Physical association of CD4 and CD45 in primary, resting CD4+ T cells

TCR Triggering Induces the Formation of Lck-RACK1-Actinin-1 Multiprotein Network Affecting Lck Redistribution

The initiation of T-cell signaling is critically dependent on the function of the member of Src family tyrosine kinases, Lck. Upon T-cell antigen receptor (TCR) triggering, Lck kinase activity induces the nucleation of signal-transducing hubs that regulate the formation of complex signaling network and cytoskeletal rearrangement. In addition, the delivery of Lck function requires rapid and targeted membrane redistribution, but the mechanism underpinning this process is largely unknown. To gain insight into this process, we considered previously described proteins that could assist in this process via their capacity to interact with kinases and regulate their intracellular translocations. An adaptor protein, receptor for activated C kinase 1 (RACK1), was chosen as a viable option, and its capacity to bind Lck and aid the process of activation-induced redistribution of Lck was assessed. Our microscopic observation showed that T-cell activation induces a rapid, concomitant, and transient co-redistribution of Lck and RACK1 into the forming immunological synapse. Consistent with this observation, the formation of transient RACK1-Lck complexes were detectable in primary CD4⁺ T-cells with their maximum levels peaking 10 s after TCR-CD4 co-aggregation. Moreover, RACK1 preferentially binds to a pool of kinase active pY394^Lck, which co-purifies with high molecular weight cellular fractions. The formation of RACK1-Lck complexes depends on functional SH2 and SH3 domains of Lck and includes several other signaling and cytoskeletal elements that transiently bind the complex. Notably, the F-actin-crosslinking protein, α-actinin-1, binds to RACK1 only in the presence of kinase active Lck suggesting that the formation of RACK1-pY394^Lck-α-actinin-1 complex serves as a signal module coupling actin cytoskeleton bundling with productive TCR/CD4 triggering. In addition, the treatment of CD4⁺ T-cells with nocodazole, which disrupts the microtubular network, also blocked the formation of RACK1-Lck complexes. Importantly, activation-induced Lck redistribution was diminished in primary CD4⁺ T-cells by an adenoviral-mediated knockdown of RACK1. These results demonstrate that in T cells, RACK1, as an essential component of the multiprotein complex which upon TCR engagement, links the binding of kinase active Lck to elements of the cytoskeletal network and affects the subcellular redistribution of Lck.

A monoclonal antibody for the CD45 receptor in the teleost fish Dicentrarchus labrax

The CD45 tyrosine phosphatase plays an important role in regulating T lymphocyte activation in vertebrate species. In this study we describe some molecular and functional features of the CD45 receptor molecule from the European sea bass Dicentrarchus labrax. Following immunization with fixed sea bass thymocytes, we obtained a murine monoclonal antibody (mAb) able to stain fish leucocytes both alive, by immunofluorescence of thymus and mucosal tissues, and fixed, by in situ immunohistochemistry of tissue sections. The selected IgG(2) mAb (DLT22) was able to recognise by western blots polypeptides mainly at 180 kDa and 130 kDa in thymus, spleen, intestine and gill leucocyte. Accordingly, a 130 kDa polypeptide immunoprecipitated with DLT22 from thymocytes and analysed by nano-RP-HPLC-ESI-MS/MS, gave peptide sequences homologous to Fugu CD45, that were employed for the homology cloning of a partial sea bass CD45 cDNA sequence. This cDNA sequence was employed to measure by quantitative PCR the transcription of the CD45 gene both in unstimulated and in in vitro stimulated leucocytes, showing that the gene transcription was specifically modulated by LPS, ConA, PHA, IL-1, and poly I:C. When splenocytes were stimulated in vitro with ConA and PHA, a cell proliferation paralleled by an increase of DLT22-positive leucocytes was also observed. These data indicate that the DLT22 mAb recognizes a putative CD45 molecule in sea bass, documenting the presence of CD45-like developing lymphocytes in thymus and CD45-associated functional stages of lymphocytes in this species, thus dating back to teleost fish the functional activities of these cell populations in vertebrates.

