Complement Factor H Modulates Splenic B Cell Development and Limits Autoantibody Production.

Complement factor H (CFH) has a pivotal role in regulating alternative complement activation through its ability to inhibit the cleavage of the central complement component C3, which links innate and humoral immunity. However, insights into the role of CFH in B cell biology are limited. Here, we demonstrate that deficiency of CFH in mice leads to altered splenic B cell development characterized by the accumulation of marginal zone (MZ) B cells. Furthermore, B cells in Cfh-/- mice exhibit enhanced B cell receptor (BCR) signaling as evaluated by increased levels of phosphorylated Bruton's tyrosine kinase (pBTK) and phosphorylated spleen tyrosine kinase (pSYK). We show that enhanced BCR activation is associated with uncontrolled C3 consumption in the spleen and elevated complement receptor 2 (CR2, also known as CD21) levels on the surface of mature splenic B cells. Moreover, aged Cfh-/- mice developed splenomegaly with distorted spleen architecture and spontaneous B cell-dependent autoimmunity characterized by germinal center hyperactivity and a marked increase in anti-double stranded DNA (dsDNA) antibodies. Taken together, our data indicate that CFH, through its function as a complement repressor, acts as a negative regulator of BCR signaling and limits autoimmunity.

Secreted IgM deficiency leads to increased BCR signaling that results in abnormal splenic B cell development.

Mice lacking secreted IgM (sIgM -/-) antibodies display abnormal splenic B cell development, which results in increased marginal zone and decreased follicular B cell numbers. However, the mechanism by which sIgM exhibit this effect is unknown. Here, we demonstrate that B cells in sIgM -/- mice display increased B cell receptor (BCR) signaling as judged by increased levels of phosphorylated Bruton's tyrosine kinase (pBtk), phosphorylated Spleen tyrosine kinase (pSyk), and nuclear receptor Nur77. Low dosage treatment with the pBtk inhibitor Ibrutinib reversed the altered B cell development in the spleen of sIgM -/- mice, suggesting that sIgM regulate splenic B cell differentiation by decreasing BCR signaling. Mechanistically, we show that B cells, which express BCRs specific to hen egg lysozyme (HEL) display diminished responsiveness to HEL stimulation in presence of soluble anti-HEL IgM antibodies. Our data identify sIgM as negative regulators of BCR signaling and suggest that they can act as decoy receptors for self-antigens that are recognized by membrane bound BCRs.

Oxidation-specific epitopes are major targets of innate immunity in atherothrombosis.

UNLABELLED: Atherosclerosis is a chronic inflammatory disease of the vascular wall that results from disturbed lipoprotein metabolism and increased oxidative stress. A major consequence of this is lipid peroxidation, which generates a number of breakdown products of membrane lipids that form so called oxidation-specific epitopes (OSE). OSE have been documented in oxidized lipoproteins and on the surface of dying cells and circulating microparticles, and their ability to trigger robust pro-inflammatory and pro-thrombotic responses has been demonstrated extensively. Recent studies have identified specific OSE as major targets of both cellular and soluble pattern recognition receptors of the innate immune system, including innate natural IgM antibodies. This allows the immune system to identify metabolic waste and mediate important physiological house keeping functions, e.g. by promoting the removal of cellular debris and by neutralizing oxidized molecules. Indeed, innate B1 cells and B1 cell derived natural IgM with specificity for OSE have been shown to protect mice from the development of atherosclerotic lesions. Moreover, OSE-specific natural IgM antibodies bind and neutralize the pro-inflammatory and pro-thrombotic effects of OSE, and low levels of OSE-specific IgM are associated with an increased risk for myocardial infarction. CONCLUSION: Understanding the molecular components and mechanisms involved in this process, will help identify individuals with increased risk for atherothrombosis and indicate novel points for therapeutic intervention.