Requirement for association of p56lck with CD4 in antigen-specific signal transduction in T cells

The T-cell antigen CD5 acts as a receptor and substrate for the protein-tyrosine kinase p56lck

CD5 is a T-cell-specific antigen which binds to the B-cell antigen CD72 and acts as a coreceptor in the stimulation of T-cell growth. CD5 associates with the T-cell receptor zeta chain (TcR zeta)/CD3 complex and is rapidly phosphosphorylated on tyrosine residues as a result of TcR zeta/CD3 ligation. However, despite this, the mechanism by which CD5 generates intracellular signals is unclear. In this study, we demonstrate that CD5 is coupled to the protein-tyrosine kinase p56lck and can act as a substrate for p56lck. Coexpression of CD5 with p56lck in the baculovirus expression system resulted in the phosphorylation of CD5 on tyrosine residues. Further, anti-CD5 and anti-p56lck coprecipitated each other in a variety of detergents, including Nonidet P-40 and Triton X-100. Anti-CD5 also precipitated the kinase from various T cells irrespective of the expression of TcR zeta/CD3 or CD4. No binding between p59fyn(T) and CD5 was detected in T cells. The binding of p56lck to CD5 induced a 10- to 15-fold increase in p56lck catalytic activity, as measured by in vitro kinase analysis. In vivo labelling with 32P(i) also showed a four- to fivefold increase in Y-394 occupancy in p56lck when associated with CD5. The use of glutathione S-transferase-Lck fusion proteins in precipitation analysis showed that the SH2 domain of p56lck could recognize CD5 as expressed in the baculovirus expression system. CD5 interaction with p56lck represents a novel variant of a receptor-kinase complex in which receptor can also serve as substrate. The CD5-p56lck interaction is likely to play roles in T-cell signalling and T-B collaboration.

The cytoplasmic domain of CD4 is sufficient for its down-regulation from the cell surface by human immunodeficiency virus type 1 Nef

Human immunodeficiency virus type 1 Nef down-regulates surface expression of murine and human CD4 but not human CD8. We recently reported that the cytoplasmic domain of CD4 is required for its down-regulation by Nef. Using a chimeric molecule composed of the extracellular and transmembrane domains of human CD8 fused to the cytoplasmic domain of human CD4, we show here that the cytoplasmic domain of CD4 is sufficient for down-regulation by Nef. Since the cytoplasmic domain of CD4 is also the site of its association with p56lck, we used a series of CD4 mutants to determine whether the regions of the cytoplasmic domain of CD4 required for down-regulation by Nef are the same as those required for p56lck binding. Our results indicate that the portion of the cytoplasmic domain required for the down-regulation of CD4 by Nef overlaps with the binding site of p56lck, but the cysteine residues which are essential for the association of CD4 with p56lck are not required. This observation raised the possibility that Nef competes with p56lck for binding to CD4. However, under conditions which are considerably milder than those permissive for coimmunoprecipitation of CD4 and p56lck, we found no evidence for an association between Nef and CD4. While a decrease in total CD4 was observed in lysates of cells expressing Nef, the levels of p56lck were not significantly affected. Pulse-chase experiments further revealed a decrease in the half-life of CD4 in Nef-expressing cells. These results show that the decrease in surface CD4 expression induced by Nef is mediated at least in part by a decrease in the half-life of CD4 protein. These results also indicate that a large portion of p56lck is free of CD4 in T cells expressing Nef, which could have a significant effect on T-cell function.

Decreased CD8-p56lck activity in peripheral blood T-lymphocytes from patients with hereditary haemochromatosis

Hereditary haemochromatosis (HH) is an autosomal recessive disease linked to certain MHC class-I specificities. The disease is characterized by increased iron absorption and, in some patients, abnormally low numbers of CD8+ T cells in the periphery. We were interested in whether CD4- and CD8-associated p56lck kinase activities were altered in patients with HH. In a study of 18 patients with HH (with and without low numbers of CD8+ cells), the level of autophosphorylation of the CD8-associated p56lck as well as its phosphotransferase activity, as determined by phosphorylation of an exogenous substrate, was significantly reduced by two- to three-fold relative to a control population of 23 healthy blood donors (P < 6 x 10(-7). CD8-p56lck activity was decreased in 16 out of 18 patients (ranging from 1.5- to 10-fold decrease). By contrast, the level of CD4-p56lck activity did not show an overall decrease relative to controls. In addition to an occasional decrease in the amount of CD8-associated lck, HH patient-derived T cells showed a consistent decrease in the relative CD8-p56lck specific activity. Immunofluorescence staining showed further that the difference could not be accounted by a discrepancy in the expression of CD8 alpha alpha or CD8 alpha beta complexes or MHC class I molecules. Decreased CD8-p56lck activity was seen both in patients undergoing intensive phlebotomy treatment and in patients in maintenance therapy (i.e. patients who had reached normal levels of iron stores), indicating that this abnormality does not appear to be corrected by iron depletion. To our knowledge, this is the first demonstration of an abnormality in a src-like receptor associated kinase in a human disease state linked to MHC class-I antigens.

Oncogenic activation of the Lck protein accompanies translocation of the LCK gene in the human HSB2 T-cell leukemia

The tyrosine protein kinase p56lck transduces signals important for antigen-induced T-cell activation. In transgenic mice, p56lck is oncogenic when overexpressed or expressed as a mutant, catalytically activated enzyme. In humans, the LCK gene is located at the breakpoint of the t(1;7)(p34;q34) chromosomal translocation. This translocation positions the beta T-cell receptor constant region enhancer upstream of the LCK gene without interrupting the LCK coding sequences, and a translocation of this sort occurs in both the HSB2 and the SUP-T-12 T-cell lines. We have found that, although the level of the p56lck protein in HSB2 cells is elevated approximately 2-fold in comparison with that in normal T-cell lines, total cellular tyrosine protein phosphorylation is elevated approximately 10-fold. Increased levels of phosphotyrosine in HSB2 cells resulted from mutations in the LCK gene that activated its function as a phosphotransferase and converted it into a dominant transforming oncogene. The oncogenic p56lck in HSB2 cells contained one amino acid substitution within the CD4/CD8-binding domain, two substitutions in the kinase domain, and an insertion of Gln-Lys-Pro (QKP) between the SH2 and kinase domains. In NIH 3T3 fibroblasts, three of these mutations cooperated to produce the fully oncogenic form of this p56lck variant. These results suggest that mutation of LCK may contribute to some human T-cell leukemias.

