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IgG Fc receptors (FcγRs) and the C5a anaphylatoxin receptor (C5aR) were identified as key regulators of type II autoimmune injury in mice. However, and with respect to C5aR, the relative importance of C5a for IgG autoantibody-induced cellular destruction remained unclear. Using an experimental model of autoimmune hemolytic anemia (AIHA), we here report marked differences in the development of AIHA between mice lacking C5aR and C5-deficient (Hc 0 ) strain, indicating a limited role of C5 in this type of C5aR-regulated disease. Ex-vivo-analyses of liver homogenates from anemic Hc 0 mice demonstrate C5a-independent C5aR activation, upregulation of FcγR expression and amplification of erythrophagocytosis by macrophages. As assessed by pharmacological inhibition studies, targeting of C5aR, but not of C5, is effective in treating experimental AIHA. Collectively, these results define a previously unrecognized disease mechanism of C5aR activation in AIHA that does not necessarily involve C5 and C5a., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Stromal interaction molecule 1 (STIM1)-dependent store operated calcium-entry (SOCE) through Orai1-mediated calcium (Ca(2+) ) influx is considered a major pathway of Ca(2+) signaling, serving T-cell, mast cell, and platelet responses. Here, we show that Orai1 is critical for neutrophil function. Orai1-deficient neutrophils present defects in fMLP and complement C5a-induced Ca(2+) influx and migration, although they respond normally to another chemoattractant, CXCL2. Up until now, no specific contribution of Orai1 independent from STIM1 or SOCE has been recognized in immune cells. Here, we observe that Orai1-deficient neutrophils exhibit normal STIM1-dependent SOCE and STIM1-deficient neutrophils respond to fMLP and C5a efficiently. Despite substantial cytokine production, Orai1(-/-) chimeric mice show impaired neutrophil recruitment in LPS-induced peritonitis. Moreover, Orai1 deficiency results in profoundly defective C5a-triggered neutrophil lung recruitment in hypersensitivity pneumonitis. Comparative evaluation of inflammation in Stim1(-/-) chimeras reveals a distinct pathogenic contribution of STIM1, including its involvement in IgG-induced C5a production. Our data establish Orai1 as key signal mediator of C5aR activation, contributing to inflammation by a STIM1-independent pathway of Ca(2+) -influx in neutrophils., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Human blood NK cells exert strong cytotoxicity against transformed cells and produce different cytokines and chemokines with an important role in modulating immune responses. However, the nature of NK-cell function depends on NK-cell interaction with other immune cells. One type of immune cells that communicate with NK cells are 6-sulfo LacNAc DCs (slanDCs), which comprise a major subpopulation of proinflammatory human blood DCs. In this study, we investigated the molecular mechanisms by which slanDCs interact with NK cells. Our in vitro studies demonstrate that LPS-stimulated slanDCs enhance activation and function of NK cells essentially via membrane-bound TNF-α (mTNF-α). LPS stimulation upregulates expression of mTNF-α in slanDCs, and surface TNF receptor 2 (TNFR2) is upregulated on NK cells after coincubation with slanDCs. IL-12 secreted by slanDCs increases surface expression of TNFR2 in NK cells. TNFR2 signaling in NK cells leads to activation of NF-kB, a transcription factor for cytokines such as GM-CSF. GM-CSF provided by NK cells is responsible for enhancing IL-12 secretion in slanDCs. In conclusion, TNFR2 and IL-12 signaling, which support one another, enables slanDCs to enhance NK-cell function through mTNF-α, thereby regulating immune responses., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Monocytes are known