Thymocyte activation induces the association of phosphatidylinositol 3-kinase and pp120 with CD5

PI3Kδ Forms Distinct Multiprotein Complexes at the TCR Signalosome in Naïve and Differentiated CD4+ T Cells

Phosphoinositide 3-kinases (PI3Ks) play a central role in adaptive immunity by transducing signals from the T cell antigen receptor (TCR) via production of PIP₃. PI3Kδ is a heterodimer composed of a p110δ catalytic subunit associated with a p85α or p85β regulatory subunit and is preferentially engaged by the TCR upon T cell activation. The molecular mechanisms leading to PI3Kδ recruitment and activation at the TCR signalosome remain unclear. In this study, we have used quantitative mass spectrometry, biochemical approaches and CRISPR-Cas9 gene editing to uncover the p110δ interactome in primary CD4⁺ T cells. Moreover, we have determined how the PI3Kδ interactome changes upon the differentiation of small naïve T cells into T cell blasts expanded in the presence of IL-2. Our interactomic analyses identified multiple constitutive and inducible PI3Kδ-interacting proteins, some of which were common to naïve and previously-activated T cells. Our data reveals that PI3Kδ rapidly interacts with as many as seven adaptor proteins upon TCR engagement, including the Gab-family proteins, GAB2 and GAB3, a CD5-CBL signalosome and the transmembrane proteins ICOS and TRIM. Our results also suggest that PI3Kδ pre-forms complexes with the adaptors SH3KBP1 and CRKL in resting cells that could facilitate the localization and activation of p110δ at the plasma membrane by forming ternary complexes during early TCR signalling. Furthermore, we identify interactions that were not previously known to occur in CD4⁺ T cells, involving BCAP, GAB3, IQGAP3 and JAML. We used CRISPR-Cas9-mediated gene knockout in primary T cells to confirm that BCAP is a positive regulator of PI3K-AKT signalling in CD4⁺ T cell blasts. Overall, our results provide evidence for a large protein network that regulates the recruitment and activation of PI3Kδ in T cells. Finally, this work shows how the PI3Kδ interactome is remodeled as CD4⁺ T cells differentiate from naïve T cells to activated T cell blasts. These activated T cells upregulate additional PI3Kδ adaptor proteins, including BCAP, GAB2, IQGAP3 and ICOS. This rewiring of TCR-PI3K signalling that occurs upon T cell differentiation may serve to reduce the threshold of activation and diversify the inputs for the PI3K pathway in effector T cells.

CD5 signalosome coordinates antagonist TCR signals to control the generation of Treg cells induced by foreign antigens

CD5 is characterized as an inhibitory coreceptor with an important regulatory role during T cell development. The molecular mechanism by which CD5 operates has been puzzling and its function in mature T cells suggests promoting rather than repressing effects on immune responses. Here, we combined quantitative mass spectrometry and genetic studies to analyze the components and the activity of the CD5 signaling machinery in primary T cells. We found that T cell receptor (TCR) engagement induces the selective phosphorylation of CD5 tyrosine 429, which serves as a docking site for proteins with adaptor functions (c-Cbl, CIN85, CRKL), connecting CD5 to positive (PI3K) and negative (UBASH3A, SHIP1) regulators of TCR signaling. c-CBL acts as a coordinator in this complex enabling CD5 to synchronize positive and negative feedbacks on TCR signaling through the other components. Disruption of CD5 signalosome in mutant mice reveals that it modulates TCR signal outputs to selectively repress the transactivation of Foxp3 and limit the inopportune induction of peripherally induced regulatory T cells during immune responses against foreign antigen. Our findings bring insights into the paradigm of coreceptor signaling, suggesting that, in addition to providing dualistic enhancing or dampening inputs, coreceptors can engage concomitant stimulatory and inhibitory signaling events, which act together to promote specific functional outcomes.

The multiple faces of CD5

Since its discovery, over 30 years ago, CD5 has been used as a marker to identify T cells, B1-a cells, and B cell chronic lymphocytic leukemia cells. Throughout the years, many studies have described the functional relevance of CD5 as a modulator of T and B cell receptor signaling. However, it has not been until recent years that CD5 has emerged as a functional receptor in other areas of the immune system. Here, we review some of the most important aspects of CD5 as a modulator of TCR and BCR signaling, cell survival receptor both in T and B cells during health and disease, as well as the newly discovered roles of this receptor in thymocyte selection, T cell effector differentiation, and immune tolerance. CD5 was found to promote T cell survival by protecting autoreactive T cell from activation-induced cell death, to promote de novo induction of regulatory T cells in the periphery, to modulate Th17 and Th2 differentiation, and to modulate immune responses by modulating dendritic cell functions. CD5 is overexpressed in Tregs and Bregs, which are fundamental to maintain immune homeostasis. The newly established roles of CD5 in modulating different aspects of immune responses identify this receptor as an immune checkpoint modulator, and therefore it could be used as a target for immune intervention in different pathologies such as cancer, autoimmune diseases or infections.

