Grb2 and Gads exhibit different interactions with CD28 and play distinct roles in CD28-mediated costimulation

The interaction between the costimulatory molecules CD80/86 and CD28 contributed to CD4+ T lymphocyte activation in flounder (Paralichthys olivaceus)

CD28 and CD80/86 are crucial co-stimulatory molecules for the T cell activation. Previous study illustrated that CD28 and CD80/86 present on T cells and antigen-presenting cells in flounder (Paralichthys olivaceus), respectively. The co-stimulatory molecules were closely associated with cell immunity. In this paper, recombinant protein of flounder CD80/86 (rCD80/86) and phytohemagglutinin (PHA) were added to peripheral blood leukocytes (PBLs) in vitro. Lymphocytes were significantly proliferated with CFSE staining, and the proportion of CD4+ and CD28+ lymphocytes significantly increased. In the meantime, genes related to the CD28-CD80/86 signaling pathway or T cell markers were significantly upregulated (p < 0.05). For further study, the interaction between CD80/86 and CD28 was confirmed. The plasmid of CD28 (pCD28-FLAG and pVN-CD28) or CD80/86 (pVC-CD80/86) was successfully constructed. In addition, pVN-ΔCD28 without the conserved motif "TFPPPF" was constructed. The results showed that bands of pCD28-FLAG bound to rCD80/86 were detected by both anti-FLAG and anti-CD80/86. pVN-CD28 complemented to pVC-CD80/86 showing positive fluorescent signals, and pVN-ΔCD28 failed to combine with pVC-CD80/86. The motif "TFPPPF" in CD28 played a crucial role in this linkage. These results indicate that CD28 and CD80/86 molecules interact with each other, and their binding may modulate T lymphocytes immune response in flounder. This study proved the existence of CD28-CD80/86 signaling pathway in flounder.

Transcriptional reprogramming via signaling domains of CD2, CD28, and 4-1BB

Costimulatory signals provided to T cells during antigen encounter have a decisive role in the outcome of immune responses. Here, we used chimeric receptors harboring the extracellular domain of mouse inducible T cell costimulator (mICOS) to study transcriptional activation mediated by cytoplasmic sequences of the major T cell costimulatory receptors CD28, 4-1BB, and CD2. The chimeric receptors were introduced in a T cell reporter platform that allows to simultaneously evaluate nuclear factor κB (NF-κB), NFAT, and AP-1 activation. Engagement of the chimeric receptors induced distinct transcriptional profiles. CD28 signaling activated all three transcription factors, whereas 4-1BB strongly promoted NF-κB and AP-1 but downregulated NFAT activity. CD2 signals resulted in the strongest upregulation of NFAT. Transcriptome analysis revealed pronounced and distinct gene expression signatures upon CD2 and 4-1BB signaling. Using the intracellular sequence of CD28, we exemplify that distinct signaling motifs endow chimeric receptors with different costimulatory capacities.

A systems and computational biology perspective on advancing CAR therapy

In the recent decades, chimeric antigen receptor (CAR) therapy signaled a new revolutionary approach to cancer treatment. This method seeks to engineer immune cells expressing an artificially designed receptor, which would endue those cells with the ability to recognize and eliminate tumor cells. While some CAR therapies received FDA approval and others are subject to clinical trials, many aspects of their workings remain elusive. Techniques of systems and computational biology have been frequently employed to explain the operating principles of CAR therapy and suggest further design improvements. In this review, we sought to provide a comprehensive account of those efforts. Specifically, we discuss various computational models of CAR therapy ranging in scale from organismal to molecular. Then, we describe the molecular and functional properties of costimulatory domains frequently incorporated in CAR structure. Finally, we describe the signaling cascades by which those costimulatory domains elicit cellular response against the target. We hope that this comprehensive summary of computational and experimental studies will further motivate the use of systems approaches in advancing CAR therapy.

Co-Stimulatory Receptor Signaling in CAR-T Cells

T cell engineering strategies have emerged as successful immunotherapeutic approaches for the treatment of human cancer. Chimeric Antigen Receptor T (CAR-T) cell therapy represents a prominent synthetic biology approach to re-direct the specificity of a patient's autologous T cells toward a desired tumor antigen. CAR-T therapy is currently FDA approved for the treatment of hematological malignancies, including subsets of B cell lymphoma, acute lymphoblastic leukemia (ALL) and multiple myeloma. Mechanistically, CAR-mediated recognition of a tumor antigen results in propagation of T cell activation signals, including a co-stimulatory signal, resulting in CAR-T cell activation, proliferation, evasion of apoptosis, and acquisition of effector functions. The importance of including a co-stimulatory domain in CARs was recognized following limited success of early iteration CAR-T cell designs lacking co-stimulation. Today, all CAR-T cells in clinical use contain either a CD28 or 4-1BB co-stimulatory domain. Preclinical investigations are exploring utility of including additional co-stimulatory molecules such as ICOS, OX40 and CD27 or various combinations of multiple co-stimulatory domains. Clinical and preclinical evidence implicates the co-stimulatory signal in several aspects of CAR-T cell therapy including response kinetics, persistence and durability, and toxicity profiles each of which impact the safety and anti-tumor efficacy of this immunotherapy. Herein we provide an overview of CAR-T cell co-stimulation by the prototypical receptors and discuss current and emerging strategies to modulate co-stimulatory signals to enhance CAR-T cell function.

CD28 and chemokine receptors: Signalling amplifiers at the immunological synapse

T cells are master regulators of the immune response tuning, among others, B cells, macrophages and NK cells. To exert their functions requiring high sensibility and specificity, T cells need to integrate different stimuli from the surrounding microenvironment. A finely tuned signalling compartmentalization orchestrated in dynamic platforms is an essential requirement for the proper and efficient response of these cells to distinct triggers. During years, several studies have depicted the pivotal role of the cytoskeleton and lipid microdomains in controlling signalling compartmentalization during T cell activation and functions. Here, we discuss mechanisms responsible for signalling amplification and compartmentalization in T cell activation, focusing on the role of CD28, chemokine receptors and the actin cytoskeleton. We also take into account the detrimental effect of mutations carried by distinct signalling proteins giving rise to syndromes characterized by defects in T cell functionality.