to engage in reciprocal crosstalk with NK cells but their influence on NK-cell-associated antibody-dependent cellular cytotoxicity (ADCC) is not well understood. We demonstrate that in humans FcγRIII (CD16)-dependent ADCC by NK cells is considerably enhanced by monocytes, and that this effect is regulated by FcγRII (CD32) crosslinking in healthy individuals. It is known that during HIV-1 infection, NK cells are known to express low levels of CD16 and exhibit reduced ADCC. We show that immune regulation of CD16-mediated NK-cell cytotoxicity by monocytes through CD32 engagement is substantially disturbed in chronic progressive HIV-1 infection. Expression of activating isoform of CD32 represented a compensatory mechanism for reduced expression of CD16 on NK cells during HIV-1 infection. As a result, the regulation of NK-cell-associated ADCC by monocytes is skewed and eventually constitutes a novel factor that contributes to HIV-1-associated immune deficiency, dysregulation and pathogenesis. Our data therefore provide evidence, for the first time, that in humans monocytes act as a rheostat for FcγRIII-mediated NK-cell functions maintaining a well-balanced immune response., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Chemokines are critical components of the immune system that participate in immune homeostasis and alterations in chemokine balance can result in severe inflammatory and autoimmune diseases. The role of chemokines and their receptors in viral infections including HIV-1 was predicted from the early studies of HIV-1 co-receptor CCR5 and its ligands and a divergent role of C-C chemokines in HIV-1 pathogenesis has been established. For example, CCL3 (MIP-1α), CCL4 (MIP-1β) and CCL5 (RANTES) have been shown to possess antiviral effects by binding to the HIV-1 co-receptor CCR5, whereas CCL2, a pro-inflammatory chemokine, supports HIV-1 replication despite being a member of same chemokine family. Furthermore, the well-established role of CCL2 in driving the Th2 immune response supports its potential role in HIV-1/AIDS. Recent reports suggest multiple pathways of CCL2 affect HIV-1 infection. In this review, we provide a comprehensive overview of the role and potential mechanisms of the HIV-1-CCL2 interplay in driving virus-induced immuno-pathology, suggesting that CCL2 could be an anti-inflammatory target in the treatment of HIV-1 infection., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The property of DC to generate effective CTL responses is influenced by TLR signaling. TLR ligands contain molecular signatures associated with pathogens, have an impact on the antigen processing and presentation by DC, and are being exploited as potential adjuvants. We hypothesized that the TLR2/6 heterodimer agonist S-[2,3-bispalmitoyiloxy-(2R)-propyl]-R-cysteinyl-amido-monomethoxyl polyethylene glycol (BPP), a synthetic derivative of the Mycoplasma macrophage activating lipopeptide-2, is a potent adjuvant for cross-priming against cellular antigens. Systemic administration of BPP-induced maturation of CD8alpha+ DC and CD8alpha- DC in the spleen and resulted in enhanced cross-presentation of intravenously co-administered antigen in mice. In addition, administration of BPP and cell-associated OVA generated an effective CTL response against OVA in vivo in a CD4+ T helper cell-dependent manner, but independent of IFN-alpha. Delivering antigenic peptides directly linked to BPP led to superior CTL immunity as compared to giving antigens and adjuvants admixed. In contrast to other TLR ligands, such as CpG, systemic activation of DC with BPP did not result in shut-down of antigen presentation by splenic DC subsets, although cross-priming against subsequently encountered antigens was reduced. Together, our data provide evidence that BPP is a potent stimulus to generate CTL via cross-priming.

