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The Veterinary Journal 171 (2006) 389–392

TheVeterinary Journal

Guest editorial

Equine eosinophils – Why do they migrate?

In this issue of The Veterinary Journal, a paper ispublished describing a potentially emerging diseasewhich leaves clinicians and scientists with lingering ques-tions regarding the role of the eosinophil in equineinflammatory diseases, especially those involving thegastrointestinal tract (Archer et al., 2006). Dr. DeborahArcher and her colleagues at the University of Liverpooldescribe idiopathic focal eosinophilic enteritis involvinggross lesions of thickened mural plaques or circumferen-tial lesions that ultimately caused small intestinalobstruction requiring emergency exploratory surgery.The histopathological findings from these cases revealedsevere transmural enteritis in which eosinophilic leuco-cytes were the predominant inflammatory cell. Other pa-pers have been recently published (Southwood et al.,2000; Swain et al., 2003) describing a similar conditionin other geographic regions.

But what is the stimulus for these cells to migrate andresult in such lesions? In the published reports, there areno identifiable common factors leading to a potentiallydefinable causative agent. However, the predominanceof eosinophils in a lesion always raises the question ofthe role of parasites and, as stated by Archer et al.(2006), parasites are often incriminated as a cause of fo-cal eosinophilic enteritis (Cohen et al., 1992).

The precise function of the eosinophil has not beendetermined in physiological or pathophysiological pro-cesses in the gastrointestinal tract of horses. Tradition-ally, their principal function is believed to bephagocytosis of immune complexes, after some type ofchemotactic stimulus, and the release of enzymes fromcytoplasmic granules of eosinophils causes degradationof target membranes (Archer, 1968). Eosinophils areexocytotic cells, and cytotoxic proteins are manufac-tured and stored as secretory granules where they canbe released onto the surface of a parasite. This processmay or may not be directed although there is evidencethat non-directional release of all granules simulta-neously may occur with eosinophil degranulation lead-ing to host cell damage (Lindau et al., 1994).

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Cytoplasmic granules have been shown to exist inthree morphological types, the biological importanceof which has not been determined (Stockert et al.,1993). Eosinophilic granules are known to contain prod-ucts associated with microbicidal activity, including ma-jor basic protein, eosinophil cationic protein, eosinophilperoxidase and eosinophil-derived neurotoxin (Oliveiraet al., 1998). Oxygen free radicals are also believed tobe involved in the microbicidal activity of eosinophils(Pincus et al., 1984), and the production of superoxideanions has been demonstrated in both equine eosinoph-ils and neutrophils (Foster and Cunningham, 1997). Ba-sic proteins have been identified in equine eosinophilsand have structural similarities to human eosinophil ba-sic proteins (Piller and Portmann, 1993). They have beenshown to have appreciable bacteriocidal activity againstEscherichia coli, most likely via eosinophil peroxidase,which in other species generates a respiratory burst (Pil-ler and Portmann, 1993).

The mechanism of chemotaxis necessary to initiatemigration of equine eosinophils has been extensivelystudied. Hydroxyacid metabolites of arachidonic acidare implicated as strong chemotactic factors for equineeosinophils; this does not implicate a specific compo-nent, but rather a group of lipoxygenase products ofarachidonic acid (Potter et al., 1985). Certain cytokinesalso cause eosinophil recruitment. Platelet activatingfactor (PAF) is a mediator of both neutrophil and eosin-ophil migration in vitro and it was concluded that eitherB4 or PAF released at sites of inflammation mediate leu-kocyte recruitment to the inflamed tissue (Foster et al.,1992). Tumor necrosis factor (TNF) also induces andprimes eosinophil recruitment in the guinea-pig (Macariet al., 1998). Other cytokines such as granulocyte/mac-rophage colony stimulating factor (GM-CSF), interleu-kin (IL)-3 or IL-5 are released at sites of inflammationand promote eosinophil survival in vitro (Her et al.,1991).

Dennis et al. (1993) partially characterized an eosino-phil chemotactic cytokine in ponies when sensitized

390 Guest editorial / The Veterinary Journal 171 (2006) 389–392

mononuclear cells were stimulated with Strongylus vul-

garis antigen (Dennis et al., 1993). This cytokine wassimilar to IL-1 in its heat instability properties, but un-like IL-1 which regulates superoxide anion or granulerelease, it had no effect on cytotoxicity or migration(Jensen and Castro, 1981; Silberstein and David,1987). The model of inflammation in mice caused byinducing pleurisy with Mycobacterium bovis bacillus Cal-

mette-Guerin (BCG) has been studied to elucidate thosefactors resulting in either eosinophil or neutrophil accu-mulation. It has been suggested that leukotrienes, PAF,IL-5 and nitric oxide (NO) may play a role in eosinophilinflux and TNFa and NO in neutrophil influx in thismodel of inflammation (Menezes-de-Lima-Junioret al., 1997).

