Collaboration of TCR-, CD4- and CD28-mediated signalling in antigen-specific MHC class II-restricted T-cells

Sex hormone-binding globulin provides a novel entry pathway for estradiol and influences subsequent signaling in lymphocytes via membrane receptor

The complex effects of estradiol on non-reproductive tissues/cells, including lymphoid tissues and immunocytes, have increasingly been explored. However, the role of sex hormone binding globulin (SHBG) in the regulation of these genomic and non-genomic actions of estradiol is controversial. Moreover, the expression of SHBG and its internalization by potential receptors, as well as the influence of SHBG on estradiol uptake and signaling in lymphocytes has remained unexplored. Here, we found that human and mouse T cells expressed SHBG intrinsically. In addition, B lymphoid cell lines as well as both primary B and T lymphocytes bound and internalized external SHBG, and the amount of plasma membrane-bound SHBG decreased in B cells of pregnant compared to non-pregnant women. As potential mediators of this process, SHBG receptor candidates expressed by lymphocytes were identified in silico, including estrogen receptor (ER) alpha. Furthermore, cell surface-bound SHBG was detected in close proximity to membrane ERs while highly colocalizing with lipid rafts. The SHBG-membrane ER interaction was found functional since SHBG promoted estradiol uptake by lymphocytes and subsequently influenced Erk1/2 phosphorylation. In conclusion, the SHBG-SHBG receptor-membrane ER complex participates in the rapid estradiol signaling in lymphocytes, and this pathway may be altered in B cells in pregnant women.

A dynamic network of estrogen receptors in murine lymphocytes: fine-tuning the immune response

The actual level of circulating estrogen (17β-estradiol, E2) has a serious impact on regulation of diverse immune cell functions, where their classical cytoplasmic receptors, ERα and ERβ, act as nuclear transcriptional regulators of multiple target genes. There is growing evidence, however, for rapid, "non-nuclear" regulatory effects of E2 on lymphocytes. Such effects are likely mediated by putative membrane-associated receptor(s) (mER), but the mechanistic details and the involved signaling pathways still remained largely unknown because of their complexity. Here, we show that in lymphocytes, mERs can signalize themselves, and upon ligation, they are able to coordinate translocation of other E2Rs to the PM. Our data firmly imply existence of a complex, dynamic network of at least seven ER forms in murine lymphocytes: cytoplasmic and membrane-linked forms of ERα, ERβ, or GPR30 and a mER that can receive extracellular E2 signals. The latter mERs are likely palmitoylated, as they are enriched in lipid-raft microdomains, and their E2 binding is also cholesterol dependent. The data also support that ligation of mERs can induce rapid regulatory signals to lymphocytes and then internalize and let the E2 liberate in lysosomes. In addition, they can dynamically control the cell-surface linkage of other cytoplasmic ERs. As demonstrated by the differential effects of mER or cytoplasmic ER ligation on the proliferation of activated T and B lymphocytes, such a dynamic E2R network can be considered as a tool to manage accommodation/fine-tuning of lymphocytes to rapidly changing hormone levels.

FLLL31, a derivative of curcumin, attenuates airway inflammation in a multi-allergen challenged mouse model

Signal transducer and activator of transcription protein 3 (STAT3), one of the major regulators of inflammation, plays multiple roles in cellular transcription, differentiation, proliferation, and survival in human diseases. Dysregulation of STAT3 is related to the severe airway inflammation associated with asthma. FLLL31 is a newly developed compound based on the herbal medicine curcumin, which specifically suppresses the activation of STAT3. However, the function of FLLL31 on inflammatory diseases, especially on the regulation of airway inflammation, has not been fully studied. In our prior investigations, we developed a mouse model that was challenged with a mixture of DRA allergens (including house dust mite, ragweed, and Aspergillums species) to mimic the severe airway inflammation observed in human patients. In this study, we performed a series of experiments on the inflammatory regulation activities of FLLL31 in both in vitro cultured cells and our in vivo DRA-challenged mouse model. Our results show that FLLL31 exhibits anti-inflammatory effects on macrophage activation, lymphocyte differentiation, and pro-inflammatory factor production. Importantly, FLLL31 significantly inhibited airway inflammation and recruitment of inflammatory cells in the DRA-challenged mouse model. Based on these results, we conclude that FLLL31 is a potential therapeutic agent that can be used against severe airway inflammation diseases.

