, and is thus likely to dominate in binding to the receptor. The two proline motifs can mediate binding to the SH3 domains of Src kinases, while the SH2i region mediates binding to the PI 3-kinase catalytic subunit.View Large Image Figure ViewerDownload Hi-res image Download (PPT) (Top) The 41 aa cytoplasmic tail of CD28 binds to three intracellular signaling molecules: PI 3-kinase, ITK, and the signaling complex GRB-2–SOS. The SH2 domains of PI 3-kinase and GRB-2 bind to the pYMNM motif, while the mechanism of ITK binding has yet to be established. Based on peptide binding, GRB-2 binds with 10- to 100-fold lower avidity than PI 3-kinase. (Bottom) The p85 subunit of PI 3-kinase consists of an SH3 domain, two proline motifs, a Bcr homology region, two SH2 domains, and an SH2 domain intervening region (SH2i). The p85 C-terminal domain of PI 3-kinase binds with 10-fold greater avidity than its N-terminal SH2 domain, and is thus likely to dominate in binding to the receptor. The two proline motifs can mediate binding to the SH3 domains of Src kinases, while the SH2i region mediates binding to the PI 3-kinase catalytic subunit. By binding to PI 3-kinase, CD28 anchors the enzyme to the inner face of the plasma membrane, where it can act on its target substrates. Various functions are dependent on the kinase, including growth factor–induced mitogenesis, glucose uptake, intervesicular transport, cytoskeletal organization, receptor trafficking, and apoptosis (31Kapeller R Cantley L.C Phosphatidylinositol 3-kinase.BioEssays. 1994; 16: 565-576Crossref PubMed Scopus (548) Google Scholar). Two of these, the regulation of mitogenesis and apoptosis, overlap with functions that have been attributed to CD28. Targets of the kinase are only partially known and the subject of much investigation. PI 3,4-P2 and PI 3,4,5-P3 can activate the δ, ε, θ, and ζ isoforms of the serine/threonine kinase, protein kinase C (PKC) (41Nakanishi H Brewer K.A Exton J.H Activation of the zeta isozyme of protein kinase C by phosphatidylinositol 3,4,5-triphosphate.J. Biol. Chem. 1993; 268: 13-16Abstract Full Text PDF PubMed Google Scholar, 31Kapeller R Cantley L.C Phosphatidylinositol 3-kinase.BioEssays. 1994; 16: 565-576Crossref PubMed Scopus (548) Google Scholar) and related AKT (20Franke T.F Yang S-I Chan T.O Datta K Kazlauskas A Morrison D.K Kaplan D.R Tsichlis P.N The protein kinase encoded by the Akt protooncogene is a target of PDGF-activated phosphatidylinositol 3-kinase.Cell. 1995; 81: 727-736Abstract Full Text PDF PubMed Scopus (1784) Google Scholar). PI 3-kinase may also activate and/or may be activated by the GTP-binding protein p21ras (25Hu Q Klippel A Muslin A.J Fantl W.J Williams L.T Ras-dependent induction of cellular responses by constitutively active phosphatidylinositol-3 kinase.Science. 1995; 268: 100-102Crossref PubMed Scopus (507) Google Scholar, 49Rodriguez-Viciana P Warne P.H Dhand R Vanhaesebroeck B Gout I Fry M.J Waterfield M.D Downward J Phosphatidyl-3-OH kinase as a direct target of Ras.Nature. 1994; 370: 527-532Crossref PubMed Scopus (1638) Google Scholar). Further downstream, PI 3-kinase is required for activation of the rapamycin-sensitive serine/threonine kinase S6 kinase (p70S6K) by PDGF-R, and for initiation of DNA synthesis (18Fantl W.J Escobedo J.A Martin G.A Turck C.W del Rosario M McCormick F Williams L.T Distinct phosphotyrosines on a growth factor receptor bind to specific molecules that mediate different signaling pathways.Cell. 1992; 69: 413-423Abstract Full Text PDF PubMed Scopus (470) Google Scholar, 6Blenis J Signal transduction via the MAP kinases proceed at your own risk.Proc. Natl. Acad. Sci. USA. 1993; 90: 5889-5892Crossref PubMed Scopus (1140) Google Scholar). To complicate matters further, CD28 can bind to two other intracellular proteins. The CD28 pYMNM site also binds to the GRB-2 SH2 domain of the GRB-2–Son of Sevenless (GRB-2–SOS) complex (52Schneider H Cai Y.