Molecular associations between the T-lymphocyte antigen receptor complex and the surface antigens CD2, CD4, or CD8 and CD5

Using CombiCells, a platform for titration and combinatorial display of cell surface ligands, to study T-cell antigen sensitivity modulation by accessory receptors

Understanding cellular decisions due to receptor-ligand interactions at cell-cell interfaces has been hampered by the difficulty of independently varying the surface density of multiple different ligands. Here, we express the synthetic binder protein SpyCatcher, designed to form spontaneous covalent bonds with interactors carrying a Spytag, on the cell surface. Using this, we show that addition of different concentrations and combinations of native Spytag-fused ligands allows for the combinatorial display of ligands on cells within minutes. We use this combinatorial display of cell surface ligands-called CombiCells-to assess T cell antigen sensitivity and the impact of T cell co-stimulation and co-inhibition receptors. We find that the T cell receptor (TCR) displayed greater sensitivity to peptides on major-histocompatibility complexes (pMHC) than synthetic chimeric antigen receptor (CARs) and bi-specific T cell engager (BiTEs) display to their target antigen, CD19. While TCR sensitivity was greatly enhanced by CD2/CD58 interactions, CAR sensitivity was primarily but more modestly enhanced by LFA-1/ICAM-1 interactions. Lastly, we show that PD-1/PD-L1 engagement inhibited T cell activation triggered solely by TCR/pMHC interactions, as well as the amplified activation induced by CD2 and CD28 co-stimulation. The ability to easily produce cells with different concentrations and combinations of ligands should accelerate the study of receptor-ligand interactions at cell-cell interfaces.

Potentiating the Antitumor Activity of Cytotoxic T Cells via the Transmembrane Domain of IGSF4 That Increases TCR Avidity

A robust T-cell response is an important component of sustained antitumor immunity. In this respect, the avidity of TCR in the antigen-targeting of tumors is crucial for the quality of the T-cell response. This study reports that the transmembrane (TM) domain of immunoglobulin superfamily member 4 (IGSF4) binds to the TM of the CD3 ζ-chain through an interaction between His177 and Asp36, which results in IGSF4-CD3 ζ dimers. IGSF4 also forms homo-dimers through the GxxVA motif in the TM domain, thereby constituting large TCR clusters. Overexpression of IGSF4 lacking the extracellular (IG4ΔEXT) domain potentiates the OTI CD8+ T cells to release IFN-γ and TNF-α and to kill OVA+-B16F10 melanoma cells. In animal models, IG4ΔEXT significantly reduces B16F10 tumor metastasis as well as tumor growth. Collectively, the results indicate that the TM domain of IGSF4 can regulate TCR avidity, and they further demonstrate that TCR avidity regulation is critical for improving the antitumor activity of cytotoxic T cells.

CD6, a Rheostat-Type Signalosome That Tunes T Cell Activation

Following T cell receptor triggering, T cell activation is initiated and amplified by the assembly at the TCR/CD3 macrocomplex of a multitude of stimulatory enzymes that activate several signaling cascades. The potency of signaling is, however, modulated by various inhibitory components already at the onset of activation, long before co-inhibitory immune checkpoints are expressed to help terminating the response. CD5 and CD6 are surface glycoproteins of T cells that have determinant roles in thymocyte development, T cell activation and immune responses. They belong to the superfamily of scavenger receptor cysteine-rich (SRCR) glycoproteins but whereas the inhibitory role of CD5 has been established for long, there is still controversy on whether CD6 may have similar or antagonistic functions on T cell signaling. Analysis of the structure and molecular associations of CD5 and CD6 indicates that these molecules assemble at the cytoplasmic tail a considerable number of signaling effectors that can putatively transduce diverse types of intracellular signals. Biochemical studies have concluded that both receptors can antagonize the flow of TCR-mediated signaling; however, the impact that CD5 and CD6 have on T cell development and T cell-mediated immune responses may be different. Here we analyze the signaling function of CD6, the common and also the different properties it exhibits comparing with CD5, and interpret the functional effects displayed by CD6 in recent animal models.

Suppression of costimulation by human cytomegalovirus promotes evasion of cellular immune defenses

CD58 is an adhesion molecule that is known to play a critical role in costimulation of effector cells and is intrinsic to immune synapse structure. Herein, we describe a virally encoded gene that inhibits CD58 surface expression. Human cytomegalovirus (HCMV) UL148 was necessary and sufficient to promote intracellular retention of CD58 during HCMV infection. Blocking studies with antagonistic anti-CD58 mAb and an HCMV UL148 deletion mutant (HCMV∆UL148) with restored CD58 expression demonstrated that the CD2/CD58 axis was essential for the recognition of HCMV-infected targets by CD8⁺ HCMV-specific cytotoxic T lymphocytes (CTLs). Further, challenge of peripheral blood mononuclear cells ex vivo with HCMV∆UL148 increased both CTL and natural killer (NK) cell degranulation against HCMV-infected cells, including NK-driven antibody-dependent cellular cytotoxicity, showing that UL148 is a modulator of the function of multiple effector cell subsets. Our data stress the effect of HCMV immune evasion functions on shaping the immune response, highlighting the capacity for their potential use in modulating immunity during the development of anti-HCMV vaccines and HCMV-based vaccine vectors.

T Cell Calcium Signaling Regulation by the Co-Receptor CD5

Calcium influx is critical for T cell effector function and fate. T cells are activated when T cell receptors (TCRs) engage peptides presented by antigen-presenting cells (APC), causing an increase of intracellular calcium (Ca2+) concentration. Co-receptors stabilize interactions between the TCR and its ligand, the peptide-major histocompatibility complex (pMHC), and enhance Ca2+ signaling and T cell activation. Conversely, some co-receptors can dampen Ca2+ signaling and inhibit T cell activation. Immune checkpoint therapies block inhibitory co-receptors, such as cytotoxic T-lymphocyte associated antigen 4 (CTLA-4) and programmed death 1 (PD-1), to increase T cell Ca2+ signaling and promote T cell survival. Similar to CTLA-4 and PD-1, the co-receptor CD5 has been known to act as a negative regulator of T cell activation and to alter Ca2+ signaling and T cell function. Though much is known about the role of CD5 in B cells, recent research has expanded our understanding of CD5 function in T cells. Here we review these recent findings and discuss how our improved understanding of CD5 Ca2+ signaling regulation could be useful for basic and clinical research.

