The Mertk receptor tyrosine kinase promotes T-B interaction stimulated by IgD B-cell receptor cross-linking.

The Mertk receptor tyrosine kinase facilitates macrophage and DC apoptotic-cell clearance and regulates immune tolerance. Mertk may also contribute to B-cell activation, because Mertk-KO mice fail to develop autoantibodies when allo-activated by T cells. We investigated this possibility with a well-characterized model in which injection of mice with goat anti-IgD antibody causes membrane IgD cross-linking that induces T-independent B cell activation and antigen presentation to T cells. Goat anti-mouse IgD antibody-injected C57BL/6 Mertk-KO mice had normal initial B cell activation and proliferation, but significantly lower T cell activation and proliferation, as well as lower IgE and IgG anti-goat IgG responses, as compared to C57BL/6 WT controls. B cell antigen processing, analyzed by evaluating B cell fluorescence following injection of monoclonal anti-IgD antibody labeled with biotin or FITC, was comparable between Mertk-KO mice and WT mice. IgD Ab primed B cells from Mertk-KO mice exhibited significantly lower ability in activating memory T cells isolated from WT mice injected with the same antigen 10 days before. These observations suggest that Mertk expression is required for optimal B-cell antigen presentation, which is, in turn, required in this model for optimal T cell activation and subsequent T cell-dependent B cell differentiation.

NKG2D ligation relieves 2B4-mediated NK-cell self-tolerance in mice.

Along with MHC class I (MHCI), 2B4 provides nonredundant NK-cell inhibition in mice. The immunoregulatory role of 2B4 has been increasingly appreciated in models of tumor and viral infection, however, the interactions among 2B4, MHCI, and other activating NK-cell receptors remain uncertain. Here, we dissect the influence of two distinct inhibitory pathways in modulating NK-cell-mediated control of tumors expressing strong activating ligands, including RAE-1γ. In vitro cytotoxicity and in vivo peritoneal clearance assays using MHCI(+) CD48(+) (RMA-neo), MHCI(+) CD48(+) RAE-1γ (RMA-RAE-1γ), MHCI(-) CD48(+) (RMA-S-neo), and MHCI(-) CD48(+) RAE-1γ (RMA-S-RAE-1γ) tumor lines demonstrated that NKG2D activation supersedes the inhibitory effect of both 2B4- and MHCI-mediated immune-tolerance systems. Furthermore, 2B4KO mice subcutaneously challenged with RMA-neo and RMA-S-neo exhibited reduced tumor growth and significantly prolonged survival compared with WT mice, implying that 2B4 is constitutively engaged in the NK-cell tolerance mechanism in vivo. Nevertheless, the inhibitory effect of 2B4 is significantly attenuated when NK cells encountered highly stressed tumor cells expressing RAE-1γ, resulting in an immune response shift toward NK-cell activation and tumor regression. Therefore, our data highlight the importance of the 2B4-mediated inhibitory system as an alternate self-tolerance mechanism, whose role can be modulated by the strength of activating receptor signaling within the tumor microenvironment.

Polyubiquitination of insulin-like growth factor I receptor (IGF-IR) activation loop promotes antibody-induced receptor internalization and down-regulation.

Ubiquitination has been implicated in negatively regulating insulin-like growth factor I receptor (IGF-IR) activity. Because of the relative stability of IGF-IR in the presence of ligand stimulation, IGF-IR ubiquitination sites have yet to be mapped and characterized, thus preventing a direct demonstration of how the receptor ubiquitination contributes to downstream molecular cascades. We took advantage of an anti-IGF-IR antibody (h10H5) that induces more efficient receptor down-regulation to show that IGF-IR is promptly and robustly ubiquitinated. The ubiquitination sites were mapped to the two lysine residues in the IGF-IR activation loop (Lys-1138 and Lys-1141) and consisted of polyubiquitin chains formed through both Lys-48 and Lys-29 linkages. Mutation of these ubiquitinated lysine residues resulted in decreased h10H5-induced IGF-IR internalization and down-regulation as well as a reduced cellular response to h10H5 treatment. We have therefore demonstrated that IGF-IR ubiquitination contributes critically to the down-regulating and antiproliferative activity of h10H5. This finding is physiologically relevant because insulin-like growth factor I appears to mediate ubiquitination of the same major sites as h10H5 (albeit to a lesser extent), and ubiquitination is facilitated by pre-existing phosphorylation of the receptor in both cases. Furthermore, identification of a breast cancer cell line with a defect in IGF-IR ubiquitination suggests that this could be an important tumor resistance mechanism to evade down-regulation-mediated negative regulation of IGF-IR activity in cancer.