Lck, Membrane Microdomains, and TCR Triggering Machinery: Defining the New Rules of Engagement

In spite of a comprehensive understanding of the schematics of T cell receptor (TCR) signaling, the mechanisms regulating compartmentalization of signaling molecules, their transient interactions, and rearrangement of membrane structures initiated upon TCR engagement remain an outstanding problem. These gaps in our knowledge are exemplified by recent data demonstrating that TCR triggering is largely dependent on a preactivated pool of Lck concentrated in T cells in a specific type of membrane microdomains. Our current model posits that in resting T cells all critical components of TCR triggering machinery including TCR/CD3, Lck, Fyn, CD45, PAG, and LAT are associated with distinct types of lipid-based microdomains which represent the smallest structural and functional units of membrane confinement able to negatively control enzymatic activities and substrate availability that is required for the initiation of TCR signaling. In addition, the microdomains based segregation spatially limits the interaction of components of TCR triggering machinery prior to the onset of TCR signaling and allows their rapid communication and signal amplification after TCR engagement, via the process of their coalescence. Microdomains mediated compartmentalization thus represents an essential membrane organizing principle in resting T cells. The integration of these structural and functional aspects of signaling into a unified model of TCR triggering will require a deeper understanding of membrane biology, novel interdisciplinary approaches and the generation of specific reagents. We believe that the fully integrated model of TCR signaling must be based on membrane structural network which provides a proper environment for regulatory processes controlling TCR triggering.

A specific type of membrane microdomains is involved in the maintenance and translocation of kinase active Lck to lipid rafts

Lck is the principal signal-generating tyrosine kinase of the T cell activation mechanism. We have previously demonstrated that induced Lck activation outside of lipid rafts (LR) results in the rapid translocation of a fraction of Lck to LR. While this translocation predicates the subsequent production of IL-2, the mechanism underpinning this process is unknown. Here, we describe the main attributes of this translocating pool of Lck. Using fractionation of Brij58 lysates, derived from primary naive non-activated CD4(+) T cells, we show that a significant portion of Lck is associated with high molecular weight complexes representing a special type of detergent-resistant membranes (DRMs) of relatively high density and sensitivity to laurylmaltoside, thus called heavy DRMs. TcR/CD4 coaggregation-mediated activation resulted in the redistribution of more than 50% of heavy DRM-associated Lck to LR in a microtubular network-dependent fashion. Remarkably, in non-activated CD4(+) T-cells, only heavy DRM-associated Lck is phosphorylated on its activatory tyrosine 394 and this pool of Lck is found to be membrane confined with CD45 phosphatase. These data are the first to illustrate a lipid microdomain-based mechanism concentrating the preactivated pool of cellular Lck and supporting its high stoichiometry of colocalization with CD45 in CD4(+) T cells. They also provide a new structural framework to assess the mechanism underpinning the compartmentalization of critical signaling elements and regulation of spatio-temporal delivery of Lck function during the T cell proximal signaling.

Engagement of the CD4 receptor affects the redistribution of Lck to the immunological synapse in primary T cells: implications for T-cell activation during human immunodeficiency virus type 1 infection

Understanding the molecular mechanisms underlying dysregulated immune responses in human immunodeficiency virus type 1 (HIV-1) infection is crucial for the control of HIV/AIDS. Despite the postulate that HIV envelope glycoprotein gp120-CD4 interactions lead to impaired T-cell responses, the precise mechanisms underlying such association are not clear. To address this, we analyzed Lck and F-actin redistribution into the immunological synapse in stimulated human primary CD4(+) T cells from HIV-1-infected donors. Similar experiments were performed with CD4(+) T cells from HIV-uninfected donors, which were exposed to anti-CD4 domain 1 antibodies, as an in vitro model of gp120-CD4 interactions, or aldithriol-inactivated HIV-1 virions before stimulation. CD4(+) T cells from HIV-infected patients exhibited a two- to threefold inhibition of both Lck and F-actin recruitment into the synapse, compared to cells from uninfected donors. Interestingly, defective recruitment of Lck was ameliorated following suppressive highly active antiretroviral therapy. Engagement of the CD4 receptor on T cells from HIV-uninfected donors before anti-CD3/CD28 stimulation led to similar defects. Furthermore, the redistribution of Lck into lipid rafts was abrogated by CD4 preengagement. Our results suggest that the engagement of CD4 by HIV gp120 prior to T-cell receptor stimulation leads to dysregulation of early signaling events and could consequently play an important role in impaired CD4(+) T-cell function.

