Susceptibility loci for the defective foreign protein-induced tolerance in New Zealand Black mice: implication of epistatic effects of Fcgr2b and Slam family genes

Distinct and synergistic roles of FcγRIIB deficiency and 129 strain-derived SLAM family proteins in the development of spontaneous germinal centers and autoimmunity

The inhibitory IgG Fc receptor (FcγRIIB) deficiency and 129 strain-derived signaling lymphocyte activation molecules (129-SLAMs) are proposed to contribute to the lupus phenotype in FcγRIIB-deficient mice generated using 129 ES cells and backcrossed to C57BL/6 mice (B6.129.RIIBKO). In this study, we examine the individual contributions and the cellular mechanisms by which FcγRIIB deficiency and 129-derived SLAM family genes promote dysregulated spontaneous germinal center (Spt-GC) B cell and follicular helper T cell (Tfh) responses in B6.129.RIIBKO mice. We find that B6 mice congenic for the 129-derived SLAM locus (B6.129-SLAM) and B6 mice deficient in FcγRIIB (B6.RIIBKO) have increased Spt-GC B cell responses compared to B6 controls but significantly lower than B6.129.RIIBKO mice. These data indicate that both FcγRIIB deficiency and 129-SLAMs contribute to elevated Spt-GC B cell responses in B6.129.RIIBKO mice. However, only 129-SLAMs contribute significantly to augmented Tfh responses in B6.129.RIIBKO mice, and do so by a combination of T cell-dependent effects and enhanced B cell and DC-dependent antigen presentation to T cells. Elevated Spt-GC B cell responses in mice with FcγRIIB deficiency and polymorphic 129-SLAMs were associated with elevated metabolic activity, improved GC B cell survival and increased differentiation of naïve B cells into GC B cell phenotype. Our data suggest that the interplay between 129-SLAM expression on B cells, T cells and DCs is central to the alteration of the GC tolerance checkpoint, and that deficiency of FcγRIIB on B cells is necessary to augment Spt-GC responses, pathogenic autoantibodies, and lupus disease.

Reduced Atherosclerosis in apoE-inhibitory FcγRIIb-Deficient Mice Is Associated With Increased Anti-Inflammatory Responses by T Cells and Macrophages

OBJECTIVE

Fcγ receptors (FcγRs) are classified as activating (FcγRI, III, and IV) and inhibitory (FcγRII) receptors. We have reported that deletion of activating FcγRs in apolipoprotein E (apoE) single knockout mice attenuated atherosclerosis. In this report, we investigated the hypothesis that deficiency of inhibitory FcγRIIb exacerbates atherosclerosis.

APPROACH AND RESULTS

ApoE-FcγRIIb double knockout mice, congenic to the C57BL/6 (apoE-FcγRIIbB6 (-/-)), were generated and atherosclerotic lesions were assessed. In contrary to our hypothesis, when compared with apoE single knockout mice, arterial lesions were significantly decreased in apoE-FcγRIIbB6 (-/-) male and female mice fed chow or high-fat diets. Chimeric mice generated by transplanting apoE-FcγRIIbB6 (-/-) marrow into apoE single knockout mice also developed reduced lesions. CD4(+) T cells from apoE-FcγRIIbB6 (-/-) mice produced higher levels of interleukin-10 and transforming growth factor-β than their apoE single knockout counterparts. As our findings conflict with a previous report using apoE-FcγRIIb129/B6 (-/-) mice on a mixed genetic background, we investigated whether strain differences contributed to the anti-inflammatory response. Macrophages from FcγRIIb129/B6 (-/-) mice on a mixed genetic background produced more interleukin-1β and MCP-1 (monocyte chemoattractant protein-1) in response to immune complexes, whereas congenic FcγRIIbB6 (-/-) mice generated more interleukin-10 and significantly less interleukin-1β. Interestingly, the expression of lupus-associated slam genes, located in proximity to fcgr2b in mouse chromosome 1, is upregulated only in mixed FcγRIIb129/B6 (-/-) mice.

CONCLUSIONS

Our findings demonstrate a detrimental role for FcγRIIb signaling in atherosclerosis and the contribution of anti-inflammatory cytokine responses in the attenuated lesions observed in apoE-FcγRIIbB6 (-/-) mice. As 129/sv genome-derived lupus-associated genes have been implicated in lupus phenotype in FcγRIIb129/B6 (-/-) mice, our findings suggest possible epistatic mechanism contributing to the decreased lesions.

Genome-wide genetic study in autoimmune disease-prone mice

Mouse models of autoimmune diseases provide invaluable insights into the cellular and molecular bases of autoimmunity. Genetic linkage studies focusing on their abnormal quantitative phenotypes in relation to the loss of self-tolerance will lead to the identification of polymorphic genes that play pivotal roles in the genetic predisposition to autoimmunity. In this chapter, we first overview the basic concepts in the statistical genetics and then provide guides to genotyping microsatellite DNA markers and to quantitative trait loci mapping using a MAPMAKER program.

Inhibitory Fcγ receptor is required for the maintenance of tolerance through distinct mechanisms

The inhibitory FcγR FcγRIIB is widely expressed on B cells, dendritic cells (DCs), and myeloid effector cells and modulates a variety of Ab-driven in vivo functions. Although it has been established that FcγRIIB plays an important role in the maintenance of peripheral tolerance, the responsible cell-specific FcγRIIB expression remains to be determined. In this study, we generated mice with selective deletion of FcγRIIB in B cells, DCs, and myeloid effector cells and evaluated these novel strains in models of tolerance and autoimmune diseases. Our results demonstrate that mice with selective deletion of FcγRIIB expression in B cells and DCs have increased Ab and T cell responses, respectively, and display enhanced susceptibility to disease in distinct models, suggesting that FcγRIIB expression in distinct cellular populations contributes to the maintenance of peripheral tolerance through different mechanisms.

Epistasis and immunity: the role of genetic interactions in autoimmune diseases

Autoimmune disorders are a complex and varied group of diseases that are caused by breakdown of self-tolerance. The aetiology of autoimmunity is multi-factorial, with both environmental triggers and genetically determined risk factors. In recent years, it has been increasingly recognized that genetic risk factors do not act in isolation, but rather the combination of individual additive effects, gene-gene interactions and gene-environment interactions determine overall risk of autoimmunity. The importance of gene-gene interactions, or epistasis, has been recently brought into focus, with research demonstrating that many autoimmune diseases, including rheumatic arthritis, autoimmune glomerulonephritis, systemic lupus erythematosus and multiple sclerosis, are influenced by epistatic interactions. This review sets out to examine the basic mechanisms of epistasis, how epistasis influences the immune system and the role of epistasis in two major autoimmune conditions, systemic lupus erythematosus and multiple sclerosis.

Epistasis: the key to understanding immunological disease?

Epistasis is fast becoming central to the understanding of the complex relationship between genotype and phenotype observed in autoimmune disease. A study in this issue of the European Journal of Immunology uses in-depth analysis of genome-wide mapping by polymorphic microsatellite markers to shed light on the genomic control of autoimmunity and self-tolerance.