The membrane skeleton controls diffusion dynamics and signaling through the B cell receptor.

Early events of B cell activation after B cell receptor (BCR) triggering have been well characterized. However, little is known about the steady state of the BCR on the cell surface. Here, we simultaneously visualize single BCR particles and components of the membrane skeleton. We show that an ezrin- and actin-defined network influenced steady-state BCR diffusion by creating boundaries that restrict BCR diffusion. We identified the intracellular domain of Igbeta as important in mediating this restriction in diffusion. Importantly, alteration of this network was sufficient to induce robust intracellular signaling and concomitant increase in BCR mobility. Moreover, by using B cells deficient in key signaling molecules, we show that this signaling was most probably initiated by the BCR. Thus, our results suggest the membrane skeleton plays a crucial function in controlling BCR dynamics and thereby signaling, in a way that could be important for understanding tonic signaling necessary for B cell development and survival.

IgE signaling suppresses FcepsilonRIbeta expression.

Activation of the high-affinity receptor for IgE, FcepsilonRI, is known to elicit its rapid down-regulation through internalization and degradation. In keeping with this, expression of all three FcepsilonRI subunits is decreased at the protein level after cross-linkage of IgE with antigen. However, we find that the FcepsilonRI beta-subunit is also selectively suppressed at the mRNA level, through a pathway primarily involving Fyn, Syk, PI3K, and NF-kappaB. IgG or calcium ionophore, stimuli known to mimic portions of the IgE signaling cascade, similarly suppressed beta-subunit expression. LPS, a NF-kappaB-activating TLR ligand, did not alter beta-subunit expression. As IgE increases FcepsilonRI expression, we examined the coordinated regulation of FcepsilonRI subunits during culture with IgE, followed by cross-linkage with antigen. IgE increased the expression of all three FcepsilonRI subunits and strikingly induced expression of the antagonistic beta(T). The ratio of beta:beta(T) protein expression decreased significantly during culture with IgE and was reset to starting levels by antigen cross-linkage. These changes in protein levels were matched by similar fluctuations in beta and beta(T) mRNAs. FcepsilonRIbeta is a key regulator of IgER expression and function, a gene in which polymorphisms correlate with allergic disease prevalence. The ability of IgE and FcepsilonRI signaling to coordinate expression of the beta and beta(T) subunits may comprise a homeostatic feedback loop-one that could promote chronic inflammation and allergic disease if dysregulated.

Analysis of somatic hypermutation in X-linked hyper-IgM syndrome shows specific deficiencies in mutational targeting.

Subjects with X-linked hyper-IgM syndrome (X-HIgM) have a markedly reduced frequency of CD27(+) memory B cells, and their Ig genes have a low level of somatic hypermutation (SHM). To analyze the nature of SHM in X-HIgM, we sequenced 209 nonproductive and 926 productive Ig heavy chain genes. In nonproductive rearrangements that were not subjected to selection, as well as productive rearrangements, most of the mutations were within targeted RGYW, WRCY, WA, or TW motifs (R = purine, Y = pyrimidine, and W = A or T). However, there was significantly decreased targeting of the hypermutable G in RGYW motifs. Moreover, the ratio of transitions to transversions was markedly increased compared with normal. Microarray analysis documented that specific genes involved in SHM, including activation-induced cytidine deaminase (AICDA) and uracil-DNA glycosylase (UNG2), were up-regulated in normal germinal center (GC) B cells, but not induced by CD40 ligation. Similar results were obtained from light chain rearrangements. These results indicate that in the absence of CD40-CD154 interactions, there is a marked reduction in SHM and, specifically, mutations of AICDA-targeted G residues in RGYW motifs along with a decrease in transversions normally related to UNG2 activity.

Endosomal trafficking of the ligated FcvarepsilonRI receptor.

