Decreased CD4 expression by polarized T helper 2 cells contributes to suboptimal TCR-induced phosphorylation and reduced Ca2+ signaling

A combined biochemical and cellular approach reveals Zn2+-dependent hetero- and homodimeric CD4 and Lck assemblies in T cells

The CD4 or CD8 co-receptors' interaction with the protein-tyrosine kinase Lck initiates the tyrosine phosphorylation cascade leading to T cell activation. A critical question is: to what extent are co-receptors and Lck coupled? Our contribution concerns Zn2+, indispensable for CD4- and CD8-Lck formation. We combined biochemical and cellular approaches to show that dynamic fluctuations of free Zn2+ in physiological ranges influence Zn(CD4)2 and Zn(CD4)(Lck) species formation and their ratio, although the same Zn(Cys)2(Cys)2 cores. Moreover, we demonstrated that the affinity of Zn2+ to CD4 and CD4-Lck species differs significantly. Increased intracellular free Zn2+ concentration in T cells causes higher CD4 partitioning in the plasma membrane. We additionally found that CD4 palmitoylation decreases the specificity of CD4-Lck formation in the reconstituted membrane model. Our findings help elucidate co-receptor-Lck coupling stoichiometry and demonstrate that intracellular free Zn2+ has a major role in the interplay between CD4 dimers and CD4-Lck assembly.

CD4 expression in effector T cells depends on DNA demethylation over a developmentally established stimulus-responsive element

The epigenetic patterns that are established during early thymic development might determine mature T cell physiology and function, but the molecular basis and topography of the genetic elements involved are not fully known. Here we show, using the Cd4 locus as a paradigm for early developmental programming, that DNA demethylation during thymic development licenses a novel stimulus-responsive element that is critical for the maintenance of Cd4 gene expression in effector T cells. We document the importance of maintaining high CD4 expression during parasitic infection and show that by driving transcription, this stimulus-responsive element allows for the maintenance of histone H3K4me3 levels during T cell replication, which is critical for preventing de novo DNA methylation at the Cd4 promoter. A failure to undergo epigenetic programming during development leads to gene silencing during effector T cell replication. Our study thus provides evidence of early developmental events shaping the functional fitness of mature effector T cells.

A population of CD4hiCD38hi T cells correlates with disease severity in patients with acute malaria

OBJECTIVE

CD4⁺ T cells are critical mediators of immunity to Plasmodium spp. infection, but their characteristics during malarial episodes and immunopathology in naturally infected adults are poorly defined. Flow cytometric analysis of PBMCs from patients with either P. falciparum or P. knowlesi malaria revealed a pronounced population of CD4⁺ T cells co-expressing very high levels of CD4 and CD38 we have termed CD4^hiCD38^hi T cells. We set out to gain insight into the function of these novel cells.

METHODS

CD4⁺ T cells from 18 patients with P. falciparum or P. knowlesi malaria were assessed by flow cytometry and sorted into populations of CD4^hiCD38^hi or CD4^norm T cells. Gene expression in the sorted populations was assessed by qPCR and NanoString.

RESULTS

CD4^hiCD38^hi T cells expressed high levels of CD4 mRNA and canonical type 1 regulatory T-cell (TR1) genes including IL10, IFNG, LAG3 and HAVCR2 (TIM3), and other genes with relevance to cell migration and immunomodulation. These cells increased in proportion to malaria disease severity and were absent after parasite clearance with antimalarials.

CONCLUSION

In naturally infected adults with acute malaria, a prominent population of type 1 regulatory T cells arises that can be defined by high co-expression of CD4 and CD38 (CD4^hiCD38^hi) and that correlates with disease severity in patients with falciparum malaria. This study provides fundamental insights into T-cell biology, including the first evidence that CD4 expression is modulated at the mRNA level. These findings have important implications for understanding the balance between immunity and immunopathology during malaria.