Human CD4 restores normal T cell development and function in mice deficient in murine CD4

The ability of a human coreceptor to function in mice was investigated by generating human CD4 (hCD4)-expressing transgenic mice on a mouse CD4-deficient (mCD4-/-) background. From developing thymocyte to matured T lymphocyte functions, hCD4 was shown to be physiologically active. By examining the expansion and deletion of specific V beta T cell families in mutated mice with and without hCD4, it was found that hCD4 can participate in positive and negative selection. Mature hCD4 single positive cells also were found in the periphery and they were shown to restore MHC class II-restricted alloreactive and antigen-specific T cell responses that were deficient in the mCD4 (-/-) mice. In addition, these hCD4 reconstituted mice can generate a secondary immunoglobulin G humoral response matching that of mCD4 wild-type mice. The fact that human CD4 is functional in mice and can be studied in the absence of murine CD4 should facilitate studies of human CD4 activity in general and human immunodeficiency virus 1 gp120-mediated pathogenesis in acquired immune deficiency syndrome specifically.

Distinct roles for CD4 and CD8 as co-receptors in T cell receptor signalling

We demonstrate that CD4 and CD8 modify signals induced through the T cell receptor for antigen (TCR alpha beta) in distinct fashions. Pretreatment of CD4+ lymph node T cells with CD4-specific monoclonal antibody results in a tenfold inhibition of DNA synthesis induced by anti-TCR alpha beta. In contrast, pretreatment of CD8+ T cells with CD8-specific mAb has no effect on DNA synthesis subsequently induced through TCR alpha beta. While inhibiting late activation signals, pretreatment with anti-CD4 does not detectably alter the pattern of anti-TCR alpha beta-induced tyrosine phosphorylation of cellular proteins, nor subsequent Ca2+ mobilization. The distinct biological consequences of anti-CD4 and anti-CD8 pretreatment correlate with the differential association of their respective ligands with the cellular protein tyrosine kinase, p56lck. While both T cell lineages contain similar levels of cellular p56lck, tenfold more is associated with CD4 than with CD8. This difference is associated with the differential effects of pretreatment with anti-CD4 and anti-CD8 on the distribution and activity of p56lck. Further, antibody-mediated aggregation of TCR alpha beta on CD4+ T cells induces the appearance of a p56lck species with decreased mobility in sodium dodecylsulfate-polyacrylamide gel electrophoresis. This effect is observed in CD4+ T cells exclusively and involves the fraction of p56lck which is not associated with CD4. The results presented here demonstrate that the signalling elements which couple the antigen receptor to second messenger-generating systems are under distinct physical and/or functional constraints in the two T cell lineages.

Oncogenic activation of the Lck protein accompanies translocation of the LCK gene in the human HSB2 T-cell leukemia

The tyrosine protein kinase p56lck transduces signals important for antigen-induced T-cell activation. In transgenic mice, p56lck is oncogenic when overexpressed or expressed as a mutant, catalytically activated enzyme. In humans, the LCK gene is located at the breakpoint of the t(1;7)(p34;q34) chromosomal translocation. This translocation positions the beta T-cell receptor constant region enhancer upstream of the LCK gene without interrupting the LCK coding sequences, and a translocation of this sort occurs in both the HSB2 and the SUP-T-12 T-cell lines. We have found that, although the level of the p56lck protein in HSB2 cells is elevated approximately 2-fold in comparison with that in normal T-cell lines, total cellular tyrosine protein phosphorylation is elevated approximately 10-fold. Increased levels of phosphotyrosine in HSB2 cells resulted from mutations in the LCK gene that activated its function as a phosphotransferase and converted it into a dominant transforming oncogene. The oncogenic p56lck in HSB2 cells contained one amino acid substitution within the CD4/CD8-binding domain, two substitutions in the kinase domain, and an insertion of Gln-Lys-Pro (QKP) between the SH2 and kinase domains. In NIH 3T3 fibroblasts, three of these mutations cooperated to produce the fully oncogenic form of this p56lck variant. These results suggest that mutation of LCK may contribute to some human T-cell leukemias.

p56lck interacts via its src homology 2 domain with the ZAP-70 kinase

p56lck, a member of the src family of protein tyrosine kinases, is an essential component in T cell receptor (TCR) signal transduction. p56lck contains a src homology 2 (SH2) domain found in a number of proteins involved in intracellular signaling. SH2 domains have been implicated in protein-protein interactions by binding to sequences in target proteins containing phosphorylated tyrosine. Using an in vitro assay, we have studied specific binding of tyrosine-phosphorylated proteins to a recombinant p56lck SH2 domain. In nonactivated Jurkat cells, two tyrosine-phosphorylated proteins were detected. Stimulation with anti-CD3 monoclonal antibodies induced the binding of seven additional tyrosine-phosphorylated proteins to the SH2 domain of p56lck. We have identified the zeta-associated tyrosine kinase, ZAP-70, as one of these proteins. Evidence suggests that binding of ZAP-70 to p56lck SH2 is direct and not mediated by zeta. The significance of this interaction was further investigated in vivo. p56lck could be coprecipitated with the zeta/ZAP-70 complex and conversely, ZAP-70 was detected in p56lck immunoprecipitates of activated Jurkat cells. The physical association of p56lck and ZAP-70 during activation supports the recently proposed functional cooperation of these two tyrosine kinases in TCR signaling.

A subset of CD4+ thymocytes selected by MHC class I molecules

To complete their maturation, most immature thymocytes depend on the simultaneous engagement of their antigen receptor [alpha beta T cell receptor (TCR)] and their CD4 or CD8 coreceptors with major histocompatibility complex class II or I ligands, respectively. However, a normal subset of mature alpha beta TCR+ thymocytes did not follow these rules. These thymocytes expressed NK1.1 and a restricted set of alpha beta TCRs that are intrinsically class I-reactive because their positive selection was class I-dependent but CD8-independent. These cells were CD4+ and CD4-8- but never CD8+, because the presence of CD8 caused negative selection. Thus, neither CD4 nor CD8 contributes signals that direct their maturation into the CD4+ and CD4-8- lineages.

Nef induces CD4 endocytosis: requirement for a critical dileucine motif in the membrane-proximal CD4 cytoplasmic domain

CD4 is crucial for antigen-driven helper T cell signaling and is used as receptor by the human immunodeficiency virus (HIV). The HIV early protein Nef causes a loss of CD4 from cell surfaces through a previously undefined posttranscriptional mechanism. Here, we demonstrate that Nef acts by inducing CD4 endocytosis, resulting in its degradation in lysosomes. CD4 down-regulation is strongly enhanced by the association of Nef with cell membranes through myristoylation. The study of chimeric molecules reveals that 20 membrane-proximal residues of the CD4 cytoplasmic domain are sufficient to confer Nef sensitivity. Within this region, a dileucine motif, reminiscent of an endocytosis and lysosomal targeting signal found in the CD3 gamma and delta chains, is crucial for CD4 response to Nef.

