Regulation of CD4 and CD8 surface expression on human thymocyte subpopulations by triggering through CD2 and the CD3-T cell receptor

Human thymocytes bearing the CD4 and/or CD8 antigens can be fractionated into cells with an immature and more mature phenotype based on their quantitative expression of the CD3 Ag (J. Immunol. 138:3108; J. Immunol. 139:1065). We show that the expression of CD4 and CD8 on thymocyte subpopulations with low CD3 (CD3L) and high CD3 (CD3H) is regulated by activation through the CD2 molecule and perturbation of the CD3-T cell receptor complex (CD3-Ti). Similar to its previously reported effects on peripheral T cells, PMA was able to induce the down-regulation of surface CD4, but not CD8, on thymocyte subpopulations. PMA could induce CD4 and CD8 phosphorylation in both CD3L and CD3H fractions. These results suggest that if changes in phosphorylation represent the mechanism by which CD4 and CD8 are able to transmit signals, this mechanism is operative in both CD3L and CD3H subpopulations. Treatment with anti-T11(2) and anti-T11(3) antibodies (CD2 activation pathway) resulted in partial down-regulation of CD4 but not CD8 surface expression on both CD3L and CD3H thymocytes. Similar treatment had no detectable effect on peripheral T cells. The down-regulation of surface CD4 induced by activation via CD2 could be inhibited by treatment of thymocytes with anti-CD3 antibodies. Treatment of thymocytes with anti-CD3 alone or following CD2 activation induced the selective down-regulation of surface CD8 within 15 minutes. These results suggest that CD2 and CD3-Ti triggering may regulate CD4 and CD8 surface expression on thymocytes. Furthermore, these results suggest that "cross-talk" between the CD2 and CD3-Ti pathway of activation may involve CD4 and CD8 molecules.     

Regulation of T cell clone function via CD4 and CD8 molecules. Anti-CD4 can mediate two distinct inhibitory activities

The functional effects resulting from CD4 and CD8 perturbation were analyzed by using a CD4+CD8+ clone and anti-CD4 and anti-CD8 monoclonal antibodies. Perturbation of CD8, but not CD4, by soluble antibody resulted in the inhibition of CD3-T cell receptor (CD3-Ti) triggering as determined by flow cytometric measurements of intracellular free Ca2+ concentrations. In addition, the CD3-T cell receptor-mediated cytotoxic function of the CD4+CD8+ clone was inhibited by anti-CD8, but not by anti-CD4. These results suggest that CD8, but not CD4, was functionally associated with CD3-Ti on the CD4+CD8+ clone. Although CD4 perturbation did not affect CD3-Ti-mediated activities, it resulted in the inhibition of the interleukin 2-dependent proliferation of this clone. Perturbation of CD8 did not affect the interleukin 2 dependent proliferation of the CD4+CD8+ clone. On the other hand, CD4 molecules of another CD4+CD8- clone unlike those of the CD4+CD8+ clone, were clearly linked to T cell receptor function. These results indicate that CD4 perturbation can result in two distinct regulatory activities; one involves the regulation of CD3-T cell receptor function, whereas the other is not directly associated with CD3-T cell antigen receptor function. The data are also consistent with the notion that CD4 and CD8 do not merely function as recognition and adhesion elements for accessory cell major histocompatibility complex molecules, but have a direct role in the regulation of T cell activation.     

Crosslinking CD3 with CD2 using sepharose-immobilized antibodies enhances T lymphocyte proliferation

T lymphocyte proliferation can be triggered through interactions with either CD3:Ti, the target of antigen-specific activation, or CD2, the target of an antigen-independent activation pathway. Sepharose-immobilized antibody reactive with CD3 was used to aggregate the T cell receptor complex resulting in T lymphocyte activation. When CD3 was simultaneously crosslinked with CD2 using Sepharose beads coupled to antibodies directed at both determinants, T cell proliferation was markedly enhanced (stimulation index = 8- to 11-fold). A smaller enhancement was induced when CD3 was crosslinked with several other functionally relevant T cell surface molecules. The relative mitogenic potency of the accessory molecules tested was CD2 greater than CD4 greater than CD8 greater than 2H4. Little or no increased proliferation resulted from crosslinking CD3 with class I or class II major histocompatibility antigens. The added proliferation induced by CD3: CD2 crosslinking did not occur in the presence of soluble antibodies directed against CD2. Human thymocytes, the majority of which express both CD3 and CD2, were similarly activated by Sepharose-immobilized antibodies. Our results suggest that specific interactions between T cell surface molecules may play a role in the regulation of lymphocyte activation.     

