Studies of stage-specific immunity against Trichostrongylus colubriformis in sheep: immunization by normal and truncated infections

The potential for vaccines against scour worms of small ruminants

This review addresses the research landscape regarding vaccines against scour worms, particularly Trichostrongylus spp. and Teladorsagia circumcincta. The inability of past research to deliver scour-worm vaccines with reliable and reproducible efficacy has been due in part to gaps in knowledge concerning: (i) host-parasite interactions leading to development of type-2 immunity, (ii) definition of an optimal suite of parasite antigens, and (iii) rational formulation and administration to induce protective immunity against gastrointestinal nematodes (GIN) at the site of infestation. Recent 'omics' developments enable more systematic analyses. GIN genomes are reaching completion, facilitating "reverse vaccinology" approaches that have been used successfully for the Rhipicephalus australis vaccine for cattle tick, while methods for gene silencing and editing in GIN enable identification and validation of potential vaccine antigens. We envisage that any efficacious scour worm vaccine(s) would be adopted similarly to "Barbervax™" within integrated parasite management schemes. Vaccines would therefore effectively parallel the use of resistant animals, and reduce the frequency of drenching and pasture contamination. These aspects of integration, efficacy and operation require updated models and validation in the field. The conclusion of this review outlines an approach to facilitate an integrated research program.

Novel immunomic technologies for schistosome vaccine development

Schistosomiasis remains one of the most common human helminthiases, despite the availability of an effective drug against the causative parasites. Drug treatment programmes have several limitations, and it is likely that a vaccine is required for effective control. While decades of vaccine development have seen the discovery and testing of several candidate antigens, none have shown consistent and acceptable high levels of protection. The migrating larval stages are susceptible to immunity, however few larval-specific antigens have been discovered. Therefore, there is a need to identify novel larval-specific antigens, which may prove to be more efficacious than existing targets. Immunomics, a relatively new field developed to cope with the recent large influx of biological information, holds promise for the discovery of vaccine targets, and this review highlights some immunomic approaches to schistosome vaccine development. Firstly, a method to focus on the immune response elicited by the important and vulnerable larval stage is described, which allows a targeted study of the immunome at different tissue sites. Then, two high-throughput arrays are discussed for the identification of protein and carbohydrate antigens. It is anticipated that these approaches will progress vaccine development against the schistosomes, as well as other parasites.

The gastrointestinal nematode Trichostrongylus colubriformis down-regulates immune gene expression in migratory cells in afferent lymph

Background

Gastrointestinal nematode (GIN) infections are the predominant cause of economic losses in sheep. Infections are controlled almost exclusively by the use of anthelmintics which has lead to the selection of drug resistant nematode strains. An alternative control approach would be the induction of protective immunity to these parasites. This study exploits an ovine microarray biased towards immune genes, an artificially induced immunity model and the use of pseudo-afferent lymphatic cannulation to sample immune cells draining from the intestine, to investigate possible mechanisms involved in the development of immunity.

Results

During the development of immunity to, and a subsequent challenge infection with Trichostrongylus colubriformis, the transcript levels of 2603 genes of cells trafficking in afferent intestinal lymph were significantly modulated (P < 0.05). Of these, 188 genes were modulated more than 1.3-fold and involved in immune function. Overall, there was a clear trend for down-regulation of many genes involved in immune functions including antigen presentation, caveolar-mediated endocytosis and protein ubiquitination. The transcript levels of TNF receptor associated factor 5 (TRAF5), hemopexin (HPX), cysteine dioxygenase (CDO1), the major histocompatability complex Class II protein (HLA-DMA), interleukin-18 binding protein (IL-18BP), ephrin A1 (EFNA1) and selenoprotein S (SELS) were modulated to the greatest degree.

Conclusions

This report describes gene expression profiles of afferent lymph cells in sheep developing immunity to nematode infection. Results presented show a global down-regulation of the expression of immune genes which may be reflective of the natural temporal response to nematode infections in livestock.

