Initiation of antigen receptor endocytosis and B lymphocyte activation lie on independent biochemical pathways

Inhibition of antigen receptor-dependent Ca(2+) signals and NF-AT activation by P2X7 receptors in human B lymphocytes

One of the first intracellular signals after antigen binding by the antigen receptor of B lymphocytes is the increased intracellular Ca(2+) concentration ([Ca(2+)]i), which is followed by several intracellular signaling events like the nuclear translocation of the transcription factor NF-AT controlling the fate of B lymphocytes after their activation. Extracellular ATP, which is released from cells under several pathological conditions, is considered a danger-associated signal serving as an immunomodulator. We investigated the interaction of antigen receptor (BCR) and P2X7 receptor (P2X7R) activation on [Ca(2+)]i signaling and on nuclear translocation of the transcription factor NF-AT in human B lymphocytes. Although the P2X7R is an ATP-gated Ca(2+)-permeable ion channel, P2X7R activation inhibits the BCR-mediated [Ca(2+)]i responses. This effect is mimicked by cell membrane depolarization induced by an increase in the extracellular K(+) concentration or by application of the Na(+) ionophore gramicidin, but is abolished by stabilization of the membrane potential using the K(+) ionophore valinomycin, by extracellular Mg(2+), which is known to inhibit P2X7R-dependent effects, or by replacing Na(+) by the less P2X7R-permeable Tris(+) ion. Furthermore, P2X7R activation by ATP inhibits the BCR-dependent translocation of the transcription factor NF-ATc1 to the nucleus. We therefore conclude that extracellular ATP via the P2X7R mediates inhibitory effects on B cell activation. This may be of relevance for understanding of the activation of the BCR under pathological conditions and for the development of therapeutic strategies targeting human B lymphocytes or P2X7 receptors.

A novel function for dendritic cell: clearance of VEGF via VEGF receptor-1

It has been reported that the plasma levels of VEGF in tumor patients decreased during dendritic cell (DC)-based immunotherapy, but the underlying mechanism remains unclear. Our current report demonstrates that VEGF levels were significantly decreased in the supernatants of DCs incubated with rhVEGF or tumor conditioned medium (TCM) while the intracellular VEGF in DCs was increased. The increased intracellular VEGF was not due to the de novo VEGF synthesis by DCs because exogenous VEGF inhibited the mRNA expression of VEGF in DCs. More direct evidence was provided to demonstrate that Cy3-labeled VEGF could be internalized by DCs specifically and efficiently. In addition, the activity of DCs to internalize VEGF was abolished by neutralizing antibody against VEGF receptor-1 (Flt-1) and inhibitors of endocytosis such as carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and genistein. This study highlights a novel function of DCs and allows a better understanding of the DC-VEGF interaction.

Role of the adaptor proteins Bam32, TAPP1 and TAPP2 in lymphocyte activation

Adaptor proteins play critical roles in lymphocyte activation by mediating intermolecular interactions and assembling signaling complexes at the activated plasma membrane. Bam32/DAPP1 and the related adaptor proteins TAPP1 and TAPP2 were identified by multiple groups about 5 years ago and considerable progress has been made in elucidating the structure, interaction partners and function of these molecules. These cytoplasmic adaptor proteins are recruited to the plasma membrane through interaction of their PH domains with the lipid products of phosphatidylinositol 3-kinases. They share a unique mode of regulation in that they bind with high affinity to phosphatidylinositol-3,4-bisphosphate and their recruitment is enhanced rather than inhibited by the lipid phosphatase SHIP. Two knockout mouse studies and several gain-and-loss of function studies in cell lines have recently been published, demonstrating multiple functions of Bam32 in B cell activation. Bam32 is required for biological responses including B cell antigen receptor (BCR)-induced proliferation and antibody responses to type II T-independent antigens. Bam32 regulates multiple BCR signaling events including activation of the mitogen activated protein kinases ERK and JNK, remodeling of the actin cytoskeleton through the GTPase Rac1 and BCR internalization. Several studies have emerged suggesting that TAPP1 and TAPP2 may play roles in B and T cell activation; however, the biological functions regulated by these molecules remain to be defined. Here we will comprehensively review the available data on the structure and function of Bam32, TAPP1 and TAPP2 and present an integrated working model for Bam32 function in B cell activation and a general model for distinct effector pathways of PI 3-kinases.

The B lymphocyte adaptor molecule of 32 kilodaltons (Bam32) regulates B cell antigen receptor internalization

The B lymphocyte adaptor molecule of 32 kDa (Bam32) is an adaptor that plays an indispensable role in BCR signaling. In this study, we found that upon BCR ligation, Bam32 is recruited to the plasma membrane where it associates with BCR complexes and redistributes and internalizes with BCRs. BCR ligation induced colocalization of Bam32 with lipid rafts, clathrin, and actin filaments. An inhibitor of Src family protein tyrosine kinases (PTKs) blocked both BCR-induced tyrosine phosphorylation of Bam32 and BCR internalization. Moreover, BCR internalization is impaired in Bam32-/- and Lyn-/- cells, and expression of Bam32 with a mutation of its tyrosine phosphorylation site (Y139F) inhibited BCR internalization. These data suggest that Bam32 functions downstream of Src family PTKs to regulate BCR internalization. Bam32 deficiency does not affect tyrosine phosphorylation of clathrin or the association of clathrin with lipid rafts upon BCR cross-linking. However, BCR-induced actin polymerization is impaired in Bam32-/- cells. Collectively, these findings indicate a novel role of Bam32 in connecting Src family PTKs to BCR internalization by an actin-dependent mechanism.

