resistance of nematode secretory products to cleavage by mast cell proteinases

Molecular and Biochemical Parasitology, 24 (1987) 137-145 137 Elsevier

MBP 00812

Resistance of nematode secretory products to cleavage by mast cell proteinases

Fakhar Qureshi 1, Malcolm W. Kennedy 1, Steve Gibson 2,*, Pu Z. Ye ~r* and Hugh R.P. Miller 2

I Wellcome Laboratories for Experimental Parasitology, University of Glasgow, Bearsden, Glasgow, U.K. and 2Moredun Research Institute, Edinburgh, U.K.

(Received 11 December 1986; accepted 5 February 1987)

Mast cell proteinases are known to be released in response to helminth infection, and are, in particular, characteristic of the immune rejection of intestinal nematode parasites. In intestinal mucosal tissue the relevant enzyme is rat mast cell proteinase II (RMCP II) and that of other tissues, including the lung, is rat mast cell proteinase I (RMCP I). The function of these enzymes is unknown, and we have examined the possibility that they directly attack the parasites. This was done by examining the cleavage patterns produced by both proteinases on 12~I-labelled excretory/secretory (ES) products of two intestinal nematodes (the infective larva of Ascaris suum, and adult Nippostrongylus brasiliensis) and one which has a pulmonary migration route (the third/fourth stage larva of A. suum). It was first established that all the labelled molecules were proteinaceous, by their susceptibility to broad spectrum proteinases, and that none were host components carried over into culture, by their antigenicity to infected hosts. All the nematode ES products were found to be remarkably resistant to.RMCP I and II, only one major component of the infective larva of A. suum being cleaved by both enzymes. This was not found to reflect a resistance to serine proteinases in general, since selected ES components were cleaved by chymotrypsin and trypsin. This would, therefore, argue that, if the enzymes play any direct role in the immune expulsion of nematodes, it is unlikely to be successfully directed at their secretions.

Key words: Rat mast cell proteinase I; Rat mast cell proteinase II; Proteinase; Nematode; Excretory/secretory antigen; Ascaris suum; Nippostrongylus brasiliensis

Introduction

Mast Cell Pro te inase Type II ( R M C P II) is released in subs tant ia l quant i t ies dur ing the expul-

Correspondence address: Dr. M.W. Kennedy, Wellcome Laboratories for Experimental Parasitology, University of Glasgow, Bearsden, Glasgow G61 1QH, Scotland, U.K.

*Present address: Pharmacia Ltd., Pharmacia House, Mid- summer Boulevard, Milton Keynes, U.K.

**Present address: Lanzhou Research Institute of Veterinary Medicine, Chinese Academy of Agricultural Sciences, Yan- chengbao, Lanzhou, Gansu Province, People's Republic of China.

Abbreviations': RMCP I and II, rat mast cell proteinases I and II; ES, excretory/secretory products; L2, second stage larva; L3/4, third/fourth stage (lung) larvae; SDS-PAGE, sodium dodecyl sulphate polyacrylamide gel electrophoresis; PBS, phosphate-buffered saline.

sive phase of intes t inal he lmin th infect ion in the rat, at which t ime enzyme is detectable in se rum and in mucosal tissue [1-3]. The role played by R M C P II in expuls ion, however , r emains myste- rious, and its possible funct ions include involve- men t in inf lammatory processes, tissue repair , or the l imi ta t ion of damage. Moreover , th rough its specificity for Type IV collagen [4], R M C P II might at tack the mucosal epi thel ial ba semen t m e m b r a n e and cont r ibu te to the villous a t rophy which is apparen t dur ing i m m u n e re jec t ion of several intes t inal nema todes [5]. Such effects on the integri ty of the ba semen t m e m b r a n e might also permi t the t rans loca t ion of p lasma components into the gut l umen , const i tu t ing the ' leak lesion ' [6] thought to cont r ibu te to worm expulsion.

We have chosen to examine ano the r possible funct ion of the enzyme, and that is to directly at-

0166-6851/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)

138

tack the parasite itself. This could be achieved by attack on, and affecting the functioning of, the two compartments accessible to immune effector mechanisms, the surface of the parasites and/or their secretions.

