Klinische Wochenschrift

Klin Wochenschr (1991) 69:981-987

9 Springer-Verlag 1991

Inflammatory Mediators in Chronic Inflammatory Bowel Diseases V. Grol3, T. Andus, H.-G. Leser, M. Roth, and J. Sch61merich Abteilung fiir Innere Medizin, Universit/it Regensburg

Summary. Crohn's disease and ulcerative colitis are chronic inflammatory bowel diseases (IBD) of unknown etiology. They are characterized by an activation of intestinal mononuclear cells. Cytokines play a crucial role in the regulation of the functions of these cells. An increased synthesis of the cytokines interleukin-I (IL-1), interleukin-6 (IL-6) and tumor necrosis factor e (TNFc0, which are primarily synthesized by activated monocytes/macrophages has been described in patients with IBD. The synthesis of interleukin-2 (IL-2) and of interferon 7 (IFNT), which are produced by lymphocytes, on the other hand, has been found to be decreased. The published data are, however, not quite consistent. In patients with IBD there is not only a stimulation of the local cytokine production in the gut. The blood levels and the synthesis of the cytokines IL-I, IL-6 and T N F e by peripheral blood mononuclear cells are also increased, in particular in patients with Crohn's disease. Drugs, which are commonly used for the treatment of IBD impair the synthesis of these cytokines in monocytes/macrophages. Key words: Inflammatory bowel diseases - Inflammatory mediators - Crohn's disease - Ulcerative colitis

Immunological Pattern in IBD The etiology of Crohn's disease and of ulcerative colitis is thus far unknown. Their pathogenesis is Abbreviations: IBD = inflammatory bowel disease; IL-1 = interleukin-1 ; IL-4 = interleukin-4; IL-5 = interleukin-5 ; IL-6 = interleukin-6; IL-7 = interleukin-7; IL-8 = interleukin-8 ; TNFc~ = t u m o r necrosis factor c~; I C A M =intercellular adhesive molecule; E L A M = e n d o t h e l i a l leukocyte adhesion molecule; G M CSF=granulocyte/macrophage-colony stimulating factor; P D G F = p l a t e l e t derived growth factor; A C T H = a d r e n o c o r t i cotropic h o r m o n ; NK-cells = natural killer cells; C S F = colony stimulating factor; U C = ulcerative colitis; C D = C r o h n ' s disease; C D A I = C r o h n ' s disease activity index

incompletely understood. Twin studies demonstrated a genetic predisposition for Crohn's disease, which is less pronounced in ulcerative colitis [11. Studies on the specificity of intestinal lymphocytes in patients with Crohn's disease or ulcerative colitis provided no evidence for the existence of a monoclonal subpopulation specific for a certain antigen [2]. In another study on the specificity of activated B-lymphocytes from mesenteric lymph nodes of patients with Crohn's disease or ulcerative colitis B-cells producing antibodies against a broad spectrum of antigens could be found [3]. Interestingly, the percentage of B-cells directed against a pool of 5 mycobacterial antigens or against a pool of fecal bacteria was significantly higher in Crohn's disease (30% and 36%, respectively) than in ulcerative colitis (13% and 17%, respectively). This might be caused by an increased penetration of bacteria into the bowel mucosa in Crohn's disease or by an increased reactivity of the intestinal immune system in Crohn's disease. An increased intestinal permeability for certain macromolecules has been described in patients with Crohn's disease [4-8]. It is, however, not clear, whether this is the cause or the consequence of the intestinal inflammatory process, since other inflammatory bowel diseases of known etiology also lead to an increased intestinal permeability [7, 8]. This might be caused, e.g., by the inflammatory mediator IFN7 [9]. In one study an increased intestinal permeability for PEG 400 has also been found in healthy relatives of patients with IBD [6]: These results, however, have been criticized [10]. Furthermore, no increased intestinal permeability for other macromolecules (mannitol, oligosaccharides) could be detected in the same population [11]. Alterations of the intestinal mucus, which has an important barrier function, have been described in patients with ulcerative colitis [12]. There were characteristically altered mucin glycoproteins. The impact of this finding, however, is not clear.