Lck-dependent Fyn activation requires C terminus-dependent targeting of kinase-active Lck to lipid rafts

Mechanisms regulating the activation and delivery of function of Lck and Fyn are central to the generation of the most proximal signaling events emanating from the T cell antigen receptor (TcR) complex. Recent results demonstrate that lipid rafts (LR) segregate Lck and Fyn and play a fundamental role in the temporal and spatial coordination of their activation. Specifically, TcR-CD4 co-aggregation-induced Lck activation outside LR results in Lck translocation to LR where the activation of LR-resident Fyn ensues. Here we report a structure-function analysis toward characterizing the mechanism supporting Lck partitioning to LR and its capacity to activate co-localized Fyn. Using NIH 3T3 cells ectopically expressing FynT, we demonstrate that only LR-associated, kinase-active (Y505F)Lck reciprocally co-immunoprecipitates with and activates Fyn. Mutational analyses revealed a profound reduction in the formation of Lck-Fyn complexes and Fyn activation, using kinase domain mutants K273R and Y394F of (Y505F)Lck, both of which have profoundly compromised kinase activity. The only kinase-active Lck mutants tested that revealed impaired physical and enzymatic engagement with Fyn were those involving truncation of the C-terminal sequence YQPQP. Remarkably, sequential truncation of YQPQP resulted in an increasing reduction of kinase-active Lck partitioning to LR, in both fibroblasts and T cells. This in turn correlated with an ablation of the capacity of these truncates to enhance TcR-mediated interleukin-2 production. Thus, Lck-dependent Fyn activation is predicated by proximity-mediated transphosphorylation of the Fyn kinase domain, and targeting kinase-active Lck to LR is dependent on the C-terminal sequence QPQP.

CD45-associated protein promotes the response of primary CD4 T cells to low-potency T-cell receptor (TCR) stimulation and facilitates CD45 association with CD3/TCR and lck

Although it is clear that the CD45 tyrosine phosphatase is required for efficient T-cell activation and T-cell development, the factors that regulate CD45 function remain uncertain. Previous data have indicated that there is an association of CD45 with CD4 and the T-cell receptor (TCR) complex controlled by the variable ectodomain of CD45 and, following activation, by high- and low-potency peptides. This suggests that controlling substrate access to CD45 may be an important regulatory mechanism during T-cell activation. In the present study we have examined the role of the transmembrane adapter-like molecule CD45-associated protein (CD45-AP) in regulating the association of CD45 with CD3/TCR and lck, and in regulating primary CD4(+) T-lymphocyte activation. In CD4(+) T cells from CD45-AP-deficient mice, coimmunoprecipitation of CD45 with the CD3/TCR complex, in addition to lck, is significantly reduced compared with wild-type T cells. Functionally, this correlates with a decreased proliferative response, a decrease in interleukin (IL)-2 production, and a decrease in calcium flux upon stimulation with a low-potency altered peptide ligand. However, the response of CD45-AP-deficient T cells to stimulation with a high-avidity agonist peptide was largely intact, except for a modest decrease in IL-2 production. These data suggest that CD45-AP promotes or stabilizes the association of CD45 with substrates and regulates the threshold of T-cell activation.