Here we examine the effects of RMCP II on the proteinaceous secretions of an infective larva (of Ascaris s u u m ) and an adult nematode (of Nip- postrongylus brasiliensis). These products, collected during short-term culture of living worms, are usually referred to as excretory/secretory (ES) materials and are taken to be representative of those released by the parasites in vivo. Culture supernatants were radio-iodinated, treated with enzyme, and the digestion products analysed by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). To ensure that all the components analysed were of parasite origin we first identified those which were antigenic to the infected host. We then confirmed that all the iodinated parasite products were proteinaceous, as defined by susceptibility to broad spectrum proteinases. Finally, the cleavage patterns produced by t reatment with the mast cell enzymes were examined.

We have three principal reasons for examining ES products. First, they are likely to be involved in nutrition of, and/or tissue penetration by, the parasites, and would be prime targets of an ef- fective immune system. Secondly, nematode ES materials are known to contain potent allergens [7-9] and their enzymatic elimination by the host might be of significance to the limitation of pa- thology. Thirdly, the surface glycoproteins of several nematodes are known to be shed in vitro [10-13], although there are exceptions [14[, and thereby contribute to ES. By examining in vitro- released materials, we can also acquire some in- formation on surface molecules, although it must be considered that their functions and biochemical properties may only be truly represented while on the surface.

Pulmonary mast cells of the rat produce a characteristic proteinase (RMCP I) which is anigeni- cally distinct from that of its mucosal homologue [15,16]. Our ignorance of the function of this enzyme is equal to that for RMCP II, and, given the number of nematode parasite species which migrate through the lungs, the same arguments ap-

ply to RMCP I as to RMCP II as regards its potential for direct attack on helminths. We have, therefore, also examined the activity of RMCP I on the secreted products of such a parasite, the lung-stage larvae of A. suum.

The secreted products of A. suum have been described previously [17], and are known to be specific to the developmental stage of the parasite, and to alter radically during its migration to the lungs. Those of adult N. brasiliensis have been described before [18], but the present report pre- sents their SDS-PAGE profiles for the first time, exploiting methods which discriminated between parasite-derived products and host components carried over into culture.

We find that the activity of RMCP I and II against A. suum larval ES is highly restricted, cleaving only one component of the ES of A. suum infective larvae, with no effect at all on that of A. suum lung-stage larvae and adult N. brasiliensis. This would, therefore, argue against a direct role for these enzymes in the immune expulsion of intestinal helminths by attack on their secretions.

Materials and Methods

Parasites and their in vitro-released parasite materials. A . suum infective (L2) larvae were obtained by artificial hatching of embryonated eggs, and the lung-stage (L3/4) forms from the lungs of infected rabbits infected seven days previously, and cultured in vitro following previous methods [17]. Both larval stages were cultured in the following protein-free medium: Medium 199, Dutch modification (Flow Laboratories, Irvine, product number 12-202-49), containing 1 mM sodium py- ruvate, 400 ng ml -~ glycyl-L-histidyl-L-lysine (Sigma G 1887), 16 ~M glutathione, 1 mg ml -~ glucose, 100 i.u. m1-1 penicillin, 100 p~g ml 1 streptomycin. The cultures were maintained at 37°C in air, the medium harvested weekly, filtered through a 0.22 ~m, low protein-retention filter (Millex GV SLGV025BS; Millipore S.A., Molsheim, France), and stored at -70°C until used. These cultures can be maintained for about 3 weeks, and the percentage of dead or moribund larvae kept below 5% by allowing the larvae to migrate through a cotton wool plug when num-

bers of immotile larvae approached this level. Adult N. brasiliensis worms were recovered

from the intestines of rats infected 7 or 8 days previously, washed six times in phosphate-buffered saline (PBS), and cultured for 4 h in PBS containing 1% glucose. Culture supernatant was filtered as above and stored at -20°C until used.

Antisera. Antiserum to A. suum was raised in rabbits (Sandy Lop; Hylyne Rabbits Ltd., North- wich, Cheshire) infected every 28 days with 3000 embryonated eggs, and serum collected 14 days following the last of four infections. Antiserum to N. brasiliensis was pooled from Lister and (Lister × BN)F~ rats which were bred at the Wellcome Laboratories for Experimental Parasitology from stock obtained from Olac (1976) Ltd., Bicester, Oxon., or Wistar rats which have been inbred in these laboratories from stock derived from Not- tingham University in 1979. These were infected twice with 4000 larvae at 28 day intervals, and bled 6 days after the second infection.

Radio-iodination. Dialysed culture medium was labelled with ~25I using the IODO-GEN method [19] as previously described [17]. The isotope was obtained from the West of Scotland Radio- nuclide Dispensary, and the iodination reagent bought from Pierce Chemical Co., Rockford, IL (product no. 28600).