-C Prasad K.V.S Shoelson S.E Rudd C.E T cell antigen CD28 binds to the GRB-2/SOS complex, regulators of p21ras.Eur. J. Immunol. 1995; 25: 1044-1050Crossref PubMed Scopus (130) Google Scholar) (Figure 2). GRB-2 is an adaptor chain that consists of one SH2 and two SH3 domains. While the SH2 domain binds to phosphotyrosine-based motifs of surface receptors, the SH3 domain binds to proline residues within SOS. In other systems, the guanine nucleotide exchange activity of SOS converts p21ras from an inactive GDP-bound state to an active GTP-bound state. Whether a similar situation exits for CD28 is unclear. Both GRB-2 and PI 3-kinase bind to the same site, although GRB-2 binds with 10- to 100-fold lower avidity (10-fold lower avidity than the N-terminal p85 SH2 domain and 100-fold lower than the C-terminal SH2 domain). The arginine (N) of the pYMNM motif dictates GRB-2 SH2 domain binding. Whether the stoichiometry of GRB-2 binding is sufficiently high to generate productive signals is uncertain, although it may also bind directly to PI 3-kinase (63Wang J Auger K.R Jarvis L Shi Y Roberts T.M Direct association of GRb2 with the p85 subunit of phosphatidylinositol 3-kinase.J. Biol. Chem. 1995; 270: 12774-12780Crossref PubMed Scopus (101) Google Scholar). The issue of stoichiometry also exists in the case of CD28 binding to ITK (3August A Gibson S Kawakami Y Kawakami T Mills G.B Dupont B CD28 is associated with and induces the immediate tyrosine phosphorylation and activation of the Tec family kinase ITK/EMT in the human Jurkat leukemic T cell line.Proc. Natl. Acad. Sci. USA. 1994; 91: 9347-9351Crossref PubMed Scopus (205) Google Scholar) (Figure 2). Nonreceptor tyrosine kinases interact with surface receptors as initially shown with CD4–p56lck (51Rudd C.E Janssen O Cai Y.-C da Silva A.J Raab M Prasad K.V.S Two-step TCRζ/CD3 and CD28 signaling in T cells SH2/SH3 domains, protein-tyrosine and lipid kinases.Immunol. Today. 1994; 15: 225-234Abstract Full Text PDF PubMed Scopus (181) Google Scholar). Similarly, ITK possesses a unique N terminus, a SH3 and SH2 domain, and a kinase domain that shares greatest homology with the Tec family of kinases. Given its restricted expression in T cells, ITK is an attractive candidate for cosignaling. As expected, CD28 ligation can recruit and activate the kinase (3August A Gibson S Kawakami Y Kawakami T Mills G.B Dupont B CD28 is associated with and induces the immediate tyrosine phosphorylation and activation of the Tec family kinase ITK/EMT in the human Jurkat leukemic T cell line.Proc. Natl. Acad. Sci. USA. 1994; 91: 9347-9351Crossref PubMed Scopus (205) Google Scholar). However, only a small percent of ITK has been reported to be recruited by the receptor, and preliminary data show that CD28-mediated IL-2 production may not be impaired in ITK-deficient mice (37Liao X.C Littman D.R Altered T cell receptor signaling and disrupted T cell development in mice lacking Itk.Immunity. 1995; 3: 757-769Abstract Full Text PDF PubMed Scopus (255) Google Scholar). Although the mechanism underlying the interaction has yet to be established, binding can occur independent of the pYMNM motif (11Cai Y.-C Céfai D Schneider H Raab M Nabavi N Rudd C.E Selective CD28 pYMNM mutations implicate phosphatidylinositol 3-kinase in CD28 mediated costimulation.Immunity. 