Neutrophils--the unexpected helpers of B-cell activation

A specific subpopulation of neutrophils, termed NBH, has been shown recently to provide help for the differentiation and function of B cells and plasma cells. These novel findings are put in the context of our current understanding of B‐cell help.

Asymmetric segregation of polarized antigen on B cell division shapes presentation capacity

During the activation of humoral immune responses, B cells acquire antigen for subsequent presentation to cognate T cells. Here we show that after mouse B cells accumulate antigen, it is maintained in a polarized distribution for extended periods in vivo. Using high-throughput imaging flow cytometry, we observed that this polarization is preserved during B cell division, promoting asymmetric antigen segregation among progeny. Antigen inheritance correlates with the ability of progeny to activate T cells: Daughter cells receiving larger antigen stores exhibit a prolonged capacity to present antigen, which renders them more effective in competing for T cell help. The generation of progeny with differential capacities for antigen presentation may have implications for somatic hypermutation and class switching during affinity maturation and as B cells commit to effector cell fates.

B cell receptor-mediated antigen gathering requires ubiquitin ligase Cbl and adaptors Grb2 and Dok-3 to recruit dynein to the signaling microcluster

The B cell receptor (BCR) mediates B cell antigen gathering and acquisition for presentation to T cells. Although the amount of antigen presentation to T cells determines the extent of B cell activation, the molecular mechanisms underlying antigen gathering remain unexplored. Here, through a combination of high-resolution imaging, genetics and quantitative mass spectrometry, we demonstrate that adaptors Grb2 and Dok-3, and ubiquitin ligase Cbl in signaling BCR microclusters mediate association with the microtubule motor dynein. Furthermore, we visualize the localization and movement of these microclusters on the underlying microtubule network. Importantly, disruption of this network or diminished dynein recruitment in Grb2-, Dok-3-, or Cbl-deficient B cells, does not influence microcluster formation or actin-dependent spreading, but abrogates directed movement of microclusters and antigen accumulation. Thus we identify a surprising but pivotal role for dynein and the microtubule network alongside Grb2, Dok-3, and Cbl in antigen gathering during B cell activation.

The cytoskeleton coordinates the early events of B-cell activation

B cells contribute to protective adaptive immune responses through generation of antibodies and long-lived memory cells, following engagement of the B-cell receptor (BCR) with specific antigen. Recent imaging investigations have offered novel insights into the ensuing molecular and cellular events underlying B-cell activation. Following engagement with antigen, BCR microclusters form and act as sites of active signaling through the recruitment of intracellular signaling molecules and adaptors. Signaling through these "microsignalosomes" is propagated and enhanced through B-cell spreading in a CD19-dependent manner. Subsequently, the mature immunological synapse is formed, and functions as a platform for antigen internalization, enabling the antigen presentation to helper T cells required for maximal B-cell activation. In this review, we discuss the emerging and critical role for the cytoskeleton in the coordination and regulation of these molecular events during B-cell activation.

Visualizing a role for the actin cytoskeleton in the regulation of B-cell activation

Appropriate activation of B cells is required for mounting protective humoral immune responses. B-cell activation is initiated following specific recognition of antigen by the B-cell receptor (BCR) and results in the generation of antibody-secreting plasma cells and long-lived memory cells. Initial imaging approaches revealed that B cells undergo dramatic molecular and morphological reorganizations following recognition of antigen. A number of these studies pointed to a role for the underlying cytoskeleton in regulating early events of B-cell activation. More recently, groundbreaking advances in imaging technologies have enabled direct visualization of the role for the cytoskeleton in regulating events at the B-cell membrane. Indeed, we have demonstrated that an ezrin-defined actin network shapes BCR diffusion and signaling both in the resting state and following antigen-induced activation. Importantly, alongside these in vitro imaging approaches, it has been demonstrated that mutations in cytoskeleton regulators such as CD19, dedicator of cytokinesis 8 (DOCK8), and Wiskott-Aldrich syndrome protein (WASp) are often associated with antibody deficiency syndromes in humans, establishing the importance of cytoskeleton reorganizations in conferring effective adaptive immunity.

Antigen presentation to B cells

B cells are capable of mounting responses to a bewildering range of potentially pathogenic antigens through the production of high-affinity antibodies and the establishment of immunological memory. Thus, regulated B-cell activation is critical for protection against a variety of bacterial and viral infections, as well as cancers. Here, we discuss a number of recent imaging studies that have provided new insights into the variety of mechanisms by which B-cell activation is initiated in the lymph node in vivo.

