SAP modulates B cell functions in a genetic background-dependent manner.

Mutations affecting the SLAM-associated protein (SAP) are responsible for the X-linked lympho-proliferative syndrome (XLP), a severe primary immunodeficiency syndrome with disease manifestations that include fatal mononucleosis, B cell lymphoma and dysgammaglobulinemia. It is well accepted that insufficient help by SAP-/- CD4+ T cells, in particular during the germinal center reaction, is a component of dysgammaglobulinemia in XLP patients and SAP-/- animals. It is however not well understood whether in XLP patients and SAP-/- mice B cell functions are affected, even though B cells themselves do not express SAP. Here we report that B cell intrinsic responses to haptenated protein antigens are impaired in SAP-/- mice and in Rag-/- mice into which B cells derived from SAP-/- mice together with wt CD4+ T cells had been transferred. This impaired B cells functions are in part depending on the genetic background of the SAP-/- mouse, which affects B cell homeostasis. Surprisingly, stimulation with an agonistic anti-CD40 causes strong in vivo and in vitro B cell responses in SAP-/- mice. Taken together, the data demonstrate that genetic factors play an important role in the SAP-related B cell functions. The finding that anti-CD40 can in part restore impaired B cell responses in SAP-/- mice, suggests potentially novel therapeutic interventions in subsets of XLP patients.

Expansion of an osteopontin-expressing T follicular helper cell subset correlates with autoimmunity in B6.Sle1b mice and is suppressed by the H1-isoform of the Slamf6 receptor.

The costimulatory receptor Slamf6 partially controls lupus-related autoimmunity in congenic Sle1b mice; for instance, the presence of the protein isoform Slamf6-H1 in Sle1b.Slamf6-H1 mice mitigates disease. Here, we report that young Sle1b mice, but not Sle1b.Slamf6-H1 or B6 mice, contain a memory T-helper cell subset identified by ]mt]2-fold increase in expression of 17 genes, chief among which is Spp1, encoding the cytokine osteopontin (OPN). These T follicular helper (TFH) cells, including OPN(+) TFH cells, expand concomitantly with severity of the disease. By contrast, Sle1b.Slamf6-H1 or Sle1b.SAP(-)/(-) mice do not develop autoantibodies and the number of T(FH) cells is 5 times lower than in age-matched Sle1b mice. By comparing Sle1b and Sle1b.OPN(-)/(-) mice, we find that the lack of OPN expression impedes early autoantibody production. Furthermore, on the adoptive transfer of Sle1b.OPN(-)/(-) CD4(+) T cells into bm12 recipients autoantibody production and germinal center formation is reduced compared to recipients of Sle1b.OPN(+/+) CD4(+) T cells. We propose a model in which OPN provides a survival signal for a precursor T(FH) cell subset, which is a key factor in autoimmunity.

SAP expression in invariant NKT cells is required for cognate help to support B-cell responses.

One of the manifestations of X-linked lymphoproliferative disease (XLP) is progressive agammaglobulinemia, caused by the absence of a functional signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) in T, invariant natural killer T (NKT) cells and NK cells. Here we report that α-galactosylceramide (αGalCer) activated NKT cells positively regulate antibody responses to haptenated protein antigens at multiple checkpoints, including germinal center formation and affinity maturation. Whereas NKT cell-dependent B cell responses were absent in SAP(-/-).B6 mice that completely lack NKT cells, the small number of SAP-deficient NKT cells in SAP(-/-).BALB/c mice adjuvated antibody production, but not the germinal center reaction. To test the hypothesis that SAP-deficient NKT cells can facilitate humoral immunity, SAP was deleted after development in SAP(fl/fl).tgCreERT2.B6 mice. We find that NKT cell intrinsic expression of SAP is dispensable for noncognate helper functions, but is critical for providing cognate help to antigen-specific B cells. These results demonstrate that SLAM-family receptor-regulated cell-cell interactions are not limited to T-B cell conjugates. We conclude that in the absence of SAP, several routes of NKT cell-mediated antibody production are still accessible. The latter suggests that residual NKT cells in XLP patients might contribute to variations in dysgammaglobulinemia.

