Genetic Interaction between Lyn, Ets1, and Btk in the Control of Antibody Levels

Abstract

Tight control of B cell differentiation into plasma cells (PCs) is critical for proper immune responses and the prevention of autoimmunity. The Ets1 transcription factor acts in B cells to prevent PC differentiation. Ets1−/− mice accumulate PCs and produce autoantibodies (autoAbs). Ets1 expression is downregulated upon B cell activation through the BCR and TLRs and is maintained by the inhibitory signaling pathway mediated by Lyn, CD22 and SiglecG, and SHP-1. In the absence of these inhibitory components, Ets1 levels are reduced in B cells in a Btk-dependent manner. This leads to increased PCs, autoAbs, and an autoimmune phenotype similar to that of Ets1−/− mice. Defects in inhibitory signaling molecules, including Lyn and Ets1, are associated with human lupus, although the effects are more subtle than the complete deficiency that occurs in knockout mice. Here, we explore the effect of partial disruption of the Lyn/Ets1 pathway on B cell tolerance and find that Lyn+/−Ets1+/− mice demonstrate greater and earlier production of IgM, but not IgG, autoAbs compared to Lyn+/− or Ets1+/− mice. We also show that Btk-dependent downregulation of Ets1 is important for normal PC homeostasis when inhibitory signaling is intact. Ets1-deficiency restores the decrease in steady state PCs and Ab levels observed in Btk−/− mice. Thus, depending on the balance of activating and inhibitory signals to Ets1, there is a continuum of effects on autoAb production and PC maintenance. This ranges from full-blown autoimmunity with complete loss of Ets1-maintaining signals to reduced PC and Ab levels with impaired Ets1 downregulation.

Self‐reactivity on a spectrum: A sliding scale of peripheral B cell tolerance

“…149 This is thought to be due in part to excessive BTK-dependent signal transduction in response to endogenous antigen stimulation in the absence of inhibitory tone. 149,150 Deletion of ETS1 in B cells is sufficient to drive spontaneous PC differentiation even with intact inhibitory tone and even in the absence of self-antigen stimulation, while overexpression of ETS1 can suppress such differentiation in Lyn-deficient and SHP-1-deficient B cells. 149,151,152 On the Lyn −/− genetic background, we observed that IgM-only B cells spontaneously differentiate into these short-lived "unswitched" plasma cells, but IgDonly B cells do not, and showed that this is associated with reduced downregulation of ETS1 by IgD-only B cells.…”

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

“…First, mice heterozygous for both Lyn and Ets1 have increased IgM autoantibodies compared to Lyn +/− or Ets1 +/− mice alone, although they do not develop full blown autoimmune disease. 106 This indicates that Lyn and Ets1 do indeed work together in a common signaling pathway to limit B cell differentiation and that partial disruption of this pathway is sufficient for an initial break in B cell tolerance. Second, the excessive downregulation of Ets1 in the absence of inhibitory signals depends on Btk.…”

“…Btk −/− mice demonstrate reduced splenic IgM-secreting cells and low serum IgM levels; this defect is normalized in the absence of Ets1. 106 These observations suggest that manipulations that shift the balance between Ets1-downregulating activating signals and Ets1-maintaining inhibitory signals may be useful therapeutic approaches to promote or dampen antibody responses as desired.…”

“…85 This occurs prior to the accumulation of autoantibodies and the development of autoimmune disease manifestations 85 and is independent of the inflammatory cytokine IL-6, 106 strongly suggesting that a Lyn-dependent signaling pathway directly controls Ets1 expression in B cells. Indeed, a similar loss of Ets1 expression was observed in B cells with mutations in SHP-1, while SHIP-deficient B cells had only a mild decrease in Ets1.…”

“…208 Thus, even in the absence of ETS1 risk alleles, Ets1 expression may be reduced in SLE B cells by other polymorphisms or defects, increasing the propensity of autoreactive B cells to differentiate and produce potentially pathogenic autoantibodies. The elevated autoantibodies in compound heterozygotes of Lyn and Ets1 106 suggest that polymorphisms in more than one component of Ets1-regulating pathways may result in enhanced B cell defects in SLE. A subset of SLE patients have a particularly striking plasma cell phenotype; 34,37 these individuals may be more likely to have disruptions in Ets1 or its regulators.…”