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. ASC was phosphorylated during inflammasome activation in a Syk- and Jnk-dependent manner, which suggested that Syk and Jnk are upstream of ASC phosphorylation. Moreover, phosphorylation of Tyr144 in mouse ASC was critical for speck formation and caspase-1 activation. Our results suggest that phosphorylation of ASC controls inflammasome activity through the formation of ASC specks.

The Neutrophil Btk Signalosome Regulates Integrin Activation during Sterile Inflammation.

Neutrophils are recruited from the blood to sites of sterile inflammation, where they are involved in wound healing but can also cause tissue damage. During sterile inflammation, necrotic cells release pro-inflammatory molecules including formylated peptides. However, the signaling pathway triggered by formylated peptides to integrin activation and leukocyte recruitment is unknown. By using spinning-disk confocal intravital microscopy, we examined the molecular mechanisms of leukocyte recruitment to sites of focal hepatic necrosis in vivo. We demonstrated that the Bruton's tyrosine kinase (Btk) was required for multiple Mac-1 activation events involved in neutrophil recruitment and functions during sterile inflammation triggered by fMLF. The Src family kinase Hck, Wiskott-Aldrich-syndrome protein, and phospholipase Cγ2 were also involved in this pathway required for fMLF-triggered Mac-1 activation and neutrophil recruitment. Thus, we have identified a neutrophil Btk signalosome that is involved in a signaling pathway triggered by formylated peptides leading to the selective activation of Mac-1 and neutrophil recruitment during sterile inflammation.

The kinase Btk negatively regulates the production of reactive oxygen species and stimulation-induced apoptosis in human neutrophils.

The function of the kinase Btk in neutrophil activation is largely unexplored. Here we found that Btk-deficient neutrophils had more production of reactive oxygen species (ROS) after engagement of Toll-like receptors (TLRs) or receptors for tumor-necrosis factor (TNF), which was associated with more apoptosis and was reversed by transduction of recombinant Btk. Btk-deficient neutrophils in the resting state showed hyperphosphorylation and activation of phosphatidylinositol-3-OH kinase (PI(3)K) and protein tyrosine kinases (PTKs) and were in a 'primed' state with plasma membrane-associated GTPase Rac2. In the absence of Btk, the adaptor Mal was associated with PI(3)K and PTKs at the plasma membrane, whereas in control resting neutrophils, Btk interacted with and confined Mal in the cytoplasm. Our data identify Btk as a critical gatekeeper of neutrophil responses.

Epigenetic modification and antibody-dependent expansion of memory-like NK cells in human cytomegalovirus-infected individuals.

Long-lived "memory-like" NK cells have been identified in individuals infected by human cytomegalovirus (HCMV), but little is known about how the memory-like NK cell pool is formed. Here, we have shown that HCMV-infected individuals have several distinct subsets of memory-like NK cells that are often deficient for multiple transcription factors and signaling proteins, including tyrosine kinase SYK, for which the reduced expression was stable over time and correlated with epigenetic modification of the gene promoter. Deficient expression of these proteins was largely confined to the recently discovered FcRγ-deficient NK cells that display enhanced antibody-dependent functional activity. Importantly, FcRγ-deficient NK cells exhibited robust preferential expansion in response to virus-infected cells (both HCMV and influenza) in an antibody-dependent manner. These findings suggest that the memory-like NK cell pool is shaped and maintained by a mechanism that involves both epigenetic modification of gene expression and antibody-dependent expansion.

Cytomegalovirus infection drives adaptive epigenetic diversification of NK cells with altered signaling and effector function.

The mechanisms underlying human natural killer (NK) cell phenotypic and functional heterogeneity are unknown. Here, we describe the emergence of diverse subsets of human NK cells selectively lacking expression of signaling proteins after human cytomegalovirus (HCMV) infection. The absence of B and myeloid cell-related signaling protein expression in these NK cell subsets correlated with promoter DNA hypermethylation. Genome-wide DNA methylation patterns were strikingly similar between HCMV-associated adaptive NK cells and cytotoxic effector T cells but differed from those of canonical NK cells. Functional interrogation demonstrated altered cytokine responsiveness in adaptive NK cells that was linked to reduced expression of the transcription factor PLZF. Furthermore, subsets of adaptive NK cells demonstrated significantly reduced functional responses to activated autologous T cells. The present results uncover a spectrum of epigenetically unique adaptive NK cell subsets that diversify in response to viral infection and have distinct functional capabilities compared to canonical NK cell subsets.

Intracellular MHC class II molecules promote TLR-triggered innate immune responses by maintaining activation of the kinase Btk.

