The lymphoid organs contain specialized microanatomic structures composed of lymphoid, myeloid and stromal cells that are vital to the generation of an effective adaptive immune response. Although the existence of these specialized structures has been known for over a century, the developmental signals that generate them and the specific roles of these structures in the immune response have remained largely elusive. Because of their position adjacent to the marginal sinuses, marginal zone B (MZB) cells are amongst the first population of cells seen by blood born antigens and are presumed to have a critical role in host defense against bacterial pathogens. Here we demonstrate that a deficiency of the tyrosine kinase (Pyk-2) results in a cell autonomous defect of MZB cell production. In response to repetitive polysaccharide antigens (T-independent type II (TI-II)) Pyk-2-deficient mice displayed marked suppression of IgM, IgG3 and IgG2a production. Furthermore, complement receptor engagement proved necessary for the specific targeting of polysaccharide antigens to MZB cells. These results suggest how innate immune responses mediated through complement coupling are translated into an adaptive response by MZB cells, and provide a potential mechanism for the T cell independence of humoral responses to polysaccharide antigens.

Cross-linking of the antigen receptor on lymphocytes by antigens or antibodies to the receptor results in activation of enzymes of the protein kinase C (PKC) family. Mice homozygous for a targeted disruption of the gene encoding the PKC-betaI and PKC-betaII isoforms develop an immunodeficiency characterized by impaired humoral immune responses and reduced cellular responses of B cells, which is similar to X-linked immunodeficiency in mice. Thus PKC-betaI and PKC-betaII play an important role in B cell activation and may be functionally linked to Bruton's tyrosine kinase in antigen receptor-mediated signal transduction.

Autoantibodies and immune complexes are major pathogenic factors in autoimmune injury, responsible for initiation of the inflammatory cascade and its resulting tissue damage. This activation results from the interaction of immunoglobulin (Ig)G Fc receptors containing an activation motif (ITAM) with immune complexes (ICs) and cytotoxic autoantibodies which initiates and propagates an inflammatory response. In vitro, this pathway can be interrupted by coligation to FcγRIIB, an IgG Fc receptor containing an inhibitory motif (ITIM). In this report, we describe the in vivo consequences of FcγRII deficiency in the inflammatory response using a mouse model of IC alveolitis. At subthreshold concentrations of ICs that fail to elicit inflammatory responses in wild-type mice, FcγRII-deficient mice developed robust inflammatory responses characterized by increased hemorrhage, edema, and neutrophil infiltration. Bronchoalveolar fluids from FcγRII−/− stimulated mice contain higher levels of tumor necrosis factor and chemotactic activity, suggesting that FcγRII deficiency lowers the threshold of IC stimulation of resident cells such as the alveolar macrophage. In contrast, complement- and complement receptor–deficient mice develop normal inflammatory responses to suprathreshold levels of ICs, while FcRγ−/− mice are completely protected from inflammatory injury. An inhibitory role for FcγRII on macrophages is demonstrated by analysis of FcγRII−/− macrophages which show greater phagocytic and calcium flux responses upon FcγRIII engagement. These data reveal contrasting roles for the cellular receptors for IgG on inflammatory cells, providing a regulatory mechanism for setting thresholds for IC sensitivity based on the ratio of ITIM to ITAM FcγR expression. Exploiting the FcγRII inhibitory pathway could thus provide a new therapeutic approach for modulating antibody-triggered inflammation.

The marginal zone of the spleen is a precisely ordered region that contains specialized subsets of B lymphocytes and macrophages. Disruption of the negative signaling inositol phosphatase, SH2-containing inositol-5-phosphatase 1 (SHIP), results in the loss of marginal zone B cells (MZBs) with reorganization of marginal zone macrophages (MZMOs) to the red pulp of the spleen. This primary macrophage defect, as revealed by selectively depleting SHIP in myeloid cells shows that MZMOs are specifically required for the retention of MZBs. The MZMO phenotype was reverted in SHIP/Bruton's tyrosine kinase (Btk) double knockout mice, thus identifying the Btk activating pathway as an essential component being regulated by SHIP. Furthermore, we identified a direct interaction between the MARCO scavenger receptor on MZMOs and MZBs. Activation or disruption of this interaction results in MZB migration to the follicle. The migration of the MZMOs was further studied after the response to Staphylococcus aureus, which induced MZMOs to move into the red pulp while MZBs migrated into the follicular zone. The marginal zone is therefore a dynamic structure in which retention and trafficking of B cells requires specific macrophage–B cell interactions.