Coreceptors and TCR Signaling - the Strong and the Weak of It

The T-cell coreceptors CD4 and CD8 have well-characterized and essential roles in thymic development, but how they contribute to immune responses in the periphery is unclear. Coreceptors strengthen T-cell responses by many orders of magnitude - beyond a million-fold according to some estimates - but the mechanisms underlying these effects are still debated. T-cell receptor (TCR) triggering is initiated by the binding of the TCR to peptide-loaded major histocompatibility complex (pMHC) molecules on the surfaces of other cells. CD4 and CD8 are the only T-cell proteins that bind to the same pMHC ligand as the TCR, and can directly associate with the TCR-phosphorylating kinase Lck. At least three mechanisms have been proposed to explain how coreceptors so profoundly amplify TCR signaling: (1) the Lck recruitment model and (2) the pseudodimer model, both invoked to explain receptor triggering per se, and (3) two-step coreceptor recruitment to partially triggered TCRs leading to signal amplification. More recently it has been suggested that, in addition to initiating or augmenting TCR signaling, coreceptors effect antigen discrimination. But how can any of this be reconciled with TCR signaling occurring in the absence of CD4 or CD8, and with their interactions with pMHC being among the weakest specific protein-protein interactions ever described? Here, we review each theory of coreceptor function in light of the latest structural, biochemical, and functional data. We conclude that the oldest ideas are probably still the best, i.e., that their weak binding to MHC proteins and efficient association with Lck allow coreceptors to amplify weak incipient triggering of the TCR, without comprising TCR specificity.

Systematic Study of the Immune Components after Ischemic Stroke Using CyTOF Techniques

Stroke induces a robust inflammatory response. However, it still lacks a systematic view of the various immune cell types due to the limited numbers of fluorophore used in the traditional FACS technique. In our current study, we utilized the novel technique mass cytometry (CyTOF) to analyze multiple immune cell types. We detected these immune cells from the ischemic brain, peripheral blood, spleen, and bone marrow at different time courses after stroke. Our data showed (1) dynamic changes in the immune cell numbers in the ischemic brain and peripheral organs. (2) The expression levels of cell surface markers indicate the inflammation response status after stroke. Interestingly, CD62L, a key adhesion molecule, regulates the migration of leukocytes from blood vessels into secondary lymphoid tissues and peripheral tissues. (3) A strong leukocyte network across the brain and peripheral immune organs was identified using the R program at day 1 after ischemia, suggesting that the peripheral immune cells dramatically migrated into the ischemic areas after stroke. This study provides a systematic, wide view of the immune components in the brain and peripheral organs for a deep understanding of the immune response after ischemic stroke.

Dual Role of CD4 in Peripheral T Lymphocytes

The interaction of T-cell receptors (TCRs) with self- and non-self-peptides in the major histocompatibility complex (MHC) stimulates crucial signaling events, which in turn can activate T lymphocytes. A variety of accessory molecules further modulate T-cell signaling. Of these, the CD4 and CD8 coreceptors make the most critical contributions to T cell sensitivity in vivo. Whereas, CD4 function in T cell development is well-characterized, its role in peripheral T cells remains incompletely understood. It was originally suggested that CD4 stabilizes weak interactions between TCRs and peptides in the MHC and delivers Lck kinases to that complex. The results of numerous experiments support the latter role, indicating that the CD4-Lck complex accelerates TCR-triggered signaling and controls the availability of the kinase for TCR in the absence of the ligand. On the other hand, extremely low affinity of CD4 for MHC rules out its ability to stabilize the receptor-ligand complex. In this review, we summarize the current knowledge on CD4 in T cells, with a special emphasis on the spatio-temporal organization of early signaling events and the relevance for CD4 function. We further highlight the capacity of CD4 to interact with the MHC in the absence of TCR. It drives the adhesion of T cells to the cells that express the MHC. This process is facilitated by the CD4 accumulation in the tips of microvilli on the surface of unstimulated T cells. Based on these observations, we suggest an alternative model of CD4 role in T-cell activation.