Multiplicity of Glucocorticoid Action in Inhibiting Allograft Rejection

Glucocorticoids (GCs) are used as immunosuppressive and antiinflammatory agents in organ transplantation and in treating autoimmune diseases and inflammatory disorders. GCs were shown to exert their antiproliferative effects directly through blockade of certain elements of an early membrane-associated signal transduction pathway, modulation of the expression of select adhesion molecules, and by suppression of cytokine synthesis and action. GCs may act indirectly by inducing lipocortin synthesis, which in turn, inhibits arachidonic acid release from membrane-bound stores, and also by inducing transforming growth factor (TGF)-β expression that subsequently blocks cytokine synthesis and T cell activation. Furthermore, by preferentially inhibiting the production of Th1 cytokines, GCs may enhance Th2 cell activity and, hence, precipitate a long-lasting state of tolerance through a preferential promotion of a Th2 cytokine-secreting profile. In exerting their antiproliferative effects, GCs influence both transcriptional and posttranscriptional events by binding their cytosolic receptor (GR), which subsequently binds the promoter region of cytokine genes on select DNA sites compatible with the GCs responsible elements (GRE) motif. In addition to direct DNA binding, GCs may also directly bind to, and hence antagonize, nuclear factors required for efficient gene expression, thereby markedly reducing transcriptional rate. The pleiotrophy of the GCs action, coupled with the diverse experimental conditions employed in assessing the GCs effects, indicate that GCs may utilize more than one mechanism in inhibiting T cell activation, and warrant careful scrutiny in assigning a mechanism by which GCs exert their antiproliferative effects. © 1998 Elsevier Science Inc.

Exploiting scavenger receptors in cancer immunotherapy: Lessons from CD5 and SR-B1

Scavenger receptors (SRs) are structurally heterogeneous cell surface receptors characterized by their capacity to remove extraneous or modified self-macromolecules from circulation, thus avoiding the accumulation of noxious agents in the extracellular space. This scavenging activity makes SRs important molecules for host defense and homeostasis. In turn, SRs keep the activation of the steady-state immune response in check, and participate as co-receptors in the priming of the effector immune responses when the macromolecules are associated with a threat that might compromise host homeostasis. Therefore, SRs built up sophisticated sensor mechanisms controlling the immune system, which may be exploited to develop novel drugs for cancer immunotherapy. In this review, we focus on the regulation of the anti-tumor immune response by two paradigmatic SRs: the lymphocyte receptor CD5 and the more broadly distributed scavenger receptor class B type 1 (SR-B1). Cancer immunity can be boosted by blockade of SRs working as immune checkpoint inhibitors (CD5) and/or by proper engagement of SRs working as innate danger receptor (SR-B1). Thus, these receptors illustrate both the complexity of targeting SRs in cancer immunotherapy and also the opportunities offered by such an approach.

CD5-mediated inhibition of TCR signaling proceeds normally in the absence of SHP-1

The CD5 transmembrane glycoprotein functions as a co-receptor in the signaling pathway linking T-cell antigen receptor (TCR) engagement to activation and differentiation. Although CD5 effects on TCR signaling have been shown to be primarily inhibitory, the underlying mechanisms remain unclear. In view of recent data revealing the ability of CD5 to associate with the SHP-1 tyrosine phosphatase, a protein that also downregulates TCR signaling, we examined the role of SHP-1 in modulating CD5 function using thymocytes from SHP-1-deficient viable motheaten (me^v) mice. The results revealed the association of SHP-1 with CD5 to be markedly increased following TCR stimulation and indicated that this interaction was enhanced by and was dependent on CD5 tyrosine phosphorylation. However, there was no difference of the tyrosine phosphorylation status of CD5 between resting and TCR-stimulated cells in SHP-1-deficient compared to wild-type thymocytes. Lack of SHP-1 activity did not affect the levels of CD5 surface expression, CD5 co-immunoprecipitable tyrosine phosphatase activity and intracellular calcium increase following co-crosslinking of the TCR and CD5. Similarly, an analysis of T-cell thymocyte populations in me^v mice expressing an H-Y transgene as well as a construct mediating T-cell restricted CD5 overexpression, revealed that the reduction in the positive selection conferred by CD5 overexpression was unaffected by SHP-1 deficiency. CD5 is not a SHP-1 substrate and SHP-1 is not required for and possibly not involved in the CD5-mediated modulation of TCR signaling.

Functional Heterogeneity in CD4(+) T Cell Responses Against a Bacterial Pathogen

To investigate how CD4⁺ T cells function against a bacterial pathogen, we generated a Listeria monocytogenes-specific CD4⁺ T cell model. In this system, two TCRtg mouse lines, LLO56 and LLO118, recognize the same immunodominant epitope (LLO_190-205) of L. monocytogenes and have identical in vitro responses. However, in vivo LLO56 and LLO118 display vastly different responses during both primary and secondary infection. LLO118 dominates in the primary response and in providing CD8 T cell help. LLO56 predominates in the secondary response. We have also shown that both specific [T cell receptor (TCR)-mediated] and non-specific stimuli (bypassing the TCR) elicit distinct responses from the two transgenics, leading us to conclude that the strength of self-pMHC signaling during development tightly dictates the cell’s future response in the periphery. Herein, we review our findings in this transfer system, focusing on the contribution of the immunomodulatory molecule CD5 and the importance of self-interaction in peripheral maintenance of the cell. We also discuss the manner in which individual TCR affinities to foreign and self-pMHC contribute to the outcome of an immune response; our assertion is that there exists a spectrum of possible T cell responses to recognition of cognate antigen during infection, adding immense diversity to the immune system’s response to pathogens.

Analysis of ancestral and functionally relevant CD5 variants in systemic lupus erythematosus patients

Objective

CD5 plays a crucial role in autoimmunity and is a well-established genetic risk factor of developing RA. Recently, evidence of positive selection has been provided for the CD5 Pro224-Val471 haplotype in East Asian populations. The aim of the present work was to further analyze the functional relevance of non-synonymous CD5 polymorphisms conforming the ancestral and the newly derived haplotypes (Pro224-Ala471 and Pro224-Val471, respectively) as well as to investigate the potential role of CD5 on the development of SLE and/or SLE nephritis.

Methods

The CD5 SNPs rs2241002 (C/T; Pro224Leu) and rs2229177 (C/T; Ala471Val) were genotyped using TaqMan allelic discrimination assays in a total of 1,324 controls and 681 SLE patients of Spanish origin. In vitro analysis of CD3-mediated T cell proliferative and cytokine response profiles of healthy volunteers homozygous for the above mentioned CD5 haplotypes were also analyzed.

Results

T-cell proliferation and cytokine release were significantly increased showing a bias towards to a Th2 profile after CD3 cross-linking of peripheral mononuclear cells from healthy individuals homozygous for the ancestral Pro224-Ala471 (CC) haplotype, compared to the more recently derived Pro224-Val471 (CT). The same allelic combination was statistically associated with Lupus nephritis.

Conclusion

The ancestral Ala471 CD5 allele confers lymphocyte hyper-responsiveness to TCR/CD3 cross-linking and is associated with nephritis in SLE patients.