Recently, several studies have been performed to elu-cidate the factors affecting adherence and migration ofequine eosinophils. These studies primarily involvedthe investigation of eosinophils with regard to skinhypersensitivity and attempted to identify a mechanismby which eosinophils are stimulated as well as the fac-tors that may stimulate target tissues such as endothe-lium. In vitro studies of cultured equine digital veinendothelial cells illustrated that adherence of either stim-ulated or unstimulated eosinophils from ponies with aknown sweet-itch hypersensitivity resulted in greateradherence of eosinophils compared with the adherenceof stimulated or unstimulated eosinophils from normalhorses (Bailey and Cunningham, 2001). This suggeststhat if the tissue endothelium itself is stimulated therewill be increased adherence over and above eosinophilstimulation by a mechanism of up-regulation of endo-thelial cell adhesion molecules. In that particular study,eosinophils were stimulated with recombinant IL-5 andthe digital vein endothelial cells pre-treated with recom-binant IL-1b (Bailey and Cunningham, 2001).

The effects of histamine on both adherence and migra-tion of equine eosinophils has also been studied and theresults implied that there is histamine regulation of eosin-ophil function as it stimulated adherence and migrationof eosinophils (Foster and Cunningham, 1998). Thereare of course a variety of other mediators that stimulateeosinophils, resulting in increased adherence to endothe-lium and migration into tissues and these have beeninvestigated for effects on the induction of equine eosin-ophil activation, adherence and migration. Substance P,for example, might have a direct effect but this may notbe relevant owing to the high concentrations requiredin vitro (Foster and Cunningham, 2003). Protein kinaseC (PKC) has also been closely observed in studiesin vitro and was shown to regulate eosinophil adherenceand subsequent superoxide production, indicating eosin-ophil function (Sepulveda et al., 2005).

More recently, the role of the chemokine eotaxin hasbeen demonstrated to have a key role in eosinophil accu-mulation and activation in the horse (Benarafa et al.,

2002a,b). Previously, these workers were able to demon-strate the expression of equine eotaxin mRNA in thejejunum of horses, suggesting that eotaxin maybe in-volved in the recruitment of eosinophils in the gastroin-testinal tract (Benarafa et al., 2000). Eotaxin has beenfurther shown to play a role in the pathogenesis ofsweet-itch, whereby its expression was up-regulatedwhen horse dermal fibroblasts were exposed to equineIL-4, suggesting a source of chemotaxis for eosinophilsin skin (Benarafa et al., 2002a,b). These studies of eotax-in support the hypothesis that mechanisms of eosinophilstimulation, migration and adherence are similar in dif-ferent tissue types such as the gastro-intestinal tract andskin. In parasitized ponies, it has been shown that acti-vation or stimulation of eosinophils is necessary forantibody-mediated adherence and larvicidal capability(Klei et al., 1992).

The equine gastrointestinal tract has been investi-gated in order to observe the resident immune cell pop-ulations. In a study quantifying the cell types in thelamina propria of jejunal biopsy specimens from normalhorses, it was shown that significantly greater numbersof eosinophils were resident in the intestinal crypts ver-sus the villous region (Packer et al., 2005). However, thisinformation was obtained from horses whose historywith regard to parasite burden or control or dietary fac-tors was unknown; the tissues were taken from randomareas in the jejunum and did not illustrate localizedareas of eosinophil migration. In a recent study ofhorses naturally infected with cyathostomes, evidencewas provided that eosinophil and mast cell infiltrationin the large intestine might be associated with cyathos-tomiasis (Collobert-Laugier et al., 2002). Absolute num-bers of eosinophils circulating do not correlate withparasite infection (Bailey et al., 1984) and there are largenumbers of resident eosinophils in the gastro-intestinaltract regardless of the anthelminthic protocol (Mooreet al., 1994). In our own work, we did not find any dif-ference in tissue eosinophil numbers between ponies par-asitized with S. vulgaris and non-parasitized ponies(Hubert et al., 2002). Although not reported, we didnot feel that there were areas of focally increased con-centrations of eosinophils in both groups of ponies.

This plethora of research is useful in illustrating toclinicians and scientists that there are specific factorswhich appear to incite eosinophil migration into tissues.The mechanisms of migration into the gastrointestinaltract are not as fully elucidated as those in dermalpathologies; however, it is evident that several importantfactors such as eotaxin, PKC and interleukins play arole in recruiting eosinophils.

The question still remains as to the aetiology of thelesions found by Archer et al. (2006). Parasite controland dietary management alone do not appear to be en-ough, and there is a paucity of research reports in the lit-erature on dietary elements that can stimulate eosinophil

Guest editorial / The Veterinary Journal 171 (2006) 389–392 391

infiltration leading to focal lesions, despite evidence thata local hypersensitivity (such as that caused by insectbites or parasites) may be initiating factors for eosino-phil migration and infiltration in the skin. In other spe-cies, there is evidence that an oral antigen challenge willincite an eosinophilic response; sensitized rats whenchallenged induced infiltration of eosinophils and thiswas also mediated by mast cells (Yang et al., 2001).The recruitment of eosinophils by mast cells has beenwell recorded: mast cell deficient rat models after sensi-tization have shown a significant lack of inflammatorycell (including eosinophil) recruitment and migration(Niwa et al., 1991). Until the precise mechanisms ofeosinophil stimulation, adherence and migration in thegastrointestinal tract are elucidated, control measuresor management practices and treatments cannot beaccurately instituted.

Jeremy HubertEquine Surgery, Equine Health Studies Program

School of Veterinary Medicine

Louisiana State University, Baton Rouge LA 70803, USAE-mail address: jhubert@vetmed.lsu.edu

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