Natural killer cell immune synapse formation and cytotoxicity are controlled by tension of the target interface

Natural killer (NK) cells can kill infected or transformed cells via a lytic immune synapse. Diseased cells may exhibit altered mechanical properties but how this impacts NK cell responsiveness is unknown. We report that human NK cells were stimulated more effectively to secrete granzymes A and B, FasL (also known as FasLG), granulysin and IFNγ, by stiff (142 kPa) compared to soft (1 kPa) planar substrates. To create surrogate spherical targets of defined stiffness, sodium alginate was used to synthesise soft (9 kPa), medium (34 kPa) or stiff (254 kPa) cell-sized beads, coated with antibodies against activating receptor NKp30 (also known as NCR3) and the integrin LFA-1 (also known as ITGAL). Against stiff beads, NK cells showed increased degranulation. Polarisation of the microtubule-organising centre and lytic granules were impaired against soft targets, which instead resulted in the formation of unstable kinapses. Thus, by varying target stiffness to characterise the mechanosensitivity of immune synapses, we identify soft targets as a blind spot in NK cell recognition. This article has an associated First Person interview with the co-first authors of the paper.

TAM receptors attenuate murine NK-cell responses via E3 ubiquitin ligase Cbl-b

TAM receptors (Tyro3, Axl, and Mer) are receptor tyrosine kinases (RTKs) that are expressed by multiple immune cells including NK cells. Although RTKs typically enhance cellular functions, TAM receptor ligation blocks NK-cell activation. The mechanisms by which RTKs block NK-cell signaling downstream of activating receptors are unknown. In this report, we demonstrate that TAM receptors attenuate NK cell responses via the activity of E3 ubiquitin ligase Casitas B lineage lymphoma b (Cbl-b). Specifically, we show that Tyro3, Axl, and Mer phosphorylate Cbl-b, and Tyro3 ligation activates Cbl-b by phosphorylating tyrosine residues 133 and 363. Ligation of TAM receptors by their ligand Gas6 suppresses activating receptor-stimulated NK-cell functions such as IFN-γ production and degranulation, in a TAM receptor kinase- and Cbl-b-dependent manner. Moreover, Gas6 ligation induces the degradation of LAT1, a transmembrane adaptor protein required for NK cell activating receptor signaling, in WT but not in Cbl-b knock-out NK cells. Together, these results suggest that TAM receptors may attenuate NK-cell function by phosphorylating Cbl-b, which in turn dampens NK-cell activation signaling by promoting the degradation of LAT1. Our data therefore support a mechanism by which RTKs attenuate, rather than stimulate, signaling pathways via the activation of ubiquitin ligases.

Effective B cell activation in vitro during viremic HIV-1 infection with surrogate T cell stimulation

Identifying HIV-1-associated B cell defects and responses to activation may direct interventions to circumvent their impaired antibody responses to infection and vaccines. Among 34 viremic HIV-1-infected and 20 seronegative control adults, we measured baseline frequencies and activation of B and T cell subsets, expression of activation-induced cytidine deaminase (AID), potential determinants of B cell activation in vivo and B and T cell responses in vitro. At baseline, HIV-1 infection was associated with increased IgM memory and decreased anergic cell frequencies, as well as increased activation in all 10 B cell subsets compared with controls. HIV-1 status, TFH activation, and BAFF were significant potential drivers of B cell activation. Despite high baseline activation among HIV-1-infected subjects, stimulation in vitro with combined surrogates for antigen (anti-IgM), cognate (CD40 ligand) and soluble T cell factors (IL-4) elicited comparable B cell activation, transitions from naïve to class-switched memory cells and AID expression in both groups. In summary, viremic HIV-1 infection perturbs circulating B cell subsets and activation at each stage of B cell maturation. However, that appropriate stimulation of B cells elicits effective activation and maturation provides impetus for advancing vaccine development to prevent secondary infections by circumventing early B cell defects.

Morphological characterization of a plant-made virus-like particle vaccine bearing influenza virus hemagglutinins by electron microscopy

Plant-made virus-like particle (VLP) vaccines that display wild-type influenza hemagglutinin (HA) are rapidly advancing through clinical trials. Produced by transient transfection of Nicotiana benthamiana, these novel vaccines are unusually immunogenic, eliciting both humoral and cellular responses. Here, we directly visualized VLPs bearing either HA trimers derived from strains A/California/7/2009 or A/Indonesia/5/05 using cryo-electron microscopy and determined the 3D organization of the VLPs using cryo-electron tomography. More than 99.9% of the HA trimers in the vaccine preparations were found on discoid and ovoid-shaped particles. The discoid-shaped VLPs presented HA trimers on their outer diameter. The ovoid-shaped VLPs contained HA trimers evenly distributed at their surface. The VLPs were stable for 12 months at 4 °C. Early interactions of the VLPs with mouse dendritic and human monocytoid (U-937) cells were visualized by electron microscopy after resin-embedding and sectioning. The VLP particles were observed bound to plasma membranes as well as inside vesicles. Mouse dendritic cells exposed to VLPs displayed classic morphological changes associated with activation including the extensive formation of dendrites. Our findings demonstrate that plant-made VLPs bearing influenza HA trimers are morphologically stable over time and raise the possibility that these VLPs may interact with and activate antigen-presenting cells in a manner similar to the intact virus.