Human NK cells can be subdivided into CD56(dim) and CD56(bright) NK cells, which exhibit different phenotypical and functional characteristics. As murine NK cells lack CD56 or a distinct correlate, direct comparative studies of NK cells in mice and humans are limited. Although CD27 is currently proposed as a feasible subset marker in mice, we assume that the usage of this marker alone is insufficient. We rather investigated the expression of the chemokine receptor CXCR3 for its suitability for distinguishing murine NK-cell subsets with simultaneous consideration of CD27. Compared with CXCR3(-) NK cells, exerting stronger cytotoxic capability, CXCR3+ NK cells displayed an activated phenotype with a lower expression of Ly49 receptors, corresponding to human CD56(bright) NK cells. Also in common with human CD56(bright) NK cells, murine CXCR3+ NK cells exhibit prolific expansion as well as robust IFN-gamma, TNF-alpha and MIP-1alpha production. We additionally demonstrated changes in both CXCR3 and CD27 expression upon NK-cell activation. In summary, CXCR3 serves as an additional applicable marker for improved discrimination of functionally distinct murine NK-cell subsets that comply with those in humans.

FcgammaRIV is a relatively new IgG Fc receptor (FcgammaR) that is reported to contribute to the pathogenesis of autoimmune diseases, although its specific role in relation to FcgammaRIII, complement and IgG2 subclasses remains uncertain. Here we define FcgammaRIV on macrophages as a receptor for soluble IgG2a/b complexes but not for cellular bound IgG2a and show that simultaneous activation of FcgammaRIV and FcgammaRIII is critical to mediate certain type II/III autoimmune responses. FcgammaRIII-deficient mice display compensatory enhanced FcgammaRIV expression, are protected from lung inflammation after deposition of IgG complexes, and show reduced sensitivity to IgG2a/b-mediated hemolytic anemia, indicating that increased FcgammaRIV alone is not sufficient to trigger these diseases in the absence of FcgammaRIII. Importantly, however, blockade of FcgammaRIV is also effective in inhibiting phagocytosis and cytokine production in IgG2b-induced anemia and acute lung injury, processes that display a further dependence on C5a anaphylatoxin receptor. Using gene deletion and functional inhibition studies, we found that FcgammaRIII and FcgammaRIV are each essential to trigger an FcRgamma-linker for activation of T-cell-dependent signal that drives C5a production in the Arthus reaction. Together, the results demonstrate a combined requirement for FcgammaRIII and FcgammaRIV in autoimmune injury, and identify the linker for activation of T cells adaptor as an integral component of linked FcgammaR and C5a anaphylatoxin receptor activation to generate inflammation.

The classic acute-phase reactant C-reactive protein (CRP) plays an important role in innate immunity. Specific CRP receptors have been described on white blood cells and were further characterized as Fcgamma receptors I and II. Here, we used biotinylated, highly purified natural CRP and recombinant human CRP from E. coli to investigate binding to white blood cells. The structural integrity of recombinant CRP was demonstrated by proof of pentamer assembly using non-denaturing gel electrophoresis. Furthermore, the functional capability was confirmed by calcium-dependent ligand binding (phosphorylcholine-coupled BSA and nuclear constituents), and by complement activation (C3 deposition). The monocytic cell line U937 expresses FcgammaRI and FcgammaRII--the proposed CRP receptors--in high density. Binding of biotinylated CRP was only detected by flow cytometry using a partially purified CRP preparation, that contained additional proteins, e.g. IgG as demonstrated by immunoblotting. Highly purified and recombinant CRP, free of IgG, were not bound. To exclude blocking of binding epitopes by labeling on recombinant CRP, biotinylation was performed at various biotin to protein ratios. In addition, competition assays demonstrated that binding of biotinylated, partially purified CRP was only inhibited by partially purified CRP and IgG, but not by highly purified and recombinant CRP. Recombinant CRP bound to U937 cells only after contamination with 0.5 microg IgG per 100 microg CRP before biotinylation. Therefore, we conclude that CRP itself is not bound to white blood cells and strongly suggest a reassessment of previous data.

In this study we present new differential characteristics of NK cells expressing CD56 surface antigen in low (CD56dim) or high (CD56bright) density. In contrast to CD56bright NK cells CD56dim cells express killer cell immunoglobulin (Ig)-like receptors (KIR) such as CD158a, CD158b, and NKB1. However, c-type lectin-like receptors (KLR) CD94/NKG2 and CD161 are present on both subsets. The ability to form conjugates with susceptible targets is approximately twice as strongly pronounced in CD56dim vs. CD56bright NK cells. Last but not least, granules of CD56dim cells contain about tenfold more perforin and granzyme A enabling potentially more effective cytolysis compared to CD56bright NK cells. On the other hand, CD56bright NK cells are superior in producing the proinflammatory cytokines IFN-gamma (28.5% vs. 20.8%, p<0.05) and TNF-alpha (28% vs. 15.8%, p<0.001). The different NK cell populations retained their specific phenotype in vitro during culture in the presence of IL-2 contradicting that they simply display different stages of maturity. Taken together our data support the view that CD56bright cells are specialized NK cells that regulate immunological response mechanisms rather by cytokine supply than by their cytotoxic potential. The poor cytolytic capacity of CD56bright NK cells can be explained by weak ability in forming conjugates with target cells and low contents of perforin and granzyme A in their granules.