T-cell modulatory properties of CD5 and its role in antitumor immune responses

The destruction of tumor cells by the immune system is under the control of positive and negative receptors that tightly regulate T-cell effector functions. The T-cell receptor (TCR) inhibitory molecule CD5 critically contributes to the regulation of antitumor immune responses. Indeed, the modulation of CD5 within the tumor microenvironment corresponds to a strategy adopted by tumor-specific cytotoxic T lymphocytes (CTLs) to optimize their cytotoxic and cytokine secretion functions. In this review, we provide insights into the immunobiology of CD5 and its role in regulating antitumor CD8 T-cell responses, and suggest the possibility of targeting CD5 to improve the efficacy of current immunotherapeutic approaches against cancer.

Immunosuppressive drug therapy

The first successful kidney transplantation between monozygotic identical twins did not require any immunosuppressive drugs. Clinical application of azathioprine and glucocorticosteroids allowed the transfer of organs between genetically disparate donors and recipients. Transplantation is now the standard of care, a life-saving procedure for patients with failed organs. Progress in our understanding of the immunobiology of rejection has been translated to the development of immunosuppressive agents targeting T cells, B cells, plasma cells, costimulatory signals, complement products, and antidonor antibodies. Modern immunopharmacologic interventions have contributed to the clinical success observed following transplantation but challenges remain in personalizing immunosuppressive therapy.

Modulation of MHC binding by lateral association of TCR and coreceptor

The structure of a T cell receptor (TCR) and its affinity for cognate antigen are fixed, but T cells regulate binding sensitivity through changes in lateral membrane organization. TCR microclusters formed upon antigen engagement participate in downstream signaling. Microclusters are also found 3-4 days after activation, leading to enhanced antigen binding upon rechallenge. However, others have found an almost complete loss of antigen binding four days after T cell activation, when TCR clusters are present. To resolve these contradictory results, we compared binding of soluble MHC-Ig dimers by transgenic T cells stimulated with a high (100 μM) or low (100 fM) dose of cognate antigen. Cells activated by a high dose of peptide bound sixfold lower amounts of CD8-dependent ligand K(b)-SIY than cells activated by a low dose of MHC/peptide. In contrast, both cell populations bound a CD8-independent ligand L(d)-QL9 equally well. Consistent with the differences between binding of CD8-dependent and CD8-independent peptide/MHC, Förster resonance energy transfer (FRET) measurements of molecular proximity reported little nanoscale association of TCR with CD8 (16 FRET units) compared to their association on cells stimulated by low antigen dose (62 FRET units). Loss of binding induced by changes in lateral organization of TCR and CD8 may serve as a regulatory mechanism to avoid excessive inflammation and immunopathology in response to aggressive infection.

The multiple roles of the CD8 coreceptor in T cell biology: opportunities for the selective modulation of self-reactive cytotoxic T cells

Short peptide fragments generated by intracellular protein cleavage are presented on the surface of most nucleated cells bound to highly polymorphic MHCI molecules. These pMHCI complexes constitute an interface that allows the immune system to identify and eradicate anomalous cells, such as those that harbor infectious agents, through the activation of CTLs. Molecular recognition of pMHCI complexes is mediated primarily by clonally distributed TCRs expressed on the surface of CTLs. The coreceptor CD8 contributes to this antigen-recognition process by binding to a largely invariant region of the MHCI molecule and by promoting intracellular signaling, the effects of which serve to enhance TCR stimuli triggered by cognate ligands. Recent investigations have shed light on the role of CD8 in the activation of MHCI-restricted, antigen-experienced T cells and in the processes of T cell selection and lineage commitment in the thymus. Here, we review these data and discuss their implications for the development of potential therapeutic strategies that selectively target pathogenic CTL responses erroneously directed against self-derived antigens.

IGSF4 is a novel TCR ζ-chain-interacting protein that enhances TCR-mediated signaling

Immunoglobulin superfamily member 4 (IGSF4) is a known ligand of CRTAM, a receptor expressed in activated NKT and CD8(+) T cells, but its function in T cell immunity has not been elucidated. In this study, we show that IGSF4 directly interacts with the T cell receptor (TCR) ζ-chain and enhances TCR signaling by enhancing ζ-chain phosphorylation. Ectopic overexpression of IGSF4 enhances TCR-mediated T cell activation. In contrast, IGSF4 knockdown shows a dramatic decrease in markers associated with T cell activation compared with those in control small interfering RNA. The transmembrane domain is essential for TCR ζ-chain association and clustering to the immunological synapse, and the ectodomain is associated with T cell interaction with antigen-presenting cells (APCs). IGSF4-deficient mice have impaired TCR-mediated thymocyte selection and maturation. Furthermore, these mice reveal attenuated effector T cell functions accompanied by defective TCR signaling. Collectively, the results indicate that IGSF4 plays a central role in T cell functioning by dual independent mechanisms, control of TCR signaling and control of T cell-APC interaction.

Diacylglycerol kinase ζ controls diacylglycerol metabolism at the immunological synapse

DGKα and DGKζ negatively regulate the DAG/RasGRP1/Ras pathway in T cells. Study of the specific contribution of each isoform to DAG metabolism during immune synapse formation by use of a combination of RNAi and videomicroscopy techniques identifies DGKζ as mainly responsible for DAG consumption at the immunological synapse.

Diacylglycerol (DAG) generation at the T cell immunological synapse (IS) determines the correct activation of antigen-specific immune responses. DAG kinases (DGKs) α and ζ act as negative regulators of DAG-mediated signals by catalyzing DAG conversion to phosphatidic acid (PA). Nonetheless, the specific input of each enzyme and their spatial regulation during IS formation remain uncharacterized. Here we report recruitment of endogenous DGKα and DGKζ to the T cell receptor (TCR) complex following TCR/CD28 engagement. Specific DGK gene silencing shows that PA production at the activated complex depends mainly on DGKζ, indicating functional differences between these proteins. DGKζ kinase activity at the TCR is enhanced by phorbol-12-myristate-13-acetate cotreatment, suggesting DAG-mediated regulation of DGKζ responsiveness. We used GFP-DGKζ and -DGKα chimeras to assess translocation dynamics during IS formation. Only GFP-DGKζ translocated rapidly to the plasma membrane at early stages of IS formation, independent of enzyme activity. Finally, use of a fluorescent DAG sensor confirmed rapid, sustained DAG accumulation at the IS and allowed us to directly correlate membrane translocation of active DGKζ with DAG consumption at the IS. This study highlights a DGKζ-specific function for local DAG metabolism at the IS and offers new clues to its mode of regulation.