In addition to initiating signaling cascades leading to mast cell mediator release, aggregation of the high affinity IgE receptor (FcvarepsilonRI) leads to rapid internalization of the cross-linked receptor. However, little is known about the trafficking of the internalized FcvarepsilonRI. Here we demonstrate that in RBL-2H3 cells, aggregated FcvarepsilonRI appears in the early endosomal antigen 1 (EEA1(+)) domains of the early endosomes within 15min after ligation. Minimal co-localization of FcvarepsilonRI with Rab5 was observed by 30min, followed by its appearance in the Rab7(+) late endosomes and lysosomes at later time points. During endosomal sorting, FcvarepsilonRIalpha and gamma subunits remain associated. In Syk-deficient RBL-2H3 cells, the rate of transport to lysosomes is markedly increased. Taken together, our data demonstrate time-dependent sorting of aggregated FcvarepsilonRI within the endosomal-lysosomal network, and that Syk may play an essential role in regulating the trafficking and retention of FcvarepsilonRI in endosomes.

NKp80 defines and stimulates a reactive subset of CD8 T cells.

NKp80, an activating homodimeric C-type lectin-like receptor (CTLR), is expressed on essentially all human natural killer (NK) cells and stimulates their cytotoxicity and cytokine release. Recently, we demonstrated that the ligand for NKp80 is the myeloid-specific CTLR activation-induced C-type lectin (AICL), which is encoded in the natural killer gene complex (NKC) adjacent to NKp80. Here, we show that NKp80 also is expressed on a minor fraction of human CD8 T cells that exhibit a high responsiveness and an effector memory phenotype. Gene expression profiling and flow cytometric analyses revealed that this NKp80(+) T-cell subset is characterized by the coexpression of other NK receptors and increased levels of cytotoxic effector molecules and adhesion molecules mediating access to sites of inflammation. NKp80 ligation augmented CD3-stimulated degranulation and interferon (IFN)gamma secretion by effector memory T cells. Furthermore, engagement of NKp80 by AICL-expressing transfectants or macrophages markedly enhanced CD8 T-cell responses in alloreactive settings. Collectively, our data demonstrate that NKp80 is expressed on a highly responsive subset of effector memory CD8 T cells with an inflammatory NK-like phenotype and promotes T-cell responses toward AICL-expressing cells. Hence, NKp80 may enable effector memory CD8 T cells to interact functionally with cells of myeloid origin at sites of inflammation.

In vivo genetic mutations define predominant functions of the human T-cell leukemia/lymphoma virus p12I protein.

The human T-cell leukemia/lymphoma virus type 1 (HTLV-1) ORF-I encodes a 99-amino acid hydrophobic membrane protein, p12(I), that affects receptors in different cellular compartments. We report here that proteolytic cleavage dictates different cellular localization and functions of p12(I). The removal of a noncanonical endoplasmic reticulum (ER) retention/retrieval signal within the amino terminus of p12(I) is necessary for trafficking to the Golgi apparatus and generation of a completely cleaved 8-kDa protein. The 8-kDa protein in turn traffics to the cell surface, is recruited to the immunologic synapse following T-cell receptor (TCR) ligation, and down-regulates TCR proximal signaling. The uncleaved 12-kDa form of p12(I) resides in the ER and interacts with the beta and gamma(c) chains of the interleukin-2 receptor (IL-2R), the heavy chain of the major histocompatibility complex (MHC) class I, as well as calreticulin and calnexin. Genetic analysis of ORF-I from ex vivo samples of HTLV-1-infected patients reveals predominant amino acid substitutions within ORF-I that affect proteolytic cleavage, suggesting that ER-associated functions of p12(I) may contribute to the survival and proliferation of the infected T cells in the host.

TREM-2 mediated signaling induces antigen uptake and retention in mature myeloid dendritic cells.

Myeloid dendritic cells (mDC) activated with a B7-DC-specific cross-linking IgM Ab (B7-DC XAb) take up and retain Ag and interact with T cell compartments to affect a number of biologic changes that together cause strong antitumor responses and blockade of inflammatory airway disease in animal models. The molecular events mediating the initial responses in mDC remain unclear. In this study we show that B7-DC XAb caused rapid phosphorylation of the adaptor protein DAP12 and intracellular kinases Syk and phospholipase C-gamma1. Pretreatment of mDC with the Syk inhibitor piceatannol blocked B7-DC XAb-induced Ag uptake with a concomitant loss of tumor protection in mice. Vaccination with tumor lysate-pulsed wild-type B7-DC XAb-activated mDC, but not TREM-2 knockout XAb-activated mDC, protected mice from lethal melanoma challenge. Multimolecular caps appeared within minutes of B7-DC XAb binding to either human or mouse mDC, and FRET analysis showed that class II, CD80, CD86, and TREM-2 are recruited in tight association on the cell surface. When TREM-2 expression was reduced in wild-type mDC using short hairpin RNA or by using mDC from TREM-2 knockout mice, in vitro DC failed to take up Ag after B7-DC XAb stimulation. These results directly link TREM-2 signaling with one change in the mDC phenotype that occurs in response to this unique Ab. The parallel signaling events observed in both human and mouse mDC support the hypothesis that B7-DC cross-linking may be useful as a therapeutic immune modulator in human patients.