1995; 3: 417-426Abstract Full Text PDF PubMed Scopus (121) Google Scholar). Whether subsets of ITK-dependent and independent peripheral T cells exist, or whether ITK plays a role in another CD28-mediated function is being investigated. By binding to multiple signaling molecules, CD28 could engage multiple signaling cascades in T cells. Distinct signaling pathways could in turn regulate the different functions attributed to this receptor. Since the receptor needs to be phosphorylated to bind to PI 3-kinase and GRB-2, an intracellular tyrosine kinase is needed to phosphorylate the site in order to provide the conditions for SH2 domain binding. Unlike growth factor receptors, CD28 lacks an intrinsic tyrosine kinase domain. What, then, is the kinase that initiates cosignaling by phosphorylating CD28? p56lck and p59fyn have been identified in expression studies as two kinases that phosphorylate CD28 and recruit PI 3-kinase and GRB-2 to the receptor (46Raab M Heyeck S.D Cai Y.-C Berg L.J Rudd C.E p56lck and p59fyn regulate CD28 recruitment of phosphatidylinositol 3 kinase, growth factor receptor–bound GRB-2 and T cell-specific protein-tyrosine kinase ITK implications for co-stimulation.Proc. Natl. Acad. Sci. USA. 1995; 92: 8891-8895Crossref PubMed Scopus (154) Google Scholar). Neither ITK nor ZAP-70 were found to phosphorylate the receptor. p56lck demonstrates a remarkable specificity for the pYMNM motif. Over 90% of phosphorylation is observed at this site, despite the presence of three other tyrosine residues in the cytoplasmic domain (see Figure 1). In agreement with a role for this kinase, J.Cam-1, a p56lck-deficient cell line, showed reduced CD28-mediated responses (39Lu Y Phillips C.A Bjorndahl J.M Trevillyan J.M CD28 signal transduction tyrosine phosphorylation and receptor association of phosphoinositide-3 kinase correlate with Ca2+-independent costimulatory activity Eur.J. Immunol. 1994; 24: 2732Google Scholar). Whether the kinase that phosphorylates CD28 is provided by CD4–p56lck, CD8–p56lck, or TCR/CD3–p59fyn, or operates independently of these receptors is unclear. Coprecipitation with CD28 has been reported in weak detergents (26Hutchcroft J.E Bierer B.E Activation-dependent phosphorylation of the T-lymphocytes surface receptor CD28 and associated proteins.Proc. Natl. Acad. Sci. USA. 1994; 91: 3260-3264Crossref PubMed Scopus (76) Google Scholar, 2August A DuPont B Activation of the src kinase lck following CD28 crosslinking in the Jurkat leukemic cell line.Biochem. Biophys. Res. Commun. 1994; 199: 1466-1473Crossref PubMed Scopus (57) Google Scholar), although when contrasted with CD4–p56lck binding, the interaction occurs with very low level of stoichiometry (46Raab M Heyeck S.D Cai Y.-C Berg L.J Rudd C.E p56lck and p59fyn regulate CD28 recruitment of phosphatidylinositol 3 kinase, growth factor receptor–bound GRB-2 and T cell-specific protein-tyrosine kinase ITK implications for co-stimulation.Proc. Natl. Acad. Sci. USA. 1995; 92: 8891-8895Crossref PubMed Scopus (154) Google Scholar). Current evidence strongly suggests that CD28 acts as an independent signaling unit. CD28 ligation can independently recruit PI 3-kinase and GRB-2 (59Truitt K.E Hicks C.M Imboden J.B Stimulation of CD28 triggers an association between CD28 and phosphatidylinositol-3-kinase in Jurkat T cells.J. Exp. Med. 1994; 179: 1071-1076Crossref PubMed Scopus (186) Google Scholar, 45Prasad K.V.S Cai Y.-C Raab M Duckworth B Cantley L Shoelson S.E Rudd C.E T cell antigen CD28 interacts with the lipid kinase phosphatidylinositol 3-kinase by a cytoplasmic Tyr(P)–Met–Xaa–Met motif.Proc. Natl. Acad. Sci. USA. 1994; 91: 2834-2838Crossref PubMed Scopus (264) Google Scholar, 24Ghiotto-Ragueneau M Battifora M Truneh A Waterfield M.D Olive D Comparision of CD28–B7.1 and B7.2 functional interaction in resting human T cells phosphotidylinositol 3-kinase association to CD28 and cytokine production.Eur. J. Immunol. 