The conserved scavenger receptor cysteine-rich superfamily in therapy and diagnosis

The scavenger receptor cysteine-rich (SRCR) superfamily of soluble or membrane-bound protein receptors is characterized by the presence of one or several repeats of an ancient and highly conserved protein module, the SRCR domain. This superfamily (SRCR-SF) has been in constant and progressive expansion, now up to more than 30 members. The study of these members is attracting growing interest, which parallels that in innate immunity. No unifying function has been described to date for the SRCR domains, this being the result of the limited knowledge still available on the physiology of most members of the SRCR-SF, but also of the sequence versatility of the SRCR domains. Indeed, involvement of SRCR-SF members in quite different functions, such as pathogen recognition, modulation of the immune response, epithelial homeostasis, stem cell biology, and tumor development, have all been described. This has brought to us new information, unveiling the possibility that targeting or supplementing SRCR-SF proteins could result in diagnostic and/or therapeutic benefit for a number of physiologic and pathologic states. Recent research has provided structural and functional insight into these proteins, facilitating the development of means to modulate the activity of SRCR-SF members. Indeed, some of these approaches are already in use, paving the way for a more comprehensive use of SRCR-SF members in the clinic. The present review will illustrate some available evidence on the potential of well known and new members of the SRCR-SF in this regard.

Physical and functional bivalency observed among TCR/CD3 complexes isolated from primary T cells

Unlike BCR and secreted Ig, TCR expression is not thought to occur in a bivalent form. The conventional monovalent model of TCR/CD3 is supported by published studies of complexes solubilized in the detergent digitonin, in which bivalency was not observed. We revisited the issue of TCR valency by examining complexes isolated from primary αβ T cells after solubilization in digitonin. Using immunoprecipitation followed by flow cytometry, we unexpectedly observed TCR/CD3 complexes that contained two TCRs per complex. Standard anti-TCR Abs, being bivalent themselves, tended to bind with double occupancy to bivalent TCRs; this property masked the presence of the second TCR per complex in certain Ab binding assays, which may partially explain why previous data did not reveal these bivalent complexes. We also found that the prevalence of bivalency among fully assembled, mature TCR/CD3 complexes was sufficient to impact the functional performance of immunoprecipitated TCRs in binding antigenic peptide/MHC-Ig fusion proteins. Both TCR positions per bivalent complex required an Ag-specific TCR to effect optimal binding to these soluble ligands. Therefore, we conclude that in primary T cells, TCR/CD3 complexes can be found that are physically and functionally bivalent. The expression of bivalent TCR/CD3 complexes has implications regarding potential mechanisms by which Ag may trigger signaling. It also suggests the possibility that the potential for bivalent expression could represent a general feature of Ag receptors.

Rescue of tumor-infiltrating lymphocytes from activation-induced cell death enhances the antitumor CTL response in CD5-deficient mice

The CD5 coreceptor is expressed on all T cells and on the B1a B cell subset. It is associated with TCR and BCR, and modulates intracellular signals initiated by both Ag receptor complexes. Human CD5 contributes to regulation of the antitumor immune response and susceptibility of specific CTL to activation-induced cell death (AICD) triggered by the tumor. In this study, we compared the T cell response to the B16F10 melanoma engrafted into CD5-deficient and wild-type C57BL/6 mice. Compared with wild-type mice, CD5 knockout animals displayed delayed tumor growth, associated with tumor infiltration by T cell populations exhibiting a more activated phenotype and enhanced antitumor effector functions. However, control of tumor progression in CD5(-/-) mice was transient due to increased AICD of CD8(+) tumor-infiltrating T lymphocytes. Remarkably, in vivo protection of T cells from TCR-mediated apoptosis by an adenovirus engineered to produce soluble Fas resulted in a dramatic reduction in tumor growth. Our data suggest that recruitment of tumor-specific T cells in the tumor microenvironment occurs at early stages of cancer development and that tumor-mediated AICD of tumor-infiltrating T lymphocytes is most likely involved in tumor escape from the immune system.

CD4 and CD8: an inside-out coreceptor model for innate immune cells

CD8 and CD4 are expressed by several cell types that do not express TCR. These include DCs, macrophages, monocytes, and NK cells. CD8(+) monocytes and macrophages are abundant at the site of pathology in many rat disease models, particularly those involving immune complex-mediated pathology. Indeed, in some disease models, CD8(+) macrophages correlate with severity of pathology or directly cause pathology or tumor cell killing. Evidence suggests CD8 or CD4 can enhance FcgammaR-dependent responses of human monocytes. Building on data that key components of TCR and FcgammaR signaling can substitute one another efficiently, we postulate that CD4 and CD8 operate with FcgammaR and potentially other receptors to enhance responses of T cells and various innate immune cells. Our model suggests CD8 on myeloid cells may contribute directly to tumor killing and tissue pathology by enhancing FcgammaR responses. Moreover, the model suggests a role for CD8 in cross-presentation of antibody-associated antigen by DCs and a new mechanism to regulate TCR sensitivity.

Visualizing intermolecular interactions in T cells

The use of appropriate fluorescent proteins has allowed the use of FRET microscopy for investigation of intermolecular interactions in living cells. This method has the advantage of both being dynamic and of working at the subcellular level, so that the time and place where proteins interact can be visualized. We have used FRET microscopy to analyze the interactions between the T cell antigen receptor and the coreceptors CD4 and CD8. This chapter reviews data on how these coreceptors are recruited to the immunological synapse, and how they interact when the T cell is stimulated by different ligands.

Inhibition of lymphocyte CD3 expression by Chlamydophila pneumoniae infection

Since lymphocytes are a major immune cell besides macrophages in the development of atherosclerosis, interaction between lymphocytes and Chlamydophila pneumoniae may contribute to the pathogenesis of chronic inflammatory diseases associated with C. pneumoniae. In this regard, we examined a possible alteration of CD3 expression of human lymphocyte Molt-4 cells by C. pneumoniae infection. The expression levels of CD3 molecules of lymphocyte Molt-4 cells were significantly decreased by C. pneumoniae infection. In contrast, heat-killed C. pneumoniae as well as mock (cell lysates) did not cause any alteration of CD3 expression of the cells. Treatment of the infected cells with NS-398 (cyclo-oxyganase-2 inhibitor) or AH-23848 (EP(4) prostanoid receptor antagonist) abolished the inhibition of CD3 expression. The enhanced prostaglandin E(2) (PGE(2)) productions in the culture supernatants of infected cells were confirmed by competitive enzyme-immunosorbent assay (ELISA). C. pneumoniae infection of enriched lymphocytes from human peripheral blood mononuclear cells also induced a decrease of CD3 expression. Thus, C. pneumoniae infection of lymphocytes induces a decrease of CD3 expression mediated by possibly PGE(2) production.