Defective T cell receptor signaling and CD8+ thymic selection in humans lacking zap-70 kinase

We have previously described a type of selective T cell deficiency (STD) characterized by persistent infections reminiscent of severe combined immunodeficiency. We show here that STD patients carry a mutation of zap-70, resulting in loss of the activity of this kinase. The thymi of zap-70-/- patients show the presence of CD4+CD8+ cells in the cortex; however, only CD4, not CD8, single-positive cells are present in the medulla. Peripheral CD4+ T cells from the zap-70-/- patients exhibit markedly reduced tyrosine phosphorylation, fail to produce interleukin-2, and do not proliferate in response to T cell receptor stimulation by mitogens or antigens. Thus, Zap-70 kinase appears to be indispensable for the development of CD8 single-positive T cells as well as for signal transduction and function of single-positive CD4 T cells.

Sequential interactions of the TCR with two distinct cytoplasmic tyrosine kinases

The T cell antigen receptor (TCR) initiates signals by interacting with cytoplasmic protein tyrosine kinases (PTKs) through a 17-residue sequence motif [called the antigen recognition activation motif (ARAM)] that is contained in the TCR zeta and CD3 chains. TCR stimulation induces the tyrosine phosphorylation of several cellular substrates, including the ARAMs. Lck kinase activity is required for phosphorylation of two conserved tyrosine residues in an ARAM. This phosphorylation leads to the recruitment of a second cytoplasmic PTK, ZAP-70, through both of the ZAP-70 Src homology 2 domains and its phosphorylation. Thus, TCR signal transduction is initiated by the sequential interaction of two PTKs with TCR ARAMs.

Tyrosine phosphorylation of CD45 phosphotyrosine phosphatase by p50csk kinase creates a binding site for p56lck tyrosine kinase and activates the phosphatase

Src family protein tyrosine kinases (PTKs) play an essential role in antigen receptor-initiated lymphocyte activation. Their activity is largely regulated by a negative regulatory tyrosine which is a substrate for the activating action of the CD45 phosphotyrosine phosphatase (PTPase) or, conversely, the suppressing action of the cytosolic p50csk PTK. Here we report that CD45 was phosphorylated by p50csk on two tyrosine residues, one of them identified as Tyr-1193. This residue was not phosphorylated by T-cell PTKs p56lck and p59fyn. Tyr-1193 was phosphorylated in intact T cells, and phosphorylation increased upon treatment with PTPase inhibitors, indicating that this tyrosine is a target for a constitutively active PTK. Cotransfection of CD45 and csk into COS-1 cells caused tyrosine phosphorylation of CD45 in the intact cells. Tyrosine-phosphorylated CD45 bound p56lck through the SH2 domain of the kinase. Finally, p50csk-mediated phosphorylation of CD45 caused a severalfold increase in its PTPase activity. Our results show that direct tyrosine phosphorylation of CD45 can affect its activity and association with Src family PTKs and that this phosphorylation could be mediated by p50csk. If this is also true in the intact cells, it adds a new dimension to the physiological function of p50csk in T lymphocytes.

Evidence for differential intracellular signaling via CD4 and CD8 molecules

Although both the CD4 and CD8 molecules enhance antigen responsiveness mediated by the T cell receptor (TCR), it is not known whether CD4 and CD8 initiate similar or different intracellular signals when they act as coreceptors. To characterize the early signals transmitted by CD4 and CD8, both CD4 and CD8 alpha were expressed in the same murine T cell hybridoma. In the double positive transfectants, CD4 and CD8 associated with equal amounts of p56lck (Lck), and both molecules enhanced interleukin 2 (IL-2) production equivalently when cross-linked with suboptimal levels of anti-TCR antibody. However, in an in vitro kinase assay, cross-linking CD4 initiated fourfold greater kinase activity compared with CD8 cross-linking. In the same assay, when CD4 or CD8 was cross-linked to the TCR, novel phosphorylated proteins were found associated with the TCR/CD4 complex but not with the TCR/CD8 complex. Consistent with this data, antiphosphotyrosine immunoblotting revealed greater tyrosine phosphorylation of intracellular substrates after TCR/CD4 cross-linking compared with TCR/CD8 cross-linking. Additionally, a specific protein kinase C inhibitor (RO318220) inhibited CD8-mediated enhancement of IL-2 production far more effectively than CD4-mediated enhancement. Thus, it appears that CD8 alpha may depend more on a protein kinase C-mediated signaling pathway, whereas CD4 may rely on greater tyrosine kinase activation. Such differential signaling via CD4 and CD8 has implications for thymic ontogeny and T cell activation.

Tyrosine phosphorylation of CD45 phosphotyrosine phosphatase by p50csk kinase creates a binding site for p56lck tyrosine kinase and activates the phosphatase

Src family protein tyrosine kinases (PTKs) play an essential role in antigen receptor-initiated lymphocyte activation. Their activity is largely regulated by a negative regulatory tyrosine which is a substrate for the activating action of the CD45 phosphotyrosine phosphatase (PTPase) or, conversely, the suppressing action of the cytosolic p50csk PTK. Here we report that CD45 was phosphorylated by p50csk on two tyrosine residues, one of them identified as Tyr-1193. This residue was not phosphorylated by T-cell PTKs p56lck and p59fyn. Tyr-1193 was phosphorylated in intact T cells, and phosphorylation increased upon treatment with PTPase inhibitors, indicating that this tyrosine is a target for a constitutively active PTK. Cotransfection of CD45 and csk into COS-1 cells caused tyrosine phosphorylation of CD45 in the intact cells. Tyrosine-phosphorylated CD45 bound p56lck through the SH2 domain of the kinase. Finally, p50csk-mediated phosphorylation of CD45 caused a severalfold increase in its PTPase activity. Our results show that direct tyrosine phosphorylation of CD45 can affect its activity and association with Src family PTKs and that this phosphorylation could be mediated by p50csk. If this is also true in the intact cells, it adds a new dimension to the physiological function of p50csk in T lymphocytes.

Signal transduction by lymphocyte antigen receptors

Despite the differences in the antigens that they recognize and in the effector functions they carry out, B and T lymphocytes utilize remarkably similar signal transduction components to initiate responses. They both use oligomeric receptors that contain distinct recognition and signal transduction subunits. Antigen receptors on both cells interact with at least two distinct families of PTKs via common sequence motifs, ARAMs, in the cytoplasmic tails of their invariant chains, which have likely evolved from a common evolutionary precursor. Coreceptors appear to serve to increase the sensitivity of both of these receptor systems through events that influence ligand binding and signal transduction. The critical role of tyrosine phosphorylation of downstream signaling components, such as phospholipase C, is the net result of changes in the balance of the action of antigen receptor-regulated PTKs and PTPases. The identification of downstream effectors, including calcineurin and Ras, that regulate cellular responses, such as lymphokine gene expression, promises the future possibility of connecting the complex pathway from the plasma membrane to the nucleus in lymphocytes. Insight gained from studies of the signaling pathways downstream of TCR and BCR stimulation is likely to contribute significantly to future understanding of mechanisms responsible for lymphocyte differentiation and for the discrimination of self from nonself in developing and mature cells.