1996; 26: 34-41Crossref PubMed Scopus (44) Google Scholar), and the interaction can be reconstituted without the need for TCR or CD4 expression (46Raab M Heyeck S.D Cai Y.-C Berg L.J Rudd C.E p56lck and p59fyn regulate CD28 recruitment of phosphatidylinositol 3 kinase, growth factor receptor–bound GRB-2 and T cell-specific protein-tyrosine kinase ITK implications for co-stimulation.Proc. Natl. Acad. Sci. USA. 1995; 92: 8891-8895Crossref PubMed Scopus (154) Google Scholar). A prime function of CD28 is up-regulation of IL-2 production. This results from effects on IL-2 promoter transcription and on mRNA stability. Although controversial, increasing evidence supports a role for PI 3-kinase in the regulation of this event. Mutation of the tyrosine within the pYMNM motif was initially reported to block anti-CD28-induced IL-2 production in a T cell hybridoma (43Pages F Ragueneau M Rottapel R Truneh A Nunes J Imbert J Olive D Binding of phosphatidyl-3-OH kinase to CD28 is required for T cell signaling.Nature. 1994; 369: 327-329Crossref PubMed Scopus (341) Google Scholar). To distinguish among the roles of PI 3-kinase, GRB-2–SOS, and ITK in costimulation, the methionine in the plus 3 position of the pYMNM motif was mutated and found to selectively disrupt PI 3-kinase binding without affecting the binding of GRB-2 or ITK (11Cai Y.-C Céfai D Schneider H Raab M Nabavi N Rudd C.E Selective CD28 pYMNM mutations implicate phosphatidylinositol 3-kinase in CD28 mediated costimulation.Immunity. 1995; 3: 417-426Abstract Full Text PDF PubMed Scopus (121) Google Scholar). Nevertheless, the same mutation inhibited IL-2 production in a system that was dependent on TCRζ–CD3 and CD28 ligation (11Cai Y.-C Céfai D Schneider H Raab M Nabavi N Rudd C.E Selective CD28 pYMNM mutations implicate phosphatidylinositol 3-kinase in CD28 mediated costimulation.Immunity. 1995; 3: 417-426Abstract Full Text PDF PubMed Scopus (121) Google Scholar). Anti-CD3 was used at 10−9 to 10−13 M, concentrations that approximate physiological ligand. Thus, p85 binding and possibly the associated PI 3-kinase catalytic subunit were implicated in costimulation. By contrast, expression of the CD28 pY-173 mutant in Jurkat cells did not block cosignaling induced by combinations of anti-CD28 plus ionomycin–phorbol ester (60Truitt K.E Shi J Gibson S Segal L.G Mills G.B Imboden J.B CD28 delivers costimulatory signals independently of its association with phosphatidylinositol 3-kinase.J. Immunol. 1995; 155: 4702-4710PubMed Google Scholar, 17Crooks M.E.C Littman D.R Carter R.H Fearon C.T Weiss A Stein P.H CD28-mediated co-stimulation in the absence of phosphatidylinositol 3-kinase association and activation.Mol. Cell. Biol. 1995; 15: 6820-6828Crossref PubMed Scopus (62) Google Scholar). Phorbol ester was found to partially dissociate PI 3-kinase binding without inhibiting costimulation (27Hutchcroft J.E Franklin D.P Tsai B Harrison-Findik D Vartocovski L Bierer B Phorbol ester treatment inhibits phosphatidylinositol 3-kinase activation by, and association with, CD28, a T-lymphocyte surface receptor.Proc. Natl. Acad. Sci. USA. 1995; 92: 8808-8888Crossref PubMed Scopus (36) Google Scholar). What might be the basis for this apparent discrepancy in experimental results? Two obvious differences are apparent, namely the mode of primary stimulation and the nature of the host cells used for transfection. CD28 required PI 3-kinase in the study that used TCRζ–CD3 ligation as the primary signal, but not in the studies that used phorbol ester and ionomycin. In this regard, TCRζ–CD3 and phorbol myristate acetate (PMA)–ionomycin signaling are almost certainly not equivalent. In support of this, the CD28 pYMNM mutant that failed to costimulate with TCRζ–CD3 could nevertheless support IL-2 production in the presence of PMA–ionomycin (12Céfai D Cai Y.