Changes in the Role of the CD45 Protein Tyrosine Phosphatase in Regulating Lck Tyrosine Phosphorylation during Thymic Development

CD45-dependent dephosphorylation of the negative regulatory C-terminal tyrosine of the Src family kinase Lck, promotes efficient TCR signal transduction. However, despite the role of CD45 in positively regulating Lck activity, the distinct phenotypes of CD45 and Lck/Fyn-deficient mice suggest that the role of CD45 in promoting Lck activity may be differentially regulated during thymocyte development. In this study, we have found that the C-terminal tyrosine of Lck (Y505) is markedly hyperphosphorylated in total thymocytes from CD45-deficient mice compared with control animals. In contrast, regulation of the Lck Y505 phosphorylation in purified, double-negative thymocytes is relatively unaffected in CD45-deficient cells. These changes in the role of CD45 in regulating Lck phosphorylation during thymocyte development correlate with changes in coreceptor expression and the presence of coreceptor-associated Lck. Biochemical analysis of coreceptor-associated and nonassociated Lck in thymocytes, and in cell lines varying in CD4 and CD45 expression, indicate that CD45-dependent regulation of Lck Y505 phosphorylation is most evident within the fraction of Lck that is coreceptor associated. In contrast, Lck Y505 phosphorylation that is not coreceptor associated is less affected by the absence of CD45. These data define distinct pools of Lck that are differentially regulated by CD45 during T cell development.

CD45 isoforms in T cell signalling and development

The CD45 phosphotyrosine phosphatase is expressed on T cells as multiple isoforms due to alternative splicing. The panoply of isoforms expressed is tightly regulated during T cell development and on mature peripheral T cell subsets following activation. We describe the analysis of comparative CD45 isoform expression levels on thymic and T cell subsets from the C57BL/6 mouse. Only four isoforms were expressed at significant protein levels: CD45R0, CD45RB, CD45RBC and CD45RABC, although trace amounts of others may be present. The expression of CD45RBC was about nine-fold higher on CD8(+) than on CD4(+) peripheral T cells, whereas CD45R0 expression was higher on CD4(+) T cells. We provide a general overview of the current models that have been proposed to explain the molecular actions of the different CD45 isoforms. Achieving a thorough understanding of the biological reasons for the existence and tight regulation of CD45 isoform expression in immune cells remains one of the outstanding challenges in the CD45 research field.

Feedback control of T-cell receptor activation

The specificity and sensitivity of T-cell recognition is vital to the immune response. Ligand engagement with the T-cell receptor (TCR) results in the activation of a complex sequence of signalling events, both on the cell membrane and intracellularly. Feedback is an integral part of these signalling pathways, yet is often ignored in standard accounts of T-cell signalling. Here we show, using a mathematical model, that these feedback loops can explain the ability of the TCR to discriminate between ligands with high specificity and sensitivity, as well as provide a mechanism for sustained signalling. The model also explains the recent counter-intuitive observation that endogenous 'null' ligands can significantly enhance T-cell signalling. Finally, the model may provide an archetype for receptor switching based on kinase-phosphatase switches, and thus be of interest to the wider signalling community.

Lateral Membrane Protein Associations of CD4 in Lymphoid Cells Detected by Cross-Linking and Mass Spectrometry

Interactions of membrane proteins are important in various aspects of cell function. However, weak membrane protein-protein interactions are difficult to study using techniques such as co-immunoprecipitations. CD4 is a cell surface protein involved in T cell activation and the binding of the human immunodeficiency virus to HIV target cells. Here we report the use of cross-linking followed by affinity purification of CD4 in combination with mass spectrometry for identification of proteins that are in the proximity of CD4. Besides the components of the CD4 receptor complex, CD4 and lck, we have identified by tandem mass spectrometry 17 tryptic peptides from transferrin receptor CD71, three peptides from protein phosphatase CD45, and one peptide from 4F2 lymphocyte activation antigen CD98. The efficiency of the cross-linking did not correlate with the level of cell surface expression of the detected molecules, excluding a possible bias of the cross-linking toward the most abundant cell surface molecules. Whereas the association of CD4 with CD45 has been reported, the associations with CD71 and CD98 have not been previously described. We used small-scale immunoprecipitation after cross-linking in combination with fluorescence resonance energy transfer (FRET) measurements to investigate the association between CD4 and CD71. Our data show that CD71 self-associates on the cell surface, that a small fraction of CD4 can be detected by copurifying it with CD71 after cross-linking, and that the level of association between CD4 and CD71 significantly increases after phorbol 12-myristate 13-acetate-induced endocytosis of CD4. This suggests that a small fraction of CD4 associates with clusters of CD71. As both molecules undergo endocytic recycling, the association and cross-linking result from their clustering in the same pit and/or vesicle. The CD4-CD98 association probably results from nonspecific cross-linking.

