Mareki Ohtsuji

Polymorphisms in IgG Fc receptor IIB regulatory regions associated with autoimmune susceptibility

Abstract Autoimmune diseases involve multiple genes. While functions of these genes are largely unknown, some may be related to an intrinsic hyperresponsiveness of B cells. B-cell responses are controlled by signaling thresholds through the B-cell antigen receptor (BCR) complex. The B1 isoform of type II IgG Fc receptors (FcγRIIB1) is exclusively expressed on B cells and serves as a negative regulator for inhibiting BCR-elicited activation. Thus, its allelic variants associated with functional deficits could be examined for possible associations with susceptibility to autoimmune diseases. We found that there are three types of polymorphisms in the reported FcγRIIB transcription regulatory regions in mouse strains. Compared to normal healthy mouse strains (group III), autoimmune disease-prone strains (group I) share three deletion sites: two in the promoter region and one in the third intron. Strains (group II) that per se are not autoimmune-prone, but have potentials to accelerate autoimmune diseases share two deletion sites in the third intron: one identical to that in group I and the other unique to group II. These polymorphisms correlated well with extents of down-regulation of FcγRIIB1 expression in germinal-center B cells upon stimulation with antigens and up-regulation of IgG antibody responses. Our data imply that these FcγRIIB polymorphisms are selected evolutionarily for natural defense against pathogens, and that such polymorphisms may, in turn, form the basis of one aspect of autoimmune susceptibility.

Late Developmental Stage-Specific Role of Tryptophan Hydroxylase 1 in Brain Serotonin Levels

Serotonin [5-hydroxytryptamine (5-HT)] is a major therapeutic target of psychiatric disorders. Tryptophan hydroxylase (TPH) catalyzes the rate-limiting reaction in the biosynthesis of 5-HT. Two isoforms (TPH1 and TPH2) having tryptophan hydroxylating activity were identified. Association studies have revealed possible TPH1 involvement in psychiatric conditions and behavioral traits. However, TPH1 mRNA was reported to be mainly expressed in the pineal gland and the periphery and to be barely detected in the brain. Therefore, contribution of TPH1 to brain 5-HT levels is not known, and the mechanisms how TPH1 possibly contributes to the pathogenesis of psychiatric disorders are not understood. Here, we show an unexpected role of TPH1 in the developing brain. We found that TPH1 is expressed preferentially during the late developmental stage in the mouse brain. TPH1 showed higher affinity to tryptophan and stronger enzyme activity than TPH2 in a condition reflecting that of the developing brainstem. Low 5-HT contents in the raphe nucleus were seen during development in New Zealand white (NZW) and SWR mice having common functional polymorphisms in the TPH1 gene. However, the 5-HT contents in these mice were not reduced in adulthood. In adult NZW and SWR mice, depression-related behavior was observed. Considering an involvement of developmental brain disturbance in psychiatric disorders, TPH1 may act specifically on development of 5-HT neurons, and thereby influence behavior later in life.

CD3 and Immunoglobulin G Fc Receptor Regulate Cerebellar Functions

ABSTRACT The immune and nervous systems display considerable overlap in their molecular repertoire. Molecules originally shown to be critical for immune responses also serve neuronal functions that include normal brain development, neuronal differentiation, synaptic plasticity, and behavior. We show here that FcγRIIB, a low-affinity immunoglobulin G Fc receptor, and CD3 are involved in cerebellar functions. Although membranous CD3 and FcγRIIB are crucial regulators on different cells in the immune system, both CD3ε and FcγRIIB are expressed on Purkinje cells in the cerebellum. Both CD3ε-deficient mice and FcγRIIB-deficient mice showed an impaired development of Purkinje neurons. In the adult, rotarod performance of these mutant mice was impaired at high speed. In the two knockout mice, enhanced paired-pulse facilitation of parallel fiber-Purkinje cell synapses was shared. These results indicate that diverse immune molecules play critical roles in the functional establishment in the cerebellum.

C1q Regulatory Region Polymorphism Down-Regulating Murine C1q Protein Levels with Linkage to Lupus Nephritis