Reduced TCR-dependent activation through citrullination of a T-cell epitope enhances Th17 development by disruption of the STAT3/5 balance

Citrullination is a post‐translational modification of arginine that commonly occurs in inflammatory tissues. Because T‐cell receptor (TCR) signal quantity and quality can regulate T‐cell differentiation, citrullination within a T‐cell epitope has potential implications for T‐cell effector function. Here, we investigated how citrullination of an immunedominant T‐cell epitope affected Th17 development. Murine naïve CD4⁺ T cells with a transgenic TCR recognising p89‐103 of the G1 domain of aggrecan (agg) were co‐cultured with syngeneic bone marrow‐derived dendritic cells (BMDC) presenting the native or citrullinated peptides. In the presence of pro‐Th17 cytokines, the peptide citrullinated on residue 93 (R93Cit) significantly enhanced Th17 development whilst impairing the Th2 response, compared to the native peptide. T cells responding to R93Cit produced less IL‐2, expressed lower levels of the IL‐2 receptor subunit CD25, and showed reduced STAT5 phosphorylation, whilst STAT3 activation was unaltered. IL‐2 blockade in native p89‐103‐primed T cells enhanced the phosphorylated STAT3/STAT5 ratio, and concomitantly enhanced Th17 development. Our data illustrate how a post‐translational modification of a TCR contact point may promote Th17 development by altering the balance between STAT5 and STAT3 activation in responding T cells, and provide new insight into how protein citrullination may influence effector Th‐cell development in inflammatory disorders.

Regulation of immune cell signaling by SHIP1: A phosphatase, scaffold protein, and potential therapeutic target

The phosphoinositide phosphatase SHIP is a critical regulator of immune cell activation. Despite considerable study, the mechanisms controlling SHIP activity to ensure balanced cell activation remain incompletely understood. SHIP dampens BCR signaling in part through its association with the inhibitory coreceptor Fc gamma receptor IIB, and serves as an effector for other inhibitory receptors in various immune cell types. The established paradigm emphasizes SHIP's inhibitory receptor-dependent function in regulating phosphoinositide 3-kinase signaling by dephosphorylating the phosphoinositide PI(3,4,5)P₃ ; however, substantial evidence indicates that SHIP can be activated independently of inhibitory receptors and can function as an intrinsic brake on activation signaling. Here, we integrate historical and recent reports addressing the regulation and function of SHIP in immune cells, which together indicate that SHIP acts as a multifunctional protein controlled by multiple regulatory inputs, and influences downstream signaling via both phosphatase-dependent and -independent means. We further summarize accumulated evidence regarding the functions of SHIP in B cells, T cells, NK cells, dendritic cells, mast cells, and macrophages, and data suggesting defective expression or activity of SHIP in autoimmune and malignant disorders. Lastly, we discuss the biological activities, therapeutic promise, and limitations of small molecule modulators of SHIP enzymatic activity.

B-cell-intrinsic function of TAPP adaptors in controlling germinal center responses and autoantibody production in mice

Control of B-cell signal transduction is critical to prevent production of pathological autoantibodies. Tandem PH domain containing proteins (TAPPs) specifically bind PI(3,4)P2, a phosphoinositide product generated by PI 3-kinases and the phosphatase SHIP. TAPP KI mice bearing PH domain-inactivating mutations in both TAPP1 and TAPP2 genes, uncoupling them from PI(3,4)P2, exhibit increased BCR-induced activation of the kinase Akt and develop lupus-like characteristics including anti-DNA antibodies and deposition of immune complexes in kidneys. Here, we find that TAPP KI mice develop chronic germinal centers (GCs) with age and show abnormal expression of B-cell activation and memory markers. Upon immunization with T-dependent Ag, TAPP KI mice develop functional but abnormally large GCs, associated with increased GC B-cell survival. Disruption of chronic GCs in TAPP KI mice by deletion of the costimulatory molecule ICOS abrogate anti-DNA and anti-nuclear antibody production in TAPP KI mice, indicating an essential role for GCs. Moreover, TAPP KI B cells are sufficient to drive chronic GC responses and recapitulate the autoimmune phenotype in BM chimeric mice. Our findings demonstrate a B-cell-intrinsic role of TAPP-PI(3,4)P2 interaction in regulating GC responses and autoantibody production and suggest that uncontrolled Akt activity in B cells can drive autoimmunity.