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.

CD5, an Undercover Regulator of TCR Signaling

T cells are critical components of adaptive immunity. As such, their activation is regulated by the T cell receptor (TCR) that constantly scan peptides associated with major histocompatibility complexes (MHC). TCR engagement initiates a series of molecular events leading to cytokine secretion, proliferation, and differentiation of T cells. As a second coincident event, activation of co-stimulatory molecules, such as CD28, synergize with the TCR in order to prolong and/or amplify intracellular signals. With the recent advances in immunotherapies targeting T cells, co-inhibitory receptors are of growing interest for immunologists due to their potential modulatory properties on T cell functions. However, special attention should be dedicated to avoid unwanted clinical outcomes (1). In particular, Manichean categorization of receptors based on incomplete functional knowledge can lead to an over-simplistic view of complex cellular regulations. Thus, analysis of the functions that characterize these receptors in diverse physiological contexts remains essential for their rational use in therapeutic protocols. Here we focus on CD5, a transmembrane receptor that regulates T cell functions and development but remains poorly characterized at the molecular level. We will review its roles in physiological conditions and suggest potential molecular effectors that could account for CD5-dependent regulation of TCR signaling.

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.

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 requirement of tyrosine residue 429 within CD5 cytoplasmic domain for regulation of T cell activation and survival

CD5 has been mainly described as a negative regulator of TCR and BCR signaling and recent evidence has shown an important role for this receptor in delivering pro-survival signals. However, the molecular mechanisms underlying these processes remain unresolved. TCR crosslinking leads to phosphorylation of three tyrosine residues within the cytoplasmic tail of CD5 (Y429, Y441 and Y463) leading to the recruitment of signaling molecules like PI3K, c-Cbl and RasGAP; nevertheless, the role of these residues in T cell survival has not yet been assessed. In this study, we show that alanine-scanning mutagenesis of such tyrosine residues, either singly or in combination, leads to an increased thymocyte cell death with or without α-CD3 stimulation. Remarkably, the T-cell death observed with each individual tyrosine mutant was Caspase 3-independent. Furthermore, Y429 mutation resulted in a hyper-phosphorylation of ERK suggesting that this tyrosine residue regulates cell survival through down modulation of TCR signaling. Mutation of Y441 or Y463 did not induce hyper-responsiveness to TCR activation, indicating that they promoted T-cell survival by a TCR signal-independent pathway. Our results show that three tyrosine-based domains within CD5 cytoplasmic tail promote T-cell survival through non-overlapping mechanisms. This study also reveals that Y429 domain of CD5, previously described as a "pseudo ITAM", is functionally an ITIM domain in T cells.

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.

The carboxy-terminal region of CD5 is required for c-CBL mediated TCR signaling downmodulation in thymocytes

CD5 functions as a negative regulator of TCR signaling during thymocyte development, however, the molecular mechanisms involved in this process remain elusive. A key molecule involved in the down modulation of TCR signaling is c-Cbl, an ubiquitin ligase that physically associates with CD5. Crosslinking of TCR in thymocytes leads to ubiquitylation and lysosomal/proteasomal degradation of TCR downstream signaling effectors and CD5 itself. The present report shows that co-engagement of CD3 with CD5 enhanced c-Cbl phosphorylation, which was not affected by the deletion of the pseudo-ITAM domain of CD5, the putative binding site for c-Cbl. However, amino acids present in the carboxy-terminal region of CD5, were necessary for this effect, indicating that ITAM-independent sites were involved in the interaction of c-Cbl with CD5. The carboxy-terminal region of CD5 was also required for Vav degradation, a well-known target for c-Cbl-dependent ubiquitylation. These results support the notion that the distal cytoplasmic domain of CD5, including Y463, plays a relevant role in the downmodulation of TCR signals in thymocytes via c-Cbl.