Antigen-induced clustering of surface CD38 and recruitment of intracellular CD38 to the immunologic synapse.

During immunologic synapse (IS) formation, human CD38 redistributes to the contact area of T cell-antigen-presenting cell (APC) conjugates in an antigen-dependent manner. Confocal microscopy showed that CD38 preferentially accumulated along the contact zone, whereas CD3-zeta redistributed toward the central zone of the IS. APC conjugates with human T cells or B cells transiently expressing CD38-green fluorescent protein revealed the presence of 2 distinct pools of CD38, one localized at the cell membrane and the other in recycling endosomes. Both pools were recruited to the T/APC contact sites and required antigen-pulsed APCs. The process appeared more efficient in T cells than in APCs. CD38 was actively recruited at the IS of T cells by means of Lck-mediated signals. Overexpression of CD38 in T cells increased the levels of antigen-induced intracellular calcium release. Opposite results were obtained by down-regulating surface CD38 expression by means of CD38 siRNA. CD38 blockade in influenza HA-specific T cells inhibited IL-2 and IFN-gamma production, PKC phosphorylation at Thr538, and PKC recruitment to the IS induced by antigen-pulsed APCs. These results reveal a new role for CD38 in modulating antigen-mediated T-cell responses during IS formation.

CD38 induces differentiation of immature transitional 2 B lymphocytes in the spleen.

CD38 is a surface receptor able to induce activation, proliferation, and survival of human and mouse lymphocytes; this molecule is expressed on the surface of both mature and immature B cells. In this work, the function of CD38 in the maturation of murine B lymphocytes in the spleen was analyzed. The results showed that CD38 is highly expressed on Transitional 2 (T2) B lymphocytes with an intermediate expression on Transitional 1 (T1) and mature follicular B cells (M). Correlating with a high expression of CD38, T2 cells are also larger and more granular than T1 or M B cells. T2 cells also showed high levels of other molecules, which indicate an activated phenotype. CD38 crosslinking induced proliferation and maturation of T2 B lymphocytes; in contrast, T1 subset died by apoptosis. Finally, CD38 stimulation of T2 B lymphocytes obtained from Btk-, Lyn-, or Fyn-deficient mice showed a defective differentiation; similarly, drugs interfering with PI3K or ERK decreased the proliferation or differentiation of this subset. This suggests that these molecules participate in the CD38 signaling pathway. As a whole, the results indicate that CD38 plays an important role in the regulation of B-cell maturation in the spleen.

Transmembrane mutations to FcgammaRIIA alter its association with lipid rafts: implications for receptor signaling.

Many immunoreceptors have been reported to associate with lipid rafts upon ligand binding. The way in which this association is regulated is still obscure. We investigated the roles for various domains of the human immunoreceptor FcgammaRIIA in regulating its association with lipid rafts by determining the resistance of unligated, or ligated and cross-linked, receptors to solubilization by the nonionic detergent Triton X-100, when expressed in RBL-2H3 cells. Deletion of the cytoplasmic domain, or destruction of the cytoplasmic palmitoylation site, had no effect on the association of the receptor with lipid rafts. A transmembrane mutant, A224S, lost the ability to associate with lipid rafts upon receptor cross-linking, whereas transmembrane mutants VA231-2MM and VVAL234-7GISF showed constitutive lipid raft association. Wild-type (WT) FcgammaRIIA and all transmembrane mutants activated Syk, regardless of their association with lipid rafts. WT FcgammaRIIA and mutants that associated with lipid rafts efficiently activated NF-kappaB, in an ERK-dependent manner. In contrast, WT FcgammaRIIA and the A224S mutant both presented efficient phagocytosis, while VA231-2MM and VVAL234-7GISF mutants presented lower phagocytosis, suggesting that phagocytosis may proceed independently of lipid raft association. These data identify the transmembrane domain of FcgammaRIIA as responsible for regulating its inducible association with lipid rafts and suggest that FcgammaRIIA-mediated responses, like NF-kappaB activation or phagocytosis, can be modulated by lipid raft association of the ligated receptor.

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.