Although T cells play a key role in the pathogenesis of many autoimmune diseases such as RA and SLE, the molecular machinery preventing the emergence of autoreactive T cells has not been fully elucidated. Our studies suggest that deregulation of IBP-controlled pathways represents an important pathogenic mechanism leading to the spontaneous development of autoimmunity. The ability of IBP to play a critical role in this process is likely due to its capacity to control the responsiveness of T cells to pMHC complexes and to ensure that the production of IL-17 and IL-21 does not occur in the absence of the proinflammatory conditions known to drive TH17 differentiation.
One of the crucial aspects underlying the spontaneous development of autoimmunity in IBP deficient mice is the inability of IBPtrap/trap
T cells to accurately sense the strength of TCR engagement. As shown in , IBPtrap/trap
DO11.10 T cells exhibit a highly abnormal pattern of responsiveness. Consistent with our previous findings that polyclonal IBP deficient T cells exposed to strong stimulatory conditions proliferate to a lesser extent than wt T cells (Fanzo et al., 2006
DO11.10 T cells exhibit defective proliferative responses upon exposure to high doses of OVA323-339
DO11.10 T cells, however, exhibit enhanced proliferative responses to low-levels of stimulation, an effect that we have also observed with polyclonal IBP deficient T cells (unpublished observations). The hyperresponsiveness of the IBPtrap/trap
T cells to low-levels of stimulation was further corroborated by their spontaneous activation in vivo
in the absence of any exposure to ovalbumin and by their enhanced expression of CD5. Increased CD5 expression was also noted on CD4SP cells from newborn IBPtrap/trap
DO11.10 mice suggesting that this phenotype was the consequence of intrinsic changes in the recognition of the DO11.10 TCR for its ligands rather than to exposure of these T cells to a pathogenic environment. Interestingly, preliminary studies from IBPtrap/trap
DO11.10 mice that we have recently generated reveal that the TCR transgenic T cells from these mice still retain the observed abnormalities indicating that the arthritis and vasculitis occurring in IBPtrap/trap
DO11.10 TCR transgenic mice are likely due to the inappropriate recognition of cross-reactive epitopes by the DO11.10 TCR rather than to the expression of a second TCR by these cells.
Importantly, our studies indicate that IBP, in addition to regulating the responsiveness of T cells to TCR engagement, controls the capacity of T cells to acquire a potentially pathogenic effector program. Indeed, one of the crucial abnormalities observed in the absence of IBP was increased production of IL-17, a cytokine with known proinflammatory effects relevant to the pathophysiology of rheumatoid arthritis in humans (Koenders et al., 2006
; Stamp et al., 2004
). We furthermore have demonstrated that the deregulated IL-17 production observed in the absence of IBP is critically dependent on IRF-4, a transcription factor recently shown to be necessary for TH17 differentiation (Brustle et al., 2007
). Interestingly, IRF-4 is an important cellular target of the HTLV-I Tax oncoprotein (Mamane et al., 2002
) and it is intriguing to speculate that deregulation of IRF-4 may also play a role in the autoimmune arthritis associated with the overexpression of Tax in mice (Habu et al., 1999
). Consistent with previous studies showing that IRF-4 can regulate the expression of ROR-γt (Brustle et al., 2007
), the enhanced production of IL-17 observed in the absence of IBP was accompanied by increased expression of ROR-γt, an effect that was dependent on the presence of IRF-4. It is, however, important to note that reconstitution of RORγt expression in IRF-4 deficient T cells only partially restores the deficient IL-17 production exhibited by these cells (Brustle et al., 2007
). Thus our finding that IRF-4 containing complexes can also target the regulatory regions of the IL-17 gene indicates that the role of IRF-4 in TH17 differentiation is multifaceted and includes both direct effects on the transcription of IL-17 gene as well as indirect effects on the expression of other IL-17 transactivators.