A new pathway of CD5 glycoprotein-mediated T cell inhibition dependent on inhibitory phosphorylation of Fyn kinase

Triggering of the T cell receptor initiates a signaling cascade resulting in the activation of the T cell. These signals are integrated alongside those resulting from the triggering of other receptors whose function is to modulate the overall response. CD5 is an immunotyrosine-based inhibition motif-bearing receptor that antagonizes the overt T cell receptor activation response by recruiting inhibitory intracellular mediators such as SHP-1, RasGAP, or Cbl. We now propose that the inhibitory effects of CD5 are also mediated by a parallel pathway that functions at the level of inhibition of Fyn, a kinase generally associated with T cell receptor-mediated activation. After CD5 ligation, phosphorylation of the negative regulatory tyrosine (Tyr(531)) of Fyn increases, and this correlates with a substantial reduction in the kinase activity of Fyn and a profound inhibition of ZAP-70 activation. The effect requires the last 23 amino acids of the cytoplasmic domain of the receptor, strongly implying the involvement of a new CD5-interacting signaling or adaptor protein. Furthermore, we show that upon CD5 ligation there is a profound shift in its distribution from the bulk fluid phase to the lipid raft environment, where it associates with Fyn, Lck, and PAG. We suggest that the relocation of CD5, which we also show is capable of forming homodimers, to the proximity of raft-resident molecules enables CD5 to inhibit membrane proximal signaling by controlling the phosphorylation and activity of Fyn, possibly by interfering with the disassembly of C-terminal Src kinase (Csk)-PAG-Fyn complexes during T cell activation.

The immunomodulatory properties of the CD5 lymphocyte receptor in health and disease

CD5 is a scavenger-like receptor expressed in association with the antigen-specific receptors on T and B-1a lymphocytes. Recent studies reveal a broader biology for CD5 that includes its role as regulator of cell death and as a receptor for pathogen-associated molecular patterns, in addition to its previously described function as an inhibitory receptor. These findings shed new light into the mechanistic role of CD5 in leukemias and effector cells to exogenous (infectious) or endogenous (autoimmune, tumoral) antigens. The newly identified properties make this receptor a potential candidate to be targeted for therapeutic intervention as well as immune modulation. This review describes the current knowledge on the function of CD5 as an immunomodulatory receptor both in health and in disease.

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.

Topological requirements and signaling properties of T cell-activating, anti-CD28 antibody superagonists

Full activation of naive T cells requires both engagement of the T cell antigen receptor (TCR; signal 1) and costimulatory signaling by CD28 (signal 2). We previously identified two types of rat CD28-specific monoclonal antibodies (mAbs): "conventional," TCR signaling-dependent costimulatory mAbs and "superagonistic" mAbs capable of inducing the full activation of primary resting T cells in the absence of TCR ligation both in vitro and in vivo. Using chimeric rat/mouse CD28 molecules, we show that the superagonists bind exclusively to the laterally exposed C"D loop of the immunoglobulin-like domain of CD28 whereas conventional, costimulatory mAbs recognize an epitope close to the binding site for the natural CD80/CD86 ligands. Unexpectedly, the C"D loop reactivity of a panel of new antibodies raised against human CD28 could be predicted solely on the basis of their superagonistic properties. Moreover, mouse CD28 molecules engineered to express the rat or human C"D loop sequences activated T cell hybridomas without TCR ligation when cross-linked by superagonistic mAbs. Finally, biochemical analysis revealed that superagonistic CD28 signaling activates the nuclear factor kappaB pathway without inducing phosphorylation of either TCRzeta or ZAP70. Our findings indicate that the topologically constrained interactions of anti-CD28 superagonists bypass the requirement for signal 1 in T cell activation. Antibodies with this property may prove useful for the development of T cell stimulatory drugs.

OX52 is the rat homologue of CD6: evidence for an effector function in the regulation of CD5 phosphorylation

The MRC OX52 monoclonal antibody is a marker of rat T lymphocytes. We have cloned by polymerase chain reaction the rat homologue of CD6, and fluorescein-activated cell sorter analysis and immunoprecipitations using OX52 in COS7 cells transfected with rat CD6 cDNA showed that CD6 is the cell-surface molecule recognized by OX52. Immunoprecipitation analysis showed that CD6 coprecipitated with CD5, which in turn, was coprecipitated equivalently with CD2, CD6, and the T cell receptor (TCR), but the fraction of CD5 associated with CD6 was highly phosphorylated in kinase assays, in marked contrast with the low level of phosphorylation of CD5 associated with TCR or CD2. Examination of protein kinases associating with these antigens showed that paradoxically, CD2 coprecipitated the highest amount of Lck and Fyn. CD6 also associated with Lck, Fyn, and ZAP-70, although at lower levels but additionally coprecipitated the Tec family kinase Itk, which is absent from CD2, CD5, and TCR complexes. Lck together with Itk was the best combination of kinases, effectively phosphorylating synthetic peptides corresponding to a cytoplasmic sequence of CD5. Overall, our results suggest that CD6 has an important role in the regulation of CD5 tyrosine phosphorylation, probably as a result of its unique feature of associating with kinases of different families.

