FcγRIIB-deficient mice generated in 129 background (FcγRIIB129−/−) if back-crossed into C57BL/6 background exhibit a hyperactive phenotype and develop lethal lupus. Both in mice and humans, the Fcγr2b gene is located within a genomic interval on chromosome 1 associated with lupus susceptibility. In mice, the 129-derived haplotype of this interval, named Sle16, causes loss of self-tolerance in the context of the B6 genome, hampering the analysis of the specific contribution of FcγRIIB deficiency to the development of lupus in FcγRIIB129−/− mice. Moreover, in humans genetic linkage studies revealed contradictory results regarding the association of “loss of function” mutations in the Fcγr2b gene and susceptibility to systemic lupus erythematosis. In this study, we demonstrate that FcγRIIB−/− mice generated by gene targeting in B6-derived ES cells (FcγRIIBB6−/−), lacking the 129-derived flanking Sle16 region, exhibit a hyperactive phenotype but fail to develop lupus indicating that in FcγRIIB129−/− mice, not FcγRIIB deficiency but epistatic interactions between the C57BL/6 genome and the 129-derived Fcγr2b flanking region cause loss of tolerance. The contribution to the development of autoimmune disease by the resulting autoreactive B cells is amplified by the absence of FcγRIIB, culminating in lethal lupus. In the presence of the Yaa lupus-susceptibility locus, FcγRIIBB6−/− mice do develop lethal lupus, confirming that FcγRIIB deficiency only amplifies spontaneous autoimmunity determined by other loci.
Membranoproliferative glomerulonephritis (MPGN) type II (dense deposit disease) is an inflammatory renal disease characterized by electron-dense deposits and complement C3 on the glomerular basement membrane. There is no effective therapy. We investigated the role of C5 activation in a model of MPGN that develops spontaneously in complement factor H-deficient mice (Cfh ؊/؊ ). At 12 months there was a significant reduction in mortality, glomerular cellularity, neutrophil numbers, and serum creatinine levels in Cfh ؊/؊ mice deficient in C5. Excessive glomerular neutrophil numbers, frequently seen in patients with MPGN during disease flares, were also observed in Cfh ؊/؊ mice after the administration of an antiglomerular basement membrane antibody. This exaggerated injurious phenotype was absent in Cfh ؊/؊ mice deficient in C5 but not in Cfh ؊/؊ mice deficient in C6, indicating a key role for C5 activation in the induction of renal lesions. Importantly, the renal injury was completely reversed in Cfh ؊/؊ mice pretreated with an anti-murine C5 antibody. These results demonstrate an important role for C5 in both spontaneous MPGN and experimentally induced nephritis in factor H-deficient mice and provide preliminary evidence that C5 inhibition therapy might be useful in human MPGN type II.complement ͉ inflammation T he complement system is a key component of innate immunity contributing to host defenses against invading pathogens through multiple mechanisms, which include opsonization, cell lysis, and inflammatory cell recruitment, an action principally mediated through the anaphylatoxin C5a. Complement activation is regulated by a complex group of membrane-bound and fluid-phase proteins (1). Factor H is an abundant serum complement regulatory protein that inhibits the alternative pathway of complement activation. It achieves this through several mechanisms, which include inhibition of the alternative pathway C3 convertase enzyme complex (C3bBb) and acting as a cofactor for the factor I-mediated proteolytic degradation of activated C3 (termed C3b) (2, 3). Its critical importance as a regulator of C3 activation in vivo is illustrated by the complement profile reported in factor H-deficient individuals, where alternative pathway activation proceeds unhindered, resulting in markedly reduced C3 levels (4).Factor H deficiency in humans (5, 6), pigs (7), and mice (8) is associated with membranoproliferative glomerulonephritis (MPGN) type II (dense deposit disease). MPGN is characterized by glomerular capillary wall thickening with increased mesangial matrix and mesangial cells (9). Intramembranous glomerular basement membrane (GBM) deposits together with C3 (10), C5 (11), and C9 (12) staining along the GBM in the absence of Ig characterize type II MPGN (13). Patients typically have low C3 levels while C5 levels remain normal (14). MPGN type II is frequently associated with the presence of C3 nephritic factor (C3NeF), an autoantibody that stabilizes the alternative pathway C3 convertase, preventing its inactivation by factor...