The SH3 domain of p56lck is involved in binding to phosphatidylinositol 3'-kinase from T lymphocytes

Many of the Src-like tyrosine kinases are thought to participate in multiprotein complexes that modulate transmembrane signalling through tyrosine phosphorylation. We have used in vitro binding studies employing bacterially expressed glutathione S-transferase-p56lck fusion proteins and cell extracts to map regions on p56lck that are involved in binding to phosphatidylinositol 3'-kinase (PI3K). Deletions within the SH3 domain of p56lck abolished binding of PI3K activity from T-cell lysates, whereas deletion of the SH2 domain caused only a slight reduction in the level of PI3K activity bound to p56lck sequences. The binding of PI3K from T-cell extracts to p56lck was not blocked by antiphosphotyrosine antibodies, but p56lck-bound PI3K activity was sensitive to phosphatase treatment. The SH3 domain of p56lck also bound the majority of PI3K activity from uninfected chicken embryo fibroblasts. However, a drastically different binding specificity was observed with use of extracts of Rous sarcoma virus v-src-transformed cells, in which the majority of PI3K activity bound to the SH2 domain of p56lck in a phosphotyrosine-dependent manner. These results suggest that are two modes of PI3K binding to p56lck, and presumably to other Src-like tyrosine kinases. In one mode, PI3K from T cells or uninfected chicken embryo fibroblasts binds predominantly to the SH3 domain of p56lck. In the other mode, involving PI3K from Rous sarcoma virus-transformed cells, binding is largely phosphotyrosine dependent and requires the SH2 domain of p56lck.

Phosphatidylinositol (PI) 3-kinase and PI 4-kinase binding to the CD4-p56lck complex: the p56lck SH3 domain binds to PI 3-kinase but not PI 4-kinase

CD4 serves as a receptor for major histocompatibility complex class II antigens and as a receptor for the human immunodeficiency virus type 1 (HIV-1) viral coat protein gp120. It is coupled to the protein-tyrosine kinase p56lck, an interaction necessary for an optimal response of certain T cells to antigen. In addition to the protein-tyrosine kinase domain, p56lck possesses Src homology 2 and 3 (SH2 and SH3) domains as well as a unique N-terminal region. The mechanism by which p56lck generates intracellular signals is unclear, although it has the potential to interact with various downstream molecules. One such downstream target is the lipid kinase phosphatidylinositol 3-kinase (PI 3-kinase), which has been found to bind to activated pp60src and receptor-tyrosine kinases. In this study, we verified that PI 3-kinase associates with the CD4:p56lck complex as judged by the presence of PI 3-phosphate generated from anti-CD4 immunoprecipitates and detected by high-pressure liquid chromatographic analysis. However, surprisingly, CD4-p56lck was also found to associate with another lipid kinase, phosphatidylinositol 4-kinase (PI 4-kinase). The level of associated PI 4-kinase was generally higher than PI 3-kinase activity. HIV-1 gp120 and antibody-mediated cross-linking induced a 5- to 10-fold increase in the level of CD4-associated PI 4- and PI 3-kinases. The use of glutathione S-transferase fusion proteins carrying Lck-SH2, Lck-SH3, and Lck-SH2/SH3 domains showed PI 3-kinase binding to the SH3 domain of p56lck, an interaction facilitated by the presence of an adjacent SH2 domain. PI 4-kinase bound to neither the SH2 nor the SH3 domain of p56lck. CD4-p56lck contributes PI 3- and PI 4-kinase to the activation process of T cells and may play a role in HIV-1-induced immune defects.

Induction of tyrosine phosphorylation and T-cell activation by vanadate peroxide, an inhibitor of protein tyrosine phosphatases

Rapid tyrosine phosphorylation of key cellular proteins is a crucial event in the transduction of activation signals to T-lymphocytes. The regulatory role of protein tyrosine phosphatases (PTPases) in this process was explored by studying the effects of a powerful PTPase inhibitor, vanadate peroxide (pervanadate), on the activation cascade of Jurkat human leukaemic T-cells. Pervanadate induced activation of the tyrosine kinases lck and fyn (4- and 3-fold respectively) and a dramatic increase in tyrosine phosphorylation of cellular proteins, notably phospholipase C gamma 1. After this event, we observed a rise in intracellular Ca2+ concentration, corresponding to an influx. This effect required surface expression of the CD45 PTPase and was not observed in CD45-deficient variants of Jurkat cells. In the CD45-negative variant, the effect of pervanadate on tyrosine phosphorylation was globally decreased and some phosphorylated substrates were specifically missing. Pervanadate also stimulated transcription of the c-fos gene and accumulation of its mRNA as well as several other hallmarks of T-lymphocyte activation such as surface expression of the CD69 antigen and the interleukin 2 receptor alpha-chain (CD25). Pervanadate synergized with signals delivered by T-cell antigen receptor engagement or by a phorbol ester to induce interleukin 2 production. Pervanadate activated NF-kappa B, as shown by an increase in DNA-binding activity of this transcription factor. We thus conclude that PTPases play a crucial role in the negative regulation of signal transduction culminating in T-lymphocyte activation. Moreover, induction of tyrosine phosphorylation appears sufficient per se to initiate a complete activation programme.

Early expression of human CD4 delays thymic differentiation in transgenic mice

CD4 is a cell surface molecule expressed mostly on cells of the T-cell lineage. Studies have shown that this molecule plays an important role in positive and negative selection of T cells in the thymus. It is not surprising therefore, that in T-cell ontogeny, CD4 starts to be expressed on thymocyte subpopulations about to undergo these selection processes. The human CD4 molecule was expressed in mouse thymus ontogeny using a promoter, MMTVD, which targets expression as early as day 14 of ontogeny, prior to expression of endogenous TCR, CD4 and CD8. Thymic ontogeny is delayed in foetal MMTVD-CD4 mice. Human CD4-expressing thymuses show a twofold reduction in cellularity at days 17 and 18 of ontogeny compared with non-transgenic control littermate thymuses, and paradoxically, MMTVD-CD4 thymuses contain more cells in the S and G2/M stages of the cell cycle than control thymuses do. At the cell surface marker level, MMTVD-CD4 thymocytes show a delay in surface expression of CD3, murine CD4 and murine CD8, along with persistent expression of IL2R alpha compared with foetal non-transgenic littermates. Biochemical studies show that, although MMTVD-CD4 thymocytes do not express surface CD3, cytoplasmic CD3 epsilon proteins as well as TCR beta incomplete and complete transcripts are present in foetal day-17 thymocytes. Low levels of surface CD3/TCR expression, however, could partly be due to the low levels of zeta mRNA and proteins detected in these cells. These results suggest that CD4 is not expressed until a certain stage of differentiation not only because it is not yet required for selection processes, but because it can lead to a reversible deregulation of thymocyte development.