Radio-immunoprecipitation. This was performed using a Staphylococcus aureus-based assay [20] in which iodinated culture medium was incubated with serum overnight at 4°C, and IgG immune complexes captured on heat-killed, formalin-fixed bacteria (Pansorbin Standardised 507861; Calbi- ochem-Behring, La Jolla, CA). The washed pel- lets were counted in a gamma counter and prepared for SDS-PAGE.

Enzymes and treatment of ES materials. All the enzymes used, apart from RMCP I and II, were purchased from Boehringer-Mannheim, Lewes, East Sussex, and included pronase (165921), trypsin (10981; EC 3.4.21.4), chymotrypsin (103306; EC 3.4.21.1), proteinase K (161519; EC 3.4.21.14), cathepsin C (200239; 3.4.14.1), plasmin (602370; EC 3.4.21.7), thrombin (Factor IIa, 602418; EC 3.4.21.5).

139

The rat mast cell proteinase RMCP I was prepared from rat peritoneal exudate cells, and RMCP II from mucosal cells of rats infected with N. brasiliensis, following previously described methods [16]. They were assayed using CBZ-L- tyrosine p-nitrophenyl ester, and stock solutions stored at -20°C at concentrations of 200 and 800 p,g ml 1 protein, respectively, and diluted for the digestion experiments as indicated.

Radio-iodinated (50-100 × 103 cpm) ES materials were reacted with various concentrations of enzyme in water or PBS-A (total volume 20 pA) from 30 min to 2.5 h at 37°C as indicated in the figure legends. The reactions were stopped by the addition of an equal quantity of electrophoresis sample buffer containing proteinase inhibitors, and immediate immersion in a boiling water bath for 10 min before loading onto the gel. The sample buffer contained 5% sodium dodecyl sulphate (BDH, Poole Dorset;10807), 5% Tris pH 7.5, 0.1 mM ethylenediaminetetraacetic acid, 1% glyc- erol, 0.1% Bromophenol Blue, and the following proteinase inhibitors: 0.2 mM phenylmethylsul- fonylfluoride (SIGMA P-7626) in isopropanol, 5 mM pepstatin A (SIGMA P-4265), 100 mM 1,10- phenanthroline (SIGMA P-9375), 5 mM leupep- tin (L-2884), 5 mM antipain (A-6271), Na-p-tosyl-L-lysine chloromethyl ketone (SIGMA T 7254) and N-tosyl-L-phenylalanine chloromethyl ketone (SIGMA T 4376).

The limited quantity of isotopically labelled material available meant that we were unable to determine precisely the protein concentration of the iodinated materials, but, for example, we cal- culated that, for A. suum L2 and L3/4 ES, the quantity of parasite protein reacted with each enzyme was of the order of 2 ng and 28 ng, respectively, per sample tube.

Polyacrylamide gel electrophoresis ( S D S-PA G E). This was carried out in a Pharmacia (Uppsala, Sweden) GE-2/4 LS slab gel apparatus according to the manufacturer's instructions. Gradient gels (5-25%) consisted of 120 mm separating gel and 15 mm stack gel, and were 0.7 mm thick. Sam- ples were prepared according to published methods [13], with 5% mercaptoethanol or 1 mg ml -l iodoacetamide for reducing and non-reducing conditions, respectively. Following electrophore-

140

sis, gels were fixed in 25% methanol, 10% acetic acid, then dried. Autoradiographs were exposed at -70°C with flashed Fuji RX X-ray film. Ap- parent molecular weights were estimated by ref- erence to standard marker proteins (Pharmacia 17-0446-01).

Results

SDS-PAGE profiles and antigenicity of nematode secretions. Before the susceptibility of ES products to mast cell proteinases could be examined, it had first to be established whether any host components contaminated the preparations. This did not apply to the products of the infective larvae of Ascaris, but did to the L3/4 stage of this parasite, and adult N. brasiliensis. One method of establishing this is to react radiolabelled ES with serum from infected animals and compare the SDS-PAGE profile of the immunoprecipitates with that of the whole preparation. This as- sumes that only non-self molecules will be precip-

