Estrogenic effect on swelling and monocytic receptor expression in an arthritic temporomandibular joint model

Abstract

Clinical presentation of temporomandibular joint (TMJ) disorders are more common in women and changes in the female hormone estrogen affect the level of swelling, pro-inflammatory cytokine release and pain in animal models of TMJ arthritis. Estrogen also modulates the expression of the CD16 receptor in vitro. This alters pro-inflammatory cytokine release in monocytes/macrophages when auto-antigens and arthritic factors bind the CD16 receptor. This study investigated the effects of various levels of estrogen on the intensity of inflammation and CD16 expression in a TMJ arthritic animal model. The experiments included rats that were intact or ovariectomized (OVX), eliminating the major source of estrogen output. A portion of the OVX animals had estrogen replaced with 17-β estradiol (E2) using Alzet^® pumps. In OVX animals E2 levels were administered for 10 days to create an artificial estrus cycle or to simulate pregnancy. Following E2 treatment the rats were given an intra-articular TMJ injection of saline or complete Freund's adjuvant (CFA). CFA injection significantly increased TMJ swelling, stress induced chromodacryorrhea and attenuated food intake, thus indicating the adjuvant induced TMJ pain/inflammation. Removing endogenous E2 through OVX reduced CFA induced TMJ inflammation, whereas CFA increased the number of TMJ monocytes expressing the CD14 receptor equally in all groups irrespective of plasma E2 levels. Paradoxically, higher levels of E2 reduced the number of TNF-α positive, CD16+ and double labeled CD14+/CD16+ cells. The findings indicate that reduced plasma E2 levels attenuated CFA induced TMJ inflammation, whereas increasing E2 levels enhanced TMJ swelling in a dose dependent manner. Estrogenic group differences in CFA induced swelling were independent of TMJ CD14+, CD14+/CD16+ or CD16+ cell numbers suggesting E2 action on the CFA immune response primarily excluded CD16 receptor action.

Introduction

Physical conditions causing temporomandibular joint (TMJ) disorders (TMD) can include muscle pain, disc displacements and joint conditions that include arthralgia, osteoarthritis and ostearthrosis. Large epidemiological studies [1], [2] indicate that women seek treatment for TMD more often than men [3], [4], [5] and women comprise over three-fourths of the clinical TMD cases. Moreover, women appear to exhibit more severe symptoms of TMD [6], [7]. Although the biological basis for this possible gender-based disparity is unclear, the time course of symptoms associated with TMD is of note in females. Peak incidence in TMD occurs during the childbearing years [8] and varies in intensity over the menstrual cycle [9]. Notably, the attenuation of symptoms in post-menopausal women is reversed when they take estrogen supplements [2], [10].

Although the above data strongly suggest that hormonal factors are involved in TMD, the effect of sex hormones on TMD inflammation and the mechanism of these hormonal effects are largely unknown. TMD, especially those associated with acute trauma, internal derangement, or osteoarthritis, often includes an inflammatory component [11], [12], [13]. Estrogen can directly act on monocytes and macrophages to regulate the production of cytokines, e.g., interleukin-1 (IL-1) [14], [15], [16], IL-6 [17] and tumor necrosis factor-α (TNF-α) [18], [19]. Estrogen can modulate monocytes/macrophage function through interaction with the beta form of the estrogen receptor [20]. Cytokines IL-1β and IL-6 have been shown to be inflammatory mediators in the TMJ synovium [21]. IL-1β and TNF-α promote cartilage reabsorption, inhibit synthesis of proteoglycans and cause inflammation, all major symptoms of TMD [22], [23], [24], [25]. Monocytes/macrophages are the major immune cell present within the synovial tissues and are also recruited in high numbers during synovial inflammation, suggesting that a majority of IL-1 and TNF-α release within the joint is from this cell type [26].

A receptor that regulates cytokine production in monocytes/macrophages due to auto-antigen stimulation is the Fc gamma receptor IIIA (CD16) [27]. The functional CD16 receptor consists of an α subunit that spans across the membrane and functions in binding auto-antigens (e.g., rheumatoid factors) and a γ subunit (FcɛRIγ) necessary for receptor assembly and signal transduction [28]. CD16 binding is preferential for small immunoglobulin G (IgG) dimer or trimer complexes that include anti-IgG antibody complexes. This is an important component of auto-antigens and arthritic factors that are potential triggers for onset or maintenance of arthritic diseases [29], [30], [31], [32]. Importantly, significantly high levels of auto-antigens have been detected in the synovial fluid of TMD patients in comparison to healthy individuals [33] and previous studies have shown that CD16 expression and cytokine release are modulated in monocytes/macrophages by estrogen treatment [34].

Knowing estrogen's impact on TMD we tested the effect of estrogen on TMJ swelling and monocytic cell numbers. Moreover, we characterized the level of CD16 receptor expression within the arthritic TMJ animal model to characterize this receptor's role in relaying estrogenic signals.