Expression of aberrant forms of CD22 on B lymphocytes in Cd22a lupus-prone mice affects ligand binding

Abstract

CD22 functions primarily as a negative regulator of B-cell receptor signaling. The Cd22a allele has been proposed as a candidate allele for murine systemic lupus erythematosus. In this study, we explored the possible expression of aberrant forms of CD22, which differ in the N-terminal sequences constituting the ligand-binding site due to synthesis of abnormally processed Cd22 mRNA, in several Cd22a mouse strains, including C57BL/6 Cd22 congenic mice. The staining pattern of splenic B cells obtained with CY34 anti-CD22 mAb, which was expected to bind poorly to the aberrant CD22, was more heterogeneous in Cd22(a) mice than in Cd22b mice. Moreover, CD22 detected on B cells of Cd22a mice was expressed more weakly and as a smaller-sized protein, compared with Cd22b mice. Significantly, analysis with a synthetic CD22 ligand demonstrated that Cd22a mice carried a larger proportion of CD22 that was not bound by cis ligands on the B-cell surface than Cd22b mice. Finally, the study of C57BL/6 Cd22 congenic mice revealed that Cd22a B cells displayed a phenotype reminiscent of constitutively activated B cells (reduced surface IgM expression and augmented MHC class II expression), as reported for B cells expressing a mutant CD22 lacking the ligand-binding domain. Our demonstration that Cd22a B cells express aberrant forms of CD22, which can potentially deregulate B-cell signaling because of their decreased ligand-binding capacity, provides further support for Cd22a as a potential candidate allele for murine systemic lupus erythematosus.

CD22 × Siglec-G Double-Deficient Mice Have Massively Increased B1 Cell Numbers and Develop Systemic Autoimmunity

“…Also, on a genetic background of autoimmune-prone mouse strains Cd22 defects can contribute to the severity of the disease. A polymorphism in the Cd22 gene has been found with one allele, the Cd22a, encoding for a defective CD22 protein, being present in several autoimmune-prone mouse strains (37). Siglec-G deficiency on a BALB/c background results in slightly elevated titers of antierythrocyte IgM and rheumatoid factor, but no high affinity IgG autoantibodies were detected (16).…”

Activation of Rheumatoid Factor–Specific B Cells Is Antigen Dependent and Occurs Preferentially Outside of Germinal Centers in the Lupus-Prone NZM2410 Mouse Model

“…For the detection of AM14 idiotype-positive B cells, membrane staining with 4-44-biotin followed by intracellular staining with 4-44-Alexa647 using the Cytofix/Cytoperm Kit (BD Biosciences) was performed as previously described (2). The NIM-R6 Ab (26), which recognizes both CD22 a and CD22 b alleles (27) was revealed with either FITC-labeled mouse anti-rat IgG1 (eBioscience) or PE-labeled mouse anti-rat-IgG1 (Southern Biotech). For detection of biotinylated Abs, streptavidin conjugated to Phycoerythrin (PE) or Allophycocyanin (APC)-CY7 was used.…”

Experimental Models of Lupus Nephritis

“…The CD22 gene is located near a lupus susceptibility site identified in NZW mice. NZW mice contain the CD22a allele, which generates an aberrantly spliced mRNA [111], producing a protein that is not normally upregulated following lipopolysaccharide stimulation [112]. CD22 may antagonize B cell activation by raising the threshold necessary to initiate BCR-induced signals.…”