B cells in Autoimmunity: Studies of Complement Receptor 1 & 2 and FcγRIIb in Autoimmune Arthritis

B cells are normally regulated to prevent activation against self-proteins through tolerance mechanisms. However, occasionally there is a break in tolerance and B cells can become self-reactive, which might lead to the development of autoimmune disease. The activation of self-reactive B cells is regulated by receptors on the B cell surface, such as Fc gamma receptor IIb (FcγRIIb), complement receptor type 1 (CR1), and CR type 2 (CR2). In this thesis I have studied the role of FcγRIIb, CR1 and CR2 on B cells in autoimmune arthritis. By using a model for rheumatoid arthritis, I discovered that the initial self-reactive B cell response in arthritis was associated with the splenic marginal zone B cell population. Marginal zone B cells express high levels of CR1/CR2 and FcγRIIb, suggesting that they normally require high regulation. Further, female mice deficient in CR1/CR2 displayed increased susceptibility to arthritis compared to CR1/CR2-sufficient female mice. When investigating whether sex hormones affected arthritis susceptibility, we found that ovariectomy, of the otherwise fairly resistant CR1/CR2-sufficient mice, reduced the expression of CR1 on B cells and rendered the mice more susceptible to arthritis. In humans, a significantly reduced CR1 and FcγRIIb expression was found on B cells in aging women, but not in men. This may contribute to the increased risk for women to develop autoimmune disease as reduced receptor expression may lead to the activation of self-reactive B cells. In agreement, lower CR1, CR2 and FcγRIIb expression was seen in patients with rheumatoid arthritis. Finally, a soluble form of FcγRIIb was used to investigate FcγRIIb’s ability to bind self-reactive IgG in an attempt to treat autoimmune arthritis. Treatment of mice with established arthritis was associated with less self-reactive IgG antibodies and consequently less disease, suggesting that soluble FcγRIIb may be used as a novel treatment in arthritis.