Mr a b c d e Mr

6 7 . 0 4 3 . 0

3 0 . 0

20-1

1 4 . 4

9 4 . 0 6 7 . 0

4 3 . 0

3 0 . 0

2 0 . 1

1 4 . 4

Fig. 1. SDS-PAGE profile of the in vitro-released materials of adult N. brasiliensis. Culture fluid was labelled with 125I using the I OD O - G E N reagent, as described in Materials and Methods, and electrophoresed on 5-25% gradient gels under non-reducing (b,c) and reducing (d) conditions. The lateral diffusion of reducing agent from track d produces partial reduction of the molecules in track c, permitting the tracing of alterations in M~ brought about by reduction. Molecular weight standards (Pharmacia 17-0446-01) were run in tracks a and e, under non-reducing and reducing conditions, respectively. The nominal molecular weights of these standard proteins are given in kDa.

itated, and the reactivity of normal uninfected rats should control for antigens derived from com- mensal organisms. The in vitro-released products of each of the nematodes examined were, therefore, radio-iodinated, reacted with serum from infected animals, and the immunoprecipitates analysed by SDS-PAGE and autoradiography.

There are no published characterisations of adult N. brasiliensis ES using these methods, and we began by carrying out such an analysis, as presented in Fig. 1. This showed that, in common with the secretions of other nematodes [13,17,21] this material comprised a restricted set of molecules, here ranging in relative mobility from 11 to 165 kDa. The latter and an additional component of Mr > 300000 were apparent in over-exposed autoradiographs. A major allergen of M r 12000 has been described [8], but no band at this pre- cise position was discernable, although this might correspond to those of 11 or 16 kDa. Reducing conditions did not significantly alter the profile, although slight shifts in mobility of some of the components were observed. The relationships between the bands under reducing and non-reducing conditions were inferred by the illustrated experiment in which a reduced track was run alongside two non-reduced tracks. The lateral diffusion of the reducing agent during electrophoresis produced molecules in a transition state in the adjacent track, and generated diagonal bands connecting the positions taken up by a given component under the two conditions. For example, two major bands in the 26 kDa region were found to resolve as one when reduced. No new bands were apparent upon reduction, and there was, therefore, no evidence for covalent linkage between any of the N. brasiliensis ES molecules, although this point demands further analysis. A significant decrease in mobility of proteins in SDS-PAGE gels following reduction is a ¢~ommon finding, and is even exhibited by the marker proteins used here; e.g. the 67 kDa marker, bovine serum albumin.

When this ES was immunoprecipitated with serum from rats multiply infected with N. brasiliensis, all of the labelled molecules were precip- itated (Fig. 2) inferring that they were all of parasite origin. Immunoprecipitation appeared to affect the proportions in which the individual

components are represented, but it was clear that all the components were antigenic. The major bands corresponded in apparent molecular weight with those described for N. brasiliensis ES from SDS-PAGE gels stained for protein [18], but the present methods identified all the labelled molecules as being parasite-derived.

An important cautionary point is also illustrated in Fig. 2, and that is that inbred animals exhibited selective recognition patterns, unlike F 1 hybrids. In the use of immunoprecipitation to M r discriminate host and parasite components in other systems, therefore, it would seem wise to double check any non-recognition with pooled serum from outbred or hybrid animals.

The in vitro-released materials of the infective 6 0 , (L2) and lung stage (L3/4) larvae of A. suum have already been described elsewhere [17]. Briefly, they are heterogeneous, specific to the developmental stage of the parasite, and potently antigenic. Serum from infected rabbits precipitates all of the labelled components, with the exception of 14 a 67 kDa molecule in L3/4 ES. There are two

141

components of this molecular weight, one of which is a parasite antigen, and the other is serum albumin from the donor rabbits, which might be internalised by the worms, or bound to their surface, and subsequently shed in vitro.

Al l radio-iodinated ES components are sensitive to pronase. The binding of 125I to the components of the ES products by the method used infers that

a b c d e f g

Mr

1 6 5

9 4

57

2 6

16

11

a b c d Mr

2 2 5

Fig. 2. Antigenicity of the in vitro-released materials of adult N. brasiliensis. 125I-labelled ES (a) was reacted with serum from normal (b) or multiply infected inbred Wistar (c) or (Lister x BN)F~ rats (d). I m m u n e complexes were immobil ised on S. aureus bacteria, and analysed by S DS -P AGE under reducing conditions. The apparent molecular masses (Mr) of the major components are given in kDa, and are relative to the s tandard proteins used in Fig. 1.

118

67

41

2 5 . 5

14

a b c d e f g

Fig. 3. All the radioiodinated components of the ES of A. s u u m L2 and L3/4 larvae are proteinaceous. L2 ES (upper panel) and L3/4 ES (lower panel) were treated with the following concentrations of pronase: none (a), 2.5 ~g ml 1 ( b ) , 25 Ixg m1-1 (c), 250 ~g ml -j (d), 2.5 mg ml 1 (e)~ 12.5 mg ml 1 (f) and 25 mg ml J (g) pronase from S. griseus in water for 30 min at 37°C, and the reaction products analysed by SDS- PAGE.