Neisserial binding to CEACAM1 arrests the activation and proliferation of CD4+ T lymphocytes

Infection with Neisseria gonorrhoeae can trigger an intense inflammatory response, yet there is little specific immune response or development of immune memory. In addition, gonorrhea typically correlates with a transient reduction in T lymphocyte counts in blood, and these populations recover when gonococcal infection is resolved. Such observations suggest that the gonococci have a suppressive effect on the host immune response. We report here that N. gonorrhoeae Opa proteins were able to bind CEACAM1 expressed by primary CD4+ T lymphocytes and suppress their activation and proliferation. CEACAM1 bound by gonococcal Opa52 associated with the tyrosine phosphatases SHP-1 and SHP-2, which implicates the receptor's ITIM (immunoreceptor tyrosine-based inhibitory motif) in this effect.

PTEN controls immunoreceptor (immunoreceptor tyrosine-based activation motif) signaling and the activation of Rac

Fcgamma receptor-mediated phagocytosis is a model for the study of immunoreceptor (immunoreceptor tyrosine-based activation motif [ITAM]) signaling and involves the activation of protein tyrosine kinases, protein tyrosine phosphatases, and downstream effectors including phosphatidylinositol-3 (PI-3) kinase. Relatively little is known of the role of lipid phosphatases in the control of ITAM signaling and inflammation. A heterologous COS7 cell system was used to examine the roles played by PI-3 kinase and the dual-specificity phosphatase, phosphatase and tensin homolog deleted on chromosome 10 (PTEN), in the signal transduction pathway leading to Fcgamma receptor IIA-mediated phagocytosis and the activation of Rac. The expression of wildtype PTEN completely abrogated the phagocytosis of immunoglobulin-G-sensitized sheep red blood cells, as compared with the catalytically inactive mutant of PTEN, which had no effect. This is the first direct evidence that PTEN, an inositol 3' phosphatase, regulates Fcgamma receptor-mediated phagocytosis, an ITAM-based signaling event. The data suggest that PTEN exerts control over phagocytosis potentially by controlling the downstream conversion of guanosine diphosphate-Rac to guanosine triphosphate-Rac following ITAM stimulation.

Determination of the tyrosine phosphorylation sites in the T cell transmembrane glycoprotein CD5

Studies of CD5-deficient mice indicate that the transmembrane glycoprotein CD5 negatively regulates antigen receptor-mediated signals in thymocytes, lymph node T cells and B1a cells. CD5 contains four tyrosine residues in its cytoplasmic domain and is phosphorylated on tyrosine residues following antigen receptor ligation. Recently it has been proposed that CD5 function is dependent on the recruitment of the tyrosine phosphatase SHP-1 to tyrosine-phosphorylated CD5 and subsequent dephosphorylation of signaling molecules. In this study we investigated the requirements for, and sites of, CD5 tyrosine phosphorylation. Using a T cell line deficient in the tyrosine kinase p56(lck) and the same cell line reconstituted with this kinase, we show that p56(lck) expression is required for efficient CD5 tyrosine phosphorylation. Using tyrosine-phosphorylated peptides corresponding to CD5 cytoplasmic sequences we also show that the Src homology 2 (SH2) domain of p56(lck) binds prominently to pY429SQP, with 30-fold less affinity to pY463DLQ and not to pY441PAL. A number of murine CD5 Y --> F and deletion mutants were expressed in Jurkat T cells. The Y441F mutant was tyrosine phosphorylated at levels comparable to wild-type, but the Y429F and Y463F mutants were phosphorylated at lower levels. Two deletion mutants, which contain only one tyrosine residue (Y378) located at the interface of the transmembrane and cytoplasmic domains, were not tyrosine phosphorylated, suggesting that Y378 is not readily available for phosphorylation. Taken together these results suggest that both Y429 and Y463 can recruit p56(lck), and that these residues are the only prominent sites for CD5 tyrosine phosphorylation.

