Mouse models of rheumatoid arthritis (RA) have inherent immunologic shortcomings. Mice do not express class II molecules on T cells, and mouse models fall short of the endocrinologic conditions that reproduce the female-biased susceptibility to RA and other autoimmune rheumatic diseases in humans.
The mouse model described by Taneja et al, which is reported elsewhere in this issue of Arthritis & Rheumatism (1), is the first to share similarities with human RA. The mice lack the classic endogenous mouse class II chains. A major difference between RA and collagen-induced arthritis (CIA) models is that, while RA shows a significant sex bias and production of rheumatoid factor (RF) and anti–cyclic citrullinated peptide (anti-CCP) autoantibodies, mouse CIA does not.
In humans, anti-CCP antibodies precede the onset of RA and are associated with DR4. The DR4-transgenic mice described by Taneja et al produce both RF and anti-CCP antibodies and large amounts of the proinflammatory cytokines tumor necrosis factor α (TNFα) and interleukin-18 (IL-18). Furthermore, the female mice had a much stronger T cell response to the DR4-restricted peptide than did the male mice, which is consistent with a recent study showing that deprivation of androgen in mice increases the cellularity of primary and peripheral lymphoid organs and increases T cell proliferation (2).
Androgen replacement therapy seems to ameliorate RA in male patients, supporting the notion that these hormones are involved in the pathophysiology of the disease (3). This new humanized mouse model of RA (1) seems to mimic the sex-bias phenotype of human disease. Sex hormones seem to modulate the immune/inflammatory responses by different mechanisms in female and male RA patients, with estrogens being enhancers of humoral immunity and androgens and progesterone being natural immunosuppressors (4) (step 1 in Figure 1). Similarly, by enhancing B cell activity via IL-10, 17β-estradiol increases the production of IgG and IgM by peripheral blood mononuclear cells (PBMCs) from patients with systemic lupus erythematosus (SLE), leading to elevated levels of polyclonal IgG, including IgG anti–double-stranded DNA (anti-dsDNA) antibody (5). In another study, testosterone suppressed the production of both IgG anti-dsDNA antibody and total IgG (6). Antibody production in B cells was also suppressed by testosterone, although the magnitude of its effect on B cells was lower than its effect on PBMCs.
Recently, the effects of 17β-estradiol and of testosterone were tested on the cultured human monocyte/macrophage cell line THP-1, which had been activated with interferon-γ, in order to investigate their roles in cell proliferation and apoptosis (7). Androgens exerted cell growth inhibition activity and increased apoptosis, whereas estrogens showed a trend toward a protective effect on cell death; both acted as modulators of the NF-κB complex (7).
The increased concentrations of estrogens and low concentrations of androgens recently described in RA synovial fluid from patients of both sexes (8) support their possible modulatory roles in synovial tissue hyperplasia and chronic synovial cell activation, resulting from the effects of estrogen on cell proliferation and apoptosis. Similar results are seen in human breast cancer cells (9).
Increased estrogen concentrations in RA synovial fluid from patients of both sexes likely result from the proinflammatory cytokines TNFα, IL-1β, and IL-6, which accelerate the metabolic conversion of estrogens from androgens by inducing the synovial tissue aromatases (step 2 in Figure 1). As a consequence, locally increased estrogen levels might exert activating effects on synovial cell proliferation, including macrophages and fibroblasts (10) (step 3 in Figure 1). Stimulation or further activation of synovial cells to produce cytokines would further reduce the availability of antiinflammatory androgens in local tissues (step 3 in Figure 1). In this regard, it may be relevant that male patients with RA seem to derive more benefit from anti-TNFα strategies than do female patients with RA (11). Because male patients have higher levels of circulating androgens than do female patients, they probably have relatively higher local aromatase-mediated production of proinflammatory estrogens. Therefore, blockade of TNF-induced up-regulation of aromatase would particularly increase the level of androgens in males, leading to a better clinical outcome (12).
Estrogen metabolites in autoimmune diseases are of interest since steroid hormones can be converted along defined pathways to downstream hormones at sites of peripheral metabolism. The conversion of dehydroepiandrosterone (DHEA) in target macrophages leads to an increase in downstream effector hormones (including estrogens), which may be an important factor for local immunomodulation in RA synovitis (13). In fact, an altered sex hormone balance in RA synovial fluid may result in lower levels of immunosuppressive androgens and higher levels of immunoenhancing estrogens, leading to favorable conditions for the development of synovitis and synovial hyperplasia.
The increased concentrations of estrogen in RA synovial fluid are more specifically the hydroxylated forms, in particular 16α-hydroxyestrone, which is a mitogenic and proliferative endogenous hormone (step 4 in Figure 1). In contrast, the hydroxylated forms inhibit the growth-promoting effects of 17β-estradiol and are present at low levels in RA synovial fluid (step 5 in Figure 1) (14). Therefore, dose-related conversion of estrogens to proinflammatory or antiinflammatory downstream metabolites might support a dual role of estrogens (pro- or antiinflammatory), for example, during estrogen replacement therapy, depending on the local concentration of 16α-hydroxyestrone or 2-hydroxyestrogens (15).
The confirmation that estrogen levels in the synovial tissue and synovial fluid play a more important role in RA than do serum levels also arises from clinical and therapeutic considerations. Recently, considerable interest has focused on whether oral contraceptives (OCs) are protective against the risk of developing RA. The results of many controlled studies have been contradictory (16), and there is no consensus regarding the relationship of OCs to the prevention or the development of RA. Furthermore, an association between estrogen receptor gene polymorphism and age at onset of RA might further explain variations in the clinical and therapeutic responses (16).
The menstrual cycle, pregnancy, the postpartum period, menopause, chronic stress, and the use of corticosteroids, OCs, and steroid hormone replacement may change the sex hormone milieu and/or peripheral conversion, altering androgen to estrogen ratios and modulating disease (17). Sex hormone balance is a crucial factor in the regulation of immune and inflammatory responses, including complex autoimmune diseases such as SLE (18). Modulation of this balance should represent part of advanced biologic treatments for RA. Sharing the sex hormone effects of the human disease, the new humanized mouse may provide a better model with which to study the pathogenesis and treatment of arthritis.