Cutting edge: Slamf8 is a negative regulator of Nox2 activity in macrophages.

Slamf8 (CD353) is a cell surface receptor that is expressed upon activation of macrophages (MΦs) by IFN-γ or bacteria. In this article, we report that a very high NADPH oxidase (Nox2) enzyme activity was found in Slamf8(-/-) MΦs in response to Escherichia coli or Staphylococcus aureus, as well as to PMA. The elevated Nox2 activity in Slamf8(-/-) MΦs was also demonstrated in E. coli or S. aureus phagosomes by using a pH indicator system and was further confirmed by a reduction in the enzyme activity after transfection of the receptor into Slamf8-deficient primary MΦs or RAW 264.7 cells. Upon exposure to bacteria or PMA, protein kinase C activity in Slamf8(-/-) MΦs is increased. This results in an enhanced phosphorylation of p40phox, one key component of the Nox2 enzyme complex, which, in turn, leads to greater Nox2 activity. Taken together, the data show that, in response to inflammation-associated stimuli, the inducible receptor Slamf8 negatively regulates inflammatory responses.

Receptor signaling lymphocyte-activation molecule family 1 (Slamf1) regulates membrane fusion and NADPH oxidase 2 (NOX2) activity by recruiting a Beclin-1/Vps34/ultraviolet radiation resistance-associated gene (UVRAG) complex.

Phagocytosis is a pivotal process by which macrophages eliminate microorganisms upon recognition by pathogen sensors. Surprisingly, the self-ligand cell surface receptor Slamf1 functions not only as a co-stimulatory molecule but also as a microbial sensor of several Gram-negative bacteria. Upon entering the phagosome of macrophages Slamf1 induces production of phosphatidylinositol 3-phosphate, which positively regulates the activity of the NOX2 enzyme and phagolysosomal maturation. Here, we report that in Escherichia coli-containing phagosomes of mouse macrophages, Slamf1 interacts with the class III PI3K Vps34 in a complex with Beclin-1 and UVRAG. Upon phagocytosis of bacteria the NOX2 activity was reduced in macrophages isolated from Beclin-1(+/-) mice compared with wild-type mice. This Slamf1/Beclin-1/Vps34/UVRAG protein complex is formed in intracellular membrane compartments as it is found without inducing phagocytosis in macrophages, human chronic lymphocytic leukemia cells, and transfectant HEK293 cells. Elimination of its cytoplasmic tail abolished the interaction of Slamf1 with the complex, but deletion or mutation of the two ITAM motifs did not. Both the BD and CCD domains of Beclin-1 were required for efficient binding to Slamf1. Because Slamf1 did not interact with Atg14L or Rubicon, which can also form a complex with Vps34 and Beclin-1, we conclude that Slamf1 recruits a subset of Vps34-associated proteins, which is involved in membrane fusion and NOX2 regulation.

Wiskott-Aldrich syndrome protein (WASP) and N-WASP are critical for peripheral B-cell development and function.

The Wiskott-Aldrich syndrome protein (WASP) is a key cytoskeletal regulator of hematopoietic cells. Although WASP-knockout (WKO) mice have aberrant B-cell cytoskeletal responses, B-cell development is relatively normal. We hypothesized that N-WASP, a ubiquitously expressed homolog of WASP, may serve some redundant functions with WASP in B cells. In the present study, we generated mice lacking WASP and N-WASP in B cells (conditional double knockout [cDKO] B cells) and show that cDKO mice had decreased numbers of follicular and marginal zone B cells in the spleen. Receptor-induced activation of cDKO B cells led to normal proliferation but a marked reduction of spreading compared with wild-type and WKO B cells. Whereas WKO B cells showed decreased migration in vitro and homing in vivo compared with wild-type cells, cDKO B cells showed an even more pronounced decrease in the migratory response in vivo. After injection of 2,4,6-trinitrophenol (TNP)-Ficoll, cDKO B cells had reduced antigen uptake in the splenic marginal zone. Despite high basal serum IgM, cDKO mice mounted a reduced immune response to the T cell-independent antigen TNP-Ficoll and to the T cell-dependent antigen TNP-keyhole limpet hemocyanin. Our results reveal that the combined activity of WASP and N-WASP is required for peripheral B-cell development and function.