Exposure of Human CD8+ T Cells to Type-2 Cytokines Impairs Division and Differentiation and Induces Limited Polarization

Effector CD8+ T cells generally produce type-1 cytokines and mediators of the perforin/granzyme cytolytic pathway, yet type-2-polarized CD8+ cells (Tc2) are detected in type-2 (T2) cytokine-driven diseases such as asthma. It is unclear whether T2 cytokine exposure during activation is sufficient to polarize human CD8+ T cells. To address this question, a protocol was developed for high-efficiency activation of human CD8+ T cells in which purified single cells or populations were stimulated with plate-bound anti-CD3 and anti-CD11a mAb for up to 8 days in T2 polarizing or neutral conditions, before functional analysis. Activation of CD8+ naïve T cells (TN) in T2 compared with neutral conditions decreased the size of single-cell clones, although early division kinetics were equivalent, indicating an effect on overall division number. Activation of TN in T2 conditions followed by brief anti-CD3 mAb restimulation favored expression of T2 cytokines, GATA3 and Eomes, and lowered expression of type-1 cytokines, Prf1, Gzmb, T-BET, and Prdm1. However, IL-4 was only weakly expressed, and PMA and ionomycin restimulation favored IFN-γ over IL-4 expression. Activation of TN in T2 compared with neutral conditions prevented downregulation of costimulatory (CD27, CD28) and lymph-node homing receptors (CCR7) and CD95 acquisition, which typically occur during differentiation into effector phenotypes. CD3 was rapidly and substantially induced after activation in neutral, but not T2 conditions, potentially contributing to greater division and differentiation in neutral conditions. CD8+ central memory T cells (TCM) were less able to enter division upon reactivation in T2 compared with neutral conditions, and were more refractory to modulating IFN-γ and IL-4 production than CD8+ TN. In summary, while activation of TN in T2 conditions can generate T2 cytokine-biased cells, IL-4 expression is weak, T2 bias is lost upon strong restimulation, differentiation, and division are arrested, and reactivation of TCM is reduced in T2 conditions. Taken together, this suggests that exposure to T2 cytokines during activation may not be sufficient to generate and retain human Tc2 cells.

Low level of Lck kinase in Th2 cells limits expression of CD4 co-receptor and S73 phosphorylation of transcription factor c-Jun

The Src-family tyrosine kinase Lck is an enzyme associated with the CD4 and CD8 co-receptors and promoting signaling through the T cell receptor (TCR) complex. The levels of Lck expression and activity change during the development and differentiation of T cells. Here we show that Lck expression is higher in Th1 cells as compared to Th2 cells. Ectopic overexpression of Lck in Th2 cells results in increased expression of CD4 co-receptor and enhanced S73 phosphorylation of transcription factor c-Jun. Our findings indicate that TCR-mediated signaling in Th2 cells may be directly attenuated by Lck protein expression level.

Th2 Cells in Health and Disease

Helper T (Th) cell subsets direct immune responses by producing signature cytokines. Th2 cells produce IL-4, IL-5, and IL-13, which are important in humoral immunity and protection from helminth infection and are central to the pathogenesis of many allergic inflammatory diseases. Molecular analysis of Th2 cell differentiation and maintenance of function has led to recent discoveries that have refined our understanding of Th2 cell biology. Epigenetic regulation of Gata3 expression by chromatin remodeling complexes such as Polycomb and Trithorax is crucial for maintaining Th2 cell identity. In the context of allergic diseases, memory-type pathogenic Th2 cells have been identified in both mice and humans. To better understand these disease-driving cell populations, we have developed a model called the pathogenic Th population disease induction model. The concept of defined subsets of pathogenic Th cells may spur new, effective strategies for treating intractable chronic inflammatory disorders.

Elongated TCR alpha chain CDR3 favors an altered CD4 cytokine profile

Background

CD4 T lymphocyte activation requires T cell receptor (TCR) engagement by peptide/MHC (major histocompatibility complex) (pMHC). The TCR complementarity-determining region 3 (CDR3) contains variable α and β loops critical for pMHC recognition. During any immune response, tuning of TCR usage through progressive clonal selection occurs. Th1 and Th2 cells operate at different avidities for activation and display distinct transcriptional programs, although polarization may be plastic, influenced by pathogens and cytokines. We therefore hypothesized that CDR3αβ sequence features may intrinsically influence CD4 phenotype during progression of a response.