Role of two conserved cytoplasmic threonine residues (T410 and T412) in CD5 signaling

CD5 is a transmembrane coreceptor that modulates activation and differentiation signals mediated by the Ag-specific receptor present on both T and B1a lymphocytes. CD5 lacks intrinsic catalytic activity, and its immunomodulatory properties result from intracellular interactions mediated by the CD5 cytoplasmic tail. The nature of these interactions is currently a matter of investigation. Here, we present a selective mutagenesis analysis of two conserved threonine residues (T410 and T412) located at the membrane-proximal cytoplasmic region of CD5. These residues are contained within consensus phosphorylation motifs for protein kinase C and are shown here to be critical for in vivo protein kinase C-mediated phosphorylation of CD5. Functional studies revealed that the integrity of T410 and T412 is also critical for CD5-mediated phosphatidylcholine-specific phospholipase C (PC-PLC) activation and phorbol ester-mediated inhibition of Ab-induced internalization of CD5. These results strongly argue in favor of a role for T410 and T412 in the signaling mediated by CD5.

Fine mapping of thymus enlargement gene 1 (Ten1) in BUF/Mna rats

Two polymeric autosomal loci, Ten1 and Ten2, regulate thymus enlargement in BUF/Mna (B) rats. Previously, we mapped Ten1 on chromosome (Chr) 1 to a 20 cM region between Myl2 and D1Mgh11, and Ten2 on Chr 13. To further characterize the precise position of Ten1, 34 and 37 microsatellite markers, that have a polymorphism between the B and WYK (W) and between the B and MITE (M) strains, were used for linkage analysis of thymus enlargement in 105 (WBF1 x B) blackcross (BC) and 78 (B x BMF1) BC rats, respectively. Our data showed that the D1Rat168, D1Rat112, D1Rat323, D1Got186, D1Got187 and D1Got188 markers each gave a peak logarithm of odds (LOD) score of 10.68 for linkage to the thymus ratio in (WBF1 x B) BC rats, and that the D1Rat168, D1Rat197, D1Got184, D1Got186 and D1Got188 markers each gave a peak LOD score of 7.82 in (B x BMF1) BC rats. The two LOD score peaks are coincident in the position of the rat genetic map. All of the markers mentioned above are located in the region between Igf2 and D1Mgh11, in which synteny is conserved with human 11q15.5 and the distal end of mouse Chr 7 or with human 11q13 and the proximal end of mouse Chr 19. Genes existing in these regions are discussed as candidate genes for Ten1.

The pseudo-immunoreceptor tyrosine-based activation motif of CD5 mediates its inhibitory action on B-cell receptor signaling

Genetic studies revealed that CD5 could be a negative regulator of the B-cell antigen receptor (BCR). We explore here the effect of human CD5 on BCR-triggered responses. B cells were obtained expressing a chimera composed of extracellular and transmembrane domains of Fcgamma type IIB receptor fused to CD5 cytoplasmic domain (CD5cyt). Coligation of the chimera with the BCR induces CD5cyt tyrosine phosphorylation. A rapid inhibition of BCR-induced calcium response is observed, as well as a partial but delayed inhibition of phospholipase Cgamma-1 phosphorylation. Activation of extracellular regulated kinase-2 is also severely impaired. Moreover, at the functional level, interleukin-2 production is abolished. Src homology 2 domain-bearing tyrosine phosphatase SHP-1 and Src homology 2 domain-bearing inositol 5'-phosphatase SHIP usually participate in negative regulation of the BCR. We show that they do not associate with the phosphorylated CD5 chimera. We finally demonstrate that the pseudo-immunoreceptor tyrosine based activation motif present in CD5cyt is involved because its deletion eliminates the inhibitory effect of the chimera, both at biochemical and functional levels. These results demonstrate the inhibitory role of CD5 pseudo-immunoreceptor tyrosine based activation motif tyrosine phosphorylation on BCR signaling. They further support the idea that CD5 uses mechanisms different from those already described to negatively regulate the BCR pathway.