CD3-zeta surface expression is required for CD4-p56lck-mediated upregulation of T cell antigen receptor-CD3 signaling in T cells.

It has been proposed that during T cell receptor antigen recognition, CD4- or CD8-p56lck molecules interact with the T cell antigen receptor-CD3 complex (TCR-CD3) to phosphorylate various undefined substrates, which then initiate signal transduction through the TCR-CD3 complex. The ability of CD4 to modulate the TCR-CD3-induced increase in intracellular Ca2+, [Ca2+]i, and substrate tyrosine phosphorylation was studied in mutants of the human leukemic T cell line HPB-ALL characterized by their low expression of the TCR-CD3 complex on the cell surface. In TCR-CD3low cells, in which CD3-zeta was found to be associated with the TCR-CD3 complex, cross-linking CD3 with CD4 resulted in a profile of calcium mobilization, CD3-zeta, and phospholipase C-gamma 1 tyrosine phosphorylation similar to that observed in HPB-ALL cells, although the magnitude of generalized substrate tyrosine phosphorylation appeared to be smaller, as compared with wild-type cells. Responses were weak or absent when CD3 was cross-linked alone. In contrast, in a mutant in which association of CD3-zeta 2 with the TCR-CD3 was defective, cross-linking of CD3 with CD4 had a weaker effect on any of the activation parameters tested. These experiments showed that the presence of CD3-zeta 2 in the TCR-CD3 complex is of critical importance for the ability of CD4 to enhance early transducing signals inside the cell. The data also suggest that CD4-associated protein tyrosine kinase p56lck could up-regulate defective CD3-mediated induction of phospholipase C activity by increasing tyrosine phosphorylation of phospholipase C-gamma 1.     

NSOM/QD-Based Direct Visualization of CD3-Induced and CD28-Enhanced Nanospatial Coclustering of TCR and Coreceptor in Nanodomains in T Cell Activation

Direct molecular imaging of nano-spatial relationship between T cell receptor (TCR)/CD3 and CD4 or CD8 co-receptor before and after activation of a primary T cell has not been reported. We have recently innovated application of near-field scanning optical microscopy (NSOM) and immune-labeling quantum dots (QD) to image Ag-specific TCR response during in vivo clonal expansion, and now up-graded the NSOM/QD-based nanotechnology through dipole-polarization and dual-color imaging. Using this imaging system scanning cell-membrane molecules at a best-optical lateral resolution, we demonstrated that CD3, CD4 or CD8 molecules were distinctly distributed as single QD-bound molecules or nano-clusters equivalent to 2–4 QD fluorescence-intensity/size on cell-membrane of un-stimulated primary T cells, and ∼6–10% of CD3 were co-clustering with CD4 or CD8 as 70–110 nm nano-clusters without forming nano-domains. The ligation of TCR/CD3 on CD4 or CD8 T cells led to CD3 nanoscale co-clustering or interaction with CD4 or CD8 co-receptors forming 200–500 nm nano-domains or >500 nm micro-domains. Such nano-spatial co-clustering of CD3 and CD4 or CD3 and CD8 appeared to be an intrinsic event of TCR/CD3 ligation, not purely limited to MHC engagement, and be driven by Lck phosphorylation. Importantly, CD28 co-stimulation remarkably enhanced TCR/CD3 nanoscale co-clustering or interaction with CD4 co-receptor within nano- or micro-domains on the membrane. In contrast, CD28 co-stimulation did not enhance CD8 clustering or CD3–CD8 co-clustering in nano-domains although it increased molecular number and density of CD3 clustering in the enlarged nano-domains. These nanoscale findings provide new insights into TCR/CD3 interaction with CD4 or CD8 co-receptor in T-cell activation.     