Modelling predicts differences in chimeric antigen receptor T-cell signalling due to biological variability

In recent decades, chimeric antigen receptors (CARs) have been successfully used to generate engineered T cells capable of recognizing and eliminating cancer cells. The structure of CARs typically includes costimulatory domains, which enhance the T-cell response upon antigen encounter. However, it is not fully known how those co-stimulatory domains influence cell activation in the presence of biological variability. In this work, we used mathematical modelling to elucidate how the inclusion of one such costimulatory molecule, CD28, impacts the response of a population of CAR T cells under different sources of variability. Particularly, we demonstrate that CD28-bearing CARs mediate a faster and more consistent population response under both target antigen variability and kinetic rate variability. Next, we identify kinetic parameters that have the most impact on cell response time. Finally, based on our findings, we propose that enhancing the catalytic activity of lymphocyte-specific protein tyrosine kinase can result in drastically reduced and more consistent response times among heterogeneous CAR T-cell populations.

Itk Promotes the Integration of TCR and CD28 Costimulation through Its Direct Substrates SLP-76 and Gads

The costimulatory receptor CD28 synergizes with the TCR to promote IL-2 production, cell survival, and proliferation; yet the obligatory interdependence of TCR and CD28 signaling is not well understood. Upon TCR stimulation, Gads, a Grb2-family adaptor, bridges the interaction of two additional adaptors, LAT and SLP-76, to form a TCR-induced effector signaling complex. SLP-76 binds the Tec-family tyrosine kinase, Itk, which phosphorylates SLP-76 Y173 and PLC-γ1 Y783. In this study, we identified TCR-inducible, Itk-mediated phosphorylation of Gads Y45 in a human T cell line and in mouse primary T cells. Y45 is found within the N-terminal SH3 domain of Gads, an evolutionarily conserved domain with no known signaling function. Gads Y45 phosphorylation depended on the interaction of Gads with SLP-76 and on the dimerization-dependent binding of Gads to phospho-LAT. We provide evidence that Itk acts through SLP-76 and Gads to promote the TCR/CD28-induced activation of the RE/AP transcriptional element from the IL-2 promoter. Two Itk-related features of SLP-76, Y173 and a proline-rich Itk SH3 binding motif on SLP-76, were dispensable for activation of NFAT but selectively required for the TCR/CD28-induced increase in cytoplasmic and nuclear c-Rel and consequent RE/AP activation. We provide evidence that unphosphorylated, monomeric Gads mediates an RE/AP-directed inhibitory activity that is mitigated upon Gads dimerization and Y45 phosphorylation. This study illuminates a new, to our knowledge, regulatory module, in which TCR-induced, Itk-mediated phosphorylation sites on SLP-76 and Gads control the transcriptional response to TCR/CD28 costimulation, thus enforcing the obligatory interdependence of the TCR and CD28 signaling pathways.

ICOS agonism by JTX-2011 (vopratelimab) requires initial T cell priming and Fc cross-linking for optimal T cell activation and anti-tumor immunity in preclinical models

Immunotherapy checkpoint inhibitors, such as antibodies targeting PD-1 and CTLA-4, have demonstrated the potential of harnessing the immune system to treat cancer. However, despite encouraging results particularly with respect to survival, only a minority of patients benefit from these therapies. In clinical studies aimed at understanding changes in the immune system following immunotherapy treatment, ICOS (Inducible T cell CO-Stimulator) was shown to be significantly up-regulated on CD4+ T cells and this was associated with clinical activity, indicating that ICOS stimulatory activity may be beneficial in the treatment of solid tumors. In this report, we describe the generation of specific, species cross-reactive, agonist antibodies to ICOS, including the humanized clinical candidate, JTX-2011 (vopratelimab). Preclinical studies suggest that the ICOS stimulating antibodies require Fc receptor cross-linking for optimal agonistic activity. Notably, the ICOS antibodies do not exhibit superagonist properties but rather require T cell receptor (TCR)-mediated upregulation of ICOS for agonist activity. Treatment with the ICOS antibodies results in robust anti-tumor benefit and long-term protection in preclinical syngeneic mouse tumor models. Additional benefit is observed when the ICOS antibodies are administered in combination with anti-PD-1 and anti-CTLA-4 therapies. Based on the preclinical data, JTX-2011 is currently being developed in the clinical setting for the treatment of solid tumors.

Single residue in CD28-costimulated CAR-T cells limits long-term persistence and antitumor durability

Chimeric antigen receptor-T (CAR-T) cell therapies can eliminate relapsed and refractory tumors, but the durability of antitumor activity requires in vivo persistence. Differential signaling through the CAR costimulatory domain can alter the T cell metabolism, memory differentiation, and influence long-term persistence. CAR-T cells costimulated with 4-1BB or ICOS persist in xenograft models but those constructed with CD28 exhibit rapid clearance. Here, we show that a single amino acid residue in CD28 drove T cell exhaustion and hindered the persistence of CD28-based CAR-T cells and changing this asparagine to phenylalanine (CD28-YMFM) promoted durable antitumor control. In addition, CD28-YMFM CAR-T cells exhibited reduced T cell differentiation and exhaustion as well as increased skewing toward Th17 cells. Reciprocal modification of ICOS-containing CAR-T cells abolished in vivo persistence and antitumor activity. This finding suggests modifications to the costimulatory domains of CAR-T cells can enable longer persistence and thereby improve antitumor response.

Bridging the Gap: Modulatory Roles of the Grb2-Family Adaptor, Gads, in Cellular and Allergic Immune Responses

Antigen receptor signaling pathways are organized by adaptor proteins. Three adaptors, LAT, Gads, and SLP-76, form a heterotrimeric complex that mediates signaling by the T cell antigen receptor (TCR) and by the mast cell high affinity receptor for IgE (FcεRI). In both pathways, antigen recognition triggers tyrosine phosphorylation of LAT and SLP-76. The recruitment of SLP-76 to phospho-LAT is bridged by Gads, a Grb2 family adaptor composed of two SH3 domains flanking a central SH2 domain and an unstructured linker region. The LAT-Gads-SLP-76 complex is further incorporated into larger microclusters that mediate antigen receptor signaling. Gads is positively regulated by dimerization, which promotes its cooperative binding to LAT. Negative regulation occurs via phosphorylation or caspase-mediated cleavage of the linker region of Gads. FcεRI-mediated mast cell activation is profoundly impaired in LAT- Gads- or SLP-76-deficient mice. Unexpectedly, the thymic developmental phenotype of Gads-deficient mice is much milder than the phenotype of LAT- or SLP-76-deficient mice. This distinction suggests that Gads is not absolutely required for TCR signaling, but may modulate its sensitivity, or regulate a particular branch of the TCR signaling pathway; indeed, the phenotypic similarity of Gads- and Itk-deficient mice suggests a functional connection between Gads and Itk. Additional Gads binding partners include costimulatory proteins such as CD28 and CD6, adaptors such as Shc, ubiquitin regulatory proteins such as USP8 and AMSH, and kinases such as HPK1 and BCR-ABL, but the functional implications of these interactions are not yet fully understood. No interacting proteins or function have been ascribed to the evolutionarily conserved N-terminal SH3 of Gads. Here we explore the biochemical and functional properties of Gads, and its role in regulating allergy, T cell development and T-cell mediated immunity.

