CIITA: a master regulator of adaptive immunity shows its innate side in the bone

CIITA (the Major Histocompatibility Class (MHC) II transactivator) has long been known as the master regulator of MHC Class II genes, which are critical for normal immune function (reviewed in (1, 2)). CIITA is a non-DNA binding co-activator that specifically regulates expression of Major Histocompatibility Class II molecules, with CIITA deficiency leading to the rare human immunodeficiency disease termed Bare Lymphocyte Syndrome (3). CIITA also regulates expression of genes encoding accessory proteins required for MHCII-restricted antigen presentation, thus CIITA is central to controlling the response to foreign antigens and the maintenance of tolerance (1, 2). In this issue of the Journal of Bone and Mineral Research, Benasciutti and colleagues demonstrate that CIITA is also a major regulator of osteoclast differentiation and bone homeostasis (4).

CIITA controls the expression of MHCII molecules that are displayed on the surface of antigen presenting cells that present peptides to T cells to initiate an adaptive immune response. The major target genes of CIITA include classical MHCII molecules (HLA-DR, -DP and –DQ), but also additional genes utilized for the presentation of antigenic peptides by MHCII molecules including invariant chain, HLA-DM and HLA-DO. Factors that activate or inhibit MHCII expression act via the promoters that drive transcription of the MHC2TA gene, which encodes CIITA (1, 2). Antigen presenting cells such as dendritic cells, B cells and macrophages as well as activated human T cells and thymic epithelial cells constitutively express MHCII, and MHCII expression is strongly induced on other cell types by INFγ. This coordinated regulation of MHCII and other genes necessary for its function is quite unique, thus CIITA has been termed the “master regulator” of MHCII and antigen presentation (1, 2).

The MHCII promoters are characterized by a group of conserved sequence elements (W, X, X2 and Y boxes) to which a stable macromolecular nucleoprotein complex referred to as the MHCII enhanceosome binds. The enhanceosome components are expressed ubiquitously and do not account for the specificity or inducibility of MHCII expression, therefore the “master regulator” of enhanceosome function is transactivation by CIITA (1, 2). Expression levels of CIITA are responsive to inflammatory stimuli and are critical for regulation of MHCII expression. The ability of CIITA to regulate MHCII and other key genes for antigen presentation has been thought to be the primary function of CIITA. However, recent studies implicate CIITA in novel roles, including a role in osteoclasts and bone homeostasis (4, 5).

The study by Benasciutti and colleagues reports that CIITA is expressed in osteoclast precursors, with increased expression in cells from osteoporotic mice. They report two mouse models in which CIITA is overexpressed which show severe osteopenia and increased numbers of osteoclasts (4). They study transgenic mice overexpressing CIITA systemically and mice with selective CIITA overexpression in bone marrow myeloid cells (a fortuitous finding in mice deficient in the thymic epithelial MHC2TA promoter (pIV^−/−)). In mice, overexpression of CIITA systemically or restricted to the myeloid lineage showed the same loss of over 50% in trabecular bone volume/total volume (BV/TV) in both genders. CIITA overexpressing mice showed increased osteoclast number, elevated serum levels of C-terminal telopeptides of collagen type I yet normal mineral apposition and bone formation rates in vivo.

Knock down of CIITA in macrophages showed decreased osteoclastogenesis (4) though the bone phenotype of CIITA hypomorphic mice has not yet been examined (6). CIITA overexpression led to increased osteoclast differentiation in vitro but normal myeloid differentiation in vitro to macrophages and dendritic cells (4). The mechanism by which CIITA overexpression increases osteoclast formation is shown to be enhanced c-fms and RANK signals. Interestingly, the authors suggest that the effects of CIITA on osteoclastogenesis are not dependent on CIITA regulation of MHC Class II expression or T cells because there is a lack of similar osteoclast phenotype in MHC Class II deficient mice, and in a transgenic mouse lacking a CIITA promoter but with thymic reexpression of CIITA (4). Thus, the study suggests an intrinsic effect of CIITA expression in osteoclast precursors that is not necessarily dependent on CIITA effects on the MHCII enhanceosome. Given the shared precursor relationship between osteoclasts and dendritic cells (professional antigen presenting cells), perhaps the shared though different usage of CTIIA should not be so surprising (7).

CIITA was previously implicated as having an indirect effect on bone during estrogen deficiency-induced bone loss due to its effects on antigen presentation. In mice, enhanced CIITA expression was demonstrated post ovariectomy, which increased MHCII expression and enhanced activation-induced T cell proliferation (8). In these studies, T cell activation was critical for ovariectomy induced bone loss, and CIITA was shown to be regulated by estrogen in vivo. CIITA regulation was dependent on IFN-γ because ovariectomy led to enhanced IFN-γ production by T_H cells, and IFN-γR^−/− mice failed to up-regulate CIITA (8). Regulation of CIITA expression in osteoclast precursors will be of interest to examine during estrogen deficiency and other states of induced bone loss.