Glucocorticoid-induced tumor necrosis factor receptor family-related protein regulates CD4(+)T cell-mediated colitis in mice.

BACKGROUND & AIMS: The glucocorticoid-induced tumor necrosis factor receptor family-related protein (GITR; also called TNFRSF18 or CD357) regulates the T cell-mediated immune response and is present on surfaces of regulatory T (Treg) cells and activated CD4(+) T cells. We investigated the roles of GITR in the development of colitis in mice. METHODS: Chronic enterocolitis was induced by the transfer of wild-type or GITR(-/-) CD4(+) T cells to GITR(-/-) × Rag(-/-) or Rag(-/-) mice. We determined the severity of colitis by using the disease activity index; measured levels of inflammatory cytokines, T cells, and dendritic cells; and performed histologic analysis of colon samples. RESULTS: Transfer of nonfractionated CD4(+) cells from wild-type or GITR(-/-) donors induced colitis in GITR(-/-) × Rag(-/-) but not in Rag(-/-) mice. Among mice with transfer-induced colitis, the percentage of Treg and T-helper (Th) 17 cells was reduced but that of Th1 cells increased. Treg cells failed to prevent colitis in GITR(-/-) × Rag(-/-) recipients; this was not the result of aberrant function of GITR(-/-) Treg or T effector cells but resulted from an imbalance between the numbers of tolerogenic CD103(+) and PDCA1(+) plasmacytoid dendritic cells in GITR(-/-) mice. This imbalance impaired Treg cell development and expanded the Th1 population in GITR(-/-) × Rag(-/-) mice following transfer of nonfractionated CD4(+) cells. CONCLUSIONS: GITR is not required on the surface of Treg and T effector cells to induce colitis in mice; interactions between GITR and its ligand are not required for induction of colitis. GITR instead appears to control dendritic cell and monocyte development; in its absence, mice develop aggravated chronic enterocolitis via an imbalance of colitogenic Th1 cells and Treg cells.

A novel isoform of the Ly108 gene ameliorates murine lupus.

Studies of human systemic lupus erythematosus patients and of murine congenic mouse strains associate genes in a DNA segment on chromosome 1 with a genetic predisposition for this disease. The systematic analysis of lupus-prone congenic mouse strains suggests a role for two isoforms of the Ly108 receptor in the pathogenesis of the disease. In this study, we demonstrate that Ly108 is involved in the pathogenesis of lupus-related autoimmunity in mice. More importantly, we identified a third protein isoform, Ly108-H1, which is absent in two lupus-prone congenic animals. Introduction of an Ly108-H1-expressing transgene markedly diminishes T cell-dependent autoimmunity in congenic B6.Sle1b mice. Thus, an immune response-suppressing isoform of Ly108 can regulate the pathogenesis of lupus.

Auto-antibody production and glomerulonephritis in congenic Slamf1-/- and Slamf2-/- [B6.129] but not in Slamf1-/- and Slamf2-/- [BALB/c.129] mice.

Several genes in an interval of human and mouse chromosome 1 are associated with a predisposition for systemic lupus erythematosus. Congenic mouse strains that contain a 129-derived genomic segment, which is embedded in the B6 genome, develop lupus because of epistatic interactions between the 129-derived and B6 genes, e.g. in B6.129chr1b mice. If a gene that is located on chromosome 1 is altered through homologous recombination in 129-derived embryonic stem cells (ES cells) and if the resultant knockout mouse is backcrossed with B6, interpretation of the phenotype of the mutant mouse may be affected by epistatic interactions between the 129 and B6 genomes. Here, we report that knockout mice of two adjacent chromosome 1 genes, Slamf1(-/-) and Slamf2(-/-), which were generated with the same 129-derived ES cell line, develop features of lupus, if backcrossed on to the B6 genetic background. By contrast, Slamf1(-/-) [BALB/c.129] and Slamf2(-/-) [BALB/c.129] do not develop disease. Surprisingly, Slamf1(-/-) [B6.129] mice develop both auto-antibodies and glomerulonephritis between 3 and 6 months of age, while disease fully develops in Slamf1(-/-) [B6.129] mice after 9-14 months. Functional analyses of CD4(+) T cells reveals that Slamf2(-/-) T cells are resistant to tolerance induction in vivo. We conclude that the Slamf2(-/-) mutation may have a unique influence on T-cell tolerance and lupus.

