Separate checkpoints regulate splenic plasma cell accumulation and IgG autoantibody production in Lyn‐deficient mice

Abstract

Accumulation of plasma cells and autoantibodies against nuclear antigens characterize both human and murine lupus. Understanding how these processes are controlled may reveal novel therapeutic targets for this disease. Mice deficient in Lyn, a negative regulator of B and myeloid cell activity, develop lupus-like autoimmune disease. Here, we show that lyn À/À mice exhibit increased splenic plasmablasts and plasma cells and produce IgM against a wide range of self-antigens. Both events require Btk, a target of Lyn-dependent inhibitory pathways. A Btk-dependent increase in the expression of the plasma cell survival factor IL-6 by lyn À/À splenic myeloid cells was also observed. Surprisingly, IL-6 was not required for plasma cell accumulation or polyclonal IgM autoreactivity in lyn À/À mice. IL-6 was, however, necessary for the production of IgG autoantibodies, which we show are focused towards a limited set of nucleic acid-containing and glomerular autoantigens in lyn À/À mice. A similar uncoupling of plasma cell accumulation from IgG autoantibodies was seen in lyn 1/À mice. Plasma cell accumulation and polyclonal IgM autoreactivity are therefore controlled separately from, and are insufficient for, the production of IgG against lupus-associated autoantigens. Regulators of either of these two checkpoints may be attractive therapeutic targets for lupus.Key words: Autoimmunity . Btk . IL-6 . Lyn . Plasma cell Supporting Information available online IntroductionThe autoimmune disease systemic lupus erythematosus (SLE) is characterized by loss of tolerance to nuclear antigens resulting in autoantibody production, immune complex deposition, inflammation, and end organ damage. Aberrant accumulation of antibody-secreting cells, which include among them autoreactive specificities, is a common feature of human SLE [1] and several genetically distinct models of murine lupus, including NZB Â NZW and Lyn-deficient mice [2][3][4][5][6][7][8][9]. This has been attributed to an unusually favorable splenic plasma cell survival niche and impaired plasma cell homing to the bone marrow in the NZB Â NZW model [5,6] Lyn inhibits responses to BCR-crosslinking by phosphorylating the ITIM of several inhibitory receptors [12]. B cells from lyn À/À mice are thus hyperresponsive to BCR stimulation [4,12]. lyn À/À mice also demonstrate numerous defects in myeloid cells, such as increased myelopoiesis [13], increased integrin signaling [14], and increased production of cytokines by activated macrophages, mast cells, and, in some circumstances, dendritic cells [15][16][17]. These cytokines include IL-6, a plasma cell survival factor [18][19][20], the expression of which has been shown to be elevated in both murine and human lupus [21][22][23]. It plays a role in the production of anti-DNA antibodies in murine lupus models both in vitro and in vivo [23,24].A major target of Lyn-dependent inhibitory pathways in B cells is the BCR signaling component Btk [25][26][27]. Btk and Lyn also have opposing effects in macrophages [15,28], dendritic cells [...

Genetic Interdependence of Lyn and Negative Regulators of B Cell Receptor Signaling in Autoimmune Disease Development

“…These observations suggest that, in the Lyn +/2 setting, age-dependent perturbations in the T cell compartment occur in the absence of early myeloid cell defects, implying that these effects may be driven by B cell defects initially, in addition to eventual perturbations in myeloid cells. Interestingly, early expansion of plasma cells in Lyn +/2 mice did not result in production of IgG ANAs in young mice, in contrast to Lyn 2/2 mice, a feature also observed by other investigators (43).…”

“…This is reminiscent of Lyn 2/2 IL-6 2/2 mice, in which significant numbers of plasma cells are present; however, because of the absence of IL-6-mediated inflammatory components, they lack IgG ANAs and remain disease-free throughout life (38,43). In contrast, in aged Lyn +/2 mice, myeloid expansion and significant T cell activation eventually come to the fore, concomitant with the production of pathogenic IgG ANAs, leading to nephritis.…”

“…2/2 mice (38,43). In previous studies, we observed that BALB/c background Lyn +/2 mice develop ANAs with age, but we did not assess disease development (35).…”

Animal Models of Molecular Pathology

“…More recent studies have shown that IL-6 is required for the development of pathogenic IgG autoAbs in Lyn À/À mice. 87,88 While it is clear that Lyn deficiency leads to systemic autoimmunity, the reasons for its decreased expression in the B cells of SLE patients are not well understood. Several mechanisms are likely to be involved such as decreased gene expression, 82 increased ubiquination, and aberrant recruitment to lipid rafts.…”

The Role of the Transcription Factor Ets1 in Lupus and Other Autoimmune Diseases

“…Lyn −/− Btk lo mice, which express a reduced level of Btk, demonstrate normal levels of Ets1 in their B cells 85 and do not accumulate plasma cells or autoantibodies. 111,112 Finally, Btk signaling to Ets1 also controls steady-state plasma cell numbers when Lyn-dependent inhibitory signaling pathways are intact. Btk −/− mice demonstrate reduced splenic IgM-secreting cells and low serum IgM levels; this defect is normalized in the absence of Ets1.…”