Results

We show that CD4 polarization involves distinct CDR3α usage: Th1 and Th17 cells favored short TCR CDR3α sequences of 12 and 11 amino acids, respectively, while Th2 cells favored elongated CDR3α loops of 14 amino acids, with lower predicted affinity. The dominant Th2- and Th1-derived TCRα sequences with14 amino acid CDR3 loops and 12 amino acid CDR3 loops, respectively, were expressed in TCR transgenics. The functional impact of these TCRα transgenes was assessed after in vivo priming with a peptide/adjuvant. The short, Th1-derived receptor transgenic T cell lines made IFNγ, but not IL-4, 5 or 13, while the elongated, Th2-derived receptor transgenic T cell lines made little or no IFNγ, but increased IL-4, 5 and 13 with progressive re-stimulations, mirrored by GATA-3 up-regulation. T cells from primed Th2 TCRα transgenics selected dominant TCR Vβ expansions, allowing us to generate TCRαβ transgenics carrying the favored, Th2-derived receptor heterodimer. Primed T cells from TCRαβ transgenics made little or no IL-17 or IFNγ, but favored IL-9 after priming with Complete Freund’s adjuvant and IL-4, 5, 9, 10 and 13 after priming with incomplete Freund’s. In tetramer-binding studies, this transgenic receptor showed low binding avidity for pMHC and polarized T cell lines show TCR avidity for Th17 > Th1 > Th2. While transgenic expression of a Th2-derived, ‘elongated’ TCR-CDR3α and the TCRαβ pair, clearly generated a program shifted away from Th1 immunity and with low binding avidity, cytokine-skewing could be over-ridden by altering peptide challenge dose.

Conclusion

We propose that selection from responding clones with distinctive TCRs on the basis of functional avidity can direct a preference away from Th1 effector responses, favoring Th2 cytokines.

Boosting regulatory T cell function by CD4 stimulation enters the clinic

Understanding tolerance mechanisms at the cellular and molecular level holds the promise to establish novel immune intervention therapies in patients with allergy or autoimmunity and to prevent transplant rejection. Administration of mAb against the CD4 molecule has been found to be exceptionally well suited for intentional tolerance induction in rodent and non-human primate models as well as in humanized mouse models. Recent evidence demonstrated that regulatory T cells (Treg) are directly activated by non-depleting CD4 ligands and suggests Treg activation as a central mechanism in anti-CD4-mediated tolerance induction. This review summarizes the current knowledge on the role of Treg in peripheral tolerance, addresses the putative mechanisms of Treg-mediated suppression and discusses the clinical potential of harnessing Treg suppressive activity through CD4 stimulation.

The TCR-mediated signaling pathways that control the direction of helper T cell differentiation

In the periphery, upon antigen recognition by alphabetaTCR, naïve CD4 T cells undergo functional differentiation and acquire the ability to produce a specific set of cytokines. At least four Th cell subsets, i.e., Th1, Th2, Th17 and iTreg cells have so far been identified and the differentiation of each subset is driven by distinct cytokine sets. Antigen recognition by TCR and the activation of the TCR-mediated signaling pathways that follows, however, are most critical for initiating Th cell differentiation. This review focuses on the TCR signal strength and the TCR-mediated signaling pathways that control the differentiation into these four Th cell subsets.

Dendritic cells control T cell tonic signaling required for responsiveness to foreign antigen

Dendritic cells (DCs) are key components of the adaptive immune system contributing to initiation and regulation of T cell responses. T cells continuously scan DCs in lymphoid organs for the presence of foreign antigen. However, little is known about the functional consequences of these frequent T cell-DC interactions without cognate antigen. Here we demonstrate that these contacts in the absence of foreign antigen serve an important function, namely, induction of a basal activation level in T cells required for responsiveness to subsequent encounters with foreign antigens. This basal activation is provided by self-recognition of MHC molecules on DCs. Following DC depletion in mice, T cells became impaired in TCR signaling and immune synapse formation, and consequently were hyporesponsive to antigen. This process was reversible, as T cells quickly recovered when the number of DCs returned to a normal level. The extent of T cell reactivity correlated with the degree of DC depletion in lymphoid organs, suggesting that a full DC compartment guarantees optimal T cell responsiveness. These findings indicate that DCs are specialized cells that not only present foreign antigen, but also promote a "tonic" state in T cells for antigen responsiveness.