Signaling Through CD5 Involves Acidic Sphingomyelinase, Protein Kinase C-ζ, Mitogen-Activated Protein Kinase Kinase, and c-Jun NH2-Terminal Kinase

The CD5 lymphocyte surface glycoprotein is a coreceptor involved in the modulation of Ag-specific receptor-mediated activation and differentiation signals. The molecular basis for its modulatory properties is not yet well understood. In the present study we describe early biochemical events triggered by CD5 stimulation, which include the phosphatidylcholine-specific phospholipase C (PC-PLC)-dependent activation of acidic sphingomyelinase (A-SMase) in normal and lymphoblastoid T and B cells. The functional coupling of PC-PLC and A-SMase is demonstrated by the abrogation of A-SMase activation by 1) xanthogenate tricyclodecan-9-yl (D609), a selective inhibitor of PC-PLC, and 2) replacement of several C-terminal serine residues (S458, S459, and S461) present in the cytoplasmic tail of CD5 that are known to be critical for PC-PLC activation. Additionally, we demonstrate that activation of protein kinase C-ζ (PKC-ζ) and members of the mitogen-activated protein kinase (MAPK) cascade (MAPK kinase and c-Jun NH2-terminal kinase), but not the NF-κB, are downstream events of the CD5 signaling pathway. A-SMase, PKC-ζ, and MAPK family members are key mediators of cell responses as diverse as proliferation, differentiation, and growth arrest and may contribute to CD5-mediated modulation of TCR or BCR signaling.

CD5 negatively regulates the T-cell antigen receptor signal transduction pathway: involvement of SH2-containing phosphotyrosine phosphatase SHP-1

The negative regulation of T- or B-cell antigen receptor signaling by CD5 was proposed based on studies of thymocytes and peritoneal B-1a cells from CD5-deficient mice. Here, we show that CD5 is constitutively associated with phosphotyrosine phosphatase activity in Jurkat T cells. CD5 was found associated with the Src homology 2 (SH2) domain containing hematopoietic phosphotyrosine phosphatase SHP-1 in both Jurkat cells and normal phytohemagglutinin-expanded T lymphoblasts. This interaction was increased upon T-cell receptor (TCR)-CD3 cell stimulation. CD5 co-cross-linking with the TCR-CD3 complex down-regulated the TCR-CD3-increased Ca2+ mobilization in Jurkat cells. In addition, stimulation of Jurkat cells or normal phytohemagglutinin-expanded T lymphoblasts through TCR-CD3 induced rapid tyrosine phosphorylation of several protein substrates, which was substantially diminished after CD5 cross-linking. The CD5-regulated substrates included CD3zeta, ZAP-70, Syk, and phospholipase Cgammal but not the Src family tyrosine kinase p56(lck). By mutation of all four CD5 intracellular tyrosine residues to phenylalanine, we found the membrane-proximal tyrosine at position 378, which is located in an immunoreceptor tyrosine-based inhibitory (ITIM)-like motif, crucial for SHP-1 association. The F378 point mutation ablated both SHP-1 binding and the down-regulating activity of CD5 during TCR-CD3 stimulation. These results suggest a critical role of the CD5 ITIM-like motif, which by binding to SHP-1 mediates the down-regulatory activity of this receptor.

Human CD5 Signaling and Constitutive Phosphorylation of C-Terminal Serine Residues by Casein Kinase II

CD5 is a lymphocyte surface glycoprotein with a long cytoplasmic domain suitable for phosphorylation and signal transduction, which is involved in the modulation of Ag-specific receptor-mediated activation and differentiation signals. In this study, we use Jurkat T cell transfectants of CD5 cytoplasmic tail mutants to reveal phosphorylation sites relevant to signal transduction. Our results show that casein kinase II (CKII) is responsible for the constitutive phosphorylation of CD5 molecules at a cluster of three serine residues located at the extreme C terminus (S458, S459, and S461). Furthermore, the yeast two-hybrid system demonstrates the specific association between the C-terminal regions of the CD5 cytoplasmic tail and the regulatory β subunit of CKII. We demonstrate that CKII associates with and phosphorylates the C-terminal region of CD5, a conserved domain known to be relevant for the generation of second lipid messengers, and thereby enables at least one component of its signaling funcion.

Cutting Edge: Differential CD5-Dependent Regulation of CD5-Associated CK2 Activity in Mature and Immature T Cells: Implication on TCR/CD3-Mediated Activation

CD5 attenuates TCR-induced signals in immature thymocytes but functions as a costimulatory molecule potentiating TCR/CD3-mediated activation in mature, peripheral T cells. We have recently shown that the serine/threonine kinase, casein kinase 2 (CK2), a major regulator of cell growth and signaling, associates with and is activated by CD5. Therefore, we tested the possibility that differential regulation of CK2 activity by CD5 may be associated with these differences in CD5 modulation of TCR signaling. Consistent with our hypothesis, CD5-specific cross-linking activated associated CK2 in thymocytes but not active in mature splenocytes. Differential regulation of CD5-associated CK2 provides, for the first time, a potential mechanism for the differential effects of CD5 signaling in immature and mature cells. We propose that CD5 modulates Ag receptor activity through developmentally regulated activation of CD5-associated CK2.