Differential remodeling of a T-cell transcriptome following CD8- versus CD3-induced signaling

CD8 engagement with class I major histocompatibility antigens greatly enhances T-cell activation, but it is not clear how this is achieved. We address the question of whether or not the antibody-mediated ligation of CD8 alone induces transcriptional remodeling in a T-cell clone, using serial analysis of gene expression. Even though it fails to induce overt phenotypic changes, we find that CD8 ligation profoundly alters transcription in the T-cell clone, at a scale comparable to that induced by antibody-mediated ligation of CD3. The character of the resulting changes is distinct, however, with the net effect of CD8 ligation being substantially inhibitory. We speculate that ligating CD8 induces weak, T-cell receptor (TCR)-mediated inhibitory signals reminiscent of the effects of TCR antagonists. Our results imply that CD8 ligation alone is incapable of activating the T-cell clone because it fails to fully induce NFAT-dependent transcription.

Restoring the association of the T cell receptor with CD8 reverses anergy in human tumor-infiltrating lymphocytes

For several days after antigenic stimulation, human cytolytic T lymphocyte (CTL) clones exhibit a decrease in their effector activity and in their binding to human leukocyte antigen (HLA)-peptide tetramers. We observed that, when in this state, CTLs lose the colocalization of the T cell receptor (TCR) and CD8. Effector function and TCR-CD8 colocalization were restored with galectin disaccharide ligands, suggesting that the binding of TCR to galectin plays a role in the distancing of TCR from CD8. These findings appear to be applicable in vivo, as TCR was observed to be distant from CD8 on human tumor-infiltrating lymphocytes, which were anergic. These lymphocytes recovered effector functions and TCR-CD8 colocalization after ex vivo treatment with galectin disaccharide ligands. The separation of TCR and CD8 molecules could be one major mechanism of anergy in tumors and other chronic stimulation conditions.

CD5 does not regulate the signaling triggered through BCR in B cells from a subset of B-CLL patients

CD5 is a transmembrane protein expressed on all T lineage cells and a subset of B cells. It is known that CD5 is physically associated with the T-cell receptor and B-cell receptor (BCR), inhibiting the signaling triggered by both of them. CD5 is also characteristic of B-chronic lymphocytic leukemia (B-CLL) B cells, although its implication in the development of this lymphoproliferative disorder has not been studied. In the present study, we examined the effect of CD5 in apoptosis, cell viability and global protein tyrosine phosphorylation mediated by BCR in B cells from B-CLL patients. As opposed to tonsil B cells, we did not observe an increase in the apoptotic or viability signals induced by anti-immunoglobulin M or SAC/interleukin-2 when CD5 was dissociated from BCR in leukemic cells of the majority of patients. We also observed that CD5 did not regulate the BCR-induced phosphotyrosine pattern in B-CLL B cells. These findings suggest that CD5 does not inhibit properly the BCR-mediated signaling in leukemic cells. This defect in inhibiting the BCR might contribute to the enhanced survival of B-CLL B cells.

CD5 provides viability signals to B cells from a subset of B-CLL patients by a mechanism that involves PKC

B-chronic lymphocytic leukaemia (B-CLL) is a heterogeneous disease characterized by an accumulation of B lymphocytes expressing CD5. To date, the biological significance of this molecule in B-CLL B cells remains to be elucidated. In this study, we have analysed the functional consequences of the binding of an anti-CD5 antibody on B-CLL B cells. To this purpose, we have measured the percentage of viability of B-CLL B cells in the presence or in the absence of anti-CD5 antibodies and also examined some of the biochemical events downstream the CD5-signalling. We demonstrate that anti-CD5 induces phosphorylation of protein tyrosine kinases and protein kinase C (PKC), while no activation of Akt/PKB and MAPKs is detected. This signalling cascade results in viability in a group of patients in which we observe an increase of Mcl-1 levels, whereas the levels of bcl-2, bcl-x(L) and XIAP do not change. We also report that this pathway leads to IL-10 production, an immunoregulatory cytokine that might act as an autocrine growth factor for leukaemic B cells. Inhibition of PKC prevents the induction of Mcl-1 and IL-10, suggesting that the activation of PKC plays an important role in the CD5-mediated survival signals in B cells from a subset of B-CLL patients.

Immunology: how do T cells recognize antigen?

T cells recognize small fragments of microorganisms (antigens) on the surface of other cells using T cell antigen receptors. The mechanism by which these receptors signal into T cells is controversial, but two recent studies provide important new clues.

Nonstimulatory peptides contribute to antigen-induced CD8-T cell receptor interaction at the immunological synapse

It is unclear if the interaction between CD8 and the T cell receptor (TCR)-CD3 complex is constitutive or antigen induced. Here, fluorescence resonance energy transfer microscopy between fluorescent chimeras of CD3zeta and CD8beta showed that this interaction was induced by antigen recognition in the immunological synapse. Nonstimulatory endogenous or exogenous peptides presented simultaneously with antigenic peptides increased the CD8-TCR interaction. This finding indicates that the interaction between the intracellular regions of a TCR-CD3 complex recognizing its cognate peptide-major histocompatibility complex (MHC) antigen, and CD8 (plus the kinase Lck), is enhanced by a noncognate CD8-MHC interaction. Thus, the interaction of CD8 with a nonstimulatory peptide-MHC complex helps mediate T cell recognition of antigen, improving the coreceptor function of CD8.

Molecular interactions at the T cell-antigen-presenting cell interface

The development of new imaging techniques has made it possible to investigate the dynamic movements of molecules involved in T-cell signalling. Fluorescence resonance energy transfer (FRET) imaging allows the investigation of protein-protein interactions in live cells, and has demonstrated that T-cell receptors (TCRs) and CD4 are brought together in the immunological synapse during antigen recognition. This interaction is inhibited by antagonist ligands. Antagonism works through competition between agonist and antagonist ligands for TCR binding, as well as through feedback via the SHP-1 tyrosine phosphatase and extracellular signal-related kinase. Early signalling events result in the clustering of co-receptors and TCRs at the synapse, and the activation of various signalling molecules. Recent data show that some T-cell signalling precedes the formation of the mature form of the immunological synapse, but that full T-cell activation depends on sustained signalling, which in turn requires the synapse.

Relevance of CD6-Mediated Interactions in T Cell Activation and Proliferation

CD6 is a cell surface receptor expressed on immature thymocytes and mature T and B1a lymphocytes. The ultimate function of CD6 has not been deciphered yet, but much evidence supports a role for CD6 in T cell activation and differentiation. In this study, we show that a fraction of CD6 molecules physically associates with the TCR/CD3 complex by coimmunoprecipitation, cocapping, and fluorescence resonance energy transfer experiments. Image analysis of Ag-specific T-APC conjugates demonstrated that CD6 and its ligand, activated leukocyte cell adhesion molecule (CD166), colocalize with TCR/CD3 at the center of the immunological synapse, the so-called central supramolecular activation cluster. The addition of a soluble rCD6 form significantly reduced the number of mature Ag-specific T-APC conjugates, indicating that CD6 mediates early cell-cell interactions needed for immunological synapse maturation to proceed. This was in agreement with the dose-dependent inhibition of CD3-mediated T cell proliferation induced by soluble rCD6. Taken together, our data illustrate the important role played by the intra- and intercellular molecular interactions mediated by CD6 during T cell activation and proliferation processes.