CD4-Lck through TCR and in the absence of Vav exchange factor induces Bax increase and mitochondrial damage

In the present study, we aimed to demonstrate that CD4 may represent a critical turning point that governs the apoptotic and survival programs in T cells, without modifying the physical association with the TCR-CD3 complex. To address this issue, we have explored the possibility that the activation of CD4 may transduce apoptotic signals unless signaling effectors neutralize them. Our data show that in Jurkat T cells CD4 engagement by Leu3a mAb results in a rapid and strong increase of Lck kinase activity, subsequent alterations of mitochondrial membrane potential, and apoptosis. Critical parameters are coassociation of CD4/Lck with TCR/CD3 and up-regulation of the proapoptotic protein Bax. Indeed, Leu3a-mediated Lck activation failed to induce apoptotic features in Jurkat cells either defective for TCR/CD3 or overexpressing the antiapoptotic protein Bcl-2. Furthermore, we demonstrate that Leu3a treatment of Jurkat cells overexpressing Vav results in the inhibition of mitochondrial damage and apoptosis; this rescue effect is accompanied with a significant decrease of Bax expression observed in apoptotic cells. Our evidence that the activation of Lck activates in T cells apoptotic pathways which are counteracted by Vav, a signaling molecule that cooperates with CD28 to boost TCR signals, suggests a novel role for costimulation in protecting T cells from CD4-mediated cell death.

Differential association of CD45 isoforms with CD4 and CD8 regulates the actions of specific pools of p56lck tyrosine kinase in T cell antigen receptor signal transduction

An investigation into the role of CD45 isoforms in T cell antigen receptor signal transduction was carried out by transfecting CD45-negative CD4(+)CD8(+) HPB-ALL T cells with the CD45R0, CD45RBC, and CD45RABC isoforms. Fluorescence resonance energy transfer analysis showed that the CD45R0 isoform, but not the CD45RBC or CD45RABC isoforms, was found as homodimers and also preferentially associated with CD4 and CD8 at the cell-surface. A comparison was therefore made of T cell antigen receptor signaling between sub-clones expressing either CD45R0 or CD45RBC. Under basal conditions CD4-associated p56(lck) tyrosine kinase activity and cellular protein tyrosine phosphorylation levels were higher in the CD45R0(+) than in the CD45RBC(+) sub-clones. Upon CD3-CD4 ligation, TCR-zeta phosphorylation, ZAP-70 recruitment to the p21/p23 TCR-zeta phosphoisomers, ZAP-70 phosphorylation, as well as p56(lck), c-Cbl and Slp-76 phosphorylation, were all markedly increased in CD45R0(+) compared with CD45RBC(+) cells. T cell antigen receptor (TCR) stimulation alone also promoted c-Cbl phosphorylation in CD45R0(+) but not in CD45RBC(+) cells. Our results are consistent with a model in which association of CD45R0 with CD4 generates a more active pool of CD4-associated p56(lck) kinase molecules. Upon CD3-CD4 co-ligation, the active p56(lck) increases the intensity of T cell antigen receptor signal transduction coupling by promoting TCR-zeta chain phosphorylation and ZAP-70 recruitment.

Role for lipid rafts in regulating interleukin-2 receptor signaling

Lipid rafts are plasma membrane microdomains characterized by a unique lipid environment enriched in gangliosides and cholesterol, leading to their insolubility in nonionic detergents. Many receptors are constitutively or inducibly localized in lipid rafts, which have been shown to function as platforms coordinating the induction of signaling pathways. In this report, the first evidence is provided for a role of these lipid microdomains in regulating interleukin-2 receptor (IL-2R) signaling. It is demonstrated that antibody- or ligand-mediated immobilization of components of lipid rafts, glycosyl-phosphatidyl-inositol-anchored proteins, and the GM1 ganglioside, respectively, inhibit IL-2-induced proliferation in T cells. IL-2Ralpha is shown to be constitutively enriched in rafts and further enriched in the presence of immobilized anti-Thy-1. In contrast, IL-2Rbeta and IL-2Rgamma, as well as JAK1 and JAK3, are found in soluble membrane fractions, and their localization is not altered by anti-Thy-1. IL-2-mediated heterotrimerization of IL-2R chains is shown to occur within soluble membrane fractions, exclusively, as is the activation of JAK1 and JAK3. As predicted by these results, the disruption of lipid raft integrity did not impair IL-2-induced signaling. Thus, the sequestration of IL-2Ralpha within lipid microdomains restricts its intermolecular interactions and regulates IL-2R signaling through impeding its association with IL-2Rbeta and IL-2Rgamma.

