Eosinophils are an essential element of a type 2 immune axis that controls thymus regeneration.

Therapeutic interventions used for cancer treatment provoke thymus damage and limit the recovery of protective immunity. Here, we show that eosinophils are an essential part of an intrathymic type 2 immune network that enables thymus recovery after ablative therapy. Within hours of damage, the thymus undergoes CCR3-dependent colonization by peripheral eosinophils, which reestablishes the epithelial microenvironments that control thymopoiesis.

BAFF signaling in health and disease.

BAFF is a critical cytokine supporting the survival of mature naïve B cells, acting through the BAFFR receptor. Recent studies show that BAFF and BAFFR are also required for the survival of memory B cells, autoimmune B cells as well as malignant chronic lymphocytic leukaemia (CLL) cells. BAFFR cooperates with other receptors, notably the B cell antigen receptor (BCR), a process which is critical for the expansion of autoimmune and CLL cells.

Critical requirement for BCR, BAFF, and BAFFR in memory B cell survival.

Memory B cells (MBCs) are long-lived cells that form a critical part of immunological memory, providing rapid antibody responses to recurring infections. However, very little is known about signals controlling MBC survival. Previous work has shown that antigen is not required for MBC survival, but a requirement for the B cell antigen receptor (BCR) has not been tested.

-specific atypical memory B cells are short-lived activated B cells.

A subset of atypical memory B cells accumulates in malaria and several infections, autoimmune disorders and aging in both humans and mice. It has been suggested these cells are exhausted long-lived memory B cells, and their accumulation may contribute to poor acquisition of long-lasting immunity to certain chronic infections, such as malaria and HIV. Here, we generated an immunoglobulin heavy chain knock-in mouse with a BCR that recognizes MSP1 of the rodent malaria parasite, .

Signalling for B cell survival.

The number of mature B cells is carefully controlled by signalling from receptors that support B cell survival. The best studied of these are the B cell antigen receptor (BCR) and BAFFR. Recent work has shown that signalling from these receptors is closely linked, involves the CD19 co-receptor, and leads to activation of canonical and non-canonical NF-κB pathways, ERK1, ERK2 and ERK5 MAP kinases, and PI-3 kinases.

TLR4 signals in B lymphocytes are transduced via the B cell antigen receptor and SYK.

Toll-like receptors (TLRs) play an important role in immune responses to pathogens by transducing signals in innate immune cells in response to microbial products. TLRs are also expressed on B cells, and TLR signaling in B cells contributes to antibody-mediated immunity and autoimmunity. The SYK tyrosine kinase is essential for signaling from the B cell antigen receptor (BCR), and thus for antibody responses.

BAFF activation of the ERK5 MAP kinase pathway regulates B cell survival.

B cell activating factor (BAFF) stimulation of the BAFF receptor (BAFF-R) is essential for the homeostatic survival of mature B cells. Earlier in vitro experiments with inhibitors that block MEK 1 and 2 suggested that activation of ERK 1 and 2 MAP kinases is required for BAFF-R to promote B cell survival. However, these inhibitors are now known to also inhibit MEK5, which activates the related MAP kinase ERK5.

Syk tyrosine kinase is critical for B cell antibody responses and memory B cell survival.

Signals from the BCR are required for Ag-specific B cell recruitment into the immune response. Binding of Ag to the BCR induces phosphorylation of immune receptor tyrosine-based activation motifs in the cytoplasmic domains of the CD79a and CD79b signaling subunits, which subsequently bind and activate the Syk protein tyrosine kinase. Earlier work with the DT40 chicken B cell leukemia cell line showed that Syk was required to transduce BCR signals to proximal activation events, suggesting that Syk also plays an important role in the activation and differentiation of primary B cells during an immune response.

Rapid CD4+ T-cell responses to bacterial flagellin require dendritic cell expression of Syk and CARD9.

Toll-like receptors (TLRs) can recognize microbial patterns and utilize adaptor molecules, such as-MyD88 or (TRIF TIR-domain-containing adapter-inducing interferon-β), to initiate downstream signaling that ultimately affects the initiation of adaptive immunity. In addition to this inflammatory role, TLR5 expression on dendritic cells can favor antigen presentation of flagellin peptides and thus increase the sensitivity of flagellin-specific T-cell responses in vitro and in vivo. Here, we examined the role of alternative signaling pathways that might regulate flagellin antigen presentation in addition to MyD88.

