Functional defects of culture-grown bone marrow-derived macrophages from autoimmune MRL/MpJ-lpr/lpr mice

Depletion of B cells in murine lupus: efficacy and resistance

In mice, genetic deletion of B cells strongly suppresses systemic autoimmunity, providing a rationale for depleting B cells to treat autoimmunity. In fact, B cell depletion with rituximab is approved for rheumatoid arthritis patients, and clinical trials are underway for systemic lupus erythematosus. Yet, basic questions concerning mechanism, pathologic effect, and extent of B cell depletion cannot be easily studied in humans. To better understand how B cell depletion affects autoimmunity, we have generated a transgenic mouse expressing human CD20 on B cells in an autoimmune-prone MRL/MpJ-Fas(lpr) (MRL/lpr) background. Using high doses of a murine anti-human CD20 mAb, we were able to achieve significant depletion of B cells, which in turn markedly ameliorated clinical and histologic disease as well as antinuclear Ab and serum autoantibody levels. However, we also found that B cells were quite refractory to depletion in autoimmune-prone strains compared with non-autoimmune-prone strains. This was true with multiple anti-CD20 Abs, including a new anti-mouse CD20 Ab, and in several different autoimmune-prone strains. Thus, whereas successful B cell depletion is a promising therapy for lupus, at least some patients might be resistant to the therapy as a byproduct of the autoimmune condition itself.

Chronic administration of dexamethasone results in Fc receptor up-regulation and inhibition of class I antigen expression on macrophages from MRL/lpr autoimmune mice

The MRL/lpr mouse develops, after approximately 8 weeks of age, a severe autoimmune syndrome with many features resembling human systemic lupus erythematosus, including autoantibodies against DNA and basement membranes resulting in immune complexes, vasculitis, and multiorgan disease. While this murine model of lupus has been used for the identification of therapeutics with potential efficacy in human autoimmune disease, the long-term impact of chronic immunosuppressive therapy on macrophage function in this paradigm is not understood. To this end, MRL/lpr mice were treated prophylactically with dexamethasone at 0.01, 0.1, and 1 mg/kg of body weight for 20 weeks or were allowed to develop autoimmune disease and, at 15 weeks of age, treated therapeutically with 1-mg/kg dexamethasone for 8 additional weeks. Analysis of surface antigens on resident peritoneal macrophages demonstrated a progressive loss in class I expression with a concomitant increase in Fc receptor expression. Neither phagocytosis nor CD11b expression was modulated with chronic steroid treatment. Furthermore, dexamethasone treatment was associated with a reduction in anti-DNA antibodies and total immunoglobulin G and yet an elevation in serum cholesterol due to an increase in high-density lipoproteins. Therefore, the MRL/lpr mouse serves not only as a small-animal model of autoimmune disease but also as one in which the negative and positive sequelae associated with chronic immunosuppression can be further understood.

Immune complex-degradation ability of macrophages in MRL/Mp-lpr/lpr lupus mice and its regulation by cytokines

Impaired clearance of circulating and/or deposited immune complexes (IC) by the mononuclear phagocytic system is one of the major factors in the pathogenesis of IC diseases. MRL/Mp-lpr/lpr (MRL/lpr) lupus mice spontaneously develop a lethal glomerulonephritis associated with IC deposition. The ability of macrophages to degrade phagocytozed IC and regulation of this degradation in MRL/lpr mice were examined. In 4-month-old MRL/lpr mice, macrophages accumulated in the affected glomeruli and these macrophages contained many phagosomes containing electron-dense bodies. When culture supernatant of human T cell line HUT102 was administered intraperitoneally into disease-bearing MRL/lpr mice, degradation of these electron-dense bodies in the macrophages in glomeruli was noted. We developed a quantitative in vitro assay for IC degradation activity of MRL/lpr resident peritoneal macrophages (RPM) using peroxidase-labelled IC derived from MRL/lpr mouse sera. The ability of the RPM to degrade IC was remarkably enhanced by the pretreatment with HUT102 cell products and the related human recombinant cytokines, lymphotoxin and IL-1 alpha. Moreover, pretreatment of RPM from non-diseased MRL/Mp-+/+ mice with the culture supernatant of spleen cells from diseased MRL/lpr mice reduced their IC degradation activity. These results suggested that the ability of macrophages to degrade IC in MRL/Mp strains of mice is under the regulation of cytokines, and the impaired ability in the disease-bearing mice may be the result of abnormalities in the cytokine system in these mice.