Domain analyses of the Runx1 transcription factor responsible for modulating T-cell receptor-beta/CD4 and interleukin-4/interferon-gamma expression in CD4(+) peripheral T lymphocytes

The Runx1 transcription factor is one of the master regulators of T-lymphocyte differentiation. There have been several reports trying to assign a domain within the Runx1 protein that is responsible for gene expression in thymocytes. The Runx1 domains involved in regulating the expression of several genes in peripheral CD4(+) T cells were analysed. It was observed that Runx1 over-expression enhanced the surface expression of CD4 and CD69 molecules via its activation domain and VWRPY domain, and decreased that of T-cell receptor-beta via its activation domain. Runx1 over-expression enhanced interferon-gamma expression via its activation and VWRPY domains, and abolished interleukin-4 expression through its activation domain. Transduction of Runx1 did not down-regulate CD4 expression until 72 hr of culture, but the repression of CD4 expression became evident after 96 hr. The main region responsible for repressing CD4 expression was the inhibitory domain of Runx1. Taken together, these results lead to a proposal that the regions in Runx1 responsible for modulating gene expression are distinct in thymocytes and in peripheral CD4(+) T cells.

T cell receptor signaling induced by an analog peptide of type II collagen requires activation of Syk

We have previously described an analog peptide of type II collagen (CII) that can suppress collagen-induced arthritis (CIA). This analog peptide represents CII(245-270), the immunodominant epitope of CII, but with substitutions at 260, 261, and 263 - CII(245-270) (A(260), B(261), and N(263)) (A9). To elucidate the mechanisms responsible for suppression, we used mice transgenic for a collagen-specific T cell receptor (TCR). When we found that APCs pulsed with A9 failed to induce T cell phosphorylation of TCR-zeta and ZAP-70, we explored alternative signaling pathways. We determined that A9 instead induced phosphorylation of spleen tyrosine kinase (Syk). The importance of Syk was confirmed by the use of chemical Syk inhibitors, which blocked both cytokine secretion and activation of GATA-3 mediated by peptide A9. In summary, T cells use an alternative pathway in response to A9 that involves Syk. This novel T cell pathway may represent an important means for altering T cell phenotypes.

Signals for the execution of Th2 effector function

Appropriate control of infection depends on the generation of lymphocytes armed with a particular array of cytokine and chemokine effector molecules. The differentiation of naïve T cells into functionally distinct effector subsets is regulated by signals from the T cell receptor (TCR) and cytokine receptors. Using gene knock-out approaches, the initiation of discrete effector programs appears differentially sensitive to the loss of individual TCR signaling components; likely due to differences in the transcription factors needed to activate individual cytokine genes. Less well understood however, are the signal requirements for the execution of effector function. With a focus on Th2 cells and the kinase ITK, we review recent observations that point to differences between the signals needed for the initiation and implementation of cytokine programs in CD4+ T cells. Indeed, Th2 effector cells signal differently from both their naïve counterparts and from Th1 effectors suggesting they may transduce activation signals differently or may be selectively receptive to different activation signals. Potential regulation points for effector function lie at the level of transcription and translation of cytokine genes. We also discuss how provision of these execution signals may be spatially segregated in vivo occurring at tissue sites of inflammation and subject to modulation by the pathogen itself.

Regulated movement of CD4 in and out of the immunological synapse

The mechanism underlying the transient accumulation of CD4 at the immunological synapse (IS) and its significance for T cell activation are not understood. To investigate these issues, we mutated a serine phosphorylation site (S408) in the cytoplasmic tail of murine CD4. Preventing phosphorylation of S408 did not block CD4 recruitment to the IS; rather, it blocked the ability of CD4 to leave the IS. Surprisingly, enhanced and prolonged CD4 accumulation at the supramolecular activation cluster in the contact area had no functional consequence for T cell activation, cytokine production, or proliferation. Protein kinase C theta (PKCtheta)-deficient T cells also displayed enhanced and prolonged accumulation of wild-type CD4 at the IS, indicating that theta is the critical PKC isoform involved in CD4 movement. These findings suggest a model wherein recruitment of CD4 to the IS allows its phosphorylation by PKCtheta and subsequent removal from the IS. Thus, an important role for PKCtheta in T cell activation involves its recruitment to the IS, where it phosphorylates specific substrates that help to maintain the dynamism of protein turnover at the IS.