IKK-induced NF-κB1 p105 proteolysis is critical for B cell antibody responses to T cell-dependent antigen.

The importance of IκB kinase (IKK)-induced proteolysis of NF-κB1 p105 in B cells was investigated using Nfkb1(SSAA/SSAA) mice, in which this NF-κB signaling pathway is blocked. Nfkb1(SSAA) mutation had no effect on the development and homeostasis of follicular mature (FM) B cells. However, analysis of mixed bone marrow chimeras revealed that Nfkb1(SSAA/SSAA) FM B cells were completely unable to mediate T cell-dependent antibody responses.

Themis2 is not required for B cell development, activation, and antibody responses.

Themis1 is a protein implicated in transducing signals from the TCR. Mice deficient in Themis1 show a strong impairment in T cell selection in the thymus and defective T cell activation. The related Themis2 protein is expressed in B cells where it associates with signaling proteins Grb2 and Vav1, and is tyrosine phosphorylated after BCR stimulation.

Phosphorylation of the adaptor ASC acts as a molecular switch that controls the formation of speck-like aggregates and inflammasome activity.

The inflammasome adaptor ASC contributes to innate immunity through the activation of caspase-1. Here we found that signaling pathways dependent on the kinases Syk and Jnk were required for the activation of caspase-1 via the ASC-dependent inflammasomes NLRP3 and AIM2. Inhibition of Syk or Jnk abolished the formation of ASC specks without affecting the interaction of ASC with NLRP3.

The BAFF receptor transduces survival signals by co-opting the B cell receptor signaling pathway.

Follicular B cell survival requires signaling from BAFFR, a receptor for BAFF and the B cell antigen receptor (BCR). This "tonic" BCR survival signal is distinct from that induced by antigen binding and may be ligand-independent. We show that inducible inactivation of the Syk tyrosine kinase, a key signal transducer from the BCR following antigen binding, resulted in the death of most follicular B cells because Syk-deficient cells were unable to survive in response to BAFF.

CLEC-2 and Syk in the megakaryocytic/platelet lineage are essential for development.

The C-type lectin receptor CLEC-2 signals through a pathway that is critically dependent on the tyrosine kinase Syk. We show that homozygous loss of either protein results in defects in brain vascular and lymphatic development, lung inflation, and perinatal lethality. Furthermore, we find that conditional deletion of Syk in the hematopoietic lineage, or conditional deletion of CLEC-2 or Syk in the megakaryocyte/platelet lineage, also causes defects in brain vascular and lymphatic development, although the mice are viable.

Restoration of pattern recognition receptor costimulation to treat chromoblastomycosis, a chronic fungal infection of the skin.

Chromoblastomycosis is a chronic skin infection caused by the fungus Fonsecaea pedrosoi. Exploring the reasons underlying the chronic nature of F. pedrosoi infection in a murine model of chromoblastomycosis, we find that chronicity develops due to a lack of pattern recognition receptor (PRR) costimulation.

Dectin-2 is a Syk-coupled pattern recognition receptor crucial for Th17 responses to fungal infection.

Innate immune cells detect pathogens via pattern recognition receptors (PRRs), which signal for initiation of immune responses to infection. Studies with Dectin-1, a PRR for fungi, have defined a novel innate signaling pathway involving Syk kinase and the adaptor CARD9, which is critical for inducing Th17 responses to fungal infection. We show that another C-type lectin, Dectin-2, also signals via Syk and CARD9, and contributes to dendritic cell (DC) activation by fungal particles.

Syk kinase signalling couples to the Nlrp3 inflammasome for anti-fungal host defence.

Fungal infections represent a serious threat, particularly in immunocompromised patients. Interleukin-1beta (IL-1beta) is a key pro-inflammatory factor in innate antifungal immunity. The mechanism by which the mammalian immune system regulates IL-1beta production after fungal recognition is unclear.

Redundant role for Zap70 in B cell development and activation.