Signal Transduction Via Co-stimulatory and Co-inhibitory Receptors

T-cell receptor (TCR)-mediated antigen-specific stimulation is essential for initiating T-cell activation. However, signaling through the TCR alone is not sufficient for inducing an effective response. In addition to TCR-mediated signaling, signaling through antigen-independent co-stimulatory or co-inhibitory receptors is critically important not only for the full activation and functional differentiation of T cells but also for the termination and suppression of T-cell responses. Many studies have investigated the signaling pathways underlying the function of each molecular component. Co-stimulatory and co-inhibitory receptors have no kinase activity, but their cytoplasmic region contains unique functional motifs and potential phosphorylation sites. Engagement of co-stimulatory receptors leads to recruitment of specific binding partners, such as adaptor molecules, kinases, and phosphatases, via recognition of a specific motif. Consequently, each co-stimulatory receptor transduces a unique pattern of signaling pathways. This review focuses on our current understanding of the intracellular signaling pathways provided by co-stimulatory and co-inhibitory molecules, including B7:CD28 family members, immunoglobulin, and members of the tumor necrosis factor receptor superfamily.

Bridging the Gap: Modulatory Roles of the Grb2-Family Adaptor, Gads, in Cellular and Allergic Immune Responses

Antigen receptor signaling pathways are organized by adaptor proteins. Three adaptors, LAT, Gads, and SLP-76, form a heterotrimeric complex that mediates signaling by the T cell antigen receptor (TCR) and by the mast cell high affinity receptor for IgE (FcεRI). In both pathways, antigen recognition triggers tyrosine phosphorylation of LAT and SLP-76. The recruitment of SLP-76 to phospho-LAT is bridged by Gads, a Grb2 family adaptor composed of two SH3 domains flanking a central SH2 domain and an unstructured linker region. The LAT-Gads-SLP-76 complex is further incorporated into larger microclusters that mediate antigen receptor signaling. Gads is positively regulated by dimerization, which promotes its cooperative binding to LAT. Negative regulation occurs via phosphorylation or caspase-mediated cleavage of the linker region of Gads. FcεRI-mediated mast cell activation is profoundly impaired in LAT- Gads- or SLP-76-deficient mice. Unexpectedly, the thymic developmental phenotype of Gads-deficient mice is much milder than the phenotype of LAT- or SLP-76-deficient mice. This distinction suggests that Gads is not absolutely required for TCR signaling, but may modulate its sensitivity, or regulate a particular branch of the TCR signaling pathway; indeed, the phenotypic similarity of Gads- and Itk-deficient mice suggests a functional connection between Gads and Itk. Additional Gads binding partners include costimulatory proteins such as CD28 and CD6, adaptors such as Shc, ubiquitin regulatory proteins such as USP8 and AMSH, and kinases such as HPK1 and BCR-ABL, but the functional implications of these interactions are not yet fully understood. No interacting proteins or function have been ascribed to the evolutionarily conserved N-terminal SH3 of Gads. Here we explore the biochemical and functional properties of Gads, and its role in regulating allergy, T cell development and T-cell mediated immunity.

Role of TRAFs in Signaling Pathways Controlling T Follicular Helper Cell Differentiation and T Cell-Dependent Antibody Responses

Follicular helper T (T_FH) cells represent a highly specialized CD4⁺ T cell subpopulation that supports the generation of germinal centers (GC) and provides B cells with critical signals promoting antibody class switching, generation of high affinity antibodies, and memory formation. T_FH cells are characterized by the expression of the chemokine receptor CXCR5, the transcription factor Bcl-6, costimulatory molecules ICOS, and PD-1, and the production of cytokine IL-21. The acquisition of a T_FH phenotype is a complex and multistep process that involves signals received through engagement of the TCR along with a multitude of costimulatory molecules and cytokines receptors. Members of the Tumor necrosis factor Receptor Associated Factors (TRAF) represent one of the major classes of signaling mediators involved in the differentiation and functions of T_FH cells. TRAF molecules are the canonical adaptor molecules that physically interact with members of the Tumor Necrosis Factor Receptor Superfamily (TNFRSF) and actively modulate their downstream signaling cascades through their adaptor function and/or E3 ubiquitin ligase activity. OX-40, GITR, and 4-1BB are the TRAF-dependent TNFRSF members that have been implicated in the differentiation and functions of T_FH cells. On the other hand, emerging data demonstrate that TRAF proteins also participate in signaling from the TCR and CD28, which deliver critical signals leading to the differentiation of T_FH cells. More intriguingly, we recently showed that the cytoplasmic tail of ICOS contains a conserved TANK-binding kinase 1 (TBK1)-binding motif that is shared with TBK1-binding TRAF proteins. The presence of this TRAF-mimicking signaling module downstream of ICOS is required to mediate the maturation step during T_FH differentiation. In addition, JAK-STAT pathways emanating from IL-2, IL-6, IL-21, and IL-27 cytokine receptors affect T_FH development, and crosstalk between TRAF-mediated pathways and the JAK-STAT pathways can contribute to generate integrated signals required to drive and sustain T_FH differentiation. In this review, we will introduce the molecular interactions and the major signaling pathways controlling the differentiation of T_FH cells. In each case, we will highlight the contributions of TRAF proteins to these signaling pathways. Finally, we will discuss the role of individual TRAF proteins in the regulation of T cell-dependent humoral responses.