Thymocyte activation induces the association of the proto-oncoprotein c-cbl and ras GTPase-activating protein with CD5

Studies of knockout mice indicate that the glycoprotein CD5, which is expressed on Tcells, most thymocytes and a subset of B cells, down-regulates TCR- and B cell receptor (BCR)-mediated signaling. CD5 is associated with the TCR and BCR, and is phosphorylated on cytoplasmic tyrosine residues following antigen receptor ligation. Cross-linking of CD5 or pervanadate stimulation of thymocytes induces the association of a 120-kDa tyrosine-phosphorylated protein with CD5. The proto-oncoprotein c-cbl associates with CD5 in pervanadate-stimulated thymocytes, and reprecipitation analysis demonstrates that the major proportion of CD5-associated pp120 is c-cbl. The GTPase-activating protein for ras (ras GAP), which is not tyrosine phosphorylated following CD5 cross-linking, associates with CD5 in pervanadate-stimulated thymocytes. Using tyrosine-phosphorylated peptides we show that ras GAP interacts in an SH2-mediated manner with the phosphorylated Y429SQP sequence of CD5. Both c-cbl and ras GAP have been proposed to suppress receptor-mediated signaling, and may contribute to CD5-mediated suppression of TCR or BCR signaling.

Signaling through CD5 activates a pathway involving phosphatidylinositol 3-kinase, Vav, and Rac1 in human mature T lymphocytes

CD5 acts as a coreceptor on T lymphocytes and plays an important role in T-cell signaling and T-cell-B-cell interactions. Costimulation of T lymphocytes with anti-CD5 antibodies results in an increase of the intracellular Ca2+ levels, and subsequently in the activation of Ca2+/calmodulin-dependent (CaM) kinase type IV. In the present study, we have characterized the initial signaling pathway induced by anti-CD5 costimulation. The activation of phosphatidylinositol (PI) 3-kinase through tyrosine phosphorylation of its p85 subunit is a proximal event in the CD5-signaling pathway and leads to the activation of the lipid kinase activity of the p110 subunit. The PI 3-kinase inhibitors wortmannin and LY294002 inhibit the CD5-induced response as assessed in interleukin-2 (IL-2) secretion experiments. The expression of an inactivated Rac1 mutant (Rac1.N17) in T lymphocytes transfected with an IL-2 promoter-driven reporter construct also abrogates the response to CD5 costimulation, while the expression of a constitutively active Rac1 mutant (Rac1-V12) completely replaces the CD5 costimulatory signal. The Rac1-specific guanine nucleotide exchange factor Vav is heavily phosphorylated on tyrosine residues upon CD5 costimulation, which is a prerequisite for its activation. A role for Vav in the CD5-induced signaling pathway is further supported by the findings that the expression of a dominant negative Vav mutant (Vav-C) completely abolishes the response to CD5 costimulation while the expression of a constitutively active Vav mutant [Vav(delta1-65)] makes the CD5 costimulation signal superfluous. Wortmannin is unable to block the Vav(delta1-65)- or Rac1.V12-induced signals, indicating that both Vav and Rac1 function downstream from PI 3-kinase. Vav and Rac1 both act upstream from the CD5-induced activation of CaM kinase IV, since KN-62, an inhibitor of CaM kinases, and a dominant negative CaM kinase IV mutant block the Vav(delta1-65)-and Rac1.V12-mediated signals. We propose a model for the CD5-induced signaling pathway in which the PI 3-kinase lipid products, together with tyrosine phosphorylation, activate Vav, resulting in the activation of Rac1 by the Vav-mediated exchange of GDP for GTP.

Modification of phosphatidylinositol 3-kinase SH2 domain binding properties by Abl- or Lck-mediated tyrosine phosphorylation at Tyr-688

In cells expressing the oncogenic Bcr-Abl tyrosine kinase, the regulatory p85 subunit of phosphatidylinositol 3-kinase is phosphorylated on tyrosine residues. We report that this phosphorylation event is readily catalyzed by the Abl and Lck protein-tyrosine kinases in vitro, by Bcr-Abl or a catalytically activated Lck-Y505F in co-transfected COS cells, and by endogenous kinases in transfected Jurkat T cells upon triggering of their T cell antigen receptor. Using these systems, we have mapped a major phosphorylation site to Tyr-688 in the C-terminal SH2 domain of p85. Tyrosine phosphorylation of p85 in vitro or in vivo was not associated with detectable change in the enzymatic activity of the phosphatidylinositol 3-kinase heterodimer, but correlated with a strong reduction in the binding of some, but not all, phosphoproteins to the SH2 domains of p85. This provides an additional candidate to the list of SH2 domains regulated by tyrosine phosphorylation and may explain why association of phosphatidylinositol 3-kinase with some cellular ligands is transient or of lower stoichiometry than anticipated.