Manuscripts published in this issue were the matter o f a Symposium held at the University o f Ulm, April 24-27, 1991

982

A phenomenon demonstrating the disturbance of the immune system in patients with IBD [13, 14] is the production of autoantibodies. Autoantibodies directed against various intestinal epithelial antigens [15-18] or extraintestinal antigens [1921] have been described in patients with IBD with various frequencies. These autoantibodies are, however, often not disease specific. An exception are - among others - antineutrophil cytoplasmic antibodies in patients with ulcerative colitis [20, 21]. An important drawback in the assessment of the relevance of these antoantibodies is the unresolved question, whether they represent epiphenomena or etiologic or pathogenetic important mechanisms. Taken the published data together there are more arguments for an autoimmune pathogenesis of ulcerative colitis than of Crohn's disease. This assumption is in accordance with the increased incidence rate of other autoimmune disorders in patients with ulcerative colitis (6.6% versus 1.9% in Crohn's disease versus 2.0% in controis) [22]. The activation of the immune system in IBD affects all cell systems associated with the mucosal immune system. There is an activation of macrophages [13]. This has been demonstrated by their release of lysosoreal enzymes [23], the production of free oxygen radicals [24], and the expression of leukocyte adhesion molecules [25]. Compared with healthy controls the ratio of CD 4-positive to CD 8-positive T-lymphocytes is not significantly changed in patients with IBD [26]. However, there is an increase in activated cells expressing IL-2 receptor [14]. There is furthermore an increase in memory T-cells [27]. In the peripheral blood more activated T-cells can be found expressing transferrin receptor [28]. Besides T-cells there is also a substantial rise in the number of B-cells in the intestinal mucosa of patients with IBD. All immunoglobulin classes, but predominantly IgG are increased [29]. The immigration of leukocytes in the inflamed bowel in patients with IBD can be visualized by scintigraphy using technetium or indium labeled leukocytes [30, 31] (Fig. 1). Taken the published data together the intestinal immune system is activated in patients with IBD. The etiologic agent, however, is not known. In ulcerative colitis there is evidence for an important role of autoimmune mechanisms, in Crohn's disease the situation seems to be more complex. An increased reactivity of the intestinal immune system or an increased penetration of luminal anti-

V. GroB et al. : Cytokines and IBD

Fig. 1. Leukocyte scintigraphy in a patient with highly active Crohn's disease

gens into the mucosa might be of importance in Crohn's disease.

Cytokines as Inflammatory Mediators Cytokines are important in orchestrating the interactions of cells involved in immunological and inflammatory reactions. To demonstrate their role the functions of IL-1, TNF~, IL-6 and IL-2 will be shortly characterized. IL-1 plays a central role in the cytokine network. During an antigen-induced immune response it is primarily synthesized by macrophages, but it is also produced by many other celltypes like endothelial cells, fibroblasts and epithelial cells in response to adequate stimuli [32]. Two different IL-1 species exist: IL-I alpha and IL-I beta [33]. They show only little homology but have similar biological functions. IL-I has accessory growth factor activity for T-cells and B-cells, stimulates the proliferation and differentiation of hematopoetic cells [34] and induces the expression of leukocyte adhesion molecules on endothelial cells [35]. IL-1 stimulates the synthesis of prostaglandins, modulates the synthesis of some acute-phase proteins in the liver and induces fever. IL-1 is further-