The CD45 tyrosine phosphatase: a positive and negative regulator of immune cell function

The CD45 phosphotyrosine phosphatase is one of the most abundant glycoproteins expressed on immune cells. Previously, the serpentine twists and turns of the CD45 research field have tended to draw attention to CD45 either as a positive or negative regulator of immune cell function. This review draws heavily on CD45 knockout mouse data to emphasize that CD45 has both positive and negative actions in regulating receptor thresholds, and these roles vary according to cell lineage and developmental stage. Previously conflicting results are reconciled in a model suggesting how CD45 regulates the p56(lck)tyrosine kinase in T cell signalling and development.

The extracellular domain of CD4 regulates the initiation of T cell activation

To assess the respective contribution of the extracellular and intracellular domains of CD4 in regulating early T cell activation events, we have used a CD4-independent murine T cell clone transfected with human CD4. Stimulation of CD4 positive clones could only be observed if CD4 molecules associated to lck were co-aggregated with the TCR complex, confirming that the simultaneous interaction of MHC class II molecules with the CD4/lck complex and the TCR is required to initiate T cell activation. To assess the involvement of the extracellular portion of CD4 in this process, we transfected a chimeric molecule (EGFRCD4) consisting of the extracellular portion of the epidermal growth factor receptor (EGFR), and of the transmembrane and cytoplasmic domains of human CD4. Although this chimeric molecule associates with lck, transfected clones were induced to proliferate by mAb specific for TCR in the absence of co-aggregation. A new regulatory role for the extracellular domain of CD4 which is independent of its interaction with MHC class II molecules is thus revealed in these experiments. Taken together, our results demonstrate that, in a CD4-independent cell line, two domains of CD4 regulate early T cell activation events: (1) its association with lck and (2) its extracellular domain, independently of its interaction with MHC class II molecules.

Immobilization of glycosylphosphatidylinositol-anchored proteins inhibits T cell growth but not function

Accumulating evidence suggests that proteins tethered to the plasma membrane through glycosylphosphatidylinositol (GPI) anchors share common biological properties. In the present study we demonstrate that GPI-anchored proteins regulate T cell growth. Specifically, anti-TCR-induced proliferation was profoundly inhibited by co-immobilized mAb specific for Thy-1, CD48 and Ly6A/E. However, neither IL-2 production nor the effector function of cytotoxic T lymphocytes was impaired in these circumstances. Analysis of the IL-2 receptor (IL-2R) signaling pathway revealed that the association of IL-2R beta and gamma chains with the Janus kinases, JAK1 and JAK3, was not perturbed in the presence of mAb specific for GPI-linked proteins. However, in these conditions, IL-2-mediated recruitment of IL-2Ralpha, beta and gamma chains, resulting in the formation of the high-affinity hetero-trimeric IL-2R, was inhibited. The resulting phosphorylation of JAK1 and JAK3, indicative of their activation states, was correspondingly reduced. These results characterize a novel state of T cell physiology in which effector function is maintained, in the absence of clonal expansion. A physiological role for GPI-anchored proteins in the maintenance of cellular homeostasis and function is discussed.