Human immunodeficiency virus type 1-associated CD4 downmodulation

This chapter discusses human immunodeficiency virus type 1 (HIV-1) associated with CD4 downmodulation. It also discusses the structure and function of CD4 and p56^lck and factors involved in hiv-1-associated cd4 downmodulation. There are, at present, at least three HIV-1 gene products known to be involved in cell surface CD4 downmodulation. These are Nef, Vpu, and gp160. Whereas Nef is expressed during the early phase of HIV-1 gene expression, both Vpu and gp160, which appear to act coordinately, are expressed during the late phase. This functional convergence of HIV-1 proteins on cell surface CD4 downmodulation, whether specific or nonspecific in activity, suggests that this event is of critical importance in the life cycle of HIV-1. Further elucidation of the mechanisms that underlie CD4 cell surface downmodulation may lead to the development of novel strategies aimed at preventing such events, and potentially to the development of new therapeutic approaches.

Tyrosine kinase activity of CD4-associated p56lck may not be required for CD4-dependent T-cell activation

The lymphoid-specific tyrosine kinase p56lck (Lck) is critical for the development and activation of T lymphocytes, and Lck kinase activity has been implicated in both T-cell antigen receptor/CD3- and CD4-mediated signaling. CD4-dependent T-cell activation has been demonstrated to be dependent upon the association of CD4 with Lck. To examine the role of the kinase activity of Lck in CD4-dependent T-cell activation, we have generated several kinase-deficient mutants of Lck. When transfected into CD4+ murine T-cell hybridoma cells, these mutants cause approximately 90% diminution in CD4-associated Lck kinase activity. Specifically, upon CD4 crosslinking there is decreased Lck autophosphorylation and decreased phosphorylation of an exogenous substrate. When CD4 is crosslinked to the T-cell antigen receptor-CD3 complex, decreased phosphorylation of associated substrates is also observed. In spite of this striking inhibition of Lck kinase function, cells expressing the kinase-deficient mutants demonstrate normal or enhanced CD4-dependent antigen responsiveness. These data demonstrate that the level of Lck kinase activity does not correlate with its CD4-associated function and suggest that the kinase activity of Lck may not be required for CD4-mediated signaling.

T cell receptor zeta/CD3-p59fyn(T)-associated p120/130 binds to the SH2 domain of p59fyn(T)

Intracellular signaling from the T cell receptor (TCR)zeta/CD3 complex is likely to be mediated by associated protein tyrosine kinases such as p59fyn(T), ZAP-70, and the CD4:p56lck and CD8:p56lck coreceptors. The nature of the signaling cascade initiated by these kinases, their specificities, and downstream targets remain to be elucidated. The TCR-zeta/CD3:p59fyn(T) complex has previously been noted to coprecipitate a 120/130-kD doublet (p120/130). This intracellular protein of unknown identity associates directly with p59fyn(T) within the receptor complex. In this study, we have shown that this interaction with p120/130 is specifically mediated by the SH2 domain (not the fyn-SH3 domain) of p59fyn(T). Further, based on the results of in vitro kinase assays, p120/130 appears to be preferentially associated with p59fyn(T) in T cells, and not with p56lck. Antibody reprecipitation studies identified p120/130 as a previously described 130-kD substrate of pp60v-src whose function and structure is unknown. TCR-zeta/CD3 induced activation of T cells augmented the tyrosine phosphorylation of p120/130 in vivo as detected by antibody and GST:fyn-SH2 fusion proteins. p120/130 represents the first identified p59fyn(T):SH2 binding substrate in T cells, and as such is likely to play a key role in the early events of T cell activation.

Phosphatidylinositol (PI) 3-kinase and PI 4-kinase binding to the CD4-p56lck complex: the p56lck SH3 domain binds to PI 3-kinase but not PI 4-kinase

CD4 serves as a receptor for major histocompatibility complex class II antigens and as a receptor for the human immunodeficiency virus type 1 (HIV-1) viral coat protein gp120. It is coupled to the protein-tyrosine kinase p56lck, an interaction necessary for an optimal response of certain T cells to antigen. In addition to the protein-tyrosine kinase domain, p56lck possesses Src homology 2 and 3 (SH2 and SH3) domains as well as a unique N-terminal region. The mechanism by which p56lck generates intracellular signals is unclear, although it has the potential to interact with various downstream molecules. One such downstream target is the lipid kinase phosphatidylinositol 3-kinase (PI 3-kinase), which has been found to bind to activated pp60src and receptor-tyrosine kinases. In this study, we verified that PI 3-kinase associates with the CD4:p56lck complex as judged by the presence of PI 3-phosphate generated from anti-CD4 immunoprecipitates and detected by high-pressure liquid chromatographic analysis. However, surprisingly, CD4-p56lck was also found to associate with another lipid kinase, phosphatidylinositol 4-kinase (PI 4-kinase). The level of associated PI 4-kinase was generally higher than PI 3-kinase activity. HIV-1 gp120 and antibody-mediated cross-linking induced a 5- to 10-fold increase in the level of CD4-associated PI 4- and PI 3-kinases. The use of glutathione S-transferase fusion proteins carrying Lck-SH2, Lck-SH3, and Lck-SH2/SH3 domains showed PI 3-kinase binding to the SH3 domain of p56lck, an interaction facilitated by the presence of an adjacent SH2 domain. PI 4-kinase bound to neither the SH2 nor the SH3 domain of p56lck. CD4-p56lck contributes PI 3- and PI 4-kinase to the activation process of T cells and may play a role in HIV-1-induced immune defects.

The SH3 domain of p56lck is involved in binding to phosphatidylinositol 3'-kinase from T lymphocytes

Many of the Src-like tyrosine kinases are thought to participate in multiprotein complexes that modulate transmembrane signalling through tyrosine phosphorylation. We have used in vitro binding studies employing bacterially expressed glutathione S-transferase-p56lck fusion proteins and cell extracts to map regions on p56lck that are involved in binding to phosphatidylinositol 3'-kinase (PI3K). Deletions within the SH3 domain of p56lck abolished binding of PI3K activity from T-cell lysates, whereas deletion of the SH2 domain caused only a slight reduction in the level of PI3K activity bound to p56lck sequences. The binding of PI3K from T-cell extracts to p56lck was not blocked by antiphosphotyrosine antibodies, but p56lck-bound PI3K activity was sensitive to phosphatase treatment. The SH3 domain of p56lck also bound the majority of PI3K activity from uninfected chicken embryo fibroblasts. However, a drastically different binding specificity was observed with use of extracts of Rous sarcoma virus v-src-transformed cells, in which the majority of PI3K activity bound to the SH2 domain of p56lck in a phosphotyrosine-dependent manner. These results suggest that are two modes of PI3K binding to p56lck, and presumably to other Src-like tyrosine kinases. In one mode, PI3K from T cells or uninfected chicken embryo fibroblasts binds predominantly to the SH3 domain of p56lck. In the other mode, involving PI3K from Rous sarcoma virus-transformed cells, binding is largely phosphotyrosine dependent and requires the SH2 domain of p56lck.

