Since the description of the first mouse knockout for an IgG Fc receptor seven years ago, considerable progress has been made in defining the in vivo functions of these receptors in diverse biological systems. The role of activating Fc gamma Rs in providing a critical link between ligands and effector cells in type II and type III inflammation is now well established and has led to a fundamental revision of the significance of these receptors in initiating cellular responses in host defense, in determining the efficacy of therapeutic antibodies, and in pathological autoimmune conditions. Considerable progress has been made in the last two years on the in vivo regulation of these responses, through the appreciation of the importance of balancing activation responses with inhibitory signaling. The inhibitory FcR functions in the maintenance of peripheral tolerance, in regulating the threshold of activation responses, and ultimately in terminating IgG mediated effector stimulation. The consequences of deleting the inhibitory arm of this system are thus manifested in both the afferent and efferent immune responses. The hyperresponsive state that results leads to greatly magnified effector responses by cytotoxic antibodies and immune complexes and can culminate in autoimmunity and autoimmune disease when modified by environmental or genetic factors. Fc gamma Rs offer a paradigm for the biological significance of balancing activation and inhibitory signaling in the expanding family of activation/inhibitory receptor pairs found in the immune system.

Antibodies against nuclear self-antigens are characteristic of systemic autoimmunity, although mechanisms promoting their generation and selection are unclear. Here, we report that B cells containing the Y-linked autoimmune accelerator (Yaa) locus are intrinsically biased toward nucleolar antigens because of increased expression of TLR7, a single-stranded RNA-binding innate immune receptor. The TLR7 gene is duplicated in Yaa mice because of a 4-Megabase expansion of the pseudoautosomal region. These results reveal high divergence in mouse Y chromosomes and represent a good example of gene copy number qualitatively altering a polygenic disease manifestation.

By virtue of its ability to couple the BCR to an inhibitory pathway, FcgammaRIIB can potentially determine the fate of B cells upon IgG immune complex engagement. We now provide evidence for FcgammaRIIB as a component of a peripheral tolerance pathway with the observation that RIIB-/- mice develop autoantibodies and autoimmune glomerulonephritis in a strain-dependent fashion. Transfer of the autoimmune phenotype is associated with the presence of donor RIIB-/- B cells, with the RIIB+/+ myeloid cells primarily derived from the recipient. These results suggest that deficiency of RIIB on B cells leads to autoimmune disease in specific genetic backgrounds, thus identifying it as a susceptibility factor under the influence of epistatic modifiers for the development of autoimmunity.

Immune complexes are potent activators of inflammatory cells, triggering effector responses through the crosslinking of Fc receptors (FcRs) such as Fc(epsilon)RI or Fc(gamma)RIII. On B cells and mast cells, immune complexes are also negative regulators of activation triggered by antigen and Fc receptors, a consequence of coligation of the B-cell antigen receptor or Fc(epsilon)RI, respectively, and the inhibitory receptor Fc(gamma)RIIB. Here we show that inhibitory signalling by Fc(gamma)RIIB does not require the SH2-domain-containing protein tyrosine phosphatase, SHP-1, in mast cells and results in the recruitment of the SH2-domain-containing inositol polyphosphate 5-phosphatase, SHIP, to the tyrosine-phosphorylated 13-amino-acid inhibitory motif of Fc(gamma)RIIB in both B cells and mast cells. SHIP, by hydrolysing the 5-phosphate of phosphatidylinositol(3,4,5)P3 and inositol(1,3,4,5)P4, suggests a mechanism by which Fc(gamma)RIIB can inhibit calcium influx and downstream responses triggered by immune receptors.

Nucleic acid-binding innate immune receptors such as Toll-like receptor 7 (TLR7) and TLR9 have been implicated in the development of some autoimmune pathologies. The Y chromosome-linked genomic modifier Yaa, which correlates with a duplication of Tlr7 and 16 other genes, exacerbates lupus-like syndromes in several mouse strains. Here we demonstrated that duplication of the Tlr7 gene was the sole requirement for this accelerated autoimmunity, because reduction of Tlr7 gene dosage abolished the Yaa phenotype. Further, we described new transgenic lines that overexpressed TLR7 alone and found that spontaneous autoimmunity developed beyond a 2-fold increase in TLR7 expression. Whereas a modest increase in Tlr7 gene dosage promoted autoreactive lymphocytes with RNA specificities and myeloid cell proliferation, a substantial increase in TLR7 expression caused fatal acute inflammatory pathology and profound dendritic cell dysregulation. These results underscore the importance of tightly regulating expression of TLR7 to prevent spontaneous triggering of harmful autoreactive and inflammatory responses.

Two signaling molecules have been implicated in the modulation of immune receptor activation by inhibitory coreceptors: an inositol polyphosphate 5'-phosphatase, SHIP, and a tyrosine phosphatase, SHP-1. To address the necessity, interaction, or redundancy of these signaling molecules, we have generated SHP-1- or SHIP-deficient B cell lines and determined their ability to mediate inhibitory signaling. Two distinct classes of inhibitory responses are defined, mediated by the selective recruitment of SHP-1 or SHIP. The Fc gammaRIIB class of inhibitory signaling is dependent on SHIP and not SHP-1; conversely, the KIR class requires SHP-1 and not SHIP. The consequence of this selective recruitment by inhibitory receptor engagement is seen in BCR-triggered apoptosis. SHP-1-mediated inhibitory signaling blocks apoptosis, while SHIP recruitment attenuates a proapoptotic signal initiated by Fc gammaRIIB.

Membrane recruitment of SHIP is responsible for the inhibitory signal generated by FcgammaRIIB coligation to the BCR. By reducing the level of PIP3, SHIP regulates the association of the tyrosine kinase Btk with the membrane through PH domain-phosphoinositol lipid interactions. Inhibition of BCR signaling by either FcgammaRIIB coligation, membrane expression of SHIP, or inhibition of P13K, conditions which result in decreased levels of PIP3, is suppressed by the expression of Btk as a membrane-associated chimera. Conversely, increasing PIP3 levels by deletion of SHIP results in increased Btk association with the membrane and hyperresponsive BCR signaling. These results suggest a central role for PIP3 in regulating the B cell stimulatory state by modulating Btk localization and thereby calcium fluxes.

Glucocorticoids are widely used to treat patients with autoimmune diseases such as systemic lupus erythematosus (SLE) 1,2 . However, regimens used to treat many such conditions cannot maintain disease control in the majority of SLE patients and more aggressive approaches such as high-dose methylprednisolone pulse therapy are used to provide transient reductions in disease activity 3,4 . The primary anti-inflammatory mechanism of glucocorticoids is thought to be NF-κB inhibition 5 . Recognition of self nucleic acids by toll-like receptors TLR7 and TLR9 on B cells and plasmacytoid dendritic cells (PDCs) is an important step in the pathogenesis of SLE 6 , promoting anti-nuclear antibodies and the production of type I interferon (IFN), both correlated with the severity of disease 1,7 . Following their activation by self-nucleic acid-associated immune complexes, PDCs migrate to the tissues 8,9 . We demonstrate, in vitro and in vivo, that stimulation of PDCs through TLR7 and 9 can account for the reduced activity of glucocorticoids to inhibit the IFN pathway in SLE patients and in two lupus-prone mouse strains. The triggering of PDCs through TLR7 and 9 by nucleic acid-containing immune complexes or by synthetic ligands activates the NF-κB pathway essential for PDC survival. Glucocorticoids do not affect NF-κB activation in PDCs, preventing glucocorticoid induction of PDC death and the consequent reduction of systemic IFN-α levels. These findings unveil a new role for self nucleic acid