Expression of the Syk family tyrosine kinase Zap70 is strongly correlated with poor clinical outcome in chronic lymphocytic leukemia, the most common human leukemia characterized by B cell accumulation. The expression of Zap70 may reflect the specific cell of origin of the tumor or may contribute to pathology. Thus, the normal role of Zap70 in B cell physiology is of great interest.

Regulatory T cells inhibit dendritic cells by lymphocyte activation gene-3 engagement of MHC class II.

Lymphocyte activation gene-3 (LAG-3) is a CD4-related transmembrane protein expressed by regulatory T cells that binds MHC II on APCs. It is shown in this study that during Treg:DC interactions, LAG-3 engagement with MHC class II inhibits DC activation. MHC II cross-linking by agonistic Abs induces an ITAM-mediated inhibitory signaling pathway, involving FcgammaRgamma and ERK-mediated recruitment of SHP-1 that suppresses dendritic cell maturation and immunostimulatory capacity.

Syk kinase is required for collaborative cytokine production induced through Dectin-1 and Toll-like receptors.

Recognition of microbial components by germ-line encoded pattern recognition receptors (PRR) initiates immune responses to infectious agents. We and others have proposed that pairs or sets of PRR mediate host immunity. One such pair comprises the fungal beta-glucan receptor, Dectin-1, which collaborates through an undefined mechanism with Toll-like receptor 2 (TLR2) to induce optimal cytokine responses in macrophages.

Syk-dependent ERK activation regulates IL-2 and IL-10 production by DC stimulated with zymosan.

Zymosan is a particulate yeast preparation that elicits high levels of IL-2 and IL-10 from dendritic cells (DC) and engages multiple innate receptors, including the Syk-coupled receptor dectin-1 and the MyD88-coupled receptor TLR2. Here, we show that induction of IL-2 and IL-10 by zymosan requires activation of ERK MAP kinase in murine DC. Surprisingly, ERK activation in response to zymosan is completely blocked in Syk-deficient DC and unaffected by MyD88 deficiency.

Syk- and CARD9-dependent coupling of innate immunity to the induction of T helper cells that produce interleukin 17.

The C-type lectin dectin-1 binds to yeast and signals through the kinase Syk and the adaptor CARD9 to induce production of interleukin 10 (IL-10) and IL-2 in dendritic cells (DCs). However, whether this pathway promotes full DC activation remains unclear. Here we show that dectin-1-Syk-CARD9 signaling induced DC maturation and the secretion of proinflammatory cytokines, including IL-6, tumor necrosis factor and IL-23, but little IL-12.

GPVI potentiation of platelet activation by thrombin and adhesion molecules independent of Src kinases and Syk.

Objective: The present study investigates the role of Src and Syk tyrosine kinases in signaling by G-protein coupled and platelet adhesion receptors.

Methods And Results: Using Syk-/- platelets or the Src kinase inhibitor PP2, we demonstrate a critical role for Src and Syk kinases in mediating lamellipodia formation on VWF, collagen, CRP, fibrinogen, and fibronectin. In all cases, the spreading defect was overcome by addition of thrombin.

A novel Syk-dependent mechanism of platelet activation by the C-type lectin receptor CLEC-2.

The snake venom rhodocytin has been reported to bind to integrin alpha2beta1 and glycoprotein (GP) Ibalpha on platelets, but it is also able to induce activation independent of the 2 receptors and of GPVI. Using rhodocytin affinity chromatography, we have identified a novel C-type lectin receptor, CLEC-2, in platelets that confers signaling responses to rhodocytin when expressed in a cell line. CLEC-2 has a single tyrosine residue in a YXXL motif in its cytosolic tail, which undergoes tyrosine phosphorylation upon platelet activation by rhodocytin or an antibody to CLEC-2, but not to collagen, thrombin receptor agonist peptide (TRAP), or convulxin.

Syk-dependent cytokine induction by Dectin-1 reveals a novel pattern recognition pathway for C type lectins.

Pattern-recognition receptors (PRRs) detect molecular signatures of microbes and initiate immune responses to infection. Prototypical PRRs such as Toll-like receptors (TLRs) signal via a conserved pathway to induce innate response genes. In contrast, the signaling pathways engaged by other classes of putative PRRs remain ill defined.