A non-conserved amino acid variant regulates differential signalling between human and mouse CD28

CD28 superagonistic antibodies (CD28SAb) can preferentially activate and expand immunosuppressive regulatory T cells (Treg) in mice. However, pre-clinical trials assessing CD28SAbs for the therapy of autoimmune diseases reveal severe systemic inflammatory response syndrome in humans, thereby implying the existence of distinct signalling abilities between human and mouse CD28. Here, we show that a single amino acid variant within the C-terminal proline-rich motif of human and mouse CD28 (P²¹² in human vs. A²¹⁰ in mouse) regulates CD28-induced NF-κB activation and pro-inflammatory cytokine gene expression. Moreover, this Y²⁰⁹APP²¹² sequence in humans is crucial for the association of CD28 with the Nck adaptor protein for actin cytoskeleton reorganisation events necessary for CD28 autonomous signalling. This study thus unveils different outcomes between human and mouse CD28 signalling to underscore the importance of species difference when transferring results from preclinical models to the bedside.

CD28 transmits co-stimulatory signals for the activation of both mouse and human T cells, but in vivo hyperactivation of CD28 has opposite effects on system immunity. Here, the authors show that a single amino acid difference between mouse and human CD28 dictates this function distinction via differential recruitment of Nck.

Expression of GADS enhances FLT3-induced mitogenic signaling

GADS is a member of a family of SH2 and SH3 domain-containing adaptors that functions in tyrosine kinase-mediated signaling cascades. Its expression is largely restricted to hematopoietic tissues and cell lines. Therefore, GADS is mainly involved in leukocyte-specific protein tyrosine kinase signaling. GADS is known to interact with tyrosine-phosphorylated SHC, BCR-ABL and KIT. The SH2 domain of GADS has a similar binding specificity to that of GRB2 but its SH3 domain displays a different binding specificity, and thus it is involved in other downstream signaling pathways than GRB2. In the present study, we examined the role of GADS in FLT3 signaling. FLT3 is a type III receptor tyrosine kinase, which is mutated in more than 30% of acute myeloid leukemia (AML) and the most common mutations is the internal tandem duplication (ITD) mutations. We observed that expression of GADS enhanced oncogenic FLT3-ITD-induced cell proliferation and colony formation in vitro. In a mouse xenograft model, GADS accelerated FLT3-ITD-dependent tumor formation. Furthermore, expression of GADS induced a transcriptional program leading to upregulation of MYC and mTORC1 target genes. GADS localizes to the cell membrane and strongly binds to ligand-stimulated wild-type FLT3 or is constitutively associated with the oncogenic mutant FLT3-ITD. We mapped the binding sites in FLT3 to pY955 and pY969 which overlaps with the GRB2 binding sites. Expression of GADS enhanced FLT3-mediated phosphorylation of AKT, ERK1/2, p38 and STAT5. Taken together, our data suggests that GADS is an important downstream component of FLT3 signaling and expression of GADS potentiates FLT3-mediated mitogenic signaling.

Membrane Rafts in T Cell Activation: A Spotlight on CD28 Costimulation

Spatiotemporal compartmentalization of signaling pathways and second messengers is pivotal for cell biology and membrane rafts are, therefore, required for several lymphocyte functions. On the other hand, T cells have the specific necessity of tuning signaling amplification depending on the context in which the antigen is presented. In this review, we discuss of membrane rafts in the context of T cell signaling, focusing on CD28-mediated costimulation.

Signals that drive T follicular helper cell formation

T follicular helper (Tfh) cells are a distinct type of CD4⁺ T cell specialized in providing help to B cells during the germinal centre (GC) reaction. As such, they are critical determinants of the quality of an antibody response following antigen challenge. Excessive production of Tfh cells can result in autoimmunity whereas too few can result in inadequate protection from infection. Hence, their differentiation and maintenance must be tightly regulated to ensure appropriate but limited help to B cells. Unlike the majority of other CD4⁺ T-cell subsets, Tfh cell differentiation occurs in three phases defined by their anatomical location. During each phase of differentiation the emerging Tfh cells express distinct patterns of co-receptors, which work together with the T-cell receptor (TCR) to drive Tfh differentiation. These signals provided by both TCR and co-receptors during Tfh differentiation alter proliferation, survival, metabolism, cytokine production and transcription factor expression. This review will discuss how engagement of TCR and co-receptors work together to shape the formation and function of Tfh cells.

Crystal Structures and Thermodynamic Analysis Reveal Distinct Mechanisms of CD28 Phosphopeptide Binding to the Src Homology 2 (SH2) Domains of Three Adaptor Proteins

Full activation of T cells and differentiation into effector T cells are essential for many immune responses and require co-stimulatory signaling via the CD28 receptor. Extracellular ligand binding to CD28 recruits protein-tyrosine kinases to its cytoplasmic tail, which contains a YMNM motif. Following phosphorylation of the tyrosine, the proteins growth factor receptor-bound protein 2 (Grb2), Grb2-related adaptor downstream of Shc (Gads), and p85 subunit of phosphoinositide 3-kinase may bind to pYMNM (where pY is phosphotyrosine) via their Src homology 2 (SH2) domains, leading to downstream signaling to distinct immune pathways. These three adaptor proteins bind to the same site on CD28 with variable affinity, and all are important for CD28-mediated co-stimulatory function. However, the mechanism of how these proteins recognize and compete for CD28 is unclear. To visualize their interactions with CD28, we have determined the crystal structures of Gads SH2 and two p85 SH2 domains in complex with a CD28-derived phosphopeptide. The high resolution structures obtained revealed that, whereas the CD28 phosphopeptide bound to Gads SH2 is in a bent conformation similar to that when bound to Grb2 SH2, it adopts a more extended conformation when bound to the N- and C-terminal SH2 domains of p85. These differences observed in the peptide-protein interactions correlated well with the affinity and other thermodynamic parameters for each interaction determined by isothermal titration calorimetry. The detailed insight into these interactions reported here may inform the development of compounds that specifically inhibit the association of CD28 with these adaptor proteins to suppress excessive T cell responses, such as in allergies and autoimmune diseases.

CD28 Costimulation: From Mechanism to Therapy

Ligation of the CD28 receptor on T cells provides a critical second signal alongside T cell receptor (TCR) ligation for naive T cell activation. Here, we discuss the expression, structure, and biochemistry of CD28 and its ligands. CD28 signals play a key role in many T cell processes, including cytoskeletal remodeling, production of cytokines, survival, and differentiation. CD28 ligation leads to unique epigenetic, transcriptional, and post-translational changes in T cells that cannot be recapitulated by TCR ligation alone. We discuss the function of CD28 and its ligands in both effector and regulatory T cells. CD28 is critical for regulatory T cell survival and the maintenance of immune homeostasis. We outline the roles that CD28 and its family members play in human disease and we review the clinical efficacy of drugs that block CD28 ligands. Despite the centrality of CD28 and its family members and ligands to immune function, many aspects of CD28 biology remain unclear. Translation of a basic understanding of CD28 function into immunomodulatory therapeutics has been uneven, with both successes and failures. Such real-world results might stem from multiple factors, including complex receptor-ligand interactions among CD28 family members, differences between the mouse and human CD28 families, and cell-type specific roles of CD28 family members.