The molecular mechanisms involved in the full activation of innate immunity achieved through Toll-like receptors (TLRs) remain to be fully elucidated. In addition to their classical antigen-presenting function, major histocompatibility complex (MHC) class II molecules might mediate reverse signaling. Here we report that deficiency in MHC class II attenuated the TLR-triggered production of proinflammatory cytokines and type I interferon in macrophages and dendritic cells, which protected mice from endotoxin shock. Intracellular MHC class II molecules interacted with the tyrosine kinase Btk via the costimulatory molecule CD40 and maintained Btk activation, but cell surface MHC class II molecules did not. Then, Btk interacted with the adaptor molecules MyD88 and TRIF and thereby promoted TLR signaling. Therefore, intracellular MHC class II molecules can act as adaptors, promoting full activation of TLR-triggered innate immune responses.

DYRK1a mediates BAFF-induced noncanonical NF-κB activation to promote autoimmunity and B-cell leukemogenesis.

B-cell-activating factor (BAFF) mediates B-cell survival and, when deregulated, contributes to autoimmune diseases and B-cell malignancies. The mechanism connecting BAFF receptor (BAFFR) signal to downstream pathways and pathophysiological functions is not well understood. Here we identified DYRK1a as a kinase that responds to BAFF stimulation and mediates BAFF-induced B-cell survival. B-cell-specific DYRK1a deficiency causes peripheral B-cell reduction and ameliorates autoimmunity in a mouse model of lupus. An unbiased screen identified DYRK1a as a protein that interacts with TRAF3, a ubiquitin ligase component mediating degradation of the noncanonical nuclear factor (NF)-κB-inducing kinase (NIK). DYRK1a phosphorylates TRAF3 at serine-29 to interfere with its function in mediating NIK degradation, thereby facilitating BAFF-induced NIK accumulation and noncanonical NF-κB activation. Interestingly, B-cell acute lymphoblastic leukemia (B-ALL) cells express high levels of BAFFR and respond to BAFF for noncanonical NF-κB activation and survival in a DYRK1a-dependent manner. Furthermore, DYRK1a promotes a mouse model of B-ALL through activation of the noncanonical NF-κB pathway. These results establish DYRK1a as a critical BAFFR signaling mediator and provide novel insight into B-ALL pathogenesis.

The right team at the right time to go for a home run: tyrosine kinase activation by the TCR.

The TCR signals via cytoplasmic tyrosine kinases. Art Weiss recounts discoveries that led to early understanding of these events.

HVCN1 modulates BCR signal strength via regulation of BCR-dependent generation of reactive oxygen species.

Voltage-gated proton currents regulate generation of reactive oxygen species (ROS) in phagocytic cells. In B cells, stimulation of the B cell antigen receptor (BCR) results in the production of ROS that participate in B cell activation, but the involvement of proton channels is unknown. We report here that the voltage-gated proton channel HVCN1 associated with the BCR complex and was internalized together with the BCR after activation. BCR-induced generation of ROS was lower in HVCN1-deficient B cells, which resulted in attenuated BCR signaling via impaired BCR-dependent oxidation of the tyrosine phosphatase SHP-1. This resulted in less activation of the kinases Syk and Akt, impaired mitochondrial respiration and glycolysis and diminished antibody responses in vivo. Our findings identify unanticipated functions for proton channels in B cells and demonstrate the importance of ROS in BCR signaling and downstream metabolism.

Integrin CD11b negatively regulates TLR-triggered inflammatory responses by activating Syk and promoting degradation of MyD88 and TRIF via Cbl-b.

Integrins are critical for the migration and function of leukocytes in inflammation. However, the interaction between integrin alpha(M) (CD11b), which has high expression in monocytes and macrophages, and Toll-like receptor (TLR)-triggered innate immunity remains unclear. Here we report that CD11b deficiency enhanced TLR-mediated responses in macrophages, rendering mice more susceptible to endotoxin shock and Escherichia coli-caused sepsis. CD11b was activated by TLR-triggered phosphatidylinositol 3-OH kinase (PI(3)K) and the effector RapL and fed back to inhibit TLR signaling by activating the tyrosine kinases Src and Syk. Syk interacted with and induced tyrosine phosphorylation of MyD88 and TRIF, which led to degradation of these adaptor molecules by the E3 ubiquitin ligase Cbl-b. Thus, TLR-triggered, active CD11b integrin engages in crosstalk with the MyD88 and TRIF pathways and subsequently inhibits TLR signaling in innate immune responses.