Analysis of tetanus toxin peptide/DR recognition by human T cell receptors reconstituted into a murine T cell hybridoma

We have previously reported that human T cell receptors (TcR) selected in the class II-restricted (HLA-DRB1*1302) response to a tetanus toxin peptide (tt830-843) frequently used the V beta 2 germ-line segment which paired with several V alpha segments and that the putative CDR3 of both alpha and beta chains showed remarkable heterogeneity. To analyze the structural basis for recognition of the tt830-843/DR complex, five of these TcR were reconstituted into a murine T cell hybridoma, 58 alpha- beta-, by expressing the human alpha and beta variable regions joined to the mouse alpha and beta constant regions, respectively. The chimeric TcR, expressing the same V beta germ-line segment (V beta 2), two expressing V alpha 21.1, two V alpha 17.1 and one V alpha 8.1 were shown to have the expected antigen specificity and DR restriction. Two lines of evidence suggested that the putative CDR3, although not conserved in these TcR, played a key role in recognition. First, two TcR with identical V germ-line segments but distinct CDR3 showed large difference in their capacity to react with the ligand. Second, interchanging the alpha and beta chains from tt830-843/DR1302-specific TcR which differed in their CDR3 sequences invariably led to loss of recognition. We also asked whether germ-line V alpha 17.1 could functionally replace V alpha 21.1, as they appear to be related in their primary sequence. However, as in the case of CDR3 exchanges, V alpha replacement abrogated TcR reactivity. Taken together, these data underline the fine interdependence of the structural components of the TcR binding site in defining a given specificity. Four of the TcR studied displaying promiscuous recognition were also tested against different DR alleles and site-directed mutants. The results of these experiments suggested that, in spite of their structural heterogeneity, anti-tt830-843 TcR may have a similar orientation with respect to the peptide/DR complex. The reconstitution system described herein should represent a valuable tool for detailed studies of human TcR specificity.

Regulation of T cell receptor expression in immature CD4+CD8+ thymocytes by p56lck tyrosine kinase: basis for differential signaling by CD4 and CD8 in immature thymocytes expressing both coreceptor molecules

Signals transduced through the T cell antigen receptor (TCR) are modulated by the src family tyrosine kinase p56lck (lck), which associates in mature T cells with the coreceptor molecules CD4 and CD8. Here we describe a novel function of lck in immature CD4+CD8+ thymocytes, that of regulating TCR expression. Activation of lck in immature CD4+CD8+ thymocytes by intrathymic engagement of CD4 maintains low TCR expression by causing most TCR components to be retained and degraded within the endoplasmic reticulum. Importantly, activation of lck in immature CD4+CD8+ thymocytes results from engagement of surface CD4 molecules, but not surface CD8 molecules, despite the nearly fourfold greater surface expression of CD8 than CD4. The competence of CD4 to activate lck in CD4+CD8+ thymocytes relates to the fact that a relatively large fraction of surface CD4 molecules (25-50%) are associated with intracellular lck molecules, whereas only 2% of surface CD8 molecules are associated with lck. The amount of lck associated with CD4 in CD4+CD8+ thymocytes is diminished by chronic CD4 engagement in the thymus, as activated lck molecules subsequently dissociate from CD4. Indeed, the amount of lck associated with CD4 in CD4+CD8+ thymocytes is markedly increased in major histocompatibility complex (MHC) class II- mice that lack the intrathymic ligand for CD4 and in which surface CD4 molecules are consequently not engaged. Thus, the present study demonstrates that (a) activation of lck in CD4+CD8+ thymocytes regulates distribution and expression of TCR components; (b) unlike CD4 molecules, CD8 molecules on CD4+CD8+ thymocytes cannot efficiently activate lck despite their significantly greater surface expression; and (c) the amount of lck associated with CD4 in the CD4+CD8+ thymocytes is inversely related to the extent of CD4 engagement by MHC class II molecules in the thymus.

Selective coupling of the T cell antigen receptor to phosphoinositide-derived diacylglycerol production in HPB-ALL T cells correlates with CD45-regulated p59fyn activity

In HPB-ALL T-cells the p59fyn tyrosine kinase is regulated by the CD45 phosphotyrosine phosphatase and plays a critical role in coupling the T cell receptor (TCR) to the generation of intracellular signals which include diacylglycerol (DAG) production and protein kinase C activation. The aim of this study was to determine the phospholipid pools from which the DAG is generated and to identify which phospholipase activities are regulated by the TCR. When CD45+ cells were pre-labeled with [3H]arachidonic acid, CD3-antigen cross-linking stimulated negligible increases in both [3H]DAG and [3H]phosphatidic acid (PA). However, CD3 monoclonal antibody (mAb) induced an increase of 300% in [3H]PA when the cells were permeabilized with streptolysin-O, and this correlated with increased levels of protein tyrosine phosphorylation. Stimulation of [3H]PA production upon CD3 cross-linking was 77% lower in permeabilized CD45- cells than in CD45+ cells, consistent with the reduced activity of p59fyn in CD45- cells. The stimulated production of PA was not mediated by activation of phospholipase D (PLD), although the presence of a G-protein-regulated PLD activity was established. The CD3-induced increase in total inositol phosphates (InsP) in permeabilized cells was similar to the stimulated production of [3H]PA production in both CD45+ and CD45- cells. Dose-response curves for InsP and PA production triggered by CD3 mAb were super-imposable and the production of InsP and PA over a range of Ca2+ concentrations was comparable. Differential labeling of phospholipids with 3H-labeled fatty acids revealed that CD3-induced PA production reflected incorporation of label into the phosphatidylinositol pool. Our data suggest that in HPB-ALL cells the production of DAG following CD3-antigen cross-linking can be fully accounted for by the selective coupling of the TCR to breakdown of phosphatidylinositol-(4,5)-bisphosphate as the result of phospholipase C gamma 1 activation. This event correlates with the activity of the CD45-regulated TCR-associated tyrosine kinase, p59fyn.