Transcriptional Regulation of Human CD5: Important Role of Ets Transcription Factors in CD5 Expression in T Cells

CD5 is a surface receptor constitutively expressed on thymocytes and mature T and B-1a cells. CD5 expression is tightly regulated during T and B cell development and activation processes. In this study we shown that the constitutive expression of CD5 on human T cells correlates with the presence of a DNase I-hypersensitive (DH) site at the 5'-flanking region of CD5. Human CD5 is a TATA-less gene for which 5'-RACE analysis shows multiple transcriptional start sites, the most frequent of which locates within an initiator sequence. Luciferase reporter assays indicate that a 282-bp region upstream of the initiation ATG displays full promoter activity in human T cells. Two conserved Ets-binding sites (at positions -239 and -185) were identified as functionally relevant to CD5 expression by site-directed mutagenesis, EMSAs, and cotransfection experiments. A possible contribution of Sp1 (-115 and -95), c-Myb (-177), and AP-1-like (-151) motifs was also detected. Further DH site analyses revealed an inducible DH site 10 kb upstream of the human CD5 gene in both T and B CD5(+) cells. Interestingly, a 140-bp sequence showing high homology with a murine inducible enhancer is found within that site. The data presented indicate that the 5'-flanking region of human CD5 is transcriptionally active in T cells, and that Ets transcription factors in conjunction with other regulatory elements are responsible for constitutive and tissue-specific CD5 expression.

Analysis of proteins copurifying with the CD4/lck complex using one-dimensional polyacrylamide gel electrophoresis and mass spectrometry: comparison with affinity-tag based protein detection and evaluation of different solubilization methods

Mass spectrometry-based identification of the components of affinity purified protein complexes after polyacrylamide gel electrophoresis (PAGE) and in-gel digest has become very popular for the detection of novel protein interactions. As an alternative, the entire protein complex can be subjected to proteolytic cleavage followed by chromatographic separation of the peptides. Based on our earlier report of a method using affinity tag-mediated purification of cysteine-containing peptides to analyse proteins present in an affinity purification of the CD4/lck receptor complex, we here evaluated the use of one-dimensional polyacrylamide gel electrophoresis for analysis of the same receptor complex purification. Using electrospray and tandem mass spectrometry analyses of tryptic peptides from in-gel digested proteins we identified the components of the CD4 receptor complex along with 23 other proteins that were all likely to be non-specifically binding proteins and mainly different from the proteins detected in our previous study. We compare the alternative strategy with the affinity tag-based method that we described earlier and show that the PAGE-based method enables more proteins to be identified. We also evaluated the use of a more stringent lysis buffer for the CD4 purification to minimise non-specific binding and identified 52 proteins along with CD4 in three independent experiments suggesting that the choice of lysis buffer had no significant effect on the extent of non-specific binding. Non-specific binding was inconsistent and involved various types of proteins underlining the importance of reproducibility and control experiments in proteomic studies.

Molecular interactions mediating T cell antigen recognition

Over the past decade, key protein interactions contributing to T cell antigen recognition have been characterized in molecular detail. These have included interactions involving the T cell antigen receptor (TCR) itself, its coreceptors CD4 and CD8, the accessory molecule CD2, and the costimulatory receptors CD28 and CTLA-4. A clear view is emerging of how these molecules interact with their ligands at the cell-cell interface. Structural and binding studies have confirmed that the proteins span small but comparable distances and that, overall, they interact very weakly. However, there have been important surprises as well: that TCR interactions with peptide-MHC are topologically constrained and characterized by considerable conformational flexibility at the binding interface; that coreceptors engage peptide-MHC with extraordinarily fast kinetics and at angles apparently precluding direct interactions with the TCR bound to the same peptide-MHC; that the structural mechanisms allowing recognition by costimulatory and accessory molecules to be weak and yet specific are very heterogeneous; and that because of differences in both binding affinity and stoichiometry, there is enormous variation in the stability of the various costimulatory receptor/ligand complexes. These studies provide the necessary framework for exploring how these molecular interactions initiate T cell activation.

The accessory molecules CD5 and CD6 associate on the membrane of lymphoid T cells

CD5 and CD6 are closely related lymphocyte surface receptors of the scavenger receptor cysteine-rich superfamily, which show highly homologous extracellular regions but little conserved cytoplasmic tails. Both molecules are expressed on the same lymphocyte populations (thymocytes, mature T cells, and B1a cells) and share similar co-stimulatory properties on mature T cells. Although several works have been reported on the molecular associations and the signaling pathway mediated by CD5, very limited information is available for CD6 in this regard. Here we show the physical association of CD5 and CD6 at the cell membrane of lymphocytes, as well as their localization at the immunological synapse. CD5 and CD6 co-immunoprecipitate from Brij 96 but not Nonidet P-40 cell lysates, independently of both the co-expression of other lymphocyte surface receptors and the integrity of CD5 cytoplasmic region. Fluorescence resonance energy transfer analysis, co-capping, and co-modulation experiments demonstrate the physical in vivo association of CD5 and CD6. Analysis of T cell/antigen-presenting cells conjugates shows the accumulation of both molecules at the immunological synapse. These results indicate that CD5 and CD6 are structurally and physically related receptors, which may be functionally linked to provide either similar or complementary accessory signals during T cell activation and/or differentiation.

The influence of the src-family kinases, Lck and Fyn, on T cell differentiation, survival and activation

The src-family kinases p56lck (Lck) and p59fyn (Fyn) are expressed in T cells and are among the first signaling molecules to be activated downstream of the T cell receptor (TCR). Evidence is emerging that although closely related, these signaling molecules have discrete functions during development, maintenance and activation of peripheral T cells. For example, during thymopoiesis Lck is uniquely able to provide all the signals required for pre-TCRbeta selection, although Fyn can substitute for a subset of these. Positive selection of CD4 single-positive (SP) cells is also critically dependent on the expression of Lck but not Fyn, while differentiation of CD8 SP cells proceeds relatively efficiently in the absence of Lck. In naïve peripheral T cells either Lck or Fyn can transmit TCR-mediated survival signals, and yet only Lck is able to trigger TCR-mediated expansion signals under conditions of lymphopenia. Stimulation of naïve T cells by antigenic stimuli is also severely compromised in the absence of Lck, but more subtly impaired by the absence of Fyn. We discuss recent experiments addressing how these two src-kinase family members interface with downstream signaling pathways to regulate these diverse aspects of T cell behavior.