Phosphatidylinositol 4-Phosphate 5-Kinases in the Regulation of T Cell Activation

Phosphatidylinositol 4,5-biphosphate kinases (PIP5Ks) are critical regulators of T cell activation being the main enzymes involved in the synthesis of phosphatidylinositol 4,5-biphosphate (PIP2). PIP2 is indeed a pivotal regulator of the actin cytoskeleton, thus controlling T cell polarization and migration, stable adhesion to antigen-presenting cells, spatial organization of the immunological synapse, and co-stimulation. Moreover, PIP2 also serves as a precursor for the second messengers inositol triphosphate, diacylglycerol, and phosphatidylinositol 3,4,5-triphosphate, which are essential for the activation of signaling pathways regulating cytokine production, cell cycle progression, survival, metabolism, and differentiation. Here, we discuss the impact of PIP5Ks on several T lymphocyte functions with a specific focus on the role of CD28 co-stimulation in PIP5K compartimentalization and activation.

CD28 costimulatory signals in T lymphocyte activation: Emerging functions beyond a qualitative and quantitative support to TCR signalling

CD28 is one of the most important co-stimulatory receptors necessary for full T lymphocyte activation. By binding its cognate ligands, B7.1/CD80 or B7.2/CD86, expressed on the surface of professional antigen presenting cells (APC), CD28 initiates several signalling cascades, which qualitatively and quantitatively support T cell receptor (TCR) signalling. More recent data evidenced that human CD28 can also act as a TCR-independent signalling unit, by delivering specific signals, which regulate the expression of pro-inflammatory cytokine/chemokines. Despite the enormous progresses made in identifying the mechanisms and molecules involved in CD28 signalling properties, much remains to be elucidated, especially in the light of the functional differences observed between human and mouse CD28. In this review we provide an overview of the current mechanisms and molecules through which CD28 support TCR signalling and highlight recent findings on the specific signalling motifs that regulate the unique pro-inflammatory activity of human CD28.

Grb2 Is Important for T Cell Development, Th Cell Differentiation, and Induction of Experimental Autoimmune Encephalomyelitis

The small adaptor protein growth factor receptor-bound protein 2 (Grb2) modulates and integrates signals from receptors on cellular surfaces in inner signaling pathways. In murine T cells, Grb2 is crucial for amplification of TCR signaling. T cell-specific Grb2(fl/fl) Lckcre(tg) Grb2-deficient mice show reduced T cell numbers due to impaired negative and positive selection. In this study, we found that T cell numbers in Grb2(fl/fl) CD4cre(tg) mice were normal in the thymus and were only slightly affected in the periphery. Ex vivo analysis of CD4(+) Th cell populations revealed an increased amount of Th1 cells within the CD4(+) population of Grb2(fl/fl) CD4cre(tg) mice. Additionally, Grb2-deficient T cells showed a greater potential to differentiate into Th17 cells in vitro. To test whether these changes in Th cell differentiation potential rendered Grb2(fl/fl) CD4cre(tg) mice more prone to inflammatory diseases, we used the murine Th1 cell- and Th17 cell-driven model of experimental autoimmune encephalomyelitis (EAE). In contrast to our expectations, Grb2(fl/fl) CD4cre(tg) mice developed a milder form of EAE. The impaired EAE disease can be explained by the reduced proliferation rate of Grb2-deficient CD4(+) T cells upon stimulation with IL-2 or upon activation by allogeneic dendritic cells, because the activation of T cells by dendritic cells and the subsequent T cell proliferation are known to be crucial factors for the induction of EAE. In summary, Grb2-deficient T cells show defects in T cell development, increased Th1 and Th17 cell differentiation capacities, and impaired proliferation after activation by dendritic cells, which likely reduce the clinical symptoms of EAE.

Recurrent activating mutations of CD28 in peripheral T-cell lymphomas

Peripheral T-cell lymphomas (PTCLs) comprise a heterogeneous group of mature T-cell neoplasms with a poor prognosis. Recently, mutations in TET2 and other epigenetic modifiers as well as RHOA have been identified in these diseases, particularly in angioimmunoblastic T-cell lymphoma (AITL). CD28 is the major co-stimulatory receptor in T cells which, upon binding ligand, induces sustained T-cell proliferation and cytokine production when combined with T-cell receptor stimulation. We have identified recurrent mutations in CD28 in PTCLs. Two residues-D124 and T195-were recurrently mutated in 11.3% of cases of AITL and in one case of PTCL, not otherwise specified (PTCL-NOS). Surface plasmon resonance analysis of mutations at these residues with predicted differential partner interactions showed increased affinity for ligand CD86 (residue D124) and increased affinity for intracellular adaptor proteins GRB2 and GADS/GRAP2 (residue T195). Molecular modeling studies on each of these mutations suggested how these mutants result in increased affinities. We found increased transcription of the CD28-responsive genes CD226 and TNFA in cells expressing the T195P mutant in response to CD3 and CD86 co-stimulation and increased downstream activation of NF-κB by both D124V and T195P mutants, suggesting a potential therapeutic target in CD28-mutated PTCLs.

Roquin--a multifunctional regulator of immune homeostasis

Roquin-1 (Rc3h1) is an E3 ubiquitin ligase originally discovered in a mutational screen for genetic factors contributory to systemic lupus erythematosus-like symptoms in mice. A single base-pair mutation in the Rc3h1 gene resulted in the manifestation of autoantibody production and sustained immunological inflammation characterized by excessive T follicular helper cell activation and formation of germinal centers. Subsequent studies have uncovered a multifactorial process by which Roquin-1 contributes to the maintenance of immune homeostasis. Through its interactions with partner proteins, Roquin-1 targets mRNAs for decay with inducible costimulator being a primary target. In this review, we discuss newly discovered functions of Roquin-1 in the immune system and inflammation, and in disease manifestation, and discuss avenues of further research. A model is presented for the role of Roquin in health and disease.