Early Events in B Cell Activation

B cell activation is initiated by the ligation of the B cell receptor (BCR) with antigen and ultimately results in the production of protective antibodies against potentially pathogenic invaders. Here we review recent literature concerned with the spatiotemporal dynamic characterization of the early molecular events of B cell activation, including the initiation of BCR triggering, the formation of BCR microclusters, and the dynamic regulation of BCR signaling. Because these events involve the considerable reorganization of molecules within the membrane, an important role for the cytoskeleton is emerging in the regulation of B cell activation. At each stage we highlight the role of the cytoskeleton, establishing its pivotal position during the initiation and regulation of B cell activation.

Early events of B cell activation by antigen

The activation of B cells confers long-lasting protection from a plethora of infectious diseases through the generation of plasma cells that produce high-affinity antibodies and memory cells. Engagement of the B cell receptor (BCR) with cognate antigen initiates intracellular signaling and subsequent internalization of antigen. Membrane-bound antigens are now considered the predominant forms that initiate B cell activation in vivo. We have shown that upon recognition of antigen on the surface of a presenting cell, the B cell undergoes a dramatic change in morphology characterized by rapid spreading followed by more prolonged contraction along the presenting surface. This two-phase response increases the amount of antigen that the B cell accumulates, internalizes, and subsequently presents to T cells. Thus, the spreading and contraction response shapes the outcome of B cell activation. We used a combination of planar lipid bilayers and total internal reflection fluorescence microscopy to investigate the early events that occur after engagement of the BCR and before B cell spreading. We observed the rapid formation of BCR-antigen microclusters, which we redefine as "microsignalosomes" because they mediate the coordinated recruitment of intracellular effectors, such as the kinases Lyn and Syk, the adaptor Vav, and phospholipase C-gamma2 (PLC-gamma2). We identified an essential role for the co-receptor CD19 in mediating spreading, and thus B cell activation, in response to membrane-bound antigen. Preliminary evidence suggests that the cellular morphology changes described in vitro are likely to occur upon recognition of antigen presented on the surface of macrophages in lymph nodes in vivo.

Regulation of integrin activation through the B-cell receptor

Effective immune surveillance is absolutely dependent on the migration of lymphocytes throughout the body and on their successful recognition of specific antigens. Both of these functions rely on the capacity of integrins that are expressed on the surface of lymphocytes to respond in a highly regulated manner to a variety of chemokines and antigens. This Commentary is primarily concerned with the role of the B-cell integrins LFA-1 and VLA-4 in the antigen-recognition process, and summarises what is currently known about the molecular mechanisms of ;inside-out' integrin activation in response to B-cell-receptor stimulation. Recent investigations have identified Vav, PI3K and small GTPases as crucial regulators of the inside-out activation of B-cell integrins. These observations are of particular interest as they allude to an underlying mechanism by which B-cell-receptor-mediated signalling is linked to cytoskeleton reorganisation and subsequent integrin activation.

New insights into the early molecular events underlying B cell activation

The appropriate activation of B cells is critical for the development and operation of immune responses and is dependent on the extensive coordination of intra- and intercellular communications in response to antigen stimulation. An accurate description of the B cell-activation process requires investigation of these interactions within their correct cellular context both at high resolution and in real time. Here, we discuss a number of recent studies that have offered insight into the early molecular events of B cell activation. We suggest that segregation within the B cell membrane triggers localized cytoskeleton reorganisation and signaling, allowing the formation of B cell receptor (BCR) microclusters. These BCR microclusters are the sites for the coordinated recruitment of the signalosome and are propagated during B cell spreading. We discuss the recent identification of a critical role for CD19 in the B cell response to membrane-bound antigen and suggest a mechanism involving BCR microclusters by which it mediates its stimulatory function. Finally, we consider research that has taken advantage of recent technological advances in multiphoton microscopy that have allowed its application to the investigation of the dynamics of membrane-bound antigen presentation and subsequent B cell activation in lymph nodes in vivo.

