Correlation of neutrophil and monocyte derived interleukin-1 receptor antagonist and interleukin-8 with colitis severity in the rabbit

Abstract

Activated neutrophils and monocytes produce interleukin (IL)-8, a pro-inflammatory chemokine, but also IL-1 receptor antagonist (IL-1ra), which is an anti-inflammatory cytokine. We were interested to see the profiles of IL-8 and IL-1ra in the colonic tissue and in the peripheral blood leukocytes (PBL) during the development of immune complex induced colitis in rabbits. IL-1ra and IL-8 in PBL were measured in 26 rabbits at time 0 h, 24 h, and 48 h after induction of colitis. The colons were removed at 48 h for measuring myeloperoxidase (MPO), ulcer area, IL-1ra and IL-8. Epithelial damage, crypt abscess formation and leukocyte infiltration of the colonic tissue were major features of this colitis model. During the development of colitis, there was an increase in circulating neutrophils and monocytes (P < 0.0001), but not lymphocytes. Likewise, elevated amounts of IL-1ra (P = 0.0001) and IL-8 (P = 0.0219) production by PBL were observed following induction of colitis. Flow cytometry revealed major source of IL-1ra was monocytes, while the main sources of IL-8 were neutrophils and monocytes. There was correlation between MPO and ulcer area (R_s = 0.6327, P < 0.0001). At 24 h, PBL from MPO^High group (n = 11) showed increased IL-1ra (P = 0.027) and IL-8 (P = 0.0128) levels vs MPO^Low group (n = 15). IL-8 production by PBL showed correlation with tissue MPO (R_s = 0.4273, P = 0.0295). The colitis in this model was associated with an increase in circulating monocytes and neutrophils, which released increased amounts of IL-8 and IL-1ra. Further, IL-8 and IL-1ra showed correlation with the severity of colitis. These observations should significantly further understandings on the role of neutrophils and monocytes in the immunopathogenesis of ulcerative colitis.

Introduction

Tissue damage or inflammation may lead to infiltration of the affected tissue by vast numbers of leukocytes, mostly neutrophils and monocytes, which can exacerbate and extend the injury by releasing pro-inflammatory mediators, proteases, and reactive oxygen derivatives [1], [2], [3]. Therefore, leukocyte migration into tissues is a hallmark of an inflammatory response. Although the role of leukocytes has long been regarded to be mainly phagocytic, recent publications indicate that neutrophils and monocytes/macrophages are capable of producing several cytokines as well [4], [5], [6]. Upon activation, neutrophils release large amounts of the pro-inflammatory chemokine, IL-8 as well as the anti-inflammatory interleukin (IL)-1 receptor antagonist, IL-1ra [7]. Further, the CD14⁺CD16⁺ monocyte phenotype is a major source of tumor necrosis factor (TNF)-α [8], which is elevated in patients with inflammatory bowel disease (IBD), both ulcerative colitis (UC) and Crohn’s disease (CD) [9], [10].

The release of cytokines from damaged cells or tissues is implicated in inflammatory processes by promoting neutrophil chemotaxis and inducing the expression of IL-8 [11]. In fact, an increased IL-8-mRNA-expression in neutrophils following major trauma [12], and the presence of high levels of leukocyte-associated IL-8 in patients with sepsis syndrome [13] have been reported. Plasma concentrations of the major pro-inflammatory cytokines, TNF-α, IL-1β, and the anti-inflammatory IL-1ra or IL-10 cytokines were predictive of mortality in the early phase of murine sepsis [14]. It has been reported that neutrophils produce IL-8 and IL-1ra upon stimulation with various mediators, such as lipopolysaccharide [10], [15], phospholipase A₂ [16], psoriasin, an Escherichia coli-cidal antimicrobial protein [17], and serum amyloid A, an acute-phase protein [18]. In the colonic inflammation, activation of neutrophils recruited to the intestinal mucosa results in the synthesis of pro-inflammatory cytokines [19]. In the colonic mucosa of patients with IBD, epithelial cells, and lamina propria mononuclear cells produce IL-1ra and IL-8, and the secreted levels of these mediators in the mucosa were correlated with macroscopic inflammation in both CD and UC [20], [21].

However, a relationship between the degree of colonic inflammation and the levels of IL-8 and IL-1ra in the peripheral blood leukocytes (PBL) is still unknown. With this in mind, we undertook the present study, focusing on the production of anti-inflammatory IL-1ra and the pro-inflammatory IL-8 by PBL as well as in the colonic mucosa in a rabbit model to address the relationship between the severity of colitis and the levels these mediators.