Five Layers of Receptor Signaling in γδ T-Cell Differentiation and Activation

The contributions of γδ T-cells to immunity to infection or tumors critically depend on their activation and differentiation into effectors capable of secreting cytokines and killing infected or transformed cells. These processes are molecularly controlled by surface receptors that capture key extracellular cues and convey downstream intracellular signals that regulate γδ T-cell physiology. The understanding of how environmental signals are integrated by γδ T-cells is critical for their manipulation in clinical settings. Here, we discuss how different classes of surface receptors impact on human and murine γδ T-cell differentiation, activation, and expansion. In particular, we review the role of five receptor types: the T-cell receptor (TCR), costimulatory receptors, cytokine receptors, NK receptors, and inhibitory receptors. Some of the key players are the costimulatory receptors CD27 and CD28, which differentially impact on pro-inflammatory subsets of γδ T-cells; the cytokine receptors IL-2R, IL-7R, and IL-15R, which drive functional differentiation and expansion of γδ T-cells; the NK receptor NKG2D and its contribution to γδ T-cell cytotoxicity; and the inhibitory receptors PD-1 and BTLA that control γδ T-cell homeostasis. We discuss these and other receptors in the context of a five-step model of receptor signaling in γδ T-cell differentiation and activation, and discuss its implications for the manipulation of γδ T-cells in immunotherapy.

Emerging cellular networks for regulation of T follicular helper cells

The cellular networks that regulate humoral immune responses have been a focus of research over the past three decades. Studies have shown that inhibition of immune responses can be attributed to both suppressor T cells and B cells. More recently, T follicular helper (Tfh) cells have been identified as a target of immune regulation. Tfh cells are a subset of highly activated T helper cells specialized for providing cognate help to B cells during germinal center reactions. In this review, we describe emerging evidence for cellular networks that alter Tfh cell phenotype and function and regulate antibody production during the germinal center reaction. We discuss how these new findings influence our understanding of Tfh cells.

ICOS receptor instructs T follicular helper cell versus effector cell differentiation via induction of the transcriptional repressor Bcl6

The nature of follicular helper CD4(+) T (Tfh) cell differentiation remains controversial, including the minimal signals required for Tfh cell differentiation and the time at which Tfh cell differentiation occurs. Here we determine that Tfh cell development initiates immediately during dendritic cell (DC) priming in vivo. We demonstrate that inducible costimulator (ICOS) provides a critical early signal to induce the transcription factor Bcl6, and Bcl6 then induces CXCR5, the canonical feature of Tfh cells. Strikingly, a bifurcation between Tfh and effector Th cells was measurable by the second cell division of CD4(+) T cells, at day 2 after an acute viral infection: IL2Rα(int) cells expressed Bcl6 and CXCR5 (Tfh cell program), whereas IL2Rα(hi) cells exhibited strong Blimp1 expression that repressed Bcl6 (effector Th cell program). Virtually complete polarization between Bcl6(+) Tfh cells and Blimp1(+) effector Th cell populations developed by 72 hr, even without B cells. Tfh cells were subsequently lost in the absence of B cells, demonstrating a B cell requirement for maintenance of Bcl6 and Tfh cell commitment via sequential ICOS signals.

A novel association between filamin A and NF-κB inducing kinase couples CD28 to inhibitor of NF-κB kinase α and NF-κB activation

CD28 costimulatory molecule plays a critical role in the activation of NF-κB. Indeed, while stimulation of T cells with either professional APCs or anti-TCR plus anti-CD28 antibodies efficiently activates NF-κB, TCR alone fails to do that. Moreover, CD28 stimulation by B7 in the absence of TCR may activate IκB kinase α (IKKα) and a non-canonical NF-κB2-like pathway, in human primary CD4(+) T cells. Despite its functional relevance in NF-κB activation, the molecules connecting autonomous CD28-mediated signals to IKKα and NF-κB activation remain still unknown. In searching for specific upstream activators linking CD28 to the IKKα/NF-κB cascade, we identify a novel constitutive association between filamin A (FLNa) and the NF-κB inducing kinase (NIK), in both Jurkat and human primary T cells. Following CD28 engagement by B7, in the absence of TCR, FLNa-associated NIK is activated and induces IKKα kinase activity. Both proline (P(208)YAP(211)P(212)) and tyrosine residues (Y(206)QPY(209)APP) within the C-terminal proline-rich motif of CD28 are involved in the recruitment of FLNa/NIK complexes to the membrane as well as in the activation of NIK and IKKα.

Dynamic regulation of T cell activation and co-stimulation through TCR-microclusters

TCR-microclusters (MC) are generated upon TCR stimulation prior to the immune synapse formation independently of lipid rafts. TCR-MCs contain receptors, kinases and adaptors, and function as the signaling unit for T cell activation. The TCR complex, but not the signaling molecules, is transported to the center to form cSMAC. The co-stimulation receptor CD28 joins the signaling region of cSMAC and recruits PKCθ and Carma1. CTLA-4 accumulates in the same region and competes with CD28 for negative regulation of T cell activation. T cell activation is therefore mediated by two spatially distinct signaling compartments: TCR signaling by the peripheral TCR-MC and co-stimulation signal by the central signaling cSMAC.

Grb2, a simple adapter with complex roles in lymphocyte development, function, and signaling

Lymphocyte development, activation, and tolerance depend on antigen receptor signaling transduced via multiple intracellular signalosomes. These signalosomes are assembled by different adapters. Given that signaling molecules can be either positive or negative regulators for a biochemical target, the complex of a target with different regulator may dictate the final signaling outcome. Grb2 is a simple adapter known to be involved in a variety of growth factor receptor signaling. However, its role in antigen receptor signaling as well as lymphocyte development and function has emerged only recently. Despite its simple molecular structure, recent experiments show that Grb2 may play a complex role in T and B-cell antigen receptor signaling. In this article, we review recent findings about the physiological role of Grb2 in T and B-cell development and activation and summarize the current mechanistic understanding of how Grb2 exerts its function following T and B-cell antigen receptor stimulation.