Phospholipase C-gamma2 and Vav cooperate within signaling microclusters to propagate B cell spreading in response to membrane-bound antigen

B cell receptor (BCR) recognition of membrane-bound antigen initiates a spreading and contraction response, the extent of which is controlled through the formation of signaling-active BCR-antigen microclusters and ultimately affects the outcome of B cell activation. We followed a genetic approach to define the molecular requirements of BCR-induced spreading and microcluster formation. We identify a key role for phospholipase C-γ2 (PLCγ2), Vav, B cell linker, and Bruton's tyrosine kinase in the formation of highly coordinated “microsignalosomes,” the efficient assembly of which is absolutely dependent on Lyn and Syk. Using total internal reflection fluorescence microscopy, we examine at high resolution the recruitment of PLCγ2 and Vav to microsignalosomes, establishing a novel synergistic relationship between the two. Thus, we demonstrate the importance of cooperation between components of the microsignalosome in the amplification of signaling and propagation of B cell spreading, which is critical for appropriate B cell activation.

Activation of the small GTPase Rac2 via the B cell receptor regulates B cell adhesion and immunological-synapse formation

The integrin leukocyte function-associated antigen-1 (LFA-1) is important in the promotion of B cell adhesion, thereby facilitating immunological synapse (IS) formation and B cell activation. Despite this significance, the associated signaling mechanisms regulating LFA-1 activation remain elusive. Here, we show that both isoforms of the small GTPase Rac expressed by primary B cells, Rac1 and Rac2, were activated rapidly downstream of Src-family kinases, guanine-nucleotide exchange factors Vav1 and Vav2, and phosphoinositide-3 kinase (PI3K) after BCR engagement. We identify Rac2, but not Rac1, as critical for B cell adhesion to intercellular adhesion molecule-1 (ICAM-1) and IS formation. Furthermore, B cells expressing constitutively active Rac2 are highly adhesive. We observe that Rac2-deficient B cells exhibit lower amounts of Rap1-GTP and severe actin polymerization defects, identifying a potential mechanism underlying their behavior. We postulate that this critical role for Rac2 in mediating B cell adhesion and IS formation might apply in all lymphocytes.

CD19 is essential for B cell activation by promoting B cell receptor-antigen microcluster formation in response to membrane-bound ligand

Here we describe the spatiotemporal architecture, at high molecular resolution, of receptors and signaling molecules during the early events of mouse B cell activation. In response to membrane-bound ligand stimulation, antigen aggregation occurs in B cell antigen receptor (BCR) microclusters containing immunoglobulin (Ig) M and IgD that recruit the kinase Syk and transiently associate with the coreceptor CD19. Unexpectedly, CD19-deficient B cells were significantly defective in initiation of BCR-dependent signaling, accumulation of downstream effectors and cell spreading, defects that culminated in reduced microcluster formation. Hence, we have defined the dynamics of assembly of the main constituents of the BCR 'signalosome' and revealed an essential role for CD19, independent of the costimulatory molecule CD21, in amplifying early B cell activation events in response to membrane-bound ligand stimulation.

The intrinsic flexibility of IgE and its role in binding FcɛRI

The interaction between IgE and its high affinity cellular receptor (FcepsilonRI) is an essential step in the development of allergic responses. Studies have identified the third constant domain of IgE (Cepsilon3) as the receptor binding region. The Cepsilon3 domain has unusual structural features; it was found to be a 'molten globule' structure in an isolated form, only assuming a well structured form upon binding to FcepsilonRI. The conformational flexibility intrinsic to the receptor binding portion of the molecule may be useful to IgE in allowing the large allosteric changes postulated to be required for FcepsilonRI engagement. If allosteric inhibitors can be developed then the dynamic properties of the Cepsilon3 domain may provide opportunities for therapeutic intervention in allergic disorders.

Catalytic folding of the Cε3 domain by its high affinity receptor

The interaction of immunoglobulin E (IgE) with its cellular receptor FcepsilonRIalpha is a central regulator of allergy. Structural studies have identified the third domain (Cepsilon3) of the constant region of epsilon heavy chain as the receptor binding region. The isolated Cepsilon3 domain is a "molten globule" that becomes structured upon binding of the FcepsilonRIalpha ligand. In this study, fluorescence and nuclear magnetic resonance spectroscopies are used to characterise the role of soluble FcepsilonRIalpha in the folding of the monomeric Cepsilon3 domain of IgE. Soluble FcepsilonRIalpha is shown to display characteristic properties of a catalyst for the folding of Cepsilon3, with the rate of Cepsilon3 folding being dependent on the concentration of the receptor.