Fine-tuning of helper T cell activation and apoptosis by antigen-presenting cells

The role of antigen-presenting cells (APC) in regulating helper T cell responses and activation-induced cell death (AICD) was investigated in vitro. T cell activation was monitored by measuring the early rise of intracellular free calcium [Ca+]ic, mRNA and cell surface expression of activation and apoptotic molecules, the production of cytokines and the activation of transcription factors. Our results demonstrate that the unique characteristics of a given APC can modify the threshold, kinetics and magnitude of the T cell response. The rapid and sustained rise of intracellular free calcium correlated well with the extent of cytokine production and the expression of activation molecules. Fas-dependent AICD could be induced by the most potent antigen-presenting cell (2PK3) only. Our results demonstrate that the response and fate of effector/memory CD4+ helper T lymphocytes is highly dependent on the individual properties of the APC they encounter.

Rafting MHC-II domains in the APC (presynaptic) plasma membrane and the thresholds for T-cell activation and immunological synapse formation

Glycosphingolipid- and cholesterol-rich membrane microdomains (rafts) in T-cells are important in triggering and regulation of T(H)-cell activation in immunological synapses (IS), which in turn may control the T-cell repertoire in lymph nodes and at the periphery. It is less known, however, how the "presynaptic side" controls formation and function of IS. We investigated here activation signals and synapse formation frequency of murine IP12-7 T(H) hybridoma cell specific to influenza virus HA-peptide upon stimulation with two B-lymphoma cells, A20 and 2PK3, pulsed with peptide antigen. Confocal microscopic colocalization and FRET data consonantly revealed clustered distribution and constitutive raft-association of a major fraction of MHC-II molecules in both APCs. Costimulatory molecules (CD80 and CD86), not associated constitutively with rafts, were expressed at much lower level in A20 cells. T-cells responded to 2PK3 APC with much higher signal strength than to A20 cells, in good correlation with the frequency of IS formation, as assessed by microscopic conjugation assay. Disruption of rafts by cholesterol depletion in 2PK3 cells largely decreased the magnitude of T(H) cell activation signals, especially at low peptide antigen doses, similarly to masking CD4 with mAb on T-cells. The frequency of IS formation was reduced by blocking LFA-1 on T-cells and CD80 on APCs, by lowering the temperature below the phase transition of the membrane or by disrupting actin cytoskeleton. These data together suggest that the surface density and affinity/stability of peptide-MHC-II complexes and the costimulatory level are primary determinants for an efficient TCR recognition and the strength of the subsequent T-cell signals, as well as of the IS formation, which additionally requires a cytoskeleton-dependent remodeling of APC surface after the initial TCR signal. The threshold of T-cell activation can be further set by rafting MHC-II domains via concentrating high affinity ligands and promoting thereby T-cells for sensing low density antigen. Our data also demonstrate that B-cells, similarly to dendritic cells, could also provide T-cells with antigen-independent weak survival signals, likely associated with integrin engagement.

Flow cytometry used for the analysis of calcium signaling induced by antigen-specific T-cell activation

BACKGROUND

In this study, the effect of antigen-presenting cells (APC), peptide concentration, and CD28 costimulation on calcium signaling, induced by antigen-specific T-cell activation, was studied by flow cytometry.