Visualization of Syk-antigen receptor interactions using green fluorescent protein: differential roles for Syk and Lyn in the regulation of receptor capping and internalization

The cross-linking of the B cell Ag receptor (BCR) is coupled to the stimulation of multiple intracellular signal transduction cascades via receptor-associated, protein tyrosine kinases of both the Src and Syk families. To monitor changes in the subcellular distribution of Syk in B cells responding to BCR cross-linking, we expressed in Syk-deficient DT40 B cells a fusion protein consisting of Syk coupled to green fluorescent protein. Treatment of these cells with anti-IgM Abs leads to the recruitment of the kinase from cytoplasmic and nuclear compartments to the site of the cross-linked receptor at the plasma membrane. The Syk-receptor complexes aggregate into membrane patches that redistribute to form a cap at one pole of the cell. Syk is not demonstrably associated with the internalized receptor. Catalytically active Syk promotes and stabilizes the formation of tightly capped BCR complexes at the plasma membrane. Lyn is not required for the recruitment of Syk to the cross-linked receptor, but is required for the internalization of the clustered BCR complexes. In the absence of Lyn, receptor-Syk complexes at the plasma membrane are long lived, and the receptor-mediated activation of the NF-AT transcription factor is enhanced. Thus, Lyn appears to function to negatively regulate aspects of BCR-dependent signaling by stimulating receptor internalization and down-regulation.

Signaling through the B cell antigen receptor regulates discrete steps in the antigen processing pathway

Antigen processing in B cells is initiated by antigen binding to the surface B cell antigen receptor (BCR). The BCR is a signaling receptor which also functions to endocytose bound antigen for subsequent intracellular processing and presentation with class II molecules. Previously, using subcellular fractionation, we showed that although the surface BCR constitutively traffics from the cell surface to the class II peptide-loading compartment (IIPLC), cross-linking the BCR regulates trafficking, resulting in a more rapid movement of the BCR to the IIPLC (Song et al., 1995, J. Immunol. 155, 4255). The rate of degradation of both the BCR and the bound antigen was also accelerated following BCR cross-linking. Here we provide evidence that the effect of cross-linking the BCR on antigen processing is in part dependent on signal cascades initiated by the BCR. We show that the protein kinase inhibitors Genistein and Chelerythrine, which block BCR signaling, reduce BCR-enhanced antigen processing in a dose-dependent manner. The kinase inhibitors have a small effect on the rate of internalization of the BCR and antigen following BCR cross-linking and significantly decrease the accelerated trafficking to the IIPLC. The increased rate of degradation of the BCR and antigen induced by BCR cross-linking is also decreased by the kinase inhibitors. BCR signaling does not appear to have a global effect on intracellular membrane trafficking as cross-linking the BCR did not alter the rate of trafficking of newly synthesized class II molecules to the IIPLC. Thus, the signaling function of the BCR appears to play a significant role in regulating discrete steps in the intracellular antigen processing pathway.

The internalization of the IgG2a antigen receptor does not require the association with Ig-alpha and Ig-beta but the activation of protein tyrosine kinases does

The B cell antigen receptor of class IgM is a multimeric protein complex containing the membrane-bound immunoglobulin molecule and a heterodimer of the two B cell-specific transmembrane proteins Ig-alpha and Ig-beta. The B cell antigen receptor fulfills a dual role on the surface of B cells. First, it is a signal transduction complex which can activate protein tyrosine kinases and induce the release of Ca2+ ions from intracellular stores. Second, its internalization mediates the specific uptake of bound antigens, which are processed intracellularly and presented as major histocompatibility complex-bound peptides on the cell surface. In case of the IgM antigen receptor, the association with the heterodimer is necessary for expression of large amounts of IgM on the surface. We show here that the IgG2a antigen receptor can be expressed on the surface of myeloma cells in two structurally different forms: either with or without the Ig-alpha/Ig-beta heterodimer. A functional comparison of the two forms of antigen receptors demonstrates that the Ig-alpha and Ig-beta molecules are required for the activation of protein tyrosine kinases after cross-linking of the B cell antigen receptor. In contrast, both forms of IgG2a are equally well internalized. This suggests that Ig-alpha and Ig-beta are essential for signal transduction through the IgG2a antigen receptor, whereas internalization can occur independently of the heterodimer.

B cell antigen receptors

Antigen receptors have a dual role on the surface of B lymphocytes, namely the binding and internalization of antigens and the activation of the antigen-recognizing B cell. The structural requirements for internalization are still a matter of controversy; but as far as signalling is concerned it is becoming increasingly apparent that the antigen receptor is functionally divided into the ligand-binding immunoglobulin (Ig) molecule and the signal-transducing Ig-alpha/Ig-beta heterodimer. This knowledge has been exploited to design variant antigen receptors which combine both functions in a single molecule.