142

Mr

2 2 5

118

6 7

41

2 5 . 5

a b c d e f g

14

Fig. 4. Selective cleavage of A . s u u m L3/4 ES molecules by trypsin. Radio- iodinated ES was treated with the following concentrations of trypsin: none (a), 10 ~g ml ~ (b), 50 ~xg ml (c), 100 p~g ml ~ (d), 500 txg ml-I (el, 1 mg m l i (f), and 5 mg ml J (g), in PBS for 2 h at 37°C, before analysis of digestion products by SDS-PAGE.

they are proteinaceous. To confirm this, all three products were treated with pronase, which is a mixture of endo- and exo-proteinases from Strep- tomyces griseus, or proteinase K, and the digestion products analysed by SDS-PAGE. Fig. 3 il- lustrates the result of t reatment of Ascaris L2 and L3/4 ES with increasing quantities of pronase, for a constant incubation time. This established the susceptibility of all products of larval A. suum without exception, and the same was found to apply to that of adult N. brasiliensis (not shown).

The activity of trypsin on the nematode ES products was also examined, and this showed selective degradation in each case. Its effect on As- caris ES is illustrated in Fig. 4.

Restricted action o f mast cell proteinases on nematode ES. The action of RMCP II on the three ES preparations was analysed as above. None of the components were found to be susceptible, with the apparent sole exception of the component of A. suum L2 ES which appears to spread from the 20-28 kDa regions of the SDS-PAGE gel (Fig. 5, upper panel). The rat mast cell proteinase of lung tissue, RMCP I, produced an

identical cleavage profile on L2 ES (not shown). The effect of this enzyme on the ES of the lung- stage Ascaris larvae is illustrated in Fig. 5, lower panel. A similar complete resistance was ob-

Mr

2 2 5

a b c d e f g

6 0

14

2 2 5

118

67

41

2 5 . 5

14

a b c d e f g

Fig. 5. Restricted cleavage of the components of A s c a r i s ES by Rat Mast Cell Proteinases. Upper panel: radioiodinated ES of A, s u u m infective larvae (L2) was incubated with the following concentrations of RMCP II: none (a), 0.4 p,g ml ~ (b), 2 ~ g m l ~ ( c ) , 4 1 s g m l ~ ( d ) , 2 0 p ~ g m l ~ ( e ) , 4 0 ~ t g m l ~(f), 200 ~g ml ] (g) in PBS-A for 2 h at 37°C, and the reaction products analysed by SDS-PAGE. Lower panel: lung-stage (L3/4) ES was treated with the following concentrat ions of RMCP I: none (a), 0.08 p~g ml L (b), 0.4 p,g ml i (c), 0.8 ~g ml ~ (d ) , 4 .01xgml ~ ( e ) , 1 0 ~ g m l L ( f ) , a n d 5 0 1 x g m l ~(g), in PBS for 2 h at 37°C, before analysis by SDS-PAGE.

143

Mr

9 4

5 7

2 6

16

11

a b c d e f g h i

Fig. 6. Resistance of N. brasiliensis ES components to cleavage by Rat Mast Cell Proteinases. ~25I-labelled ES from adult worms was subjected to the following concentrations of RMCP II: none (a), 0.2 Ixg ml -l (b), 0.4 ~g m1-1 (c), 2.0 p.g ml -t (d), 4/a.gml i ( e ) , 2 0 1 s g m l l ( f ) , 4 0 p . g m l ~(g) ,200p.gml ~(h), 400 ~g ml ~ (i), in PBS for 2.5 h at 37°C, before analysis by SDS-PAGE.

served for the secretions of adult N. brasiliensis, the digestion profile of which is illustrated in Fig. 6, in which the mucosal mast cell enzyme, RMCP II, was used.

Discussion

While the stimulus for the immune expulsion of intestinal nematode parasites, such as N. brasiliensis and Trichinella spiralis, is clearly a specific immunological event [22,23], it is equally clear that non-specific effector mechanisms are involved [23,24]. A thymus-dependent mucosal mastocytosis is a characteristic of the expulsion phase of such infections [6,25,26], and the release of RMCP II suggests a central role for the mucosal mast cell in the induction or control of the inflammatory response or in a direct effect on the parasite itself.