An enigmatic tail of CD28 signaling

CD28 costimulation regulates a wide range of cellular processes, from proliferation and survival to promoting the differentiation of specialized T-cell subsets. Since first being identified over 20 years ago, CD28 has remained a subject of intense study because of its profound consequences on T cell function and its potential for therapeutic manipulation. In this review we highlight the signaling cascades initiated by the major signaling motifs in CD28, focusing on PI-3 kinase-dependent and -independent pathways and how these are linked to specific cellular outcomes. Recent studies using gene targeted knockin mice have clarified the relative importance of these motifs on in vivo immune responses; however, much remains to be elucidated. Understanding the mechanism behind costimulation holds great potential for development of new clinically relevant reagents, a fact beginning to be realized with the advent of drugs that prevent CD28 ligation and signaling.

PD-1 signaling in primary T cells

SUMMARY

Programmed death-1 (PD-1) is a cell surface molecule that regulates the adaptive immune response. Engagement of PD-1 by its ligands PD-L1 or PD-L2 transduces a signal that inhibits T-cell proliferation, cytokine production, and cytolytic function. While a great deal is known concerning the biologic roles PD-1 plays in regulating the primary immune response and in T-cell exhaustion, comparatively little is known regarding how PD-1 ligation alters signaling pathways. PD-1 ligation is known to inhibit membrane-proximal T-cell signaling events, while ligation of the related inhibitory molecule cytotoxic T-lymphocyte antigen-4 appears to target more downstream signaling pathways. A major obstacle to an in-depth understanding of PD-1 signaling is the lack of physiologic models in which to study signal transduction. This review focuses on: (i) signaling pathways altered by PD-1 ligation, (ii) factors recruited upon PD-1 phosphorylation, and (iii) exploring the hypothesis that PD-1 ligation induces distinct signals during various stages of immune-cell differentiation. Lastly, we describe models to dissect the function of the PD-1 cytoplasmic tail using primary cells in the absence of agonist antibodies.

Dynamic regulation of T-cell costimulation through TCR-CD28 microclusters

SUMMARY

T-cell activation requires contact between T cells and antigen-presenting cells (APCs) to bring T-cell receptors (TCRs) and major histocompatibility complex peptide (MHCp) together to the same complex. These complexes rearrange to form a concentric circular structure, the immunological synapse (IS). After the discovery of the IS, dynamic imaging technologies have revealed the details of the IS and provided important insights for T-cell activation. We have redefined a minimal unit of T-cell activation, the 'TCR microcluster', which recognizes MHCp, triggers an assembly of assorted molecules downstream of the TCR, and induces effective signaling from TCRs. The relationship between TCR signaling and costimulatory signaling was analyzed in terms of the TCR microcluster. CD28, the most valuable costimulatory receptor, forms TCR-CD28 microclusters in cooperation with TCRs, associates with protein kinase C theta, and effectively induces initial T-cell activation. After mature IS formation, CD28 microclusters accumulate at a particular subregion of the IS, where they continuously assemble with the kinases and not TCRs, and generate sustained T-cell signaling. We propose here a 'TCR-CD28 microcluster' model in which TCR and costimulatory microclusters are spatiotemporally formed at the IS and exhibit fine-tuning of T-cell responses by assembling with specific players downstream of the TCR and CD28.

Two pathways of costimulation through CD28

CD28 is recognized as the primary costimulatory molecule involved in the activation of naïve T cells. However, the biochemical signaling pathways that are activated by CD28 and how these pathways are integrated with TCR signaling are still not understood. We have recently shown that there are at least two independent activation pathways induced by CD28 costimulation. One is integrated with TCR signaling in the context of the immunological synapse and is mediated through transcriptional enhancement and the second is mediated through the induction of mRNA stability. Here, we review the immunological consequences and biochemical mechanisms associated with CD28 costimulation and discuss the major questions that need to be resolved to understand the molecular mechanisms that transduce CD28 costimulation.

Roquin differentiates the specialized functions of duplicated T cell costimulatory receptor genes CD28 and ICOS

During evolutionary adaptation in the immune system, host defense is traded off against autoreactivity. Signals through the costimulatory receptor CD28 enable T cells to respond specifically to pathogens, whereas those through the related costimulatory receptor, ICOS, which arose by gene duplication, are critical for affinity maturation and memory antibody responses. ICOS ligand, unlike the pathogen-inducible CD28 ligands, is widely and constitutively expressed in the immune system. Here, we show that crosstalk between these two pathways provides a mechanism for obviating the normal T cell dependence on CD28. Several CD28-mediated responses-generation of follicular helper T cells, germinal center formation, T helper 1 cell-dependent extrafollicular antibody responses to Salmonella and bacterial clearance, and regulatory T cell homeostasis-became independent of CD28 and dependent on ICOS when the E3 ubiquitin ligase Roquin was mutated. Mechanisms to functionally compartmentalize ICOS and CD28 signals are thus critical for two-signal control of normal immune reactions.

Expression of the Grb2-related RET adapter protein Grap-2 in human medullary thyroid carcinoma

Inappropriate signaling of the RET receptor tyrosine kinase causes different forms of human thyroid malignancy. We have previously identified the adaptor Grap-2 as RET binding protein. To verify involvement of Grap-2 in RET oncogenic signaling we performed sequence and expression analysis of Grap-2 in 15 human MTC samples. All tumors displayed marked Grap-2 mRNA and protein expression without a linear correlation. Beyond one conservative base pair substitution we detected no further alteration in genomic Grap-2 sequence. Consistent Grap-2 expression suggests a specific role for this adaptor in human MTC, while qualitative alterations do not appear to influence RET signaling.

Spatiotemporal regulation of T cell costimulation by TCR-CD28 microclusters and protein kinase C theta translocation

T cell activation is mediated by microclusters (MCs) containing T cell receptors (TCRs), kinases, and adaptors. Although TCR MCs translocate to form a central supramolecular activation cluster (cSMAC) of the immunological synapse at the interface of a T cell and an antigen-presenting cell, the role of MC translocation in T cell signaling remains unclear. Here, we found that the accumulation of MCs at cSMAC was important for T cell costimulation. Costimulatory receptor CD28 was initially recruited coordinately with TCR to MCs, and its signals were mediated through the assembly with the kinase PKCtheta. The accumulation of MCs at the cSMAC was accompanied by the segregation of CD28 from the TCR, which resulted in the translocation of both CD28 and PKCtheta to a spatially unique subregion of cSMAC. Thus, costimulation is mediated by the generation of a unique costimulatory compartment in the cSMAC via the dynamic regulation of MC translocation.