Much of the pathology of systemic lupus erythematosus (SLE) is caused by deposition of immune complexes (ICs) into various tissues, including renal glomeruli. Because clearance of ICs depends largely on early complement component C1q, homozygous C1q deficiency is a strong genetic risk factor in SLE, although it is rare in SLE patients overall. In this work we addressed the issue of whether genetic polymorphisms affecting C1q levels may predispose to SLE, using the (NZB × NZW)F1 model. C1q genes are composed of three genes, C1qa, C1qc, and C1qb, arranged in this order, and each gene consists of two exons separated by one intron. Sequence analysis of the C1q gene in New Zealand Black (NZB), New Zealand White (NZW), and BALB/c mice showed no polymorphisms in exons and introns of three genes. However, Southern blot analysis revealed unique insertion polymorphism of a total of ∼3.5 kb in the C1qa upstream region of NZB mice. C1q levels in sera and culture supernatants of LPS-stimulated peritoneal macrophages and C1q messages in spleen cells were all lower in disease-free young NZB and (NZB × NZW)F1 mice than in age-matched non-autoimmune NZW and BALB/c mice. Quantitative trait loci analysis using (NZB × NZW)F1 × NZW backcrosses showed that NZB microsatellites in the vicinity of the C1q allele on chromosome 4 were significantly linked to low serum C1q levels and the development of nephritis. These data imply that not only C1q deficiency but also regulatory region polymorphisms down-regulating C1q levels may confer the risk for lupus nephritis by reducing IC clearance and thus promoting IC deposition in glomeruli.

Genetic Dissection of the Effects of Stimulatory and Inhibitory IgG Fc Receptors on Murine Lupus

Immune complex (IC)-mediated tissue inflammation is controlled by stimulatory and inhibitory IgG Fc receptors (FcγRs). Systemic lupus erythematosus is a prototype of IC-mediated autoimmune disease; thus, imbalance of these two types of FcγRs is probably involved in pathogenesis. However, how and to what extent each FcγR contributes to the disease remains unclear. In lupus-prone BXSB mice, while stimulatory FcγRs are intact, inhibitory FcγRIIB expression is impaired because of promoter region polymorphism. To dissect roles of stimulatory and inhibitory FcγRs, we established two gene-manipulated BXSB strains: one deficient in stimulatory FcγRs (BXSB.γ−/−) and the other carrying wild-type Fcgr2b (BXSB.IIBB6/B6). The disease features were markedly suppressed in both mutant strains. Despite intact renal function, however, BXSB.γ−/− had IC deposition in glomeruli associated with high-serum IgG anti-DNA Ab levels, in contrast to BXSB.IIBB6/B6, which showed intact renal pathology and anti-DNA levels. Lymphocytes in BXSB.γ−/− were activated, as in wild-type BXSB, but not in BXSB.IIBB6/B6. Our results strongly suggest that both types of FcγRs in BXSB mice are differently involved in the process of disease progression, in which, while stimulatory FcγRs play roles in effecter phase of IC-mediated tissue inflammation, the BXSB-type impaired FcγRIIB promotes spontaneous activation of self-reactive lymphocytes and associated production of large amounts of autoantibodies and ICs.

Transgene-mediated hyper-expression of IL-5 inhibits autoimmune disease but increases the risk of B cell chronic lymphocytic leukemia in a model of murine lupus

IL‐5 preferentially activates B1 cells to produce natural antibodies cross‐reactive to self antigens. To determine the role of IL‐5 in antibody‐mediated autoimmune disease, we generated systemic lupus erythematosus (SLE)‐prone (NZB×NZW)F1 mice congenic for IL‐5 transgene (TG‐F1). The transgene unexpectedly reduced the incidence of lupus nephritis. Anti‐DNA antibodies in sera and those produced by splenic B cells in vitro were markedly decreased in TG‐F1 mice, while total polyclonal Ig levels were comparable to those in IL‐5 transgene‐negative (NZB×NZW)F1 (non‐TG‐F1) littermates. Flow cytometry‐sorted splenic B1 cells showed a significant reduction of anti‐DNA antibody synthesis in response to IL‐5, while proliferative responses to IL‐5 did not significantly differ between TG‐F1 and non‐TG‐F1 mice. As TG‐F1 mice aged, frequencies of peripheral B1 cells progressively increased, and the mice frequently developed B cell chronic lymphocytic leukemia (B‐CLL). Our results suggest that dysregulated, continuous high expression of IL‐5 in SLE‐prone mice may directly or indirectly mediate a skewed signaling of proliferation/differentiation of self‐antigen‐activated B1 cells, leading to suppression of autoimmune disease, but instead to aberrant expansion of B1 cells, giving rise to B‐CLL. Thus, this model may provide a clue to the pathogenesis of both SLE and B‐CLL.

A Monoclonal Antibody to the α2 Domain of Murine Major Histocompatibility Complex Class I that Specifically Kills Activated Lymphocytes and Blocks Liver Damage in the Concanavalin A Hepatitis Model

We earlier found that a rat monoclonal antibody (mAb) RE2 can induce rapid death of murine activated, but not resting, lymphocytes and lymphocyte cell lines, in a complement-independent manner, a cell death differing from typical apoptosis or necrosis. We here found that this cell death is independent of pathways involving Fas, caspase, and phosphoinositide-3 kinase. With the advantage of producing human B cell line transfectants with stable expression of human/mouse xeno-chimeric MHC class I genes, we found that RE2 epitope resides on the murine class I α2 domain. However, the α3 domain plays a key role in transducing the death signal, which mediates extensive aggregation of the MHC class I-integrin-actin filament system, giving rise to membrane blebs and pores. In mouse models with T/NKT cell activation-associated fulminant hepatitis, administration of mAb RE2 almost completely inhibited the development of liver cell injuries. Taken collectively, this form of cell death may be involved in homeostatic immune regulation, and induction of this form of cell death using the mAbs may be potentially therapeutic for subjects with immunological diseases mediated by activated lymphocytes.