Murine low-affinity receptors for IgG, FcgammaRII and FcgammaRIII, differ by their distinct capacities in mediating down-regulation or activation of cellular effector functions, respectively. In this study, antibodies detecting the mouse Ly-17.1 / 2 alloantigen system are demonstrated to be specific for FcgammaRII with no cross-reactivities to other FcgammaR, including FcgammaRIII. Using these FcgammaRII-specific monoclonal antibodies (mAb), the significance of FcgammaRII inhibition of FcgammaRIII was examined in two models of autoantibody [autoimmune hemolytic anemia (AIHA)]- and IgG immune complex-induced (Arthus reaction) inflammation in C57BL / 6 mice in comparison with FcgammaRII(- / -) and FcgammaRIII(- / -) mice. Our results demonstrate that both FcgammaRIII and FcgammaRII contributed to the binding of erythrocytes opsonized with the pathogenic IgG1 autoreactive anti-murine red blood cell antibody 105-2H. However, the functional blocking with anti-FcgammaRII mAb in C57BL / 6 mice and the lack of FcgammaRII expression in FcgammaRII(- / -) mice, which both lowered the threshold level of FcgammaRIII-triggered phagocytosis in vitro, did not results in enhanced disease development of 105-2H mAb-induced AIHA in vivo. This was in sharp contrast to cutaneous Arthus reaction, where FcgammaRIII-mediated activation was inhibited by FcgammaRII. Together these results show that murine AIHA is markedly different from other FcgammaR-dependent inflammatory diseases where FcgammaRIII is normally counterregulated by FcgammaRII.

The deposition of immune complexes, followed by activation of complement and/or Fc receptors and generation of chemoattractants, is the most common feature of human glomerulonephritis. Recently we have shown that primary cultured human glomerular mesangial cells (HMC), which are normally negative for IgG Fc receptors, can be stimulated to express the low-affinity FcgammaRIII-A receptor isoform. In this study we further demonstrate that activation of HMC through IFN-gamma resulted in the functional expression of the high-affinity Fc receptor for IgG (FcgammaRI, CD64). IFN-gamma-dependent induction of classical FcgammaRIa1 mRNA as well as a2, b2 splice variants were evident after 24 h in proliferating HMC and after 48 h in resting HMC. Transcription of FcgammaRI mRNA was also induced by IL-10 in proliferating HMC, whereas other cytokines such as IL-3, transforming growth factor-beta1 and granulocyte-macrophage colony-stimulating factor were not effective. Cell surface expression of FcgammaRI could be detected by flow cytometric analysis after IFN-gamma stimulation and was accompanied by the augmentation of MHC class II and the up-regulation of intercellular adhesion molecule-1 expression. Triggering of HMC by cross-linking FcgammaRI with F(ab')2 fragments of the anti-CD64 monoclonal antibody 22 led to enhanced synthesis of mRNA for the chemokines IL-8 and monocyte chemoattractant protein-1, indicating that the FcgammaRI of HMC is functionally active. These in vitro data suggest that engagement of both FcgammaRI and FcgammaRIII-A on activated HMC through IgG immune complexes may result in an increased chemoattraction of leukocytes into the glomerulus, contributing to the development of glomerulonephritis.

Activation of the 40-kDa low-affinity receptor for IgG (Fc gammaRIIa, CD32) leads to tyrosine phosphorylation, increase of cytosolic free calcium concentration ([Ca2+]i), and production of superoxide anions (O2-) in neutrophils (PMN). It has been established that protein tyrosine kinases (PTK) and phosphatases (PTP) are essential for the regulation of intracellular signaling. CD45 is a type I receptor-type protein tyrosine phosphatase (RPTP) with two PTP domains. Recently it has been demonstrated that co-cross-linking of CD45 modulates the signal transduction pathway of Fc gammaRIIa in PMN. In contrast, the functional characteristics of CD148 (HPTP eta/DEP-1), a new RPTP with only one PTP domain, is unknown. CD148 is expressed on PMN in slightly lower density than CD45, and in higher density than on lymphocytes. [Ca2+]i measured with fluo-3-loaded PMN by flow cytometry and O2- production determined by lucigenin-dependent chemiluminescence were inhibited by co-cross-linking of CD45 with Fc gammaRIIa in comparison to isotype control monoclonal antibody (mAb). In contrast, pre-incubation with CD148 mAb 143-41 abolished O2- generation, but did not inhibit [Ca2+]i rise. In summary, both clustered human RPTP, CD45 and CD148, inhibit Fc gammaRIIa-induced O2- production in PMN, but they differ in regulation of [Ca2+]i. Therefore, it is suggested that co-cross-linking of Fc gammaRII with CD45 and CD148 leads to dephosphorylation of different substrates. These distinct functional capacities may be important for differential regulation of Fc gammaR signaling by currently unknown ligands.

