Matrix metalloproteinase-9 and its natural inhibitor TIMP-1 expressed or secreted by peripheral blood mononuclear cells from patients with systemic lupus erythematosus

Abstract

Matrix metalloproteinase-9 (MMP-9) was involved in inflammation and immune system dysfunctions. Besides immunologic abnormalities, systemic lupus erythematosus (SLE) also presents chronic inflammatory components. Therefore, a role of MMP-9 in SLE pathology might be supposed. To verify this hypothesis, SLE patients and healthy donors were compared for the MMP-9 and MMP-9 mRNA levels in peripheral blood mononuclear cells (PBMCs), the spontaneous secretion of MMP-9 and TIMP-1 and the MMP-9 activity. Thus, we found that fresh PBMCs from SLE patients expressed a significantly higher activity of MMP-9 and spontaneously released higher levels of MMP-9, as compared to healthy donors, while the secreted TIMP-1 level was the same for both groups. When the patients were sub-grouped based on disease status, the most increased pro-MMP-9 activity inside the PBMCs was identified for relapse SLE sub-group. A similar observation for SLE patients with positive serum fibrinogen was found. Following culture, the PBMCs from remission SLE patients secreted significantly higher MMP-9 level, than the PBMCs from relapse SLE patients. PBMCs from relapse SLE patients secreted the highest levels of TIMP-1, although this difference was not statistically significant. Taken together, these observations suggested the multiple roles of MMP-9 and TIMP-1 in progress of inflammation and tissue damage and/or in repair, depending on clinical stages of SLE.

Introduction

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease characterized by the prolonged production of high-affinity autoantibodies resulting in direct and immune complex-mediated tissue damage [1]. SLE evolves over time, the course of the disease being characterized by periods of remission and relapse. Despite a large number of studies, the etiology of SLE is not completely understood. In the past years, many abnormalities were demonstrated in SLE. Among these, the levels of some molecules involved in mediation of inflammatory processes were reported to be increased in this disease [2], [3], [4], [5]. Among the effects of the inflammation mediators, the induction of matrix metalloproteinases (MMPs) has been demonstrated [6].

MMPs are a family of zinc-dependent enzymes, whose activation mechanism involves the disruption of the cysteine-Zn²⁺bond by a conformational change or proteolysis followed by the excision of the propeptid [7]. The MMPs are regulated mainly by natural tissue inhibitors (TIMPs) [8]. MMPs and TIMPs are expressed in many different cell types, including those of the immune system [6]. MMPs are important in many normal biological processes, including embryonic development, angiogenesis, and wound healing, as well as in pathological processes such as inflammation and autoimmunity [6], [9], [10], [11]. In the immune system, gelatinase B (MMP-9), a member of the MMPs family, cleaves denatured collagens (gelatins) and type IV collagen, the major component of the basement membranes. This cleavage helps lymphocytes and other leukocytes to enter and leave the blood and lymph circulations. In addition, MMP-9 cleaves some proteins (such as myelin basic protein and type II gelatins) leading to remnant epitopes that can generate autoimmunity, and processes cytokines and chemokines, resulting in skewed immune functions. Therefore, MMP-9 has been considered as a prototypical example of the regulation of immune functions by proteolysis. The expression and the secretion of MMP-9 by activated lymphocytes and monocytes are tightly regulated by cytokines, chemokines, eicosanoids and peptidoglycans [11].

Starting from these observations and taking into account the activation state of peripheral blood mononuclear cells (PBMCs) from SLE patients and the inflammatory component of this disease, we investigated the role of MMP-9 in SLE pathology. This idea was also derived from previous observations that demonstrated elevated levels of MMP-3 in SLE patients' sera or plasma [12], [13], [14], [15], [16], [17], [18]. Once activated, MMP-3 was shown to be a potent activator of pro-MMP-9 enzyme [19]. In addition, the hypothesis of the present study was in accordance with the study of Faber-Elmann et al. that demonstrated a significantly elevated activity of MMP-9, but not of MMP-2, in sera of SLE patients, the origin of this increase being unknown [20].

Our data demonstrated for the first time that PBMCs from SLE patients presented some abnormalities in the control of MMP-9 expression and secretion. Inside fresh PBMCs from SLE patients, the total activity of pro-MMP-9 was higher than in PBMCs from healthy donors. PBMCs isolated from both SLE patients and healthy donors, spontaneously released in vitro MMP-9 and its natural inhibitor, TIMP-1. While the level of secreted TIMP-1 was comparable between patients and healthy donors, the level of secreted MMP-9 was the highest in culture media of PBMCs from SLE patients. Interesting observations resulted when the patients were compared according to their disease stage. Thus, the increased level of pro-MMP-9 inside the PBMCs seems to be a characteristic of relapse SLE patients, while the high level of secreted MMP-9 seems to characterize the remission SLE patients. Generally, PBMCs of SLE patients presenting positive inflammatory markers exhibited an increased pro-MMP-9 activity. No correlation with medication was found, at least in our experimental conditions.