Flow cytometric characterization of microglia in the offspring of PolyI:C treated mice

The neuropathology of schizophrenia has been reported to be closely associated with microglial activation. In a previous study, using the prenatal PolyI:C schizophrenia animal model, we showed an increase in cell numbers and a reduction in microglial branching in 30-day-old PolyI:C descendants, which suggests that there is microglial activation during adolescence. To provide more information about the activation state, we aimed to examine the expression levels of Iba1, which was reported to be up-regulated in activated microglia. We used a flow cytometric approach and investigated CD11b and CD45, two additional markers for the identification of microglial cells. We demonstrated that intracellular staining against Iba1 can be used as a reliable flow cytometric method for identification of microglial cells. Prenatal PolyI:C treatment had long-term effects on CD11b and CD45 expression. It also resulted in a trend towards increased Iba1 expression. Imbalance in CD11b, CD45, and Iba1 expression might contribute to impaired synaptic surveillance and enhanced activation/inflammatory activity of microglia in adult offspring.

Pharmacological preconditioning with adenosine A(1) receptor agonist suppresses cellular immune response by an A(2A) receptor dependent mechanism

Under stressful conditions such as ischemia, sepsis, and severe trauma, adenosine levels are elevated and protect the tissue by interaction with G coupled receptors. In a model of peritonitis, we previously found that pharmacological preconditioning (PPC) of mice with a selective adenosine A1 receptor (A1R) agonist, 2-chloro-N(6)-cyclopentyladenosine (CCPA), induced the A2AR which reduces cytokine secretion and leukocyte recruitment. In our present study we determined whether mice PPC will moderate cellular immune response by the same mechanism. Similar to the effect on inflammation, PPC reduced the response to lymphocyte mitogens and allogeneic MLR response. The inhibitory effect of PPC on the immune response was A1R and A2AR dependent as illustrated by experiments with antagonists of these receptors and mice with knock down (KO) receptors. In MLR with PPC splenocytes we found reduced levels of pro-inflammatory cytokines (IFN-γ, IL-15, TNF-α) and elevation of IL-10, as well as elevation of regulatory T-cell. Our data indicate that PPC is able to remarkably suppress cellular immune response due to the sensitization A2AR. This effect of PPC sheds light on the protective role of adenosine in ischemic preconditioning and makes this treatment candidate for the prevention of graft rejection.

Microfilaments make mast cells migrate (rather than degranulate)

Expression of the high-affinity receptor for IgE (FcεRI) provides mast cells with the ability to react in a proinflammatory manner to antigens (Ags). In particular, the immediate secretion of preformed mediators from secretory lysosomes (degranulation) is typical for FcεRI-mediated mast cell activation. In addition to the FcεRI, the stem cell factor receptor, KIT, is expressed at high levels on the surface of mast cells. KIT activation controls mast cell differentiation and survival in vivo and potently stimulates the chemotaxis of these cells. Although FcεRI and KIT initiate many of the same early signaling events in mast cells, FcεRI activation results in potent degranulation and a poor chemotactic response while KIT activation triggers very little degranulation and a strong chemotactic response. Novel data published in this issue of the European Journal of Immunology [Smrž et al. Eur. J. Immunol. 2013. 43: 1873-1882] demonstrate that actin de- and repolymerization, involved in both degranulation and chemotaxis, make all the difference: Pharmacological suppression of F-actin formation converts activated KIT into a strong degranulator. The possible implications for mast cell physiology and pathophysiology are discussed in this Commentary.

CTLA-4-Tg/CD-28-KO Mice Exhibit Reduced T Cell Proliferation in vivo Compared to CD-28-KO Mice in a Graft-versus-host Disease Model

Activated T cells express inhibitory receptors such as CTLA-4 that can downregulate immune responses. Blockade of or genetic deficiency in CTLA-4 can result in autoimmunity. Therefore, strategies to increase the inhibitory function of CTLA-4 may be attractive in settings of undesirable T cell responses such as autoimmunity or transplant rejection. We have tested the hypothesis that transgenic constitutive expression of CTLA-4 can further attenuate immune responses when compared with normal inducible expression. Our results indicate that transgenic expression of CTLA-4 in mouse T cells (CTLA-4-Tg T cells) results in reduced cell cycle progression and increased apoptosis of TCR-stimulated T cells. CTLA-4-Tg T cells display reduced T cell proliferation in an in vivo model of graft versus host disease (GVHD). These results further our understanding of how CTLA-4 can be manipulated to inhibit immune responses and may help development of new therapeutic strategies for clinical settings of autoimmunity and transplantation.