Previous studies on T cell activation via CD43 antigen stimulation were limited to the use of L10, a monoclonal antibody (mAb) recognizing a sialic acid-independent epitope on the CD43 molecule. Here we study the CD43 mAb MEM-59, which recognizes a neuraminidase-sensitive epitope on the CD43 molecule, for its ability to activate T lymphocytes. The antibody by itself is able to stimulate proliferation of peripheral blood mononuclear cells (PBMC) in a monocyte-dependent fashion, and to act synergistically with the mitogen phorbol 12-myristate 13-acetate. It is demonstrated that the monocyte dependence of MEM-59-induced proliferation of peripheral blood lymphocytes (PBL) cannot be attributed to cross-linking via Fc receptors on monocytes alone: F(ab')2 fragments of MEM-59 are at least as effective as intact IgG in the induction of PBMC proliferation. The effects of MEM-59 reported here are distinct in important ways from those reported for L10. Our proliferation data are extended by the observation that MEM-59 mAb induces mobilization of intracellular Ca2+ in PBMC and in the T cell line Jurkat, while the CD3/TcR-negative Jurkat derived-mutant J.TR3-T3.5 exhibits defective signaling compared to the parent cell line. Moreover, CD3 and CD43 are shown to be present jointly in a large complex in a mild detergent lysate of the T cell line HPB-ALL. These data indicate a physical and functional association between CD3/TcR and CD43 pathways, suggesting a role for CD43 as a co-stimulatory molecule in CD3/TcR signaling, especially in T cell-antigen-presenting cell interactions.

CD14, a glycolipid-anchored membrane glycoprotein, acts as a high affinity lipopolysaccharide receptor on leukocytes. We previously reported that the Mono-Mac-6 cell line releases two different soluble forms of CD14 (sCD14) (Labeta et al., Eur. J. Immunol. 1993. 23: 2144). Here we show that the two sCD14, which we now refer to as sCD14 alpha (low M(r)) and sCD14 beta (high M(r)), are also synthesized and released by normal human monocytes and present in normal plasma. Their mechanism of release was examined by using the Mono-Mac-6 cell line, chinese hamster ovary cell (CHO)/CD14+ transfectants and plasma from paroxysmal nocturnal hemoglobinuria (PNH) patients. It was found that: (1) sCD14 beta is released faster than sCD14 alpha and that the release of the latter is a lengthy process. (2) Monensin blocked the biosynthesis of membrane-bound CD14 (mCD14) and sCD14, additionally, a 50-kDa CD14 polypeptide accumulated in the cell lysate, suggesting that the different forms of CD14 may have a common precursor. (3) Monensin also blocked the release of sCD14 alpha from surface-labeled cells, suggesting that conversion of mCD14 to sCD14 alpha involves a mechanism of endocytosis followed by exocytosis. Interestingly, (4) sCD14 alpha and sCD14 beta were detected in PNH plasma, indicating that sCD14 alpha may also derive from an endogenous pathway. (5) Phospholipase C-released CD14 was identical in size to mCD14, thus differed from sCD14 beta by approximately 2000, indicating that release of sCD14 beta involves further processing. (6) CHO cells transfected with a CD14 cDNA coding for an eight C-terminal amino acids shorter product released an sCD14 beta-like form; thus absence of the eight C-terminal amino acids prevented mCD14 expression but not the secretion of sCD14 beta. The characterization of sCD14 alpha and sCD14 beta reported here may be useful for better understanding of variations in sCD14 levels in pathological conditions and the contribution of each sCD14 in sepsis and other, as yet unknown functions.

TcR1+ T cells in peripheral blood have been shown to express in an exclusive fashion either the Ti gamma A or the delta TCS1 epitope. Here, we characterize a subset of TcR1+ T cells in fresh thymus co-expressing the Ti gamma A and delta TCS1 epitopes. TcR1dim and TcR1bright clones can be distinguished. Biochemical and molecular studies on both types of clones generated from these thymocytes reveal a unique T cell receptor structure characterized by the use of a V gamma 9/C gamma 2-encoded 55-kDa gamma chain nondisulfide linked to a V delta 1-encoded delta chain.