The acquisition of a TH17 phenotype normally requires progression through a series of developmental stages during which TH17 cells are sequentially exposed to key cytokines (Bettelli et al., 2007a
; Ivanov et al., 2007
). Presence of IL-6 is crucial for the initiation of this process and leads to the induction of IL-21 production, which, in turn, acts in an autocrine manner and further amplifies commitment of TH cells toward the TH17 lineage. Our studies demonstrate that IBP deficient T cells can produce IL-21 even during the initial stimulation by antigen and in the absence of any TH17 skewing conditions. This abnormality may have profound pathophysiologic consequences since aberrant synthesis of IL-21 may not only reinforce the abnormal IL-17 production exhibited by these cells, but may also enable these CD4+
T cells to inappropriately promote the terminal differentiation of B cells at extrafollicular sites and, thus, provide help to autoreactive B cells (William et al., 2002
). Consistent with this notion, the absence of IBP leads to the aberrant localization of Blimp1 positive plasma cells within peripheral lymphoid organs and the presence of autoantibodies. By controlling IL-21 production, IBP may thus play a critical role in ensuring that T cell help to B cells is provided only to appropriately selected B cell populations (Vinuesa et al., 2005
). Unlike what we observed with IL-17 and IL-21, IBP deficiency did not affect the production of IL-22 in agreement with studies demonstrating that the regulation of IL-22 differs from that of IL-17 and that IL-22 may primarily function as a downstream target of IL-23 (Ouyang et al., 2008
Similarly to IL-17, the ability of IBP to control IL-21 production is dependent on its interaction with IRF-4. At a mechanistic level, IBP does not affect either the expression or the subcellular localization of IRF-4. Consistent with the finding that SWAP-70 contains three nuclear localization signals and can be detected in the nucleus of activated B cells (Masat et al., 2000
), IBP could be detected in the nuclear compartment of T cells where it “sequestered” IRF-4, preventing it from binding and transactivating the IL-21 promoter. The interaction of IBP and IRF-4 requires its carboxy-terminus, which is also important for the ability of IBP to activate Rho GTPases (Gupta et al., 2003a
). Experiments are now in progress to determine whether the two functions of IBP map to distinct subdomains within its carboxy-terminal region. Unlike other master regulators of TH differentiation like GATA3 and RORγt, the expression of IRF-4 is upregulated by TCR stimulation and is not restricted to a specific TH effector lineage (Matsuyama et al., 1995
; Rengarajan et al., 2002b
). The presence of mechanisms that restrain the access of IRF-4 to selected regulatory regions may thus be particularly important to ensure that its activity can be controlled in a TH lineage-specific manner.
In contrast to the IBPtrap/trap DO11.10 mice, young IBPtrap/trap Balb/c mice do not develop obvious signs of arthritis, although evidence of a systemic autoimmune disease can be observed in these mice upon aging (unpublished observations). Interestingly, however, T cells derived from IBPtrap/trap Balb/c mice do exhibit abnormal production of both IL-21 and IL-17 upon in vitro stimulation (unpublished observations). These findings suggest that the pathogenic effector function observed in the absence of IBP becomes unmasked in the setting of a restricted TCR repertoire (i.e. in the DO11.10 TCR transgenic background) and/or of declining T cell numbers (i.e. upon aging). Given that the absence of IBP leads to an exaggerated ability to undergo homeostatic proliferation (unpublished observations), the pathogenicity of the IBP deficient T cells may become evident only after these T cells receive a signal to expand in order to replenish or maintain the mature T cell pool. It is also likely that interactions with other cellular constituents are required for IBP deficient T cells to fully acquire their pathogenic potential. Indeed, we have found that IBP deficient T cells exhibit markedly elevated ICOS expression in vivo but not in vitro suggesting that the deregulated ICOS expression observed in these mice is under complex regulation. We thus envision a scenario whereby the dysregulated production of IL-17 and IL-21 by IBP deficient T cells, via the effects of these cytokines on other cellular compartments, leads to additional aberrations within T cells eventually resulting in the development of autoimmunity.
Absence of IBP can thus result in the development of either a lupus-like syndrome or rheumatoid arthritis-like joint disease. The finding that IBP deficiency can lead to these distinct autoimmune manifestations is consistent with the fact that familial aggregation of RA and SLE has been reported (Alarcon-Segovia et al., 2005
; Criswell et al., 2005
). Deregulation of IL-17 and IL-21 production furthermore has been detected in murine models of RA and SLE as well as in patients affected by these two disorders (Afzali et al., 2007
; Herber et al., 2007
; McInnes and Schett, 2007
; Sawalha et al., 2007
). Blockade of the IL-21/IL-21R pathway has recently been reported to be efficacious in ameliorating disease in murine models of both lupus and RA (Herber et al., 2007
; Young et al., 2007
). The capacity of IBP to control both TCR responsiveness and the production of IL-17 and IL-21 thus suggest that deregulation of IBP-mediated pathways could function as a common pathogenic mechanism involved in the development of these two distinct autoimmune disorders.