Fc gammaRIIB is an inhibitory receptor that terminates activation signals initiated by antigen cross-linking of the BCR through the recruitment of SHIP. Fc gammaRIIB can also signal independently of BCR coligation to directly mediate an apoptotic response, requiring only an intact transmembrane domain. Failure to recruit SHIP, either by deletion of SHIP or mutation of Fc gammaRIIB, results in enhanced Fc gammaRIIB-triggered apoptosis. Thus, in the germinal center, where ICs are retained by FDCs, Fc gammaRIIB may be an active determinant in the negative selection of B cells whose BCRs have reduced affinity for antigen as a result of somatic hypermutation. Selection of B cells may represent the sum of opposing signals generated by the interaction of ICs with the BCR and Fc gammaRIIB through pathways modulated by SHIP.

Our findings position lysosomal cholesterol hydrolysis as a critical process that prevents metabolic inflammation by enabling efficient macrophage apoptotic cell clearance.

Rationale Inflamed atherosclerotic plaques can be visualized by non-invasive PET-CT imaging with 18FDG, a glucose analog but the underlying mechanisms are poorly understood. Objective Here, we directly investigated the role of Glut1-mediated glucose uptake in ApoE−/− mouse model of atherosclerosis. Methods and Results We first show that the enhanced glycolytic flux in atheromatous plaques of ApoE−/− mice was associated with the enhanced metabolic activity of hematopoietic stem and multi-potential progenitors (HSPCs) and higher Glut1 expression in these cells. Mechanistically, the regulation of Glut1 in ApoE−/− HSPCs was not due to alterations in hypoxia-inducible factor 1α (HIF1α) signaling or the oxygenation status of the bone marrow but was the consequence of the activation of the common β subunit of the granulocyte macrophage colony-stimulating factor/interleukin-3 receptor driving glycolytic substrate utilization by mitochondria. By transplanting BM from WT, Glut1+/−, ApoE−/− and ApoE−/−Glut1+/− mice into hypercholesterolemic ApoE deficient mice, we found that Glut1 deficiency reversed ApoE−/− HSPC proliferation and expansion, which prevented the myelopoiesis and accelerated atherosclerosis of ApoE−/− mice transplanted with ApoE−/− BM and resulted in reduced glucose uptake in the spleen and aortic arch of these mice. Conclusions We identified that Glut1 connects the enhanced glucose uptake in atheromatous plaques of ApoE−/− mice with their myelopoiesis through regulation of HSPC maintenance and myelomonocytic fate and suggest Glut1 as potential drug target for atherosclerosis.

The Wiskott-Aldrich syndrome (WAS) is a rare immunodeficiency disease affecting mainly platelets and lymphocytes. Here, we show that the WAS gene product, WASp, is tyrosine phosphorylated upon aggregation of the high affinity IgE receptor (FcO ORI) at the surface of RBL-2H3 rat tumor mast cells. Lyn and the Bruton's tyrosine kinase (Btk), two protein tyrosine kinases involved in FcO ORI-signaling phosphorylate WASp and interact with WASp in vivo. Interestingly, expression of a GTPase defective mutant form of CDC42, that interacts with WASp, is accompanied by a substantial increase in WASp tyrosine phosphorylation. This study suggests that activated CDC42 recruits WASp to the plasma membrane where it becomes phosphorylated by Lyn and Btk. We conclude that WASp represents a connection between protein tyrosine kinase signaling pathways and CDC42 function in cytoskeleton and cell growth regulation in hematopoietic cells.z 1998 Federation of European Biochemical Societies.