Addition of an indoleamine 2,3,-dioxygenase inhibitor to B cell-depletion therapy blocks autoreactive B cell activation and recurrence of arthritis in K/BxN mice.

The inflammatory autoimmune disease rheumatoid arthritis (RA) has classically been thought to be mediated by T cells, either by direct infiltration of tissues or indirectly through release of inflammatory cytokines (1, 2). Increasingly, it is becoming apparent that B cells also play a critical role in driving inflammatory autoimmunity in RA (3). In addition to producing pathogenic autoantibodies, B cells can trigger autoimmune responses through the presentation of self-reactive antigens to T cells and the production of inflammatory cytokines. The most convincing evidence supporting the role for B cells in RA is the recent success of B cell-mediated therapies (4). However, the factors important in initiating and maintaining autoreactive B cell responses remain unknown. An exciting new strategy to treat RA relies on B cell depletion using a chimeric monoclonal Ab directed against the B cell-specific cell surface marker CD20 (Rituximab) (4). The addition of Rituximab to the treatment regimen led to reduced autoantibody levels and clinical improvement in the majority of patients, with some showing a complete resolution of inflammation (5). Similarly, B cell depletion has been shown to be effective in several mouse models of arthritis (6, 7). Unfortunately, the primary limitation of B cell depletion therapy in both humans and mice is that eventually the B cells return and the repopulation of the B cell repertoire correlates with the return of arthritis symptoms in many individuals (8, 9). A co-therapeutic strategy to inhibit the activation of autoreactive B cells upon repopulation would help to lengthen the effectiveness of the therapeutic window and could improve clinical outcomes in RA patients. Recently, our laboratory has identified indoleamine-2,3-dioxygenase (IDO) as an important factor in driving the initial stages of B cell-mediated autoimmune responses (10). IDO is an IFN-γ inducible enzyme that catalyzes the initial and rate-limiting step in tryptophan degradation (11). In humans, elevated tryptophan degradation has been shown to correlate with disease activity in RA patients (12). Likewise, we have shown that IDO activity is highest during the acute phase of disease in the K/BxN mouse model of inflammatory joint disease (10). Inhibition of IDO activity in K/BxN mice with the pharmacological inhibitor, 1-methyl-tryptophan (1MT) led to reduced levels of inflammatory cytokines, diminished autoantibody titers, and an attenuated course of disease. This alleviation of arthritis was not due to a reduction in regulatory T cells or an altered T helper cell phenotype, but rather resulted from a diminished autoreactive B cell response (10). This work demonstrated a previously unappreciated role for IDO in stimulating B cell responses; however the role that IDO played in B cell activation remained unknown. Here, we use Ig transgenic (tg) mice to define the stage at which B cell activation is influenced by IDO. We demonstrate that IDO activity is involved in the differentiation of autoreacitve B cells into antibody secreting cells, but is not required for the initial stages of B cell activation or germinal center formation. This suggests that IDO plays a role in establishing the autoreactive B cell profile at the onset of the autoimmune response. As such, inhibitors of IDO activity should be most useful therapeutically at the initiation of autoreactive B cell responses. We propose that inhibition of IDO activity at this critical stage will prevent the establishment of the autoreactive B cell profile, thereby reducing subsequent joint inflammation and damage. To test this hypothesis, we combine 1MT with B cell depletion therapy using antibodies to CD20. We demonstrate that the addition of 1MT inhibits the differentiation of autoantibody secreting cells following B cell depletion therapy and prevents the recurrence of autoimmune arthritis. These data suggest that inhibition of the IDO pathway could be an effective strategy to use in conjunction with B cell depletion therapy in the treatment of RA.