Distinct roles for neutrophils and dendritic cells in inflammation and autoimmunity in motheaten mice.

The motheaten mouse has long served as a paradigm for complex autoimmune and inflammatory disease. Null mutations in Ptpn6, which encodes the nonreceptor protein-tyrosine phosphatase Shp1, cause the motheaten phenotype. However, Shp1 regulates multiple signaling pathways in different hematopoietic cell types, so the cellular and molecular mechanism of autoimmunity and inflammation in the motheaten mouse has remained unclear. By using floxed Ptpn6 mice, we dissected the contribution of innate immune cells to the motheaten phenotype. Ptpn6 deletion in neutrophils resulted in cutaneous inflammation, but not autoimmunity, providing an animal model of human neutrophilic dermatoses. By contrast, dendritic cell deletion caused severe autoimmunity, without inflammation. Genetic and biochemical analysis showed that inflammation was caused by enhanced neutrophil integrin signaling through Src-family and Syk kinases, whereas autoimmunity resulted from exaggerated MyD88-dependent signaling in dendritic cells. Our data demonstrate that disruption of distinct Shp1-regulated pathways in different cell types combine to cause motheaten disease.

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. Genetic rescue studies demonstrated that Syk transduces BAFFR survival signals via ERK and PI3 kinase. Surprisingly, BAFFR signaling directly induced phosphorylation of both Syk and the BCR-associated Igα signaling subunit, and this Syk phosphorylation required the BCR. We conclude that the BCR and Igα may be required for B cell survival because they function as adaptor proteins in a BAFFR signaling pathway leading to activation of Syk, demonstrating previously unrecognized crosstalk between the two receptors.

TCR Signal Strength and Antigen Affinity Regulate CD8+ Memory T Cells.

CD8+ T cells play a critical role in adaptive immunity, differentiating into CD8+ memory T cells that form the basis of protective cellular immunity. Vaccine efficacy is attributed to long-term protective immunity, and understanding the parameters that regulate development of CD8+ T cells is critical to the design of T cell-mediated vaccines. We show in this study using mouse models that two distinct parameters, TCR signal strength (regulated by the tyrosine kinase ITK) and Ag affinity, play important but separate roles in modulating the development of memory CD8+ T cells. Unexpectedly, our data reveal that reducing TCR signal strength along with reducing Ag affinity for the TCR leads to enhanced and accelerated development of CD8+ memory T cells. Additionally, TCR signal strength is able to regulate CD8+ T cell effector cytokine R production independent of TCR Ag affinity. Analysis of RNA-sequencing data reveals that genes for inflammatory cytokines/cytokine receptors are significantly altered upon changes in Ag affinity and TCR signal strength. Furthermore, our findings show that the inflammatory milieu is critical in regulating this TCR signal strength-mediated increase in memory development, as both CpG oligonucleotide treatment or cotransfer of wild-type and Itk-/- T cells eliminates the observed increase in memory cell formation. These findings suggest that TCR signal strength and Ag affinity independently contribute to CD8+ memory T cell development, which is modulated by inflammation, and suggest that manipulating TCR signal strength along with Ag affinity, may be used to tune the development of CD8+ memory T cells during vaccine development.

The ROP16III-dependent early immune response determines the subacute CNS immune response and type III Toxoplasma gondii survival.

Toxoplasma gondii is an intracellular parasite that persistently infects the CNS and that has genetically distinct strains which provoke different acute immune responses. How differences in the acute immune response affect the CNS immune response is unknown. To address this question, we used two persistent Toxoplasma strains (type II and type III) and examined the CNS immune response at 21 days post infection (dpi). Contrary to acute infection studies, type III-infected mice had higher numbers of total CNS T cells and macrophages/microglia but fewer alternatively activated macrophages (M2s) and regulatory T cells (Tregs) than type II-infected mice. By profiling splenocytes at 5, 10, and 21 dpi, we determined that at 5 dpi type III-infected mice had more M2s while type II-infected mice had more pro-inflammatory macrophages and that these responses flipped over time. To test how these early differences influence the CNS immune response, we engineered the type III strain to lack ROP16 (IIIΔrop16), the polymorphic effector protein that drives the early type III-associated M2 response. IIIΔrop16-infected mice showed a type II-like neuroinflammatory response with fewer infiltrating T cells and macrophages/microglia and more M2s and an unexpectedly low CNS parasite burden. At 5 dpi, IIIΔrop16-infected mice showed a mixed inflammatory response with more pro-inflammatory macrophages, M2s, T effector cells, and Tregs, and decreased rates of infection of peritoneal exudative cells (PECs). These data suggested that type III parasites need the early ROP16-associated M2 response to avoid clearance, possibly by the Immunity-Related GTPases (IRGs), which are IFN-γ- dependent proteins essential for murine defenses against Toxoplasma. To test this possibility, we infected IRG-deficient mice and found that IIIΔrop16 parasites now maintained parental levels of PECs infection. Collectively, these studies suggest that, for the type III strain, rop16III plays a key role in parasite persistence and influences the subacute CNS immune response.

