The role of CD40 ligand and tumor necrosis factor alpha signaling in the transgenic K/BxN mouse model of rheumatoid arthritis

Humoral immunity and long-lived plasma cells

A selected fraction of plasmablasts enters the compartment of nondividing, long-lived plasma cells to maintain humoral antibody memory. In accord with a current model for lymphocyte homeostasis, the lifetime of long-lived plasma cells is probably regulated by competition for a limited number of survival niches present in splenic red pulp, bone marrow and inflamed tissue. Plasma cells secreting autoantibodies specific for some, but not all, self-antigens are probably 'allowed' to enter the compartment of long-lived plasma cells and provide antibody-mediated 'autoimmune memory' that is resistant to conventional therapies.

Critical roles for interleukin 1 and tumor necrosis factor alpha in antibody-induced arthritis

In spontaneous inflammatory arthritis of K/BxN T cell receptor transgenic mice, the effector phase of the disease is provoked by binding of immunoglobulins (Igs) to joint surfaces. Inflammatory cytokines are known to be involved in human inflammatory arthritis, in particular rheumatoid arthritis, although, overall, the pathogenetic mechanisms of the human affliction remain unclear. To explore the analogy between the K/BxN model and human patients, we assessed the role and relative importance of inflammatory cytokines in K/BxN joint inflammation by transferring arthritogenic serum into a panel of genetically deficient recipients. Interleukin (IL)-1 proved absolutely necessary. Tumor necrosis factor (TNF)-alpha was also required, although seemingly less critically than IL-1, because a proportion of TNF-alpha-deficient mice developed robust disease. There was no evidence for an important role for IL-6. Bone destruction and reconstruction were also examined. We found that all mice with strong inflammation exhibited the bone erosion and reconstruction phenomena typical of K/BxN arthritis, with no evidence of any particular requirement for TNFalpha for bone destruction. The variability in the requirement for TNF-alpha, reminiscent of that observed in treated rheumatoid arthritis patients, did not appear genetically programmed but related instead to subtle environmental changes.

Lessons from animal models of arthritis

There is increasing thought that autoantibodies to systemic self-antigens may provide a principal effector mechanism for the initiation and propagation of joint inflammation. The recent identification of arthritis transfer with antibodies to the self-antigen glucose-6-phosphate isomerase has boosted this interest. Fc receptor involvement in arthritis has been evaluated, identifying pro-inflammatory and inhibitory Fc gamma receptor subtypes, and demonstrating a link between Fc gamma receptor expression, cytokine production, cartilage destruction, and mouse strain susceptibility to immune complex arthritis. Further proof of a key role of interleukin (IL)-1 in arthritis was provided by the occurrence of spontaneous arthritis in IL-1 receptor antagonist knockout mice and elicitation of full-blown arthritis in tumor necrosis factor (TNF)-deficient mice. IL-18 (part of the IL-1 family) is a crucial upstream cytokine that, with IL-12, induces IL-1 and TNF and promotes arthritis and T-cell differentiation. IL-18 neutralization improved arthritis outcome, but its central role in host defense against bacterial infections may complicate therapeutic IL-18 targeting. T helper 1 (Th1) cells may aggravate arthritis and joint destruction through the production of IL-17, which shows joint destructive potential independent of IL-1. Studies have also focused on the control of receptor activator of nuclear factor kappaB ligand, modulation with IL-4, and regulation of downstream mediators in tissue destruction. Gene therapeutic approaches proved efficacious and will provide future ways to control arthritis.