The transmembrane protein CD16 (Fc gamma RIII) is detected on activated macrophages, natural killer (NK) cells and a small subset of T lymphocytes. From CD3-CD56+ CD16+bright NK cells and CD3+ CD56+ CD16+dim non-major histocompatibility complex (MHC)-restricted cytotoxic T lymphocyte (CTL) clones were generated reflecting the stable, but different, CD16 expression of the respective peripheral blood subpopulations. To compare the role of CD16 on NK cells and non-MHC-restricted CTL, Fc gamma RIII activation and its mechanisms were investigated using monoclonal antibodies (mAb). Cross-linking of CD16 induced Ca2+ influx in CD16+bright NK clones. In contrast, there was no Ca2+ mobilization after CD16 activation in CD16+dim CTL, which revealed a good response to cross-linking of CD3 antigen. Pretreatment with CD16 mAb alone or cross-linked CD16 mAb did not block the CD3 response of CD16+dim CTL. Again, CD16 cross-linking induced more interferon-gamma transcription in NK cell clones than in non-MHC-restricted CTL clones. Also a higher tumor necrosis factor-alpha production of NK clones after CD16 cross-linking compared to CD16+dim CTL could be observed. These data suggest that after CD16 activation CD16+dim CTL and CD16+bright NK cells use different second messengers. In addition, signal transduction via CD3 and CD16 appears to function independently in CD16+dim non-MHC-restricted CTL.

Polymorphonuclear neutrophils (PMN) express constitutively two low-affinity Fc gamma receptors, Fc gamma RII and Fc gamma RIII. Fc gamma RII is a transmembrane molecule, and Fc gamma RIII is linked via a glycosylphosphatidylinositol (GPI) anchor to the membrane. The role of each of these receptors in activation of PMN is still unclear. We used specific cross-linking of Fc gamma RII via Fab fragments of IV.3 (anti-Fc gamma RII, CDw32) and of Fc gamma RIII using F(ab')2 fragments of 3G8 (anti-Fc gamma RIII, CD16) to activate PMN. Stimulation of Fc gamma RIII was significantly more effective in inducing a respiratory burst than cross-linking of Fc gamma RII. A synergistic effect was observed after simultaneous activation of Fc gamma RIII. We could demonstrate that both Fc gamma R mobilize calcium as intracellular signal in spite of their different membrane linkage. The kinetic of calcium mobilization after Fc gamma R stimulation is delayed in comparison to formyl-methionyl-leucyl-phenylalanine activation. In addition Fc gamma R-induced increase of cytoplasmic calcium is pertussis toxin insensitive. When monoclonal IgG1 kappa complexes were used for stimulation calcium mobilization and hydrogen peroxide (H2O2) production could also be demonstrated. Inhibition studies of this activation using monoclonal antibodies suggested that this immune complex activation was predominantly mediated via Fc gamma RIII. Only in Fc gamma RIII-deficient PMN from paroxysmal nocturnal hemoglobinuria patients could a decreased H2O2 production be demonstrated to be Fc gamma RII dependent. In normal PMN the GPI-anchored Fc gamma RIII structure is the predominant receptor.

Activated T lymphocytes can provide all of the signals necessary to induce the proliferation of resting B lymphocytes. The activation signal is presumably initiated through direct T-B lymphocyte contact. The role of the leukocyte function antigen-1 (LFA-1) and T4 molecules in the activation of purified, small B lymphocytes by mitomycin C-treated T lymphocytes was examined by using monoclonal antibodies that react with and inhibit the function of these molecules. Anti-LFA-1 antibody binding significantly inhibited T-B lymphocyte interactions that result in B lymphocyte proliferation. In contrast, the presence of anti-T4 antibodies at concentrations as high as 100 micrograms/ml did not inhibit this interaction. These results indicate that the B lymphocyte activation signal may not be mediated through the interaction of T4 molecules with major histocompatibility complex class II antigens of the B lymphocyte but is a cell-cell contact-dependent event that is facilitated by LFA-1 molecules.

The human T cell lymphoma Jurkat can be activated by stimuli directed either against the T cell antigen receptor-CD3 antigen complex (TcR/CD3) or the CD2 molecule. Stimulation of cells via the TcR/CD3-complex or via the CD2 molecule increases inositol phosphates and cytoplasmic free calcium. Pretreatment of Jurkat cells with cholera toxin leads to a decrease of TcR/CD3 expression on the surface of the cells, while the expression of CD2 is unaffected. In contrast to this distinct effect on the receptor expression, signaling via both pathways is inhibited by cholera toxin. The most convincing explanation for the cholera toxin-mediated inhibition of signaling is that cholera toxin interrupts the signaling pathways at a point where both, stimulation via TcR/CD3 and via CD2, use the same route. The earliest common point of the two signaling pathways, at least in the Jurkat cell line, seems to be the CD3 complex because after its down-regulation (and functional inactivation) both pathways of activation are interrupted.