V. Grog et al. : Cytokines and IBD

more a potent inducer of several other cytokines like TNF, IL-6 and IL-2 [36]. The physiological role of IL-1 is proinflammatory. The subcutaneous implantation of a slowly IL-I releasing product into mice led to a granulomatous inflammation characterized by macrophage infiltration and fibrosis [37]. T N F a exerts marked proinflammatory effects [38]. It promotes the immigration of monocytes, macrophages and granulocytes by inducing leukocyte adhesion molecules (ICAM, ELAM). It furthermore increases the phagocytic capacity and cytotoxic activity of these cells. TNF~ induces the synthesis of other cytokines (IL-1, IL-6, GM-CSF, PDGF, TGF) and of itself in various other cells and thus indirectly modulates its effects [39]. IL-6 is another inflammatory mediator primarily synthesized by monocytes and macrophages. It is important for the terminal maturation of B-cells into immunoglobulin secreting cells [40]. IL-6 acts synergistically together with IL-I during antigendependent T-cell activation and subsequent proliferation of activated T-cells [41-44]. IL-6 promotes the proliferation of hematopoetic stem cells, stimulates the synthesis of ACTH by the pituitary gland, acts as endogenous pyrogen, and induces the synthesis of acute-phase proteins in the liver [45]. Besides activated monocytes and macrophages numerous other cell types like endothelial cells or fibroblasts are able to produce IL-6 when stimulated by other cytokines like IL-1 or T N F a [45]. There is evidence from patients with IL-6 producing tumors (e.g. cardiac myxomas), that a permanently increased IL-6 synthesis might be connected with autoimmune disorders [40, 46]. IL-2 is synthesized by activated T-lymphocytes [47]. A maximal induction requires both antigen and IL-1. On the other hand T-cell activation leads to the expression of IL-2 receptors on the cell surface. The interaction of IL-2 with its receptor induces T-cell proliferation [48]. Besides its central function as growth factor for mature T-cells IL-2 acts as differentiation factor for B-cells under certain conditions [49]. It can furthermore stimulate the growth of NK-cells [50] and promote the cytotoxic activity of monocytes [51]. The subcutaneous implantation of a slowly IL-2 releasing product into mice led to a local vasculitis and lymphocyte proliferation.

Intestinal Mucosal Cytokine Synthesis in IBD An increased synthesis of various cytokines has been demonstrated in the intestinal mucosa of patients with IBD. An increased synthesis of IL-I

983 Table 1. Mucosal cytokine synthesis in IBD

Cytokine

Without

with

stimulation

IL-1 IL-2 interferon 7 CSF

~" 0 0 t

IL-4 IL-5 IL-6 IL-7 IL-8 TNF~ TNFfl TGF~ TGFfl

9 ? "~ ? T T ? ~" =

Data accordingto [62] could be demonstrated in isolated intestinal mononuclear cells [52] and in organ cultures of colonic biopsy specimen [53]. Similarly, the synthesis of TNF by intestinal mononuclear cells was found to be increased in IBD with a more pronounced stimulation in Crohn's disease than in ulcerative colitis [54]. There is furthermore evidence for an increased IL-6 synthesis [55, 56]. In contrast to IL-I, T N F and IL-6 the synthesis of IL-2 has been described to be decreased in the intestinal mucosa of patients with IBD [57], as well as in peripheral blood mononuclear cells of patients with Crohn's disease [58]. This is, however, in contrast with the finding of increased IL-2 concentrations and soluble IL-2 receptor levels in the plasma of patients with active Crohn's disease [59, 6O]. Similarly to IL-2 the synthesis of IFN7 has also been described to be decreased in the intestinal mucosa of patients with IBD [58, 61]. The available data on intestinal mucosal cytokine synthesis are summarized in Table 1 [62]. They show an increased synthesis of cytokines primarily synthesized by monocytes/macrophages and an impaired synthesis of cytokines synthesized by lymphocytes. As mentioned above, the data are, however, not quite consistent and have been criticized.

Systemic Cytokine Levels and Cytokine Synthesis in IBD Increased cytokine levels have been described in the peripheral blood of patients with IBD. Elevated cytokine concentrations are more frequently found in patients with Crohn's disease than in pa-

V. GroB et al. : Cytokines and IBD

984

Table 3. Serum levels of IL-6 and extraintestinal manifestations

in Crohn's disease IL-6

32

> 3 2 U/ml

< 8 U/ml

Means_+ SEM

With extraintestinal manifestations

E

16%

17%

9.9_+2.0 U/ml

2%

40%

5.9 +0.9 U/ml

Without extraintestinal manifestations

= Z

10000

O000Q 00000 OQO00 00000

eeoo

. . . . . . . .

oeoo ooooe eoeoo eoeoo

1000

::...

*ooee oeeeo ooooe

eoeoe eooee oo~o 9

control

UC

oeeoo oooee ooeee

100

C0

Fig. 2. Serum levels of IL-6 in patients with Crohn's disease and ulcerative colitis

10

IL-6 [U/ml]

i T

510

[3 ~ Q

54

•

256

o

x x

I

1

Table 2. Serum levels of IL-6 and disease activity in Crohn's

disease van Hees index

IL-6 (U/ml)

210