METHODS

We used two experimental approaches, which differed in their time scale and in the duration of the T cell-APC interaction, to measure the increase of intracellular free calcium levels ([Ca(2+)](i)) in activated T cells: (1) Fluo-3-loaded T cells were activated by cocentrifugation with peptide-loaded APC and the kinetics of fluorescence intensity changes was monitored continuously and (2) peptide-loaded APC and T cells were mixed, cocultured, and the fluorescence intensity was measured at various time intervals.

RESULTS

The calcium signal of T cells was dependent on the APC as demonstrated by the ratio of cells exhibiting high versus low fluorescence intensity and by the magnitude of the calcium signal in the activated population. Short-term interaction of T cells with less potent APC or with efficient APC in the presence of low antigen concentration resulted in decreased calcium signaling. CD28-mediated costimulation enhanced the magnitude and sustained the increase of intracellular calcium levels. In line with the strong and sustained calcium signals, the activation of the calcium-dependent transcription factors NF-AT, AP-1, and NF-kappaB was induced.

CONCLUSIONS

Flow cytometric methods, feasible for the rapid and flexible analysis of calcium signaling upon antigen-specific T-cell activation, were established. Kinetics of the increase of mean fluorescence intensity reflected the calcium response of the total cell population whereas statistical analysis of fluorescence intensity at selected time points provided information on the activation state of single cells.

Age-related increase of frequency of a new, phenotypically distinct subpopulation of human peripheral blood T cells expressing lowered levels of CD4

Aging is associated with modifications of T-cell phenotype and function, leading to impaired activation in response to both new and recall antigens. It is not known if T-cell activation results in elimination of a number of the CD4 molecules from the cell surface, as is the case with CD3/T-cell receptor complexes, or how aging influences the process. The T cells of young and elderly donors with reduced expression of CD4 were examined to see whether these cells exhibit other phenotypic features suggesting their active state. It was found that T lymphocytes expressing CD4 can be divided into 2 semidiscrete subpopulations: the major (CD4(+)) population, in which the level of expression of CD4 is constant and high, and a minor population (CD4(lo)), in which the expression of CD4 can be up to an order of magnitude lower than on the CD4(+) cells. The proportion of CD4(lo) cells is age dependent and highly variable in the apparently healthy human population, with the expression of CD4 ranging from around 10% of all peripheral blood lymphocytes in the young to more than 30% in the elderly. Lowered expression of CD4 is correlated with a reduced expression of CD3, as well as with a decreased amount of CD28 and CD95Fas. Activation of CD4(lo) cells is suggested by their expression of CD25 and increased amounts of HLA-DR. Phenotypic characteristics of the CD4(lo) T-cell subpopulation suggest that it might be formed by (perhaps chronically) activated, temporarily apoptosis-resistant cells, possibly accumulating in the elderly. (Blood. 2001;98:1100-1107)

CD44 supports T cell proliferation and apoptosis by apposition of protein kinases

T cell activation is supposed to require two signals via the TCR and a co-stimulatory molecule. However, the signaling cascade of co-stimulatory molecules has remained elusive. Here we provide evidence that CD44, which is constitutively associated with Ick and fyn, supports proliferation as well as apoptosis mainly, if not exclusively, by enhancing signal transduction via the TCR/CD3 complex. Antigenic stimulation of a T helper line in the presence of a CD44 receptor globulin was accompanied by a significant decrease in IL-2 production. To evaluate the underlying mechanism, CD44 was cross-linked via an immobilized antibody (IM-7). Cross-linking of CD44 induces proliferation of peripheral T cells and apoptosis of thymocytes and a T helper line in the presence of subthreshold levels of anti-CD3. Several proteins are rapidly tyrosine phosphorylated; erk and c-jun are strongly activated; expression of CD69 and CD25 is up-regulated on mature T cells; and expression of CD95 and CD95L is up-regulated on the T helper line. All these phenomena become less dependent of CD44 in the presence of high amounts of anti-CD3. Furthermore, cross-linking of CD44 is only effective when supporting co-localization of CD44 with the TCR/CD3 complex, since mixtures of beads coated with either anti-CD3 (low dose) or anti-CD44 do not induce T cell activation. These findings imply the rearrangement of adhesion molecules with apposition of protein kinases as a critical event for the initiation of signaling via the TCR/CD3 complex.