T-cell regulation: Thy-1 - hiding in full view

Thy-1 is a highly abundant glycoprotein on the surface of thymocytes and neurons. A null mutation in the gene encoding Thy-1 deregulates T-cell receptor signaling and causes abnormal thymocyte development; does this mean that Thy-1 has a signaling function?

Analysis of the tyrosine phosphorylation and calcium fluxing of human CD6 isoforms with different cytoplasmatic domains

CD6 is a cell surface glycoprotein that functions both as a co-stimulatory and adhesion receptor on T cells. Recently we have described CD6 isoforms (CD6a, b, c, d, e) that arise via alternative splicing of exons encoding the cytoplasmic region of the molecule. CD6 becomes phosphorylated on tyrosine (Tyr) residues following stimulation through the T cell receptor (TCR) complex. Since the phosphorylation of Tyr residues renders some cell surface receptors competent for interactions with proteins of intracellular signaling pathways, we wanted to determine which region(s) and residues in the cytoplasmic domain of CD6 were important for phosphorylation on Tyr residues. We engineered and stably expressed chimeric receptors that consisted of the extracellular region of mouse CD6 and the cytoplasmic regions of either naturally occurring isoforms of human CD6, truncated proteins, or point mutants. We were able to demonstrate that of the nine Tyr residues in the cytoplasmic domain of the largest isoform CD6a, the two C-terminal Tyr residues (Tyr 629/662) are critical for the phosphorylation of CD6 following TCR cross-linking. Isoform CD6e, which is missing a region that contains two proline-rich motifs, is not phosphorylated. We further analyzed the ability of the different CD6 isoforms and truncated receptors to mobilize intracellular calcium after CD6/TCR co-ligation. All CD6 isoforms, including CD6e, as well as the truncation mutant delta 555, which is missing approximately the C-terminal half of the cytoplasmic domain, are able to increase Ca2+ influx. Taken together, these results suggest that the region of CD6 which is critical for Ca2+ mobilization is located N-terminal from amino acid 555 and is therefore different from the region located at the C terminus of CD6, which is necessary for tyrosine phosphorylation.

A negative role for phosphoinositide 3-kinase in T-cell antigen receptor function

BACKGROUND

A delicate balance between positive and negative regulatory mechanisms during T-cell activation determines the specificity and magnitude of an immune response. Phosphoinositide 3-kinase (PI 3-kinase) is activated by a diverse set of receptors that determine T-cell function, including the T-cell antigen receptor (TCR), the costimulatory receptor CD28, and negative regulators of T-cell activation such as CTLA-4. PI 3-kinase is also regulated by the haematopoietic cytokines that determine T-cell differentiation and lymphocyte proliferation. PI 3-kinase can thus dynamically influence the outcome of the immune reactions at various stages. In this study, we investigated the importance of PI 3-kinase in TCR-directed T-cell activation using activated or inhibitory versions of PI 3-kinase.

RESULTS

Certain aspects of TCR responses such as the induction of transcriptional activity of AP1 and serum response factor were not affected by expression of the mutant forms of PI 3-kinase. We found, however, that PI 3-kinase profoundly influenced the transactivation capacity of 'nuclear factor of activated T cells' (NF-AT) elicited by the TCR: expression of an activated form of PI 3-kinase inhibited TCR-mediated NF-AT responses, whereas expression of a dominant negative mutant of PI 3-kinase potently enhanced TCR-controlled NF-AT induction. These effects of PI 3-kinase were not mediated by previously identified PI 3-kinase effectors, such as protein kinase B, a positive regulator of PI 3-kinase, or the GTPase Rac, and are therefore likely to involve a novel, as yet unknown, effector molecule.

CONCLUSIONS

Our results establish that PI 3-kinase can both positively and negatively regulate T-cell function, and uncover a previously unrecognized function for PI 3-kinase in T cells as a selective negative regulator of TCR-signalling events and therefore as a determinant of T-cell homeostasis.