Mast cells as targets for the therapy of inflammatory bowel disease

The etiology and pathogenesis of inflammatory bowel disease (IBO) is poorly understood. However, numerous studies have demonstrated that immunological and inflammatory responses are activated during this disease. A better understanding of these events will help identify appropriate therapeutic interventions. Mast cell hyperplasia is a prominent feature of inflamed intestinal tissue in IBO. Intestinal mast cells are heterogeneous and at least two populations are present in the human intestine. The authors' objective is to explore mast cell properties, activation and mediators that are involved in the induction, maintenance and perpetuation of inflammatory lesions in the intestine. Although some therapies used in lBD can modulate mast cell activities, whether these actions are important in the beneficial effects of the drugs is unknown. Future drug development targeted to the inhibition of mast cells might be of therapeutic value. However, a cascade of different cellular events are involved in IBO development. The complexity of the disease raises difficulties in the development of therapies. Multiple drugs, selective for different phases of the disease or acting on different cells, might be most appropriate, rather than a single, all-encompassing therapeutic agent. CanJ Gastroenterol 1990;4(7):285,288

throughout the hody and are abundant in the skin and gastrointestinal mucosa.Their occurrence in strategic areas, often close to nerves anJ vascu lature, anJ their capacity to release a vari ety of potent mediators m response to many inflammatory stimuli contribute to pathologica l conditions.In the past, mast cells were mainly considered in relation to immunoglobulin E (IgE) antibody responses, but now it is well recognized that mast cells can be activated by a wide range of factors.Activated mast cells are implicated in many physiological and pathological processes including inflammation, fibrosis, infectious diseases and neoplasia (l-3).
Celiac disease, ulcerative coli tis and Crohn's disease are associated with mast cell hyperplasia and activation (4)(5)(6).This hyperplasia might be caused by a local pro liferation and/or differentiation, or by chemotaxis of mast cells or their progenitors from distant sites.A recent study sht,wed that mast cells display chemoraxis towards laminin, a major component of basement membranes and inflammation (7).This might explain t he accumulation of mast cells at sites of tissue injury where laminin may be exposed, degraded and synthesized.This chemotaxis can be increased when mast cells are stimulated with antigen.It is interesting that intestinal epithelial antigens can stimulate mediator secretion from intestinal mast cells m inflammatory bowel disease (IBO) patients but not from non-IBO patients (8).The role of mast cells in pathogenesis or tissue repair in IBO remains to be clarified, bur they appear

MAST CELL HETEROGENEITY AND ACTIVATION
There are numemus studies on the heterogeneity of mast cells regarding their morphology, function, and mediator content (9,10).In rodents, there are at least two subpopulatiom of mast cells: connective tissue and mucosa!mast cells.The development of techni4ues to purify each subpopulation in rodents h as allowed them to be characterized in detail (9).Heterogeneity is also evident in human mast cells, but unfortunately the two (or more) subpopulations are difficult to separate because of their physical similarities ( 10).
Subpopulations of human mast cells differ in their :sensitivity to formaldehyde blockade of dye binding, as well as their proteoglycan and protease content (Table l ).One subpopulation, abundant in certain connective tissue sites, contains two proteases, crypcase and chymase, whereas the other subpopulation, most abundant at mucosa[ surfaces contains only tryptase (11).Boch populations are present in human intestine; the latter is prominent in the mucosa, whereas the former is prominent in the muscle an<l connective tissue layers.Further characterization and analysis of human mast cell subpopulations is essential to determine if they are functionally different and to what extent they are similar to the mast cell populations more widely studied in the rat.
Heterogeneity of mast cells is also evident at the functional level (Table 1 ).There are site-specific differences in the responses of human mast cells to various stimuli.Recent observations of anatomic associations and interactions between peripheral nerves and mast cells in the gastrointestinal tracts of rats an<l humans may shed new light on site-specific mechanisms of the initiation and modulation of pathogenesis in IBO.
Many inflammatory cells and cytokines are implicated in the development of IBO.Interactions among these cells and mast cells can lead to mast cell activation or down-regulation.
Mast cell activating factors have been identified in macrophages, neutrophils, T lymphocytes and eosinophils.Furthermore, because of their similarities to mast cells and their accessibility, human basophils have been studied for their responsiveness to recombinant cytokines.Only interleukin-3 and granulocyte macrophage colony stimulating factor (GMCSF) stimulated the release of histamine ( l ).

MAST CELL MEDIATORS AND THE PATHOGENESIS OF lBD
Mast cell mediators have a w1Je range of actions which have been reviewed extensively (10,11,13 ).Although a number of inflammatory mediators have heen identified m human intestinal mast cells (Table l), studies of these cells are still in their infancy.Many of the cell types present in the intestine are targets for mast cell mediators.Consequences of mast cell mediator release could include: vasodilation, increased vascular permeability, activation of sensory nerves (histamine), enhanced mucus and flu1J secretion and smooth muscle contraction (histamine, prostaglandin D2, sul, fidopepcide leukotrienes); vasoconstriction with resulting ischcmic necrosis of mucosa (alpha-TNF, platelet activating factor-accther, leukotriene C4); recruitment and activation of neutrophils an<l eosinophib leading to epithelial damage (polypeptide chemotactic factors, leukotriene-B4, platelet activating factor-acether, histamine, a lpha-TNF); changes m neuropepcide (substance P, vasoactivc intestinal peptide) levels resulting m altered vasomotor function and lymphocyte activity (tryptase and chymase).
Rodent mast cell lines synthesize a variety of cytokines including interleukins l, 3, 4, 5 and 6, gamma-interferon, GMCSF and alpha-TNF (14) which, if released by human intestinal mast cells, would allow them to initiate or modulate diverse immune and inflammatory functions.For example, mcerleukin-3 and -4 may have aucocnne functions as they induce mast cell proliferation, while alpha-TNF, recently identified as a product of human mru,t cells and/or basophils (15), has multiple activ1tie:.relevant w the develop-