TCRαβ-Independent CD28 Signaling and Costimulation Require Non-CD4-Associated Lck

Whether the sequelae of signals generated through CD28 either directly or in circumstances of costimulation require proximal events mediated by p56lck remains contentious. We demonstrate that CD4−, but not CD4+ clonal variants respond to CD28-specific mAb with both early and late indicators of activation. Forced expression of A418/A420-mutated CD4 or wild-type CD4 in the CD4− variant recapitulated the CD28-mediated responses of the CD4− and CD4+ variants, respectively. The implicated involvement of non-CD4-associated Lck is formally demonstrated by overexpressing S20/S23 Lck or wild-type Lck in CD4+ variants. The former, but not latter, rescues direct CD28 signaling, and supports costimulation. The results demonstrate that constitutive levels of non-CD4-associated Lck functionally limit CD28-mediated signaling.

Interactions of CD45-associated protein with the antigen receptor signaling machinery in T-lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase playing an essential role during T-cell activation. This function relates to the ability of CD45 to regulate p56(lck), a cytoplasmic protein tyrosine kinase necessary for T-cell antigen receptor (TCR) signaling. Previous studies have demonstrated that CD45 is constitutively associated in T-lymphocytes with a transmembrane molecule termed CD45-AP (or lymphocyte phosphatase-associated phosphoprotein). Even though the exact role of this polypeptide is unclear, recent analyses of mice lacking CD45-AP have indicated that its expression is also required for optimal T-cell activation. Herein, we wished to understand better the function of CD45-AP. The results of our studies showed that in T-cells, CD45-AP is part of a multimolecular complex that includes not only CD45, but also TCR, the CD4 and CD8 coreceptors, and p56(lck). The association of CD45-AP with TCR, CD4, and CD8 seemed to occur via the shared ability of these molecules to bind CD45. However, binding of CD45-AP to p56(lck) could take place in the absence of other lymphoid-specific components, suggesting that it can be direct. Structure-function analyses demonstrated that such an interaction was mediated by an acidic segment in the cytoplasmic region of CD45-AP and by the kinase domain of p56(lck). Interestingly, the ability of CD45-AP to interact with Lck in the absence of other lymphoid-specific molecules was proportional to the degree of catalytic activation of p56(lck). Together, these findings suggest that CD45-AP is an adaptor molecule involved in orchestrating interactions among components of the antigen receptor signaling machinery. Moreover, they raise the possibility that one of the functions of CD45-AP is to recognize activated Lck molecules and bring them into the vicinity of CD45.

CD4-Mediated Inhibiton of IL-2 Production in Activated T Cells

The role of CD4 in T cell activation has been attributed to its capacity to increase the avidity of interaction with APC and to shuttle associated Lck to the TCR/CD3 activation complex. The results presented in this study demonstrate that ligation of CD4 inhibits ongoing responses of preactivated T cells. Specifically, delayed addition of CD4-specific mAb is shown to inhibit Ag- or mAb-induced responses of both primary T cells and T cell clonal variants. The Ag responses of the latter are independent of the adhesion provided by CD4; thus the observed inhibition is not due to blocking CD4-MHC interactions. Further, analysis of the clonal variants demonstrates that CD4-associated Lck is not essential for the inhibition observed, as anti-CD4 inhibits responses of clonal variants, expressing a form of CD4 unable to associate with Lck (double cysteine-mutated CD4). The inhibition is counteracted by the addition of exogenous IL-2, demonstrating that the block is not due to a lesion in IL-2 utilization, rather its production. It is demonstrated that the delayed addition of anti-CD4 results in a rapid reduction in steady-state levels of IL-2 mRNA in both primary T cells and clonal variants.

Cell surface GPI-anchoring of CD45 isoforms

We have designed a new cell surface expression plasmid to study the structural and membrane-topological requirements for functioning of different isoforms of CD45, a leucocyte specific member of the protein tyrosine phosphatase (PTPase) family of proteins. Use of this vector in cell transfection experiments enabled us to produce multiple CD45 isoforms (ABC, B, Null), with their extracellular segment intact, and the entire membrane spanning and intracellular C-terminal domain replaced by a GPI-membrane-anchor and VSV-tag. Our strategy facilitated the identification and analysis of chimeric proteins and selection of cell clones from low transfection efficiency experiments. We demonstrate here that simple expression of GPI-anchored CD45 isoforms on transfected Cos-1 cells does not facilitate binding to CD22+ lymphoid cells. This suggests that not only the mere presence of CD45 extracellular domains but also their assembly into higher order structures at the cell surface, is necessary in order to engage in the recognition and/or signalling processes normally used by B- and T-cells.