Regulation of CD4-p56lck-associated phosphatidylinositol 3-kinase (PI 3-kinase) and phosphatidylinositol 4-kinase (PI 4-kinase)

CD4 serves as a receptor for MHC class II antigens and as a receptor for the human immunodeficiency virus (HIV-1) viral coat protein gp120. It is coupled to the protein-tyrosine kinase p56lck, an interaction necessary for an optimal response of certain T cells to antigen. Although anti-CD4 crosslinking may increase lck activity, the effects of HIV-1 gp120 have been controversial. Activated protein-tyrosine kinases are known to associate with certain intracellular proteins possessing src-homology regions (SH-2 domains) such as phosphatidylinositol 3-kinase (PI 3-kinase). In this paper, we demonstrate that the CD4:p56lck complex associates with significant amounts of phosphatidylinositol (PI) kinase activity. High pressure liquid chromatographic (HPLC) analysis of the reaction products demonstrated the presence of phosphatidylinositol 3-phosphate (PI 3-P) and phosphatidylinositol 4-phosphate (PI 4-P), thus indicating that PI 3 and PI 4 kinases associate with CD4-p56lck. The p85 subunit of PI 3-kinase was also detected in anti-CD4 immunoprecipitates by immunoblotting with anti-p85 antiserum. Significantly, p56lck binding to CD4 appears to be necessary for the detection of lipid kinase activity associated with p56lck. Also, anti-HIV gp120 and anti-CD4 crosslinking induced a 10-15-fold increase in levels of both PI 3- and PI 4-kinase activity in anti-CD4 precipitates. Stimulation of CD4-p56lck-linked PI kinases by crosslinked HIV-1 gp120 may play a role in HIV-1-induced immune defects.

CD4 function in thymocyte differentiation and T cell activation

The ectodomains of the T cell surface glycoproteins CD4 and CD8 bind to membrane-proximal domains of MHC class II and class I molecules, respectively, while both cytoplasmic domains interact with the protein tyrosine kinase (PTK) p56lck (lck) through a shared cysteine-containing motif. Function of CD4 and CD8 requires their binding to the same MHC molecule as that recognized by the T cell antigen receptor (TCR). In vitro studies indicate that CD4-associated lck functions even in the absence of kinase activity. In vivo experiments show that, whereas helper T cell development is impaired in CD4-deficient mice, high level expression of a transgenic CD4 that cannot bind lck rescues development of this T cell subset. These studies suggest that CD4 is an adhesion molecule whose localization is regulated through protein-protein interactions of the associated PTK and whose function is to increase the stability of the TCR signalling complex by binding to the relevant MHC. The function of CD4 in development has been further studied in the context of how double positive (CD4+CD8+) thymocytes mature into either CD4+ T cells with helper function and TCR specificity for class II or into CD8+ T cells with cytotoxic function and specificity for class I. Studies using CD4-transgenic mice indicate that development of single positive T cells involves stochastic downregulation of either CD4 or CD8, coupled to activation of a cytotoxic or helper program, respectively, and subsequent selection based on the ability of the TCR and remaining co-receptor to engage the same MHC molecule.

Protein tyrosine kinase p56lck controls allelic exclusion of T-cell receptor beta-chain genes

During T-cell development, site-specific DNA rearrangements mediating assembly of beta- and alpha-chain genes of the T-cell receptor (TCR) are developmentally ordered. In particular, assembly and expression of a complete beta-chain gene blocks further rearrangements at the beta-locus (a process referred to as allelic exclusion) and drives the generation and expansion of CD4+8+ cells. Although the mechanism used by TCR beta chains to deliver such signals is unknown, studies in transgenic animals have suggested that the lymphocyte-specific protein tyrosine kinase p56lck may impinge on a similar signalling pathway. The hypothesis that TCR beta chains deliver intracellular signals via p56lck makes an explicit prediction: that interference with p56lck function will mitigate the effects of a simultaneously expressed TCR beta chain. Here we confirm this prediction through examination of allelic exclusion in mice expressing both a functional TCR beta chain transgene and a catalytically inactive form of p56lck.

B-cell antigen receptor motifs have redundant signalling capabilities and bind the tyrosine kinases PTK72, Lyn and Fyn

BACKGROUND

The 13 cell antigen receptor (BCR) is a multimeric protein complex consisting of an antigen recognition structure (membrane immunoglobulin) and two associated proteins, lg-alpha and Ig-beta It has been proposed that signalling through the BCR involves Ig-alpha and Ig-beta. Both of these proteins contain within their cytoplasmic domains an amino-acid motif that is present in a number of immune recognition receptors, including the BCR, T-cell antigen receptor and Fc receptor complexes. This motif, termed the antigen-receptor homology motif (ARH1), appears to have signal transduction ability.

RESULTS

We now show that the presence of cytoplasmic regions containing the ARM motif from either Ig-alpha or Ig-beta is sufficient to confer signalling capability on an otherwise non-functional fusion protein. Both Ig-alpha- and Ig-beta-containing chimeras induced, in an apparently redundant fashion, signalling events seen upon membrane immunoglobulin crosslinking, including tyrosine phosphorylation of particular proteins, phosphoinositicle breakdown and calcium mobilization. Furthermore, crosslinking of the chimeras resulted in tyrosine phosphorylation of the Ig-alpha and Tg-beta tails and their association with the tyrosine kinases PTK72, p53/56(lyn) and p59(fyn).

CONCLUSIONS

These observations indicate that Ig-alpha and Ig-beta are responsible for coupling membrane immunoglobulin to intracellular signalling components. Moreover, they demonstrate that a number of tyrosine kinases associate directly with the cytoplasmic domains of both Ig-alpha and Ig-beta. Stimulation of the chimeras, which results in tyrosine phosphorylation of the ig-alpha and Ig-beta tails, is a prerequisite for some of these associations. The implications of these findings for the mechanism by which the BCR initiates the signalling reactions are discussed.

A heterodimer of HEB and an E12-related protein interacts with the CD4 enhancer and regulates its activity in T-cell lines

A T-lymphocyte-specific enhancer located 13 kb upstream of the murine CD4 gene was recently shown to be required for the developmentally regulated expression of CD4. We have previously identified three nuclear protein binding sites in this enhancer; one of these sites, CD4-3, is essential for expression and contains two E-box core motifs (CANNTG) adjacent to each other in the sequence TAACAGGTGTCAGCTGGT. In electrophoretic mobility shift assays using the CD4-3 oligonucleotide as a probe, three nuclear protein complexes, termed CD4-3A, -B, and -C, were detected with nuclear extracts from T-cell lines. CD4-3A, which involves nuclear protein binding to the 5' E-box, was detected only with nuclear extracts from lymphoid cells. Specific antisera were used to show that the CD4-3A complex contains a heterodimer or heterooligomer of basic helix-loop-helix transcriptional factors, E12 or a related factor and HEB, which is expressed predominantly in thymus. Consistent with this finding, in vitro-translated E12 and HEB proteins, as homodimers or heterodimers, bound preferentially to the 5' E-box. Point mutations in the 5' E-box, but not in the 3' E-box, abolished CD4 enhancer activity. Furthermore, overexpression of Id, a protein that forms inactive heterodimers with E12/E47, blocked CD4 enhancer activity in T cells. These results suggest that a heterodimer composed of HEB and E12 or a closely related protein plays a critical role in CD4 enhancer function by interacting with the 5' E-box motif of the CD4-3 site in vivo.

