C1q deficiency promotes the production of transgenic-derived IgM and IgG3 autoantibodies in anti-DNA knock-in transgenic mice

Abstract

C1q-deficient mice have been shown to develop a lupus-like disease and to display an impaired clearance of apoptotic cells that are enriched in lupus autoantigens. However, the role of C1q in the regulation of autoreactive B cells remains debatable. To explore this we crossed MRL/Mp C1q-deficient mice with knock-in transgenic (Tg) mice expressing an anti-ssDNA antibody (VH3H9R and VH3H9R/VLkappa8R). Analysis of the VH3H9R mice showed that in the absence of C1q higher titres of Tg-derived IgM and IgG3 anti-ssDNA antibodies were detectable. In contrast, in the VH3H9R/VLkappa8R C1q-deficient animals no increase in Tg antibody levels was observed. In both models the lack of C1q induced a marked reduction of marginal zone B cells and this was paralleled by a significant increase in the percentage of plasmocytes. Thus, one could postulate that in the absence of C1q the failure to clear efficiently dying cells provides an additional stimulus to the autoreactive Tg B cells resulting in their emigration from the marginal zone B cell compartment with subsequent increase in plasmocytes. However, the lack of C1q led to an increased production of Tg IgM and IgG3 antibodies only in VH3H9R mice indicating that additional genetic susceptibility factors are required to break self-tolerance.

Fig. 1

Expression of idiotype⁺ (A) IgM, (B) IgG3, (C) IgG2a and (D) IgG2b immunoglobulins in circulation. The combination of V_H3H9R/V_Lκ8R was assessed using a specific anti-idiotype mAb (1.209). Ten month-old V_H3H9R/V_Lκ8R.MRL/Mp (closed diamonds) and V_H3H9R.MRL/Mp (closed triangles) mice displayed detectable levels of idiotype⁺ IgM, IgG3, IgG2a and IgG2b Abs. IgM and IgG3 levels were markedly increased in V_H3H9R.MRL/Mp.C1qa^−/− mice. C1q-deficient mice are represented with open symbols. Each symbol represents one mouse, horizontal bars indicate median.

Fig. 2

AutoAb profiles. Serology at 10 months of age in V_H3H9R/V_Lκ8R.MRL/Mp (diamonds) and V_H3H9R.MRL/Mp (triangles) mice. C1q-deficient mice are represented with open symbols. Each symbol represents one mouse, horizontal bars indicate median. (A) IgM anti-ssDNA Abs; (B) IgM anti-dsDNA Abs; (C) IgM anti-histone Abs; (D) IgM anti-chromatin Abs measured by ELISA as described in Section 2.

Fig. 3

Flow cytometric analysis of splenic and peritoneal cells. In C1q-deficient mice there was a reduction of (A) marginal zone B cells (percentage of CD21^highCD23^low, CD19⁺ gated splenic B cells) and this was associated with an increase of (B) plasma cells (percentage of CD138⁺, B220⁻CD90⁻ gated splenic cells). Each symbol represents one mouse; V_H3H9R/V_Lκ8R.MRL/Mp mice (diamonds) and V_H3H9R.MRL/Mp mice (triangles); C1q-deficient mice are represented with open symbols.