lck-Independent Inhibition of T Cell Antigen Response by the HIV gp120

Binding of the HIV envelope glycoprotein gp120 to CD4 inhibits T cell activation. We have used a murine T cell clone transfected with either wild-type human CD4 or mutated forms of CD4 to characterize the pathways involved in this inhibitory effect of gp120. Ag-induced proliferation of T cell clones transfected with human CD4 was completely inhibited in the presence of gp120, even though stimulation of this clone is independent of a CD4/MHC class II interaction. In addition, our results demonstrate that the inhibition by gp120 is not due to the sequestration of lck from TCR and does not require activation of lck by gp120. This suggests that CD4 can regulate the initiation of T cell activation independently of its interaction with lck. Moreover, we demonstrate that the nonresponsiveness induced by gp120 can be reversed by soluble CD4 when added early after onset of stimulation and that gp120 exerts its inhibitory effect when cells are in the G0 ≥ 1 phase of the cell cycle.

The down-modulation of CD4 induced by the GM1 ganglioside is regulated by phosphatases and kinases: evidence from enzyme inhibitors and anti-CD45 antibodies

The involvement of protein kinases and phosphatases in the down-modulation of expression of CD4 molecules on peripheral blood lymphocytes (PBL) by gangliosides was studied. Exposure of PBL either to genistein or to H7 practically abolished the down-modulation of CD4 induced by GM1 and diminished their susceptibility to CD4+ down-modulation by exposure to GD1a. Staurosporine had no effect on the down-modulation of CD4 by either GM1 or GD1a. Orthovanadate treatment drastically inhibited the down-regulation of CD4 induced by GM1 but had no effect on down-modulation of CD4 induced by GD1a. Exposure to monoclonal antibodies (mAbs) against CD45 and CD45RA but not against CD45RO abrogated the down-modulation of CD4 by GM1. The down-modulation of CD4 elicited by GD1a, GD1b, or GT1b was not inhibited by anti-CD45RA and anti-CD45RO mAbs. MAbs against CD3, CD2, or HLA-DR had no effect on the GM1-induced down-modulation of CD4. In view of the differences obtained between GM1 and GD1a it was of interest to check whether these gangliosides competed for cellular binding sites. When PBL were first treated with anti-CD45RA and GM1 or with orthovanadate and GM1, which had a negligible effect on CD4 expression, and subsequently treated with GD1a the expression of CD4 was down-modulated. This demonstrated that GD1a binds to sites on the cell membrane to which GM1 does not bind. The present study indicates that the capacity of GM1 to down-modulate CD4 depended on the CD45 and particularly CD45RA molecules, while other gangliosides may utilize different cell surface structures to down-modulate the expression of CD4.

The receptor function of CD2 in human CD2 transgenic mice is based on highly conserved associations with signal transduction molecules

The activation of human T cells via CD2 in response to mitogenic monoclonal antibodies (mAbs) typically requires that one mAb is specific for an epitope within the N-terminal Ig domain of CD2 and the other for a partially hidden epitope. We have examined the proliferative response of human T cells and human CD2 (huCD2) transgenic murine T cells to two novel CD2 monoclonal antibodies, AICD2.M1 and AICD2.M2, and have partially mapped the epitopes of these and other mitogenic CD2-specific monoclonal antibodies by way of recognition of CD2:CD58 chimeric proteins possessing either the N-terminal or the membrane proximal immunoglobulin domains of CD2. To understand the molecular basis of proliferation in huCD2 transgenic murine T cells, the interactions of huCD2 with signaling proteins in murine T cells were analyzed. The transgenic huCD2 molecule was found to interact with the murine tyrosine kinases p56lck and p59fyn and the CD3-epsilon and zeta chains of the TCR/CD3 signaling complex and to coimmunoprecipitate tyrosine phosphatase activity. These molecular associations resemble the situation in human T cells and suggest that human CD2 couples to the same signal transduction pathways in humans and transgenic mice.