Mapping of a protective helper T cell epitope of human influenza A virus hemagglutinin

The synthetic peptide comprising the 317-341 region of human influenza A virus (H1N1 subtype) hemagglutinin elicits peptide-specific antibody and helper T cell responses and confers protection against lethal virus infection. Molecular mapping of the 317-329 region, which encompasses the epitope recognized by peptide-specific T cells, revealed that the minimal size required for T cell activation was the 317-326 segment. The most likely peptide alignment, which placed 320Leu to pocket 1 of the I-E(d) peptide binding groove, was predicted by molecular mechanics calculations performed with the parental and with the Ala-substituted analogs. In line with the prediction data, the results of the peptide binding assay, where the relative binding efficiency to I-E(d) molecules expressed on the surface of antigen-presenting cells was monitored, identified the 320-326 core sequence interacting with the major histocompatibility class II peptide binding groove. Functional analysis of Ala-substituted variants by functional assays and by calculating the surface-accessible areas of the single peptidic amino acids in the I-E(d)-peptide complexes demonstrated that 324Pro is a primary contact residue for the T cell receptor. Our results show that this type of analysis offers a suitable tool for molecular mapping of helper T cell epitopes and thus provides valuable data for subunit vaccine design.

Signaling complex formation of CD44 with src-related kinases

The complex formation of murine CD44 with the src-like protein tyrosine kinases, lck and lyn, was investigated. In accordance with previous observations, stable CD44-lck and CD44-lyn complexes were detected in nonstimulated lymphoid T- and B-cells, respectively. In addition, a direct modulation of lck and lyn by CD44 was observed as revealed by the CD44-dependent translocation of these enzymes to the Triton X-100 resistant cell fraction. To clarify which receptor domain is responsible for the association, peptide binding assays were performed. Interestingly, the synthetic peptide pCD44 (ILAVCIAVNSRRR), which corresponds to the plasma membrane-cytoplasmic interface region of murine CD44, exhibited a high capacity to bind lck and lyn. A single amino acid modification in the position of the cysteine residue completely abolished this interaction, while the truncation of the three tandem arginines significantly decreased it. Remarkably, similar sequences were found in a number of other molecules including subunits of receptors recognizing antigens, immunoglobulins, extracellular matrix components, accessory molecules, cytokines and also in certain viral gene products. Synthetic peptides corresponding to the homologous regions found in CD28 and FcepsilonRIbeta were also studied and comparable lck-lyn-binding potentials were detected. These data suggest a novel interaction between src-family kinases and CD44, CD28, FcepsilonRIbeta, and provide a simple model for the association of src-like kinases with transmembrane proteins.

The density of the class II MHC T cell receptor ligand influences IFN-gamma/IL-4 ratios in immune responses in vivo

Activation of CD4+ T cells depends on T cell receptor recognition of MHC class II/peptide and on costimulation provided by CD28/B7. It has been shown that different levels of costimulation can influence T helper cell differentiation into Th1 versus Th2 phenotypes. Similar arguments have been made for different levels of peptide/MHC density on antigen-presenting cells, but to date supportive evidence has only come from in vitro studies. Here, using transgenic mice with reduced MHC class II expression on both B cells and dendritic cells, we demonstrate that T helper cell differentiation in vivo is also influenced by the density of expression of MHC class II. Although priming and expansion of antigen-specific T cells were normal in these mice, T cell responses were dominated by the Th1-associated cytokine IFN-gamma, with reduced levels of the Th2 cytokine IL-4 compared to controls. These results provide direct evidence that the efficiency of antigen presentation in vivo can determine effector cell phenotype.