Scientists have identified the tryptophan catabolizing enzyme indoleamine 2,3-dioxygenase (IDO) as a potential therapeutic target in systemic autoimmune diseases such as lupus. Studies in mice by researchers at Georgia Health Sciences University and colleagues have shown that IDO is needed to prevent apoptotic cell-driven autoimmunity in healthy animals and limit inflammatory pathology associated with systemic autoimmunity in a mouse model of lupus. Tracy L. McGaha, Ph.D., et al., report their findings in PNAS, in a paper titled "Tolerance to apoptotic cells is regulated by indoleamine 2, 3-dioxygenase.”
Scavenging macrophages play a key role in maintaining tolerance to self-antigens. Circulating apoptotic cells are trapped in the marginal zone of the spleen and removed by a specialized population of macrophages MZMs that expresses either the scavenger macrophage receptor with collagenous structure (Marco) marker or the metallophillic macrophage marker (Moma-1), the team explains. In fact, recent studies by the Georgia team has shown that depleting these splenic macrophages changes the immune response to apoptotic cells, increasing proinflammatory cytokine production and enhancing phagocytosis and phagocyte activation.
One potential mechanism by which macrophages and dendritic cells promote tolerance is mediated by IDO, production of which can be induced in macrophages and DCs by a variety of stimuli. Activity of IDO has been shown to suppress effector T-cell responses, promote regulatory T-cell (Treg) differentiation, and activation, and inhibit IL-6 production. In mice, administration of apoptotic cells promotes anti-inflammatory responses and immune tolerance. But in animals depleted of MZMs, challenge with apoptotic cells leads to a proinflammatory cytokine response and autoimmunity.
To see whether apoptotic cells might induce IDO production in phagocytes in vivo, the team assessed at IDO expression in the spleen marginal zone (MZ) of mice after the administration of apoptotic thymocytes. The results showed that while there was no detectible IDO production under basal conditions, 18 hours after the administration of apoptotic cells there was marked IDO expression in the MZ. However, in animals depleted of MZMs there was no evidence of IDO production following apoptotic cell challenge.
Additional studies demonstrated that IDO is integral to inhibiting inflammatory responses to apoptotic cells. Animals were given an IDO inhibitor (DIMT) in their drinking water and subsequently challenged with apoptotic cells. Compared with control animals that didn’t receive the IDO inhibitor, the DIMT-treated animals demonstrated marked reductions in TGF-β and IL-10 levels and highly significant increases in proinflammatory cytokine production including three-fold higher TNF-α levels.
Studies in both wild-type and IDO1-deficient (IDO1-/-) mice also confirmed that IDO was requisite for the normal regulation of T-cell responses after apoptotic cell challenge. Interestingly, CD8+ DCs constituted the bulk of the TGF-β response after challenge with apoptotic cells. However, the researchers remark, “because CD8+ DCs are not themselves the source of IDO expression, this finding indicates that IDO might act in a paracrine fashion, influencing TGF-β production after challenge with apoptotic cells."
The team moved on to investigate the impact of IDO on autoimmune diseases progression lupus-prone Murphy Roths large mice (MRLlpr/lpr). In contrast with normal mice that display little basal IDO activity in the spleen, young presymptomatic (MRLlpr/lpr) exhibit signficiant expression of IDO in spleen MZ. Interestingly, when presymptomatic (MRLlpr/lpr) mice were treated with the IDO inhibitor D1MT, they developed elevated IgG αdsDNA titres, but not total serum IgG concentrations, “indicating that IDO selectively affected development of autoantibodies,” the team reports.
Elevated autoantibody levels in D1MT-treated MRLlpr/lpr mice correlated with increased IgG immune-complex deposition in the kidneys. Histological analyses showed that IDO inhibition was accompanied by changes in glomerular architecture, with increased presence of collagen, hypercellularity, and mesangial thickening. Young D1MTM-treated MRLlpr/lpr animals also demonstrated the rapid onset of skin pathology manifesting as structural alterations, including hair-follicle loss, hyperplasia of the epidermal and dermal layers, and the appearance of hyaline cysts. “Thus, taking these data together, we find that inhibition of IDO in presymptomatic MRLlpr/lpr mice caused accelerated loss of self-tolerance and development of end-organ disease,” Dr. McGaha and colleagues state.
Prior research has shown that genetic knockdown of IDO doesn’t itself, but what wasn’t yet clear was whether IDO was needed to maintain self-tolerance to chronic challenge with apoptotic cells. To test this, the investigators injected IDO−/− and wild-type C57BL/6 mice with apoptotic thymocytes twice a week for five months. While the wild-type mice demonstrated only low and self-limiting responses to the cells, the IDO-deficient animals showed progressive increases in serum αdsDNA IgG and the development of lethal autoimmunity, which was associated with renal pathology.
“Taken together, these results identify IDO as a critical factor in the maintenance of tolerance to self-antigens released by apoptotic cells,” the authors conclude. “The observations that IDO inhibitor treatment led to increased autoantibody titers and target organ pathology in MRLlpr/lpr mice supports the hypothesis that IDO is a key factor that slows autoimmune disease progression ... These findings identify defective IDO-dependent immune suppression as a potential target for therapy of systemic autoimmune disease.”