CD28 stimulation triggers NF-kappaB activation through the CARMA1-PKCtheta-Grb2/Gads axis

CD28 stimulation contributes to activation of the IL-2 promoter by up-regulating the activity of several transcription factors, including nuclear factor kappaB (NF-kappaB)/Rel family members. However, the signal-transducing cascades linking the CD28 molecule and activation of NF-kappaB remain unclear. Protein kinase C (PKC) , CARMA1 and Bcl10 have recently been reported to integrate TCR-mediated NF-kappaB activation. However, since the data in these studies were drawn from experiments in which T cells were usually stimulated with both TCR and CD28, the relative contributions of TCR- and CD28-mediated signals to initiation of the NF-kappaB pathway remain elusive. To examine the role of these molecules in NF-kappaB activation through CD28-mediated stimulation, Bcl10 was over-expressed in Jurkat cells and their NF-kappaB activation by CD28- or TCR-cross-linking was evaluated. We found that CD28 stimulation alone can induce NF-kappaB activation in Bcl10-over-expressing Jurkat cells, whereas TCR stimulation alone has only little effect. In addition, we found that Bcl10-induced NF-kappaB activation through CD28-mediated stimulation could be blocked by the dominant-negative form of PKC or CARMA1. Furthermore, genetic studies revealed that Grb2/Gads binding, but not phosphatidylinositol 3-kinase binding, is important in CD28-mediated NF-kappaB activation. These findings indicate that the PKC-CARMA1-Bcl10 signaling pathway participates in the CD28 co-stimulatory signal independently of the TCR-signaling pathway, which leads us to propose that the activation of the NF-kappaB-signaling pathway via PKC-CARMA1-Bcl10 may be markedly dependent on CD28 stimulation rather than TCR stimulation.

CD28 ligation in the absence of TCR promotes RelA/NF-kappaB recruitment and trans-activation of the HIV-1 LTR

CD28 is one of the most important co-stimulatory receptors necessary for full T lymphocyte activation. CD28 can act as a TCR-independent signalling unit by delivering specific signals which may induce HIV transcription and replication. However, the mechanisms by which CD28 regulates HIV expression remain largely unknown. Here we show that the TCR-independent CD28 signals lead to the trans-activation of HIV-1 LTR in an NF-kappaB-dependent manner. In particular, we found that CD28 engagement by B7 induces the specific recruitment of RelA/NF-kappaB subunit to the HIV-1 LTR promoter both in vitro and in ex vivo infected cells. The results obtained by mutating specific tyrosine residues within the CD28 cytoplasmic tail as well as by using LY294002 inhibitory drug evidenced that the recruitment and activation of the phosphatidylinositol 3-kinase/Akt signalling pathway is crucial in mediating CD28-induced HIV transcription through RelA/NF-kappaB.

CD28 and Grb-2, relative to Gads or Grap, preferentially co-operate with Vav1 in the activation of NFAT/AP-1 transcription

The co-receptor CD28 binds to several intracellular proteins including PI3 kinase, Grb-2, Gads and ITK. Grb-2 and PI3 kinase binding has been mapped to the pYMNM motif within the cytoplasmic tail of CD28 and has been shown to play a role in co-stimulation. In this study, we demonstrate that amongst the Grb-2 family adapter proteins, CD28 precipitated Grb-2 and specifically co-operated in the up-regulation of NFAT/AP-1 transcription. By contrast, Gads and Grap either failed or only weakly collaborated with CD28 ligation. Further, the loss of Grb-2 binding interferes with the ability of Vav1 to co-operate with CD28. Anti-CD28 ligation alone was capable for co-operating with Grb-2 or Grb-2-Vav1. Our findings define a pathway involving CD28 binding to Grb-2 and its co-operativity with Vav1 in the regulation of T-cell co-stimulation.

The NKG2D receptor: sensing stressed cells

The activating killer cell lectin-like receptor NKG2D plays a key role in the natural killer (NK) cell-mediated lysis of tumours and infected cells. Unlike other receptors, the ligands recognised by NKG2D are 'induced-self' ligands on stressed cells. This system requires precise regulation because inappropriate expression of NKG2D ligands might compromise NK cell activation. For therapeutic purposes it is essential to understand the mechanisms that regulate the expression and function of the NKG2D system. This review focuses on the importance of the signalling pathways involved in the regulation of the NKG2D receptor and its ligand expression in arming the immune response against infected or tumour cells and for the identification of new molecular targets and therapeutic strategies.

Signalling to suit function: tailoring phosphoinositide 3-kinase during T-cell activation

Members of the CD28 family of co-receptors are crucial determinants of the outcome of T-cell activation. These receptors interact with ligands in the B7 family and either costimulate or co-inhibit signals through antigen-specific receptors. The T-cell-costimulatory molecules CD28 and inducible costimulator recruit and activate class 1A phosphoinositide 3-kinase (PI3K). Interestingly, the co-inhibitory molecules cytotoxic T lymphocyte antigen-4 and B and T lymphocyte attenuator also interact with class 1A PI3K. However, all co-inhibitory receptors share an ability to oppose activation of the key PI3K effector protein kinase B (also known as Akt). Recent evidence suggests that distinct mechanisms exist to limit Akt activation by different co-inhibitory receptors. This article examines how differential positive or negative regulation of the PI3K-Akt signalling pathway by CD28 family receptors enables functional differences between the receptors.

IMGT Colliers de Perles and IgSF domain standardization for T cell costimulatory activatory (CD28, ICOS) and inhibitory (CTLA4, PDCD1 and BTLA) receptors

T cell activation depends on the specific recognition by their T cell receptors (TR) of antigenic peptides bound to major histocompatibility complex (pMHC). Optimal T cell responses occur when T cells not only receive antigen-specific signals through the TR but also non-antigen-specific costimulatory activatory or inhibitory signals through costimulatory receptors. The activatory CD28/B7-1 (or B7-2), inhibitory CTLA4/B7-1 (or B7-2), activatory ICOS/B7H2 and inhibitory PDCD1/B7H1 (or B7DC) pathways involve the interaction of the V-LIKE-DOMAIN of the receptor with a B7 family member. The BTLA/HVEM pathway involves the interaction of the BTLA receptor C-LIKE-DOMAIN with HVEM, a TNFR family member. The human and mouse CD28, CTLA4, ICOS, PDCD1 and BTLA genes, alleles and alternative transcripts and the IMGT Colliers de Perles of the IgSF domains, based on the IMGT unique numbering, are described according to the IMGT-ONTOLOGY concepts of IMGT, the international ImMunoGeneTics information system, http://imgt.cines.fr).