Ute Wellmann

Siglec-G is a B1 cell-inhibitory receptor that controls expansion and calcium signaling of the B1 cell population.

B1 cells are an important cell population for the production of natural antibodies and for antibacterial immunoglobulin responses. Here we identified the mouse protein Siglec-G as a B1 cell inhibitory receptor. Siglec-G was expressed in a B cell–restricted way, with large amounts present in B1 cells. When overexpressed, Siglec-G inhibited B cell receptor–mediated calcium signaling. Siglec-G-deficient mice had massive expansion of the B1a cell population, which began early in development and was B cell intrinsic. Siglec-G-deficient mice had higher titers of natural IgM antibodies but not a higher penetrance of IgG autoantibodies. Siglec-G-deficient B1 cells showed a strongly enhanced calcium signaling. Our results demonstrate that Siglec-G-dependent negative regulation exists in B1 cells, which may explain the naturally muted signaling response of B1 cells.

CD22 x Siglec-G double-deficient mice have massively increased B1 cell numbers and develop systemic autoimmunity.

CD22 and Siglec-G are inhibitory coreceptors for BCR-mediated signaling. Although CD22-deficient mice show increased calcium signaling in their conventional B2 cells and a quite normal B cell maturation, Siglec-G–deficient mice have increased calcium mobilization just in B1 cells and show a large expansion of the B1 cell population. Neither CD22-deficient, nor Siglec-G–deficient mice on a pure C57BL/6 or BALB/c background, respectively, develop autoimmunity. Using Siglec-G × CD22 double-deficient mice, we addressed whether Siglec-G and CD22 have redundant functions. Siglec-G × CD22 double-deficient mice show elevated calcium responses in both B1 cells and B2 cells, increased serum IgM levels and an enlarged population of B1 cells. The enlargement of B1 cell numbers is even higher than in Siglecg−/− mice. This expansion seems to happen at the expense of B2 cells, which are reduced in absolute cell numbers, but show an activated phenotype. Furthermore, Siglec-G × CD22 double-deficient mice show a diminished immune response to both thymus-dependent and thymus-independent type II Ags. In contrast, B cells from Siglec-G × CD22 double-deficient mice exhibit a hyperproliferative response to stimulation with several TLR ligands. Aged Siglec-G × CD22 double-deficient mice spontaneously develop anti-DNA and antinuclear autoantibodies. These resulted in a moderate form of immune complex glomerulonephritis. These results show that Siglec-G and CD22 have partly compensatory functions and together are crucial in maintaining the B cell tolerance.

Altered selection processes of B lymphocytes in autoimmune NZB/W mice, despite intact central tolerance against DNA.

Anti‐DNA autoantibodies are the hallmark of systemic lupus erythematosus and the (NZB×NZW)F1 (NZB/W) murine model. To investigate potential defects in B cell tolerance, we followed the development of anti‐DNA‐specific B cells in 2–5‐month‐old mice transgenic for an unmutated μH chain in the normal C57BL/6 and in the NZB/W background. When the transgenic H chain was combined witha random κ L chain repertoire about 60% of the antibodies bound to DNA. The analysis of the B cell repertoire in the spleen showed extensive receptor editing and a deletion of DNA reactivity in the C57BL/6 as well as in the autoimmune NZB/W background. NZB/W compared to C57BL/6 transgenic mice had a higher frequency of anti‐DNA B cells among follicular B cells that were not censored by central tolerance mechanisms. Furthermore, positive selection of B cells with a recurrent rearrangement into the marginal zone compartment was more pronounced in NZB/W mice. Serum levels of transgenic IgM and of anti‐DNA autoantibodies indicate a polyclonal activation of hyperactive B cells in the transgenic NZB/W mice. We propose different B cell receptor signaling thresholds for the NZB/W compared to C57BL/6 B cells. This could explain the quantitative differences in the B cell repertoire as well as the hyperactivity of B cells from NZB/W mice.

High frequency of autoantibody-secreting cells and long-lived plasma cells within inflamed kidneys of NZB/W F1 lupus mice

Autoantibodies to double‐stranded (ds) DNA represent a serological hallmark of systemic lupus erythematosus (SLE) and may critically contribute to the pathogenesis of lupus nephritis. Self‐reactive antibodies might be partially produced by long‐lived plasma cells (PCs), which mainly reside within the bone marrow and spleen. In contrast to short‐lived PCs, long‐lived PCs are extremely resistant to therapy and may sustain refractory disease courses. Recently, antibody‐secreting cells were found within the inflamed kidneys of New Zealand black/white (NZB/W) F1 lupus mice as well as of patients with SLE. To analyze the longevity of the IgG‐producing cells present in nephritic kidneys of NZB/W F1 mice we performed in vivo BrdU‐labeling. We identified a higher frequency of long‐lived than short‐lived renal PCs, indicating that survival niches for long‐lived PCs also exist within inflamed kidneys. Using ELISPOT assays, we found that on average 31% of renal IgG‐producing cells reacted with dsDNA and 24% with nucleolin. Moreover, the frequencies of IgG‐secreting cells specific for the autoantigens dsDNA and nucleolin were higher in the kidneys compared with those in the spleen and bone marrow.

Autoreactive B cells get activated in extrafollicular sites

Autoreactive B cells are prevented from producing autoantibodies that may cause pathogenicity in autoimmune diseases by the induction of tolerance. When autoreactive B cells escape regulation in autoimmune‐prone individuals, large amounts of autoantibodies are produced with somatic mutations in their variable regions. In this issue of the European Journal of Immunology, a new and very useful model is presented that induces activation and hypermutation of autoreactive B cells upon injection of chromatin‐containing immune complexes. The differentiation and hypermutation of autoreactive B cells takes place at extrafollicular sites.