Expansion of immunoglobulin-secreting cells and defects in B cell tolerance in Rag-dependent immunodeficiency.

The contribution of B cells to the pathology of Omenn syndrome and leaky severe combined immunodeficiency (SCID) has not been previously investigated. We have studied a mut/mut mouse model of leaky SCID with a homozygous Rag1 S723C mutation that impairs, but does not abrogate, V(D)J recombination activity. In spite of a severe block at the pro-B cell stage and profound B cell lymphopenia, significant serum levels of immunoglobulin (Ig) G, IgM, IgA, and IgE and a high proportion of Ig-secreting cells were detected in mut/mut mice. Antibody responses to trinitrophenyl (TNP)-Ficoll and production of high-affinity antibodies to TNP-keyhole limpet hemocyanin were severely impaired, even after adoptive transfer of wild-type CD4(+) T cells. Mut/mut mice produced high amounts of low-affinity self-reactive antibodies and showed significant lymphocytic infiltrates in peripheral tissues. Autoantibody production was associated with impaired receptor editing and increased serum B cell-activating factor (BAFF) concentrations. Autoantibodies and elevated BAFF levels were also identified in patients with Omenn syndrome and leaky SCID as a result of hypomorphic RAG mutations. These data indicate that the stochastic generation of an autoreactive B cell repertoire, which is associated with defects in central and peripheral checkpoints of B cell tolerance, is an important, previously unrecognized, aspect of immunodeficiencies associated with hypomorphic RAG mutations.

GITR engagement preferentially enhances proliferation of functionally competent CD4+CD25+FoxP3+ regulatory T cells.

Naturally occurring regulatory T cells (Treg) express high levels of glucocorticoid-induced tumour necrosis factor receptor (GITR). However, studies of the role of GITR in Treg biology has been complicated by the observation that upon activation effector CD4(+) T (Teff) cells also express the receptor. Here, we dissect the contribution of GITR-induced signaling networks in the expansion and function of FoxP3(+) Treg. We demonstrate that a high-affinity soluble Fc-GITR-L dimer, in conjugation with alphaCD3, specifically enhances in vitro proliferation of Treg, which retain their phenotypic markers (CD25 and FoxP3) and their suppressor function, while minimally affecting Teff cells. Furthermore, Fc-GITR-L does not impair Teff susceptibility to suppression, as judged by cocultures employing GITR-deficient and GITR-sufficient CD4(+) T-cell subsets. Notably, this expansion of Treg could also be seen in vivo, by injecting FoxP3-IRES-GFP mice with Fc-GITR-L even in the absence of antigenic stimulation. In order to test the efficacy of these findings therapeutically, we made use of a C3H/HeJ hemophilia B-prone mouse model. The use of liver-targeted human coagulation factor IX (hF.IX) gene therapy in this model has been shown to induce liver toxicity and the subsequent failure of hF.IX expression. Interestingly, injection of Fc-GITR-L into the hemophilia-prone mice that were undergoing liver-targeted hF.IX gene therapy increased the expression of F.IX and reduced the anticoagulation factors. We conclude that GITR engagement enhances Treg proliferation both in vitro and in vivo and that Fc-GITR-L may be a useful tool for in vivo tolerance induction.

SLAM family receptors and the SLAM-associated protein (SAP) modulate T cell functions.