A common pathway for T lymphocyte activation involving both the CD3-Ti complex and CD2 sheep erythrocyte receptor determinants

T lymphocyte activation with monoclonal antibodies directed against the CD2 (T,p50) sheep red blood cell receptor antigen and against CD3 (T,p19,29) has been investigated. Co-stimulation of purified T lymphocytes with anti-CD3 (SP34) and anti-CD2 (9-1), which detects a unique epitope on the CD2 molecule, results in T cell activation and cell proliferation. Each antibody alone is unable to mediate this effect. Co-stimulation of purified T cells with two different anti-CD2 antibodies, 9-1 and 9.6, which detect two different epitopes on the CD2 molecule, are also mitogenic. In contrast, the combination of anti-CD3 (SP34) and anti-CD2 (9.6) cannot induce T cell activation. These data suggest that the CD2 epitope defined by the 9-1 antibody is functionally important for T cell activation via the CD3/Ti complex. Furthermore, it is demonstrated that anti-CD3 (SP34) induces epitopic modulation of the CD2 molecule, resulting in enhanced expression of the CD2, 9-1 epitope. This epitope modulation of the CD2 (9-1) epitope by anti-CD3 (SP34) occurs instantaneously at 4 degrees C and in the presence of NaN3. The functional interaction between CD3 and CD2 occurs in spite of any evidence of complex formation between these two molecules. These data suggest that the T cell differentiation antigens CD3 and CD2 are jointly involved in antigen-specific T cell activation. The data are consistent with a model for antigen-specific T cell activation involving both the CD3/Ti complex and subsequent activation of the CD2 complex T cell activation by co-stimulation with anti-CD3 (SP34) and anti-CD2 (9-1) is substantially enhanced by the addition of exogenous, purified interleukin 1 (IL 1). These data would suggest that the CD2 complex, as well as the putative IL 1 receptor, are involved in separate and complementary receptor-ligand interactions, resulting in the amplification of antigen-specific T cell responses.     

Cross-linking of CD4 in a TCR/CD3-juxtaposed inhibitory state: a pFRET study.

Instances when T cell activation via the T cell receptor/CD3 complex is suppressed by anti-CD4 Abs are generally attributed either to the topological separation of CD4-p56lck from CD3, or their improper apposition. Photobleaching fluorescence resonance energy transfer measurements permitted direct analysis of these alternatives on human peripheral blood lymphocytes. Distinction between changes of relative antigen densities or positioning was made possible by simultaneously recording donor and acceptor fluorescence in the energy transfer experiment performed on homogeneous populations of flow-sorted cells. We show here that CD4 stays in the molecular vicinity of CD3, while anti-CD3 stimulation is suppressed by anti-CD4 or cross-linked HIV gp120. Our data suggest that cross-linking of CD4 through particular epitopes is capable of inhibiting activation driven by Abs binding to specific sites on CD3 without major topological sequestration of the Ags, in such a way that additional positive signals will also be affected. Thus, these and other related cases of negative signaling via CD4 may be interpreted in terms of functional uncoupling rather than a wide physical separation of CD4 from the T cell receptor-CD3 complex.     

Comparison of phosphorylation and internalization of the antigen receptor/CD3 complex, CD8, and class I MHC-encoded proteins on T cells. Role of intracytoplasmic domains analyzed with hybrid CD8/class I molecules

We analyzed the phosphorylation and the dynamics of TCR/CD3, CD8 and MHC class I molecules during the activation of a CD8+ cytotoxic T lymphocyte clone and of CD8- T helper hybridomas transfected with the gene coding for the native (J. Gabert, C. Langlet, R. Zamoyska, J.R. Parnes, A.M. Schmitt-Verhulst, and B. Malissen. 1987. Reconstitution of MHC class I specificity by transfer of the T cell receptor and Lyt-2 genes. Cell 50:545) or truncated CD8 alpha molecule. The CD3 components gamma and epsilon and the CD8 alpha subunit were phosphorylated after activation of the CTL clone with the protein kinase C activator PMA. Class I MHC molecules were phosphorylated irrespective of PMA activation. Constitutive phosphorylation of the MHC class I products was found to be intrinsic to the transmembrane/cytoplasmic portion of the molecules because it was transferred to the CD8 alpha hybrid molecules composed of extracellular CD8 and MHC class I transmembrane and intracytoplasmic domains (CD8-e/MHC-t-i). Measurements of the dynamics of these cell surface molecules by using radiolabeled mAb revealed distinct behaviors: TCR/CD3 complex ligand internalization was increased (around 50% after 40 to 60 min) after PMA activation, whereas the ligand of class I MHC molecules was internalized at constant rate irrespective of PMA activation. Ligand bound to native CD8 molecules was poorly internalized, irrespective of the activation of the T cells with PMA. The same ligand bound to the CD8-e/MHC-t-i hybrid molecule was internalized at the same rate as a class I MHC molecule ligand, indicating that the behavior of the hybrid molecule was characteristic of the transmembrane/cytoplasmic portion of MHC class I molecules.     