Blimp-1 induced by IL-4 plays a critical role in suppressing IL-2 production in activated CD4 T cells

Although an inhibitory function of IL-4 in CD4 T cell IL-2 production has long been recognized, a mechanism mediating the inhibition remains unclear. In this study we demonstrate that IL-4 displays a potent suppressive function in IL-2 production of activated CD4 T cells through STAT6. IL-4-induced IL-2 suppression required IL-2 because IL-2 neutralization restored the production of IL-2 even in the presence of IL-4. In vivo, enhanced IL-2 production was found in nematode-infected IL-4- or STAT6-deficient animals, whereas immunization in the presence of IL-4 substantially diminished IL-2 production by Ag-specific CD4 T cells. IL-2 mRNA expression was reduced when T cells were stimulated in the presence of IL-4, whereas IL-2 mRNA decay was unaltered, suggesting that IL-4 mediates the suppression at a transcriptional level. Blimp-1 induced by IL-4 stimulation in activated CD4 T cells was found to be necessary to mediate the IL-2 inhibition as IL-4-mediated IL-2 suppression was less pronounced in activated CD4 T cells deficient in Blimp-1. Taken together, our results demonstrate a potential link with IL-4, Blimp-1, and IL-2 production, suggesting that Blimp-1 may play an important role in controlling IL-2 production in activated T cells and in adaptive T cell immunity.

Th1 and Th2 cells form morphologically distinct immunological synapses

The arrangement of molecules at the interface between T cells and APCs is known as the immunological synapse (IS). We conducted experiments with supported planar bilayers and transfected fibroblast APC to examine the IS formed by polarized Th1 and Th2 cells. Th1 cells formed typical "bull's-eye" IS with a ring of adhesion molecules surrounding MHC/TCR interactions at all Ag concentrations tested, while Th2 cells formed multifocal IS at high concentrations of Ag. At low Ag concentrations, the majority of Th2 cells formed IS with a compact, central accumulation of MHC/TCR, but ICAM-1 was not excluded from the center of the IS. Additionally, CD45 was excluded from the center of the interface between Th1 cells and APC, while CD45 was found at the center of the multifocal IS formed by Th2 cells. Finally, phosphorylated signaling molecules colocalized with MHC/TCR to a greater extent in Th2 IS. Together, our results indicate that the IS formed by Th1 and Th2 cells are distinct in structure, with Th2 cells failing to form bull's-eye IS.

Normal TCR signal transduction in mice that lack catalytically active PTPN3 protein tyrosine phosphatase

PTPN3 (PTPH1) is a cytoskeletal protein tyrosine phosphatase that has been implicated as a negative regulator of early TCR signal transduction and T cell activation. To determine whether PTPN3 functions as a physiological negative regulator of TCR signaling in primary T cells, we generated gene-trapped and gene-targeted mouse strains that lack expression of catalytically active PTPN3. PTPN3 phosphatase-negative mice were born in expected Mendelian ratios and exhibited normal growth and development. Furthermore, numbers and ratios of T cells in primary and secondary lymphoid organs were unaffected by the PTPN3 mutations and there were no signs of spontaneous T cell activation in the mutant mice with increasing age. TCR-induced signal transduction, cytokine production, and proliferation was normal in PTPN3 phosphatase-negative mice. This was observed using both quiescent T cells and recently stimulated T cells where expression of PTPN3 is substantially up-regulated. We conclude, therefore, that the phosphatase activity of PTPN3 is dispensable for negative regulation of TCR signal transduction and T cell activation.

Distinctly different sensitivity in the induction and reversal of anergy of Th1 and Th2 cells

T cell anergy is one of the mechanisms of immunological tolerance. We examined in this study the distinct responses of Th1 and Th2 cells to in vitro anergic stimulation using Th1 and Th2 cells from two strains of T cell receptor transgenic mice. Proliferation of the Th2 cells was difficult to suppress by anergic stimulation, while that of Th1 cells was significantly inhibited even by weak stimulation. However, IL-4 production by Th2 cells was definitely reduced by anergic stimulation, although the inhibition level of IL-4 was lower than that of IFN-gamma production by Th1 cells. We also examined the reversal of anergy in both subsets. While both the anergized Th1 and Th2 cells responded to IL-2 stimulation, only the anergy of the Th2 cells could be reversed. This result indicates that progression of the cell cycle was not sufficient for anergy reversal in Th1 cells. Our findings indicate that the induction and reversal of T cell anergy might be affected by the distinct signaling features of Th1 and Th2 cells.