One or more of the signaling lymphocytic activation molecule (SLAM) family (SLAMF) of cell surface receptors, which consists of nine transmembrane proteins, i.e., SLAMF1-9, are expressed on most hematopoietic cells. While most SLAMF receptors serve as self-ligands, SLAMF2 and SLAMF4 use each other as counter structures. Six of the receptors carry one or more copies of a unique intracellular tyrosine-based switch motif, which has high affinity for the single SH2-domain signaling molecules SLAM-associated protein and EAT-2. Whereas SLAMF receptors are costimulatory molecules on the surface of CD4+, CD8+, and natural killer (NK) T cells, they also involved in early phases of lineage commitment during hematopoiesis. SLAMF receptors regulate T lymphocyte development and function and modulate lytic activity, cytokine production, and major histocompatibility complex-independent cell inhibition of NK cells. Furthermore, they modulate B cell activation and memory generation, neutrophil, dendritic cell, macrophage and eosinophil function, and platelet aggregation. In this review, we will discuss the role of SLAM receptors and their adapters in T cell function, and we will examine the role of these receptors and their adapters in X-linked lymphoproliferative disease and their contribution to disease susceptibility in systemic lupus erythematosus.

Homozygous DNA ligase IV R278H mutation in mice leads to leaky SCID and represents a model for human LIG4 syndrome.

DNA ligase IV (LIG4) is an essential component of the nonhomologous end-joining (NHEJ) repair pathway and plays a key role in V(D)J recombination. Hypomorphic LIG4 mutations in humans are associated with increased cellular radiosensitivity, microcephaly, facial dysmorphisms, growth retardation, developmental delay, and a variable degree of immunodeficiency. We have generated a knock-in mouse model with a homozygous Lig4 R278H mutation that corresponds to the first LIG4 mutation reported in humans. The phenotype of homozygous mutant mice Lig4(R278H/R278H) (Lig4(R/R)) includes growth retardation, a decreased life span, a severe cellular sensitivity to ionizing radiation, and a very severe, but incomplete block in T and B cell development. Peripheral T lymphocytes show an activated and anergic phenotype, reduced viability, and a restricted repertoire, reminiscent of human leaky SCID. Genomic instability is associated with a high rate of thymic tumor development. Finally, Lig4(R/R) mice spontaneously produce low-affinity antibodies that include autoreactive specificities, but are unable to mount high-affinity antibody responses. These findings highlight the importance of LIG4 in lymphocyte development and function, and in genomic stability maintenance, and provide a model for the complex phenotype of LIG4 syndrome in humans.

Cholesterol and sphingolipids as lipid organizers of the immune cells' plasma membrane: their impact on the functions of MHC molecules, effector T-lymphocytes and T-cell death.

The possible regulatory mechanisms by which glycosphingolipid- and cholesterol-rich membrane microdomains, caveolar and non-caveolar lipid rafts, control the immune response are continuously expanding. In the present overview we will focus on how these membrane-organizing lipids are involved, in collaboration with tetraspanin proteins, in the formation of distinct MHC-I and MHC-II microdomains at the cell surface and will analyze the possible roles of MHC compartmentation in the processes of antigen presentation and regulation of various stages of the cellular immune response. Some basic, lipid raft- and tetraspan mediated mechanisms involved in the formation and function of immunological synapses between various APCs and T-cells will also be discussed. Finally, a new aspect of immune regulation by sphingolipids will be briefly described, namely how can the death or stress signals, leading to ceramide accumulation, result in raft-associated regulatory platforms controlling cell death or antigen-induced, TCRmediated signaling of T-lymphocytes. The influence of these signals and their cross-talk on the fate (death or survival) of T-cells and the outcome of T-cell response will also be discussed.

Death or survival: membrane ceramide controls the fate and activation of antigen-specific T-cells depending on signal strength and duration.