CD3 delta establishes a functional link between the T cell receptor and CD8

T cells expressing T cell receptor (TCR) complexes that lack CD3 delta, either due to deletion of the CD3 delta gene, or by replacement of the connecting peptide of the TCR alpha chain, exhibit severely impaired positive selection and TCR-mediated activation of CD8 single-positive T cells. Because the same defects have been observed in mice expressing no CD8 beta or tailless CD8 beta, we examined whether CD3 delta serves to couple TCR.CD3 with CD8. To this end we used T cell hybridomas and transgenic mice expressing the T1 TCR, which recognizes a photoreactive derivative of the PbCS 252-260 peptide in the context of H-2K(d). We report that, in thymocytes and hybridomas expressing the T1 TCR.CD3 complex, CD8 alpha beta associates with the TCR. This association was not observed on T1 hybridomas expressing only CD8 alpha alpha or a CD3 delta(-) variant of the T1 TCR. CD3 delta was selectively co-immunoprecipitated with anti-CD8 antibodies, indicating an avid association of CD8 with CD3 delta. Because CD8 alpha beta is a raft constituent, due to this association a fraction of TCR.CD3 is raft-associated. Cross-linking of these TCR-CD8 adducts results in extensive TCR aggregate formation and intracellular calcium mobilization. Thus, CD3 delta couples TCR.CD3 with raft-associated CD8, which is required for effective activation and positive selection of CD8(+) T cells.

Allogeneic and syngeneic class I MHC complexes drive the association of CD8 and TCR on 2C T cells

In most cases, cytotoxic T cell activation is dependent on the interaction of the T cell receptor (TCR) and CD8 with MHC class I molecules. In the CD8(+) T cell system based on the mouse cytotoxic T cell clone 2C, recognition of the allogeneic MHC L(d) exhibits a less significant role for CD8 than recognition of the syngeneic MHC K(b). Here, we examined whether this difference is related to the relative abilities of the two pepMHC complexes to drive the association of CD8 and TCR on the T cell surface. We show that both the syngeneic and allogeneic pepMHC induced association of CD8 and TCR, as revealed by fluorescence resonance energy transfer (FRET). Thus, the orientation of the syngeneic and allogeneic ligands when bound to the same TCR both allow CD8 to be recruited to the TCR complex. The conserved diagonal orientation of TCRs on different pepMHC ligands may facilitate such associations. The FRET results are consistent with the known binding properties and the CD8 involvement of the two different TCR:pepMHC interactions.

Mouse MHC class I tetramers that are unable to bind to CD8 reveal the need for CD8 engagement in order to activate naive CD8 T cells

Although the role of CD8 as a supplier of lck is unchallenged, its role in contributing to the formation of a stable complex between class I molecules and the TCR, as well as its role as an adhesion molecule, is less clear. To address the role of CD8/MHC-I interactions, we generated tetramers composed of H2-K(b) molecules with mutations in the alpha 3 domain of H2-K(b) that abolish CD8 binding. We show that the ability of tetramers to stain and activate CD8 T cells is strongly dependent on binding of CD8 to the same class I molecule engaged by the TCR. We characterize a mutation in the alpha 3 domain that results in H2-K(b) molecules capable of staining specific CD8 T cells with little ensuing activation. Although CD8 to some extent serves an adhesive function, this contribution is modest and does not substitute for lack of binding of CD8 to the class I molecule engaged by the TCR. We show that CD8 and the TCR associate in a process independent of binding of CD8 to class I. Our data support the notion that CD8 is required to form a stable complex between class I and the TCR.

Direct observation of ligand recognition by T cells

The activation of T cells through interaction of their T-cell receptors with antigenic peptide bound to major histocompatibility complex (MHC) on the surface of antigen presenting cells (APCs) is a crucial step in adaptive immunity. Here we use three-dimensional fluorescence microscopy to visualize individual peptide-I-E(k) class II MHC complexes labelled with the phycobiliprotein phycoerythrin in an effort to characterize T-cell sensitivity and the requirements for forming an immunological synapse in single cells. We show that T cells expressing the CD4 antigen respond with transient calcium signalling to even a single agonist peptide-MHC ligand, and that the organization of molecules in the contact zone of the T cell and APC takes on the characteristics of an immunological synapse when only about ten agonists are present. This sensitivity is highly dependent on CD4, because blocking this molecule with antibodies renders T cells unable to detect less than about 30 ligands.

AP2 adaptor complex-dependent internalization of CD5: differential regulation in T and B cells

CD5 is a key regulator of Ag receptor-mediated activation, selection, and differentiation in both T and B cells. Accumulating evidence indicates that lymphocyte activation and selection are sensitive to variations in levels of CD5 on the cell surface. We now show that CD5 expression on the surface of B and T cells is regulated posttranslationally by direct interaction with the mu(2) subunit of the AP2 adaptor complex that links transmembrane proteins to clathrin-coated pits. CD5 is rapidly internalized from the cell surface in lymphoid cell lines, mature splenic T and B cells, and peritoneal CD5(+) B cells following monovalent or bivalent ligation of the receptor. We mapped the mu(2) subunit binding site on CD5 to Y(429) and determined that the integrity of this site was necessary for CD5 internalization. Cross-linking of the Ag receptor with intact Abs inhibited CD5 internalization in B cells, but had the opposite effect in T cells. However, if F(ab')(2) Abs were used to stimulate the Ag receptor in B cells, the effect on CD5 internalization was now similar to that observed in T cells, indicating that signals through the Ag receptor and FcR regulate CD5 endocytosis in B cells. This was confirmed using an FcgammaRIIB1-deficient B cell line. The ability to differentially alter posttranslational CD5 expression in T and B cells is likely to be key in regulation of Ag receptor signaling and generation of tolerance in T and B lymphocytes.

Inhibition of T cell receptor-coreceptor interactions by antagonist ligands visualized by live FRET imaging of the T-hybridoma immunological synapse

The diverse effects of TCR agonists and antagonists on T cell activation are believed to be modified by the differential recruitment of CD4 or CD8 coreceptors to the TCR-MHCp complex. We used three-dimensional live cell imaging of fluorescence resonance energy transfer (FRET) between CD3zeta and CD4 fused to variants of the green fluorescent protein to investigate TCR-CD4 interactions during T cell activation. We demonstrate that recognition of agonist MHCp complexes triggers intermolecular interaction between CD4 and TCR, detectable across the T-hybridoma-APC contact area. This interaction is blocked by the presence of antagonist ligands without decreasing the recruitment of zeta and CD4 or preventing their partial colocalization in the immunological synapse.