Navigating the leukocyte signaling maze guided by Ariadne's thread.

Ariadne is the legendary Minoan goddess of the Labyrinth. The term 'Ariadne's thread' is used to describe the understanding of complex issues. Immunologists attending the 5th Leukocyte Signal Transduction Workshop discussed the Ariadne's thread woven about intracellular signaling pathways.

Dectin-1 directs T helper cell differentiation by controlling noncanonical NF-kappaB activation through Raf-1 and Syk.

The C-type lectin dectin-1 activates the transcription factor NF-kappaB through a Syk kinase-dependent signaling pathway to induce antifungal immunity. Here we show that dectin-1 expressed on human dendritic cells activates not only the Syk-dependent canonical NF-kappaB subunits p65 and c-Rel, but also the noncanonical NF-kappaB subunit RelB. Dectin-1, when stimulated by the beta-glucan curdlan or by Candida albicans, induced a second signaling pathway mediated by the serine-threonine kinase Raf-1, which integrated with the Syk pathway at the point of NF-kappaB activation. Raf-1 antagonized Syk-induced RelB activation by promoting sequestration of RelB into inactive p65-RelB dimers, thereby altering T helper cell differentiation. Thus, dectin-1 activates two independent signaling pathways, one through Syk and one through Raf-1, to induce immune responses.

Fc receptor gamma-chain, a constitutive component of the IL-3 receptor, is required for IL-3-induced IL-4 production in basophils.

The Fc receptor common gamma-chain (FcRgamma) is a widely expressed adaptor bearing an immunoreceptor tyrosine-based activation motif (ITAM) that transduces activation signals from various immunoreceptors. We show here that basophils lacking FcRgamma developed normally and proliferated efficiently in response to interleukin 3 (IL-3) but were very impaired in IL-3-induced production of IL-4 and in supporting T helper type 2 differentiation. Through its transmembrane portion, FcRgamma associated constitutively with the common beta-chain of the IL-3 receptor and signaled by recruiting the kinase Syk. Retrovirus-mediated complementation demonstrated the essential function of the ITAM of FcRgamma in IL-3 signal transduction. Our results identify a previously unknown mechanism whereby FcRgamma functions to 'route' selective cytokine-triggered signals into the ITAM-mediated IL-4 production pathway.

Tuning T helper cell differentiation by ITK.

CD4+ effector T cells effectuate T cell immune responses, producing cytokines to orchestrate the nature and type of immune responses. The non-receptor tyrosine kinase IL-2 inducible T cell kinase (ITK), a mediator of T cell Receptor signaling, plays a critical role in tuning the development of these effector cells. In this review we discussed the role that signals downstream of ITK, including the Ras/MAPK pathway, play in differentially controlling the differentiation of TH17, Foxp3+ T regulatory (Treg) cells, and Type 1 regulatory T (Tr1) cells, supporting a model of ITK signals controlling a decision point in the effector T cell differentiation process.

Augmentation of Human Monocyte Responses to Lipopolysaccharide by the Protein S and Mer/Tyro3 Receptor Tyrosine Kinase Axis.

Resolution of the inflammatory response requires coordinated regulation of pro- and anti-inflammatory mediator production, together with clearance of recruited inflammatory cells. Many different receptors have been implicated in phagocytosis of apoptotic cells (efferocytosis), including Mer, a receptor tyrosine kinase that can mediate recognition and subsequent internalization of apoptotic cells. In this manuscript, we examine the expression and function of the Tyro3/Axl/Mer (TAM) family of receptors by human monocytes. We demonstrate that the Mer ligand, protein S, binds to the surface of viable monocytes via phosphatidylserine-dependent and -independent mechanisms. Importantly, we have identified a novel role for receptor tyrosine kinase signaling in the augmentation of monocyte cytokine release in response to LPS. We propose that low-level phosphatidylserine exposure on the plasma membrane of viable monocytes allows protein S binding that leads to TAM-dependent augmentation of proinflammatory cytokine production. Our findings identify a potentially important role for TAM-mediated signaling during the initiation phase of inflammation.