Physical associations between CD45 and CD4 or CD8 occur as late activation events in antigen receptor-stimulated human T cells.

Activation of human PBL T cells with solid phase anti-CD3 mAb or during the course of an MLR response gives rise to the association of CD4 or CD8 molecules with the protein tyrosine phosphatase, CD45, on the cell surface. This paired association of cell-surface molecules occurs late in the activation cycle and appears to be dependent upon Ti-CD3-mediated signaling because mitogen-driven activation does not induce formation of the complex. Maximal association occurred 72 to 96 h after exposure to anti-CD3 mAb on both CD4+ and CD8+ T cells. In contrast, association between CD8 and CD45 during an MLR response did not occur until day 6 of a MLR whereas CD4-CD45 association was detected by 72 h of culture. The kinetics of association between CD4 or CD8 and CD45 was measured by fluorescence resonance energy transfer and confirmed by immunoprecipitation of dithiobis succinimidylpropionate or disuccinimidyl suberate cross-linked 125I-labeled resting or activated T cells. The molecules that co-precipitated with either CD4 or CD8 and had an apparent kDa of 180 to 205 could be immunodepleted with anti-CD45 mAb. Furthermore, CD4 or CD8 immunoprecipitates from 96-h activated T cells contained significant levels of protein tyrosine phosphatase activity whereas corresponding immunoprecipitates from resting or recently activated T cells showed little protein tyrosine phosphatase activity. This association may allow CD45 to engage and dephosphorylate lck or another CD4- or CD8-associated substrate in order to reset the receptor complex to receive a new set of stimuli. Our observations suggest that synergistic signaling provided as a consequence of CD4 or CD8 association with the TCR after antigenic stimulation may develop on a different temporal scale than that observed after soluble anti-CD4+ anti-CD3 heteroconjugate antibody cross-linking.     

Comodulation of CD3 and CD4. Evidence for a specific association between CD4 and approximately 5% of the CD3:T cell receptor complexes on helper T lymphocytes

The aggregation of a specific class of lymphocyte surface molecules results in patching, capping, and surface modulation of the aggregated ligand. Both CD4, an associative recognition structure found on helper T lymphocytes, and CD3, a component of the T cell receptor complex, are members of this functional subgroup. When 125I-labeled monoclonal antibodies reactive with either CD4 (19Thy 5D7) or CD3 (RW24B6) were bound to T lymphocytes, the subsequent addition of goat anti-mouse Ig resulted in their rapid, temperature-dependent internalization. Whereas the binding of 125I-19Thy 5D7 (anti-CD4) was inhibited by greater than 90% in the presence of unlabeled 19Thy 5D7, no inhibition occurred in the presence of unlabeled antibody reactive with CD3 (RW28C8). We took advantage of the fact that these antibodies were of different isotypes (19Thy 5D7:IgG2a; RW28C8:IgGl) to determine whether the internalization of CD3 induced the comodulation of CD4. T lymphocytes preincubated with 125I-19Thy5D7 (anti-CD4) and unlabeled RA28C8 (anti-CD3) were treated with goat anti-mouse IgGl under conditions shown to quantitatively internalize CD3. After 1 h at 37 degrees C, T lymphocytes had internalized 10.5 +/- 2.6% (n = 3) of their antibody-bound cell surface CD4. After similar incubations with media alone or with goat anti-mouse IgGl in the absence of prebound RW28C8 (anti-CD3), no internalization of CD4 could be detected. Control antibodies reactive with CD45R (2H4, IgGl) also failed to induce the internalization of CD4. Similar results were obtained by using a helper T cell clone (T4C1) that internalized 9.6 +/- 2.8% (n = 3) of its antibody-bound cell surface CD4 in response to CD3 modulation. In a reciprocal experiment, 125I-anti-CD3 (RW24B6, IgG2b) was preincubated with T4Cl cells together with unlabeled anti-CD4 (12T4D11, IgG1) prior to the addition of goat anti-mouse IgGl. The quantitative modulation of CD4 induced the co-internalization of 4.6 +/- 0.6% (n = 3) of cell surface CD3. These results suggest that approximately 5% of the CD3:T cell receptor complexes on helper T lymphocytes are specifically associated with CD4. Furthermore, our results suggest that an average of two CD4 molecules associate with each CD3:T cell receptor complex.     