-C Hu H Rudd C.E CD28 co-stimulatory regimes differ in their dependence on PI 3-kinase common cosignals induced by CD80 and CD86.Int. Immunol., in press. 1996; Google Scholar). Therefore, phorbol ester may bypass the need for the kinase, perhaps by acting on downstream targets of PI 3-kinase. In this regard, both D-3 lipids and PMA activate PKC. PI 3,4-P2/PI 3,4,5-P3 activate the δ, ε, θ, ζ isoforms, while PMA can activate of the α, β, γ, δ, ε isoforms. In addition to this, PMA is known to act pleiotropically by binding to cysteine-rich diacylglycerol binding motifs within proteins such as the haematopoetic proto-oncogene Vav, and by activating downstream signaling molecules that are independently regulated by CD28. These targets include p21ras, MAPK, and SAPK/JNK (see below). The use of different primary signals may also account for the discrepancy in results obtained using the inhibitor of PI 3-kinase, wortmannin. While wortmannin has generally been reported to inhibit anti-CD3 plus anti-CD28 costimulation in peripheral T cells (65Ward S.G Wilson A Turner L Westwick J Sansom D.M Inhibition of CD28-mediated T cell costimulation by the phosphoinositides 3-kinase inhibitor wortmannin.Eur. J. Immunol. 1995; 25: 526-532Crossref PubMed Scopus (85) Google Scholar, 61Ueda Y Levine B.L Huang M.L Freeman G.J Nadler L.M June C Ward S.G Both CD28 ligands CD80 (B7-1) and CD86 (B7-2) activate phosphatidylinositol 3-kinase, and wortmannin reveals heterogeneity in the regulation of T cell IL-2 secretion.Int. Immunol. 1995; 7: 957-966Crossref PubMed Scopus (79) Google Scholar, 24Ghiotto-Ragueneau M Battifora M Truneh A Waterfield M.D Olive D Comparision of CD28–B7.1 and B7.2 functional interaction in resting human T cells phosphotidylinositol 3-kinase association to CD28 and cytokine production.Eur. J. Immunol. 1996; 26: 34-41Crossref PubMed Scopus (44) Google Scholar), it fails to inhibit IL-2 production using a regime of anti-CD28 plus PMA–ionomycin (25Hu Q Klippel A Muslin A.J Fantl W.J Williams L.T Ras-dependent induction of cellular responses by constitutively active phosphatidylinositol-3 kinase.Science. 1995; 268: 100-102Crossref PubMed Scopus (507) Google Scholar, 27Hutchcroft J.E Franklin D.P Tsai B Harrison-Findik D Vartocovski L Bierer B Phorbol ester treatment inhibits phosphatidylinositol 3-kinase activation by, and association with, CD28, a T-lymphocyte surface receptor.Proc. Natl. Acad. Sci. USA. 1995; 92: 8808-8888Crossref PubMed Scopus (36) Google Scholar, 61Ueda Y Levine B.L Huang M.L Freeman G.J Nadler L.M June C Ward S.G Both CD28 ligands CD80 (B7-1) and CD86 (B7-2) activate phosphatidylinositol 3-kinase, and wortmannin reveals heterogeneity in the regulation of T cell IL-2 secretion.Int. Immunol. 1995; 7: 957-966Crossref PubMed Scopus (79) Google Scholar, 60Truitt K.E Shi J Gibson S Segal L.G Mills G.B Imboden J.B CD28 delivers costimulatory signals independently of its association with phosphatidylinositol 3-kinase.J. Immunol. 1995; 155: 4702-4710PubMed Google Scholar). Given these differences, consideration should be given to the appropriateness of PMA–ionomycin as a primary signal and substitute for TCR ligation. Individual T cell
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