PHARMACOLOGICAL MANIPULATION OF MAST CELL FUNCTION
Figure l illustrates the multiple sites where drugs used to treat intestinal inflammation may down-regulate mast cell function.Several drugs may affect mast cell development and proli feration.Knowledge of human mast cell Jevelopmem is limited, but stud ies with rodent mast cells demonstrate the importance of the T lymphocyte-Jerived cytokines interlcukin-3 and -4 in the developmcm of mast cells from precursors from bone marrow and other sites (9).These cytokines are probably also required for human mast cell development in association with accessory cells such as fibroblasts ( 16).Endogenous inhibitors of mast cell proliferation also exist, cg, GMCSF.gamma-interferon and beta-transforming growth factor ( l 7, 18).
Corticosteroids reJuce the number::.of mast cells in tissue~, eg, in the inte~tine of Nipposcrongylus hrasiliensis-infected rats where mast cells are markedly depleted within 24 h of steroid treatment ( 19).These effect::.may be due to several actions of corticosteroids such as their abilities to reJuce interleukin 3 production by T lymphocytes (20), to act Jirectly on mast celb l o inhibtt proper granule formation (2 l ), and ro stimulate programmed cell Jeath in mast cell~ anJ their subsequent phagocyro~is by macrophages (personal communication).
The immunosuppressanr cyclosporine A a lso interferes with the synthesb of cyrokines by T lymphocytes, which may account for its inhibition of mast cell development (22).In contrast, inhibitors of DNA synrhesi~ such as 6-mercaptopurine and awthioprine which have had limited rriab for treatment of IBD may diminish mm,tocytosis by a direct effect on mast cell precursors ( 23).
The multiple pathways in volved in mast cell stimulus-secretion coupling have been pharmacologically manipulated.Cromolyn sodium inhibits in vitro lgE-dependent release of histamine anJ eicosanoids from human lung and intestinal mast cells (24 ).Sulphasalazinc inhibits histamine release from rat and mouse peritoneal mast cells (25) and dimmishes rat mast cell cyrorox1c1ty (unpublished observations).Cyclosporine A, in addition lO its effects on mast cell development, both down-regulates the expression of genes for inflammatory polypeptides exp ressed by murmc mast cells ( 14) and inhabits murme hisramme release from rat mast cells and human basophils (26).
Products of 5-lipnxygenase action such as leukotriene B4 anJ sulfidopeptide leukotrienes may play a role in IBO: 5-lipoxygenase inh1b1tors prevent several of the pathological change::.in animal models of IBO (27).The 5lipoxygenase pathway in human mast cells b susceptible to one such inhibitor, termed AA-861 (28).Although many cell types produce 5-lipnxygenase products, the effect of 5-lipoxygenase inhibitors on mast cells resident in human mtestme wuld contribute ro their potential clinical use.Antagonists of mast cell mediators may also he employed therapeutically.While antihismmines directly block the effects of histamine on target tissues, severa l of these compound~ also inhibit mediator release from human mast cells (29).Human inrestinal mast cells may be a source of platelet activa1ing facrnr-acethcr ( 30), anJ antagonists of this lipid mediator show promise for therapy of !Bl) in clinical trials and animal models.
Allhough 1h1s section has concentrated on mast celb as targets for therapeulic agent~.the drugs mentioned affect the funuinns of various cell types.For example, cromolyn soJium down-regulates the activities nf monocytes, ensinophils, macrophages, platelets and neutrophils.Similarly, hy affecting the release of cytokines from T lymphocytes, corticosteroids and cyclosporine A wilt have complex effec ts on many aspects of immune and inflammatory systems which depend on these cytokines.

Figure 1 )
Figure1) Down-regulation of mast cells by drugs used rn the therapy of rnflammarory bowel disease.5-LP5-Lipoxygenase; P AF Placelei-acuvacing factor FUTURE DIRECTIONS le is likely that mast cells play importam roles in !BD, eg, in motility changes, diarrhea, inflammation, tissue Jamage and rep;1 ir.Techniques lll study f\JSSONNl'TIEer al the propertie~ and funct1om of mast cells in different compartments of the human intestine must be improved to facilitate understanding of their roles in mtestmal m1ury, repa ir and normal function.Further characterization of responses of human mast ce ll sub-Pllpulat1ons to different therapeutic agents will he importa nt in understanding the actions of existing therapies and m the development of new o n es.

TABLE 1
Heterogeneity of human mast cells