Signal transduction via CD4, CD8, and CD28 in mature and immature thymocytes. Implications for thymic selection.

The positive and negative selection of immature thymocytes that shapes the mature T cell repertoire appears to occur at an intermediate stage of development when the cells express low levels of TCR/CD3. These cells are also CD4+CD8+ and CD28+ (dull), and signals delivered by these three accessory molecules have been implicated in the selection process. We have examined the regulatory function of these accessory molecules on responses of immature thymocytes stimulated through the TCR/CD3 complex. Cross-linking CD4 or CD8 with CD3 strongly enhanced signal transduction via CD3 as assessed by protein tyrosine phosphorylation and calcium mobilization. Subsequent cell proliferation could be induced by soluble anti-CD28 mAb, which was comitogenic for cells stimulated with CD3 x CD4 or CD3 x CD8 cross-linking, but was without effect on cells stimulated with CD3 x CD3 cross-linking. A potential role for CD28 signal transduction in thymic maturation is suggested by the demonstration that the BB-1 molecule, a natural ligand for CD28, is expressed on thymic stromal cells. Taken together, our data suggest a model of thymic development in which CD4 or CD8 may enhance TCR/CD3 signaling upon coligation by an MHC molecule. If the CD28 surface receptor is simultaneously stimulated by a BB-1 expressing stromal cell, this set of interactions could lead to proliferation and positive selection. In the absence of CD28 stimulation the enhanced TCR/CD3 signals might lead to apoptosis and negative selection.     

Stimulation via the CD3 and CD28 molecules induces responsiveness to IL-4 in CD4+CD29+CD45R- memory T lymphocytes

Although both IL-2 and IL-4 can promote the growth of activated T cells, IL-4 appears to selectively promote the growth of those helper/inducer and cytolytic T cells which have been activated via their CD3/TCR complex. The present study examines the participation of CD28 and certain other T cell-surface molecules in inducing T cell responsiveness to IL-4. Purified small high density T cells were cultured in the absence of accessory cells with various soluble anti-human T cell mAb with or without soluble anti-CD3 mAb and their responsiveness to IL-4 was studied. None of the soluble anti-T cell mAb alone was able to induce T cell proliferation in response to IL-4. A combination of soluble anti-CD3 with anti-CD28 mAb but not with mAb directed at the CD2, CD5, CD7, CD11a/CD18, or class I MHC molecules induced T cell proliferation in response to IL-4. Anti-CD2 and anti-CD5 mAb enhanced and anti-CD18 mAb inhibited this anti-CD3 + anti-CD28 mAb-induced T cell response to IL-4. In addition, anti-CD2 in combination with anti-CD3 and anti-CD28 mAb induced modest levels of T cell proliferation even in the absence of exogenous cytokines. IL-1, IL-6, and TNF were each unable to replace either anti-CD3 or anti-CD28 mAb in the induction of T cell responsiveness to IL-4, but both IL-1 and TNF enhanced this response. The anti-CD3 + anti-CD28 mAb-induced response to IL-4 was exhibited only by cells within the CD4+CD29+CD45R- memory T subpopulation, and not by CD8+ or CD4+CD45R+ naive T cells. When individually cross-linked with goat anti-mouse IgG antibody immobilized on plastic surface, only anti-CD3 and anti-CD28 mAb were able to induce T cell proliferation. These results indicate that the CD3 and CD28 molecules play a crucial role in inducing T cell responsiveness to IL-4 and that the CD2, CD5, and CD11a/CD18 molecules influence this process.     