Sphingomyelinase (SMase)-mediated release of ceramide in the plasma membrane of T-lymphocytes induced by different stimuli such as ligation of Fas/CD95, irradiation, stress, inflammation or anticancer drugs primarily involves mitochondrial apoptosis signaling, but under specific conditions non-apoptotic Fas-signaling was also reported. Here we investigated, using a quantitative simulation model with exogenous C2-ceramide (and SMase), the dependence of activation and fate of T-cells on the strength and duration of ceramide accumulation. A murine, influenza virus hemagglutinin-specific T-helper cell (IP12-7) alone or together with interacting antigen presenting B-cells (APC) was used. C2-ceramide induced apoptosis of TH cells above a 'threshold' stimulus (>25 microM in 'strength' or >30 min in duration), while below the threshold C2-ceramide was non-apoptotic, as confirmed by early and late apoptotic markers (PS-translocation, mitochondrial depolarization, caspase-3 activation, DNA-fragmentation). The modest ceramide stimuli strongly suppressed the calcium response and inhibited several downstream signal events (e.g. ERK1/2-, JNK-phosphorylation, CD69 expression or IL-2 production) in TH cells during both anti-CD3 induced and APC-triggered activation. Ceramide moderately affected the Ca2+ -release from internal stores upon antigen-specific engagement of TCR in immunological synapses, while the influx phase was remarkably reduced in both amplitude and rate, suggesting that the major target(s) of ceramide-effects are membrane-proximal. Ceramide inhibited Kv1.3 potassium channels, store operated Ca2+ -entry (SOC) and depolarized the plasma membrane to which contribution of spontaneously formed ceramide channels is possible. The impaired function of these transporters may be coupled to the quantitative, membrane raft-remodeling effect of ceramide and responsible, in a concerted action, for the suppressed activation. Our results suggest that non-apoptotic Fas stimuli, received from previously activated, FasL+ interacting lymphocytes in the lymph nodes, may negatively regulate subsequent antigen-specific T-cell activation and thus modulate the antigen-specific T-cell response.

Cholesterol sensitivity of detergent resistance: a rapid flow cytometric test for detecting constitutive or induced raft association of membrane proteins.

BACKGROUND: Lipid rafts are cholesterol- and glycosphingolipid-rich microdomains in the cellular plasma membranes that play critical roles in compartmentalization (concentration, coupling, and isolation) of receptors and signal molecules. Therefore, detecting constitutive or induced raft associations of such proteins is of central interest in cell biology. This has mostly been done with time- and cell-consuming immunobiochemical techniques affected by several sources of artifacts. A flow cytometric analysis of immunocytochemical staining under differential circumstances of detergent treatment offers a new alternative to this method. METHODS: Membrane microdomains are resistant to nonionic detergents due to extensive, strong interactions between their molecular constituents. We used this feature to develop a rapid flow cytometric assay of differential detergent resistance based on immunocytochemical labeling of extracellular domain epitopes in membrane proteins. Data evaluation is based on comparative detection of their detergent solubility without and with cholesterol depletion of cell membranes, resolved by moderate concentrations of nonionic detergents. RESULTS: Nonionic detergents Triton X-100 and Nonidet-40 (0.05-0.1%) in cold or Brij-98 (0.1-0.5%) at 37 degrees C efficiently resolved detergent solubility or resistance of many lymphocyte cell surface proteins. Kinetic data revealed that a short (5-10 min) detergent treatment is sufficient for this assay. Comparison of detergent solubility in untreated and cholesterol-depleted cells differentiated membrane proteins associated with or excluded from raft microdomains, respectively. Confocal microscopy showed that this mild detergent treatment leaves the cytoskeleton of the cells intact, with a detectable expression of raft marker detergent-resistant proteins attached to it. An induced association with rafts of immunoglobulin E receptors upon antigen cross-linking was also easily detectable in rat mast cells by this approach. CONCLUSIONS: A protocol is proposed for a rapid (5-10 min) test of detergent resistance of membrane proteins in cells. The approach requires only a small amount of cells (10(4)/sample) and offers a good resolution of detergent solubility or resistance of membrane proteins, also in terms of the underlying mechanisms, with an advantage of applicability for all conventional bench-top flow cytometers.

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.