CD8beta endows CD8 with efficient coreceptor function by coupling T cell receptor/CD3 to raft-associated CD8/p56(lck) complexes

The extraordinary sensitivity of CD8+ T cells to recognize antigen impinges to a large extent on the coreceptor CD8. While several studies have shown that the CD8beta chain endows CD8 with efficient coreceptor function, the molecular basis for this is enigmatic. Here we report that cell-associated CD8alphabeta, but not CD8alphaalpha or soluble CD8alphabeta, substantially increases the avidity of T cell receptor (TCR)-ligand binding. To elucidate how the cytoplasmic and transmembrane portions of CD8beta endow CD8 with efficient coreceptor function, we examined T1.4 T cell hybridomas transfected with various CD8beta constructs. T1.4 hybridomas recognize a photoreactive Plasmodium berghei circumsporozoite (PbCS) peptide derivative (PbCS (4-azidobezoic acid [ABA])) in the context of H-2K(d), and permit assessment of TCR-ligand binding by TCR photoaffinity labeling. We find that the cytoplasmic portion of CD8beta, mainly due to its palmitoylation, mediates partitioning of CD8 in lipid rafts, where it efficiently associates with p56(lck). In addition, the cytoplasmic portion of CD8beta mediates constitutive association of CD8 with TCR/CD3. The resulting TCR-CD8 adducts exhibit high affinity for major histocompatibility complex (MHC)-peptide. Importantly, because CD8alphabeta partitions in rafts, its interaction with TCR/CD3 promotes raft association of TCR/CD3. Engagement of these TCR/CD3-CD8/lck adducts by multimeric MHC-peptide induces activation of p56(lck) in rafts, which in turn phosphorylates CD3 and initiates T cell activation.

Regulation of the association between PSTPIP and CD2 in murine T cells

Prominent in T cells and natural killer cells, CD2 binding protein 1 (CD2BP1) plays an important role in CD2-mediated adhesion and signal transduction. In the current study, we investigated CD2 and PSTPIP (proline, serine, threonine phosphatase interacting protein, murine homologue of CD2BP1) interactions in purified mouse splenic T cells. PSTPIP associated with CD2 in both resting and activated T cells. Following various stimuli, such as concanavalin A, anti-TCRbeta, anti-CD3epsilon, anti-CD3epsilon/phorbol myristate acetate (PMA), IL-2, or PMA/ionomycin, PSTPIP and CD2 expression, as well as their association, increased in a time-dependent fashion. While PSTPIP expression and CD2 expression were comparable across most groups, the PSTPIP-CD2 association stimulated by anti-CD3epsilon alone was significantly greater than with other stimuli. Stimulation by anti-CD3epsilon plus anti-CD28 induced even greater PSTPIP-CD2 association than anti-CD3epsilon treatment alone, indicating that CD28 initiated signals are involved in regulating this interaction. There was no direct association between CD3epsilon or CD28 and PSTPIP. Tyrosine phosphorylated PSTPIP bound poorly to CD2 compared to dephosphorylated PSTPIP, and protein tyrosine phosphatase was shown to affect both phosphorylation of PSTPIP and the CD2-PSTPIP association. In addition to CD2, PSTPIP associated with CD4, CD8, CD54, and CD62L. CD2 and CD4 ligation reciprocally regulated their association with PSTPIP. These findings indicate that T cell activation, particularly through the CD3 and CD28 signal transduction pathways, regulates PSTPIP-CD2 interactions. PSTPIP likely has additional broader effects through interactions with CD4, CD8, CD54, and CD62L, and this may influence T cell responses to antigen.

The regulation of CD5 expression in murine T cells

BACKGROUND

CD5 is a pan-T cell surface marker that is also present on a subset of B cells, B-1a cells. Functional and developmental subsets of T cells express characteristic CD5 levels that vary over roughly a 30-fold range. Previous investigators have cloned a 1.7 Kb fragment containing the CD5 promoter and showed that it can confer similar lymphocyte-specific expression pattern as observed for endogenous CD5 expression.

RESULTS

We further characterize the CD5 promoter and identify minimal and regulatory regions on the CD5 promoter. Using a luciferase reporter system, we show that a 43 bp region on the CD5 promoter regulates CD5 expression in resting mouse thymoma EL4 T cells and that an Ets binding site within the 43 bp region mediates the CD5 expression. In addition, we show that Ets-1, a member of the Ets family of transcription factors, recognizes the Ets binding site in the electrophoretic mobility shift assay (EMSA). This Ets binding site is directly responsible for the increase in reporter activity when co-transfected with increasing amounts of Ets-1 expression plasmid.We also identify two additional evolutionarily-conserved regions in the CD5 promoter (CD5X and CD5Y) and demonstrate the respective roles of the each region in the regulation of CD5 transcription.

CONCLUSION

Our studies define a minimal and regulatory promoter for CD5 and show that the CD5 expression level in T cells is at least partially dependent on the level of Ets-1 protein. Based on the findings in this report, we propose a model of CD5 transcriptional regulation in T cells.

Changes in the T cell receptor macromolecular signaling complex and membrane microdomains during T cell development and activation

Initiation and propagation of T cell receptor signaling pathways involves the mobilization and aggregation of a variety of signaling intermediates with the T cell receptor and associated molecules into specialized signaling complexes. Accumulating evidence suggests that differential regulation of the formation and composition of the T cell receptor macromolecular signaling complex may affect the different biological consequences of T cell activation. The regulatory mechanisms involved in the assembly of these complexes remains poorly understood, but in part is affected by the avidity of the T cell receptor ligand, co-stimulatory signals, and by the differentiation state of the T cell.

Multiple co-stimulatory signals are required for triggering proliferation of T cells from human secondary lymphoid tissue

Vaccine-based therapies are being developed for a variety of cancers and their efficacy will be determined by their ability to stimulate T cells in the secondary lymphoid tissue. We found that T cells isolated from human secondary lymphoid organs (LT-T), in contrast to peripheral blood T cells (PB-T) are hyporesponsive to cross-linked anti-CD3 mAb (CD3c) even in the presence of exogenous IL-2. Using mAb to trigger CD2 and CD28 co-stimulatory molecules, we found that such dual co-stimulation of LT-T induces profound and sustained responses including CD25 expression, IL-2 secretion and proliferation. Different levels of co-stimulation produced a hierarchical pattern of responses in LT-T, which correlated with the degree of CD3-TCR down-regulation. Mature antigen-presenting cells (APC) restored the capacity of LT-T to proliferate to stimulation of the CD3-TCR complex. Blocking studies demonstrated that optimal proliferation was critically dependent on co-stimulation via CD2 and CD28 engaged by their ligands on the APC. Therefore, LT-T have increased co-stimulatory requirements as compared to PB-T, i.e. multiple co-stimulatory signals coupled to CD3-TCR triggering. Furthermore, LT-T were found to be dependent on APC for survival, in contrast to PB-T. Clearly, LT-T do not behave in a comparable way to PB-T and in vitro experiments assessing novel cancer vaccines should therefore use LT-T as the most appropriate population of responder T cells.