Extensive evidence indicates that genetic predisposition is a central element in susceptibility to systemic lupus erythematosus both in humans and animals. We have previously shown that a congenic line carrying a 129-derived chromosome 1 interval on the C57BL/6 background developed humoral autoimmunity. To further dissect the contribution to autoimmunity of this 129 interval, we have created six subcongenic strains carrying fractions of the original 129 region and analyzed their serological and cellular phenotypes. At 1 year of age the congenic strain carrying a 129 interval between the microsatellites D1Mit15 (87.9 cM) and D1Mit115 (99.7 cM) (B6.129chr1b) had high levels of autoantibodies, while all the other congenic lines were not significantly different from the C57BL/6 controls. The B6.129chr1b strain displayed only mild proliferative glomerulonephritis despite high levels of IgG and C3 deposited in the kidneys. FACS analysis of the spleens revealed that the B6.129chr1b mice had a marked increase in the percentage of activated T cells associated with a significant reduction in the proportion of CD4+CD25high regulatory T cells. Moreover, this analysis showed a significantly reduced percentage of marginal zone B cells that preceded autoantibody production. Interestingly the 129chr1b-expressing bone marrow-derived macrophages displayed an impaired uptake of apoptotic cells in vitro. Collectively, our data indicate that the 129chr1b segment when recombined on the C57BL/6 genomic background is sufficient to induce loss of tolerance to nuclear Ags. These findings have important implication for the interpretation of the autoimmune phenotype associated with gene-targeted models.
PET radioligands targeted to translocator protein (TSPO) offer a highly sensitive and specific means of imaging joint inflammation in rheumatoid arthritis (RA). Through high expression of TSPO on activated macrophages, TSPO PET has been widely reported in several studies of RA as a means of imaging synovial macrophages in vivo. However, this premise does not take into account the ubiquitous expression of TSPO. This study aimed to investigate TSPO expression in major cellular constituents of RA pannus-monocytes, macrophages, fibroblastlike synoviocytes (FLS cells), and CD4-positive (CD4+) T lymphocytes (T cells)-to more accurately interpret TSPO PET signal from RA synovium. Three RA patients and 3 healthy volunteers underwent PET of both knees using the TSPO radioligandC-PBR28. Through H-PBR28 autoradiography and immunostaining of synovial tissue in 6 RA patients and 6 healthy volunteers, cellular expression of TSPO in synovial tissue was evaluated. TSPO messenger RNA expression andH-PBR28 radioligand binding was assessed using in vitro monocytes, macrophages, FLS cells, and CD4+ T cells. C-PBR28 PET signal was significantly higher in RA joints than in healthy joints (average SUV, 0.82 ± 0.12 vs. 0.03 ± 0.004; < 0.01). Further, H-PBR28-specific binding in synovial tissue was approximately 10-fold higher in RA patients than in healthy controls. Immunofluorescence revealed TSPO expression on macrophages, FLS cells, and CD4+ T cells. The in vitro study demonstrated the highest TSPO messenger RNA expression andH-PBR28-specific binding in activated FLS cells, nonactivated M0 macrophages, and activated M2 reparative macrophages, with the least TSPO expression being in activated and nonactivated CD4+ T cells. To our knowledge, this study was the first evaluation of cellular TSPO expression in synovium, with the highest TSPO expression and PBR28 binding being found on activated synovial FLS cells and M2 macrophages. TSPO-targeted PET may therefore have a unique sensitivity in detecting FLS cells and macrophage-predominant inflammation in RA, with potential utility for assessing treatment response in trials using novel FLS-cell-targeted therapies.
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