Dynamic recruitment of human CD2 into lipid rafts. Linkage to T cell signal transduction

CD2 mediates T cell adhesion via its ectodomain and signal transduction utilizing its 117-amino acid cytoplasmic tail. Here we show that a significant fraction of human CD2 molecules is inducibly recruited into lipid rafts upon CD2 cross-linking by a specific pair of mitogenic anti-CD2 monoclonal antibodies (anti-T11(2) + anti-T11(3)) or during cellular conjugate formation by CD58, the physiologic ligand expressed on antigen-presenting cells. Translocation to lipid microdomains is independent of the T cell receptor (TCR) and, unlike inducible TCR-raft association, requires no tyrosine phosphorylation. Structural integrity of rafts is necessary for CD2-stimulated elevation of intracellular free calcium and tyrosine phosphorylation of cellular substrates. Whereas murine CD2 contains two membrane-proximal intracellular cysteines, partitioning CD2 into cholesterol-rich lipid rafts constitutively, human CD2 has no cytoplasmic cysteines. Mapping studies using CD2 point mutation, deletion, and chimeric molecules suggest that conformational change in the CD2 ectodomain participates in inducible raft association and excludes the membrane-proximal N-linked glycans, the transmembrane segment, and the CD2 cytoplasmic region (residues 8-117) as necessary for translocation. Translocation of CD2 into lipid rafts may reorganize the membrane into an activation-ready state prior to TCR engagement by a peptide associated with a major histocompatibility complex molecule, accounting for synergistic T cell stimulation by CD2 and the TCR.     

A regulatory role for the CD4 and CD8 molecules in T cell activation

The role of the CD4 and CD8 molecules in T cell activation is presently a matter of controversy. Although their role as associative binding elements to MHC class II or class I is well documented, their influence on the triggering process in unclear. Because antibodies to CD4 or CD8 block T cell activation in the absence of their respective ligands, a negative signaling by these molecules has been suggested. However, recent experimental evidence argues against a negative regulatory effect of these molecules, since, e.g., simultaneous cross-linking of TCR and CD4 leads to enhanced T cell activation. Therefore, a current model suggests that the association of TCR and CD4 in the membrane gives a positive signal essential for triggering. In this report we present evidence that this model is likely to be too simple. Anti-CD4 and CD8 antibodies inhibit alternative, nonreceptor pathways of T cell triggering via Tp103 and Tp44 in the absence of class II positive accessory or target cells. These antibodies also inhibit bypass activation of T cells by phorbol ester and calcium ionophore in an accessory cell-free system. Furthermore, if the CD4 or CD8 molecules are removed from the cell surface by antibody-induced modulation, the proliferative and cytotoxic response of T cell clones is enhanced. This enhancement is also observed if resting peripheral blood T cells are used as responder cells. These data show that the CD4 or CD8 molecules have a complex regulatory function in T cell activation beyond the requirement for co-cross-linking with the TCR.     

Mechanism of peripheral T cell activation by coengagement of CD44 and CD2.

A number of CD44 antibodies are known to augment peripheral T cell proliferation stimulated with suboptimal concentrations of activating pairs of CD2 mAb. These findings have implicated the CD44 adhesion receptor in the activation of peripheral T cells via CD2. We have investigated early events after CD44 and CD2 coengagement on peripheral T cells. CD44 and CD2 coengagement resulted in enhanced [Ca2+]i mobilization. However, the increase in [Ca2+]i mobilization did not occur until at least 3 min after CD2 and CD44 coengagement, suggesting that other events precede the elevation in [Ca2+]i. Using a T cell/fibroblast adhesion assay, we could demonstrate a dramatic increase in T cell adhesiveness after about 1 min after CD44 and CD2 coengagement. The increase in T cell adhesiveness was comparable to that induced by PMA. In the absence of antibodies or treatment with mAb directed to other T cell surface Ag, there was little if any adhesion between unstimulated peripheral T cells and fibroblasts. Enhancement of T cell adhesiveness through CD44 engagement was not mediated by a direct effect on lymphocyte-function associated Ag-3, the known ligand of CD2. However, cross-linking of CD44 resulted in epitopic modulation of CD2 as demonstrated by the increased expression of the T11(3) activation epitope. Furthermore, anti-CD44 could substitute for anti-T11(2) in the activation of peripheral T cells via CD2. These results suggest that CD44 ligation has profound effects on CD2-mediated events by inducing epitopic modulation of CD2.     