FTY720 exerts a survival advantage through the prevention of end-stage glomerular inflammation in lupus-prone BXSB mice

FTY720 is a novel investigational agent targeting the sphingosine 1-phosphate (S1P) receptors with an ability to cause immunosuppression by inducing lymphocyte sequestration in lymphoid organs. Systemic lupus erythematosus (SLE) is refractory autoimmune disease characterized by the production of a wide variety of autoantibodies and immune complex (IC)-mediated lupus nephritis. Among several SLE-prone strains of mice, BXSB is unique in terms of the disease-associated monocytosis in periphery and the reduced frequency of marginal zone B (MZ B) cells in spleen. In the present study, we examined the effect of FTY720 on lupus nephritis of BXSB mice. FTY720 treatment resulted in a marked decrease in lymphocytes, but not monocytes, in peripheral blood, and caused relocalization of marginal zone B (MZ B) cells into the follicle in the spleen. These changes did not affect the production of autoantibodies, thus IgG and C3 were deposited in glomeruli in FTY720-treated mice. Despite these IC depositions, FTY720-treated mice showed survival advantage with the improved proteinuria. Histological analysis revealed that FTY720 suppressed mesangial cell proliferation and inflammatory cell infiltration. These results suggest that FTY720 ameliorates lupus nephritis by inhibiting the end-stage inflammatory process following IC deposition in glomeruli.

Presumptive role of 129 strain-derived Sle16 locus in rheumatoid arthritis in a new mouse model with Fcγ receptor type IIb-deficient C57BL/6 genetic background.

OBJECTIVE Fcγ receptor type IIb (FcγRIIb) is a major negative regulator of B cells, and the lack of FcγRIIb expression has been reported to induce systemic lupus erythematosus (SLE) in mice of the C57BL/6 (B6) genetic background. The 129 strain-derived Sle16 locus on the telomeric region of chromosome 1 including polymorphic Fcgr2b confers the predisposition to systemic autoimmunity when present on the B6 background. We undertook this study to examine the effect of the Sle16 locus on autoimmune disease in FcγRIIb-deficient B6 mice. METHODS We established 2 lines of FcγRIIb-deficient B6 congenic mouse strains (KO1 and KO2) by selective backcrossing of the originally constructed FcγRIIb-deficient mice on a hybrid (129×B6) background into a B6 background. Although both lack FcγRIIb expression, the KO1 and KO2 strains carry different lengths of the 129 strain-derived telomeric chromosome 1 segment flanked to the null-mutated Fcgr2b gene; the KO1 strain carries a 129 strain-derived ∼6.3-Mb interval distal from the null-mutated Fcgr2b gene within the Sle16 locus, while this interval in the KO2 strain is of B6 origin. RESULTS Unexpectedly, both strains failed to develop SLE; instead, the KO1 strain, but not the KO2 strain, spontaneously developed severe rheumatoid arthritis (RA) with an incidence reaching >90% at age 12 months. CONCLUSION The current study shows evidence that the epistatic interaction between the Fcgr2b-null mutation and a polymorphic gene(s) in the 129 strain-derived interval located in the distal Sle16 locus contributes to RA susceptibility in a new mouse model with the B6 genetic background, although the participation of other genetic polymorphisms cannot be totally excluded.

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

In contrast to normal mice, autoimmune‐prone New Zealand Black (NZB) mice are defective in susceptibility to tolerance induced by deaggregated bovine γ globulin (DBGG). To examine whether this defect is related to the loss of self‐tolerance in autoimmunity, susceptibility loci for this defect were examined by genome‐wide analysis using the F2 intercross of nonautoimmune C57BL/6 (B6) and NZB mice. One NZB locus on the telomeric chromosome 1, designated Dit (Defective immune tolerance)‐1, showed a highly significant linkage. This locus overlapped with a locus containing susceptibility genes for autoimmune disease, namely Fcgr2b and Slam family genes. To investigate the involvement of these genes in the defective tolerance to DBGG, we took advantage of two lines of Fcgr2b‐deficient B6 congenic mice: one carries autoimmune‐type, and the other carries B6‐type, Slam family genes. Defective tolerance was observed only in Fcgr2b‐deficient mice with autoimmune‐type Slam family genes, indicating that epistatic effects of both genes are involved. Thus, common genetic mechanisms may underlie the defect in foreign protein antigen‐induced tolerance and the loss of self‐tolerance in NZB mouse‐related autoimmune diseases.