It is becoming apparent that mast cell proteinases belong to a family of proteolytic enzymes which are associated with immune effector mechanisms. These include complement Factor D [27,28], and proteinases synthesised by cytotoxic T and natural killer cells [29-32]. These enzymes can be grouped together by virtue of sequence homology and unusual structural features which

are not found in other serine proteinases [28,29]. Some of these unifying characteristics are associated with the active site, and these might play some role in the greater substrate specificity of these enzymes [30]. Such stringency might argue in favour of their involvement in tightly regulated lytic pathways [33], but not for a general role in direct attack of pathogens whose products are likely to be diverse. On the other hand, too little is known about the function, biochemistry and structure of nematode secretions for the possibility to be ignored. Mucosal mast cell proteinases could, for example, be aimed at essential and conserved features of these products.

With respect to attack on the helminth surface, it has been suggested that complement C3, C4, and C5, might act as substrates for RMCP-like enzymes [28,34,35] and the latter could thereby be involved in attack on intestinal helminths. While the unusual nature of the surface lipid layer of nematodes [36] might mitigate against direct complement effects, complement binding to the nematode surface is known to mediate cellular attack and attrition of such parasites [37].

Our present observations on nematode ES products revealed a remarkable degree of resistance to cleavage by mast cell proteinases. Al- though we do not present the data here, we also found this to be true of several other proteinases associated with inflammatory reactions, for example, cathepsin, thrombin, plasmin and elastase, and this raises the question of whether nematode secretions are adapted to resist the action of such enzymes. On the other hand, we did find that certain of the ES components were susceptible to other serine proteinases, such as trypsin and chymotrypsin, but could identify no mam- malian enzyme able to degrade all of the ES molecules in the preparations used.

An important implication of the differential proteinase susceptibility of the ES components of Ascaris to cleavage by several proteinases, including trypsin (Fig. 4), is that it testifies to their individuality; differential susceptibility is not con- sistent with degradation products of the larger molecules, or components with shared homolo- gous peptide sequences. We have also found this with a range of site-restricted proteinases, such as S. aureus V8 proteinase, which we will present

144

elsewhere. This conclusion is supported by the differential recognition of these antigens by antibody from infected inbred animals [17,38].

Only the 20-28 kDa component of A. suum L2 ES was susceptible to RMCP I and II, and SDS- PAGE analysis suggested that digestion of this molecule was extensive, since no cleavage products were discernible. This might imply that the molecule contained repeats of the peptide sequence at which these enzymes cleave, resulting in the generation of multiple fragments of too small a size to resolve within the gel.

One possible explanation for resistance to proteolytic cleavage is that the various ES preparations contained proteinase inhibitors or enzymes capable of inactivating proteinases. Our prelimi- nary results have shown that while Ascaris ES contains elastase and trypsin-like activity, there is no evidence of inhibitors of RMCP I and II (D.P. Knox and M.W. Kennedy, unpublished). More- over, our present finding that nematode ES components were selectively susceptible to trypsin and both mast cell proteinases would not be consist- ent with protection of resistant molecules by an- tagonists. Furthermore, the isotope methods em- ployed here allowed the exposure of very small quantities of nematode products to molar ex- cesses of enzymes, probably greatly overwhelm- ing any putative inhibitory activity. A detailed search for enzymes and inhibitors in materials released by parasites is clearly warranted in view of

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the potential importance of such activities in the host-parasite relationship, and we are currently examining this point in greater detail.

The most likely explanation for the resistance of ES components to digestion by the mast cell enzymes is that the molecules concerned do not contain the peptide sequence at which the proteinases cleave. Alternatively, the recognition sequence could be protected in some way, such as by carbohydrate residues (the ES components of Ascaris are extensively glycosylated (F. Qureshi and M.W. Kennedy, unpublished observations) as are those of the related nematode, Toxocara-

canis [39]), or by the conformation of the molecules. This could be tested by denaturing and/or deglycosylating ES before enzyme treatment, but it would remain to explain the action of other serine proteinases, for example trypsin, on the native molecules.

Acknowledgements

We thank The Wellcome Trust, the Medical Research Council and the Scottish Home and Health Department for support, Fiona Seath, Anne McIntosh and David McLaughlin for their expert technical assistance and Peter Rickus for photographic work. We are also indebted to George Newlands and Dr. David Haig for N. brasiliensis ES, and to Elizabeth Hall for antiserum to this parasite.

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resistance of nematode secretory products to cleavage by mast cell proteinases

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