Nonequivalent effects of PKC activation by PMA on murine CD4 and CD8 cell-surface expression

The membrane glycoproteins CD4 (L3T4) and CD8 (Lyt2) are expressed on distinct populations of mature murine T lymphocytes, and are thought to be receptors for monomorphic determinants expressed on MHC class II and class I molecules, respectively. Although they differ in their ligand specificity, it has been presumed that CD4 and CD8 perform equivalent functions in the T cells that bear them. Since activation of protein kinase C (PKC) is known to cause rapid down-regulation of various receptors, including the T cell receptor complex (TcR complex), we treated cells with phorbol 12-myristate 13-acetate (PMA), a PKC activator, to determine whether cell-surface expression of CD4 and CD8 would be similarly affected by this intracellular mediator. Brief or relatively prolonged treatment with PMA induced mature murine T cells to reduce their surface expression of the TcR complex and of CD4, but not of CD8. Similarly, PMA rapidly induced transfected L cells to down-regulate surface CD4 expression, but had no effect on surface CD8 expression. Most significantly, PMA treatment induced CD4+CD8+ immature thymocytes to rapidly reduce their surface CD4 expression, but, again, it had no immediate effect on the surface expression of CD8. These results indicate that CD4 and TcR complex cell-surface expression are both sensitive to PKC activation by brief treatment with PMA, whereas CD8 expression is not, and suggest that CD4 and CD8 surface expression levels are regulated by distinct intracellular mechanisms.     

Intercellular adhesion molecule-2, a second counter-receptor for CD11a/CD18 (leukocyte function-associated antigen-1), provides a costimulatory signal for T-cell receptor-initiated activation of human T cells.

Activation of T cells often requires both activation signals delivered by ligation of the TCR and those resulting from costimulatory interactions between certain T cell surface accessory molecules and their respective counter-receptors on APC. CD11a/CD18 complex on T cells modulate the activation of T cells by interacting with its counter-receptors intracellular adhesion molecule (ICAM-1) (CD54) and/or ICAM-2 on the surface of APC. The costimulatory ability of ICAM-1 has been demonstrated. Using a soluble ICAM-2 Ig fusion protein (receptor globulin, Rg) we demonstrate the costimulatory effect of ICAM-2 during the activation of CD4+ T cells. When coimmobilized with anti-TCR-1 mAb ICAM-2 Rg induced vigorous proliferative response of CD4+ T cells. This costimulatory effect of ICAM-2 was dependent on its coimmobilization with mAb directed at the CD3/TCR complex but not those directed at CD2 or CD28. Both resting as well as Ag-primed CD4+ T cells responded to the costimulatory effects of ICAM-2. The addition of mAb directed at the CD11a or CD18 molecules almost completely inhibited the responses to ICAM-2 Rg. These results are consistent with the role of CD11a/CD18 complex as a receptor for ICAM-2 mediating its costimulatory effects. Stimulation of T cells with coimmobilized anti-TCR-1 and ICAM-2 resulted in the induction of IL-2R (CD25), and anti-Tac (CD25) mAb inhibited this response suggesting the contribution of endogenously synthesized IL-2 during this stimulation. These results demonstrate that like its homologue ICAM-1, ICAM-2 also exerts a strong costimulatory effect during the TCR-initiated activation of T cells. The costimulatory effects generated by the CD11a/CD18:ICAM-2 interaction may be critical during the initiation of T cell activation by ICAM-1low APC.     

Association of human lymph node homing receptor (Leu 8) with the TCR/CD3 complex.