While dendritic cells (DC) are the professional antigen presenting cells of the immune system, osteoclasts have also been shown to be capable of antigen presentation. Osteoclasts (OC) can uptake soluble antigens and express MHC class I and II, as well as costimulatory molecules such as CD80, CD86, and CD40(9). Osteoclasts can present allogeneic antigens and activate both CD4⁺ and CD8⁺ alloreactive T cells in an MHC-restricted fashion (9). The role of CIITA in antigen presention in osteoclasts has not yet been examined. However, osteoclasts are much weaker in T cell activation than dendritic cells and show differences in cytokine expression in response to activation (9). Thus, OCs and DCs are both capable of regulation of T-cell immunity but with distinct differences.

Interestingly, a prior in vitro study by Kim et al showed the opposite effect regarding the effect of CIITA on osteoclastogenesis (10). Overexpressed CIITA was proposed to decrease osteoclastogenesis by sequestration of CBP/p300, with decreased cfos transactivation and NFATc1 (10). Prior studies also demonstrated conflicting results with CIITA effects on T cell polarization. IL-4, a Th2 cytokine was shown to be aberrantly upregulated in Th1 cells derived from CIITA deficient mice. Transfection experiments suggested that CIITA might suppress IL-4 expression by competing with NFAT for binding to the general coactivator CBP (rev in (11)). However, other studies did not show that CIITA represses IL-4 expression in Th2 cells but rather enhanced Th2 bias during CD4+ T cell activation (rev in (11)). Perhaps these discrepancies in observed effects of CIITA deficiency or overexpression are due to the unique regulatory mechanism of CIITA. The co-activator function of CIITA is highly dependent on expression of other transcription factors, co-activators and/or repressors, which may vary with exact conditions of the study or cell type.

Several diseases including rheumatoid arthritis, multiple sclerosis and atheroscelerosis/myocardial infarction are associated a specific MHC2TA polymorphism (12). In these studies the polymorphism was demonstrated to lead to decreased CIITA expression after interferon-γ (IFN-γ) stimulation, but the mechanism involved in disease pathogenesis is not yet understood (12). Nonetheless, the same polymorphism has been demonstrated to be of importance in a recent osteoporosis/fracture study. The importance of CIITA in regulation of bone homeostasis is highlighted by the demonstration that the CIITA rs3087456(G) allele was protective against incident fractures in elderly women. The observed protective effect of the CIITA rs3087456(G) allele is associated with lower expression of CIITA and MHCII (13), which is consistent with the finding in mice that overexpression of CIITA promotes osteoclastogenesis.

The maintenance of bone homeostasis is highly regulated by innate immune factors. In this light it is of interest to note that by structure, CIITA is a member of the NOD-LRR or CATERPILLER protein family which is best known for regulation of innate stimuli such as IL-1 and TLR receptors (reviewed in (14)). NOD-LRR proteins such as NALP 1 or 3 are critical intermediates in the inflammasome pathway that regulate activation of the inflammatory cytokine IL-1. CIITA contains NOD-LRR domains and one isoform of CIITA contains a CARD domain important in protein-protein interactions (2, 14). The NOD-LRR proteins evolutionarily precede adaptive immunity raising the possibility that CIITA may have evolved earlier for innate functions. Benasciutti et al propose that because MHC-based allorecognition predates skeletal evolution, that bone co-opted the use of CIITA (4). However, if CIITA has underlying innate functions that are more primitive, it is equally possible that CIITA was actually coopted for regulation of both MHCII and bone. Understanding how CIITA regulates osteoclastogenesis will be of significant interest particularly to determine if the MHCII enhanceosome is involved. It will need to be determined if CIITA regulation of osteoclastogenesis requires the NOD-LRR domains and if CIITA can be shown to have other innate regulatory functions.

Polymorphisms of CIITA are now linked to a variety of disease states associated with aging and autoimmunity as well as fracture risk in elderly women (12, 13). In pathological states, it is likely that inflammatory cytokine upregulation of CIITA plays a role in bone loss during inflammatory and autoimmune disease. Similarly, significant epigenetic regulation of MHC2TA has been demonstrated to alter CIITA expression (15), which may also influence bone homeostasis. Thus, CIITA may not be as specific a master regulator as was once thought. However, CIITA is a critical regulator with multiple functions in adaptive and innate immunity and should also be seen as an important factor that links the immune and skeletal systems.