A heterodimer of HEB and an E12-related protein interacts with the CD4 enhancer and regulates its activity in T-cell lines

A T-lymphocyte-specific enhancer located 13 kb upstream of the murine CD4 gene was recently shown to be required for the developmentally regulated expression of CD4. We have previously identified three nuclear protein binding sites in this enhancer; one of these sites, CD4-3, is essential for expression and contains two E-box core motifs (CANNTG) adjacent to each other in the sequence TAACAGGTGTCAGCTGGT. In electrophoretic mobility shift assays using the CD4-3 oligonucleotide as a probe, three nuclear protein complexes, termed CD4-3A, -B, and -C, were detected with nuclear extracts from T-cell lines. CD4-3A, which involves nuclear protein binding to the 5' E-box, was detected only with nuclear extracts from lymphoid cells. Specific antisera were used to show that the CD4-3A complex contains a heterodimer or heterooligomer of basic helix-loop-helix transcriptional factors, E12 or a related factor and HEB, which is expressed predominantly in thymus. Consistent with this finding, in vitro-translated E12 and HEB proteins, as homodimers or heterodimers, bound preferentially to the 5' E-box. Point mutations in the 5' E-box, but not in the 3' E-box, abolished CD4 enhancer activity. Furthermore, overexpression of Id, a protein that forms inactive heterodimers with E12/E47, blocked CD4 enhancer activity in T cells. These results suggest that a heterodimer composed of HEB and E12 or a closely related protein plays a critical role in CD4 enhancer function by interacting with the 5' E-box motif of the CD4-3 site in vivo.

A kinase-independent function of Lck in potentiating antigen-specific T cell activation

The lymphocyte-specific cytoplasmic protein-tyrosine kinase p56lck (Lck) is essential for T cell development and activation. Its association with the co-receptor molecules, CD4 and CD8, is required for potentiation of antigen-specific signals through the T cell antigen receptor. To study the mechanism of action of Lck, hybrid molecules consisting of the extracellular and transmembrane domains of CD4 fused to Lck or other Src family kinases were analyzed in an antigen-specific, CD4-dependent T cell hybridoma. Surprisingly, a chimera with a deletion of the Lck kinase domain was more active than the full-length protein. In contrast, point mutations in residues required for SH2 or kinase function resulted in moderately decreased activity, while a combination of these mutations rendered the chimera largely inactive. Different domains of CD4-associated Lck therefore have distinct functions that can independently contribute to T cell activation.

src-related protein tyrosine kinases and their surface receptors

The CD4-p56lck and CD8-p56lck complexes have served as a paradym for an expanding number of interactions between src-family members (p56lck, p59fyn, p56lyn, p55blk) and surface receptors. These interactions implicate src-related kinases in the regulation of a variety of intracellular events, from lymphokine production and cytotoxicity to the expression of specific nuclear binding proteins. Different molecular mechanisms appear to have evolved to facilitate the receptor-kinase interactions, including the use of N-terminal regions, SH2 regions and kinase domains. Variation exists in stoichiometry, affinity and the nature of signals generated by these complexes in cells. The CD4-p56lck complex differs from receptor-tyrosine kinases in a number of important ways, including mechanisms of kinase domain regulation and recruitment of substrates such as PI 3-kinase. Furthermore, they may have a special affinity for receptor-substrates such as the TcR zeta, MB1/B29 or CD5 receptors, and act to recruit other SH2-carrying proteins, such as ZAP-70 to the receptor complexes. Receptor-src kinase interactions represent the first step in a cascade of intracellular events within the protein-tyrosine kinase/phosphatase cascade.

Helper T-cell development in the absence of CD4-p56lck association

The CD4 and CD8 glycoproteins are expressed on helper and cytoxic T lymphocytes, respectively, and have important functions in the differentiation and activation of these cells. These molecules are thought to participate in signal transduction by binding to the same class II or class I major histocompatibility complex molecules that are engaged by the T-cell antigen receptor. The cytoplasmic domains of both CD4 and CD8 interact with the protein tyrosine kinase p56lck (refs 14-17), an essential participant in thymocyte maturation and T-cell activation. This interaction is required for effective in vitro responses to antigen, suggesting that signalling through p56lck is a major function of CD4 and CD8. Here we investigate the role of the CD4-p56lck interaction during T-lymphocyte development by expressing wild-type and truncated products of CD4 transgenes in mice that lack endogenous CD4 and hence have defective helper-cell development. We find that transgenic CD4, which cannot associate with p56lck, can nevertheless rescue the helper-cell lineage when overexpressed. This result indicates that the contribution of CD4 to lineage development need not involve signalling through p56lck, and provides insight into the general function of CD4 and CD8.

Nef from primary isolates of human immunodeficiency virus type 1 suppresses surface CD4 expression in human and mouse T cells

The human immunodeficiency virus type 1 (HIV-1) nef gene was originally described as a negative regulator of transcription from the viral long terminal repeat promoter. This observation has been disputed, and the function of Nef remains unclear. In vivo experiments have indicated that an intact nef gene is required for disease progression in macaques infected with simian immunodeficiency virus, suggesting a role for Nef in the pathogenesis of AIDS. We and others have previously shown that expression of Nef in cells bearing surface CD4 results in a sustained decrease in surface CD4 expression. This was demonstrated for Nef from two laboratory strains of HIV-1, Bru and SF2. Because both of these isolates were passaged in vitro prior to molecular cloning and in vitro passage can result in mutations which might alter nef gene function, we have analyzed two primary isolates of Nef for their ability to suppress cell surface CD4 expression. The nef genes of HIV-1 isolates from two patients with fewer than 200 CD4+ T cells per mm3 of blood were introduced into human and mouse T-cell lines by retrovirus-mediated gene transfer. Expression of Nef from both isolates correlated with a decrease in surface expression of both human and mouse CD4. To determine whether the ability to suppress surface CD4 expression is a general function of Nef, we also tested an artificially generated consensus nef gene derived from analysis of 54 patient isolates of HIV-1. Expression of the consensus Nef protein also correlated with decreased cell surface CD4 expression in both mouse and human T-cell lines. These results suggest that the ability to suppress cell surface CD4 expression is an intrinsic feature of HIV-1 Nef.