Cross-linking of the human homologue of the murine MEL-14 lymph node homing receptor (Selectin-1, LECAM-1, Leu 8) on both T and B cells results in modification of cell function. To investigate this phenomenon, we performed studies to determine if the Leu 8 molecule influences T cell activation via the TCR/CD3 complex. In initial studies, we treated T cells with immobilized anti-CD3 (OKT3 mAb) in the presence or absence of immobilized Leu 8 mAb. We found that although Leu 8 mAb alone had no effect on T cell proliferation, this antibody markedly augmented immobilized OKT3 mAb-induced proliferation. In further studies, we immunoprecipitated surface radioiodinated T cell lysates with OKT3 and Leu 8 mAb to determine if molecules in the TCR/CD3 complex associate with Leu 8 molecules. Although Leu 8 mAb immunoprecipitated only a single protein of approximately 80 kDa from T cell lysates treated with Nonidet P-40 under reducing condition, it coimmunoprecipitated additional proteins of 48, 42, 28, 24, and 22 kDa from T cell lysates treated with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate. These additional proteins were identified as the alpha-, beta-, gamma-, delta-, and epsilon-chains of the TCR/CD3 complex by one-dimensional and two-dimensional diagonal SDS-PAGE. Densitometric scanning showed that, on average, 18% of the TCR/CD3 complex associates with Leu 8. In a final study, we showed by immunoblotting analysis using anti-zeta peptide antibody that Leu 8 mAb coimmunoprecipitates the zeta-chain of CD3. These results indicate that the human lymph node homing receptor homologue (Leu 8) participates in the activation of T cells, probably via its association with the TCR/CD3 complex.     

Human immunodeficiency virus-specific circulating CD8 T lymphocytes have down-modulated CD3zeta and CD28, key signaling molecules for T-cell activation

Although human immunodeficiency virus (HIV)-infected subjects without AIDS have a high frequency of HIV-specific CD8 T lymphocytes, cellular immunity is unable to control infection. Freshly isolated lymphocytes often do not lyse HIV-infected targets in 4-h cytotoxicity assays. A large fraction of circulating CD8 T cells from HIV-infected donors down-modulate CD3zeta, the signaling component of the T-cell receptor complex, which is reexpressed in vitro coincident with the return of cytotoxic function. To investigate further the link between CD3zeta down-modulation and possible CD8 T-cell functional defects, we used flow cytometry to characterize further the properties of the CD3zeta-down-modulated subset. HIV-specific CD8 T cells, identified by tetramer staining, are CD3zeta(-). CD8 T cells with down-modulated CD3zeta also do not express the key costimulatory receptor CD28 and have the cell surface phenotype of activated or memory T cells (HLA-DR(+) CD62L(-)). After T-cell activation, CD3zeta-down-modulated cells express the activation marker CD69 but not the high-affinity interleukin 2 (IL-2) receptor alpha-chain CD25 and produce gamma interferon but not IL-2. Therefore HIV-specific CD8 T cells have down-modulated key signaling molecules for T-cell activation and costimulation and require exogenous cytokine stimulation. The typical impairment of HIV-specific CD4 T helper cells, which would normally provide specific CD8 T-cell stimulation, means that in vivo CTL function in vivo is compromised in most HIV-infected individuals. In AIDS patients, the functional defect is more severe, since CD3zeta is not reexpressed even after IL-2 exposure.     

Developmental regulation of transmembrane signaling via the T cell antigen receptor/CD3 complex in human T lymphocytes.

We have examined transmembrane signaling events via the TCR/CD3 complex (TCR/CD3) at various stages of T cell development for evidence of developmental regulation. Engagement of TCR/CD3 induced defective activation of phospholipase C (PLC) in thymocytes relative to peripheral blood T lymphocytes. The defect in PLC activation via TCR/CD3 was restricted to immature thymocytes (CD3low, CD4+CD8+). Mature thymocytes (CD3high, CD4+CD8-/CD8+CD4-) were similar to PBL in signaling via TCR/CD3. Both immature and mature thymocytes expressed a similar profile of PLC isoenzyme mRNA species, indicating that the defect in signaling in immature thymocytes was not due to altered expression of PLC isoenzymes. Activation of tyrosine phosphorylation pathways implicated in the coupling of TCR/CD3 to PLC was impaired in immature thymocytes, as evidenced by depressed phosphorylation of CD3 zeta subunit after stimulation with anti TCR/CD3 mAb. This was associated with lower levels of p59fyn tyrosine kinase and minimal or undetectable stimulus-induced kinase activation in immature thymocytes relative to mature thymocytes. We conclude that the capacity to signal via TCR/CD3 is regulated during T cell development by mechanisms acting at the level of TCR/CD3-associated tyrosine phosphorylation pathways.