Relationships between immune indicators of parasitic gastroenteritis, nematode burdens and faecal dry matter in sheep

Immune-mediated scouring in sheep is a mucosal hypersensitivity response caused by ingesting infective parasite larvae. In this experiment we tested the hypothesis that levels of parasite-specific antibodies and also interleukin-5 (IL-5) would be negatively correlated with worm numbers, but also faecal dry matter (FDM), in parasite-resistant Merino sheep. Forty Merino rams were challenged with Trichostrongylus colubriformis and Teladorsagia circumcincta larvae every day for 6 weeks, after which they were euthanised and total worm burdens determined. Faecal samples were taken for measurement of worm egg counts and FDM. Serum was collected from the rams before and after the larval challenge commenced, and levels of ovine immunoglobulin light chain, IgG, IgM, IgA and IgE specific for T. colubriformis and T. circumcincta as well as IL-5 were determined by sandwich enzyme-linked immunosorbent assay. IL-5 and all serum antibodies apart from T. colubriformis-specific light chain were significantly increased by the larval challenge. However, none of the antibodies, or IL-5, was correlated with FDM. Negative correlations were observed between the number of adult T. circumcincta and antibody levels; however, there was little relationship between antibodies and numbers of T. colubriformis. It was concluded that serum antibody levels are a poor indicator of the susceptibility of sheep to immune-mediated scouring. Because of these results, sheep breeders should continue to select for low worm egg count and focus on phenotypic indicators of scouring, such as dags, as a means to reduce diarrhoea.

Increased expression of interleukin-5 (IL-5), IL-13, and tumor necrosis factor alpha genes in intestinal lymph cells of sheep selected for enhanced resistance to nematodes during infection with Trichostrongylus colubriformis

Cytokine gene expression in cells migrating in afferent and efferent intestinal lymph was monitored for extended time periods in individual sheep experimentally infected with the nematode Trichostrongylus colubriformis. Animals from stable selection lines with increased levels of either genetic resistance (R) or susceptibility (S) to nematode infection were used. Genes for interleukin-5 (IL-5), IL-13, and tumor necrosis factor alpha (TNF-alpha), but not for IL-4, IL-10, or gamma interferon (IFN-gamma), were consistently expressed at higher levels in both afferent and efferent lymph cells of R sheep than in S sheep. However, only minor differences were observed in the surface phenotypes and antigenic and mitogenic responsiveness of cells in intestinal lymph between animals from the two selection lines. The IL-4 and IL-10 genes were expressed at higher levels in afferent lymph cells than in efferent lymph cells throughout the course of the nematode infection in animals of both genotypes, while the proinflammatory TNF-alpha gene was relatively highly expressed in both lymph types. These relationships notwithstanding, expression of the IL-10 and TNF-alpha genes declined significantly in afferent lymph cells but not in efferent lymph cells during infection. Collectively, the results showed that R-line sheep developed a strong polarization toward a Th2-type cytokine profile in immune cells migrating in lymph from sites where the immune response to nematodes was initiated, although the IFN-gamma gene was also expressed at moderate levels. Genes or alleles that predispose an animal to develop this type of response appear to have segregated with the R selection line and may contribute to the increased resistance of these animals.

Long-term collection and characterization of afferent lymph from the ovine small intestine

Reliable methods for long-term collection of afferent lymph draining from the small intestine of sheep are described and validated. The procedure was used successfully in normal sheep, in animals infected experimentally with the parasitic intestinal nematode Trichostrongylus colubriformis and in animals infected naturally with Mycobacterium avium subsp. paratuberculosis, the causative agent of Johne's disease. Our approach enabled afferent lymph draining from the small intestine to be collected continuously for up to 4 months, without any detrimental effects on the animals. Based on cytokine gene expression profiles of afferent intestinal lymph cells, the two infections induced contrasting regional immune responses, namely, Th2-type immunity in the case of T. colubriformis infection and Th1-type immunity in natural cases of Johne's disease. Some immune parameters differed markedly between the two disease models, highlighting the potential value of this approach to gain real-time insights into distinctive host-pathogen interactions as they occur in vivo within the regional immune system of the gastrointestinal tract.

Individual variation and effect of priming dose level on establishment, growth and fecundity ofCooperia oncophorain re-infected calves

We investigated the effect of bovine anamnestic immunity on a range of parasitological variables. To this end, calves were primed with a single oral dose of 30 000 or 100 000 infective larvae (L3) ofCooperia oncophora, drenched with anthelmintic, maintained worm free in the following 2·5 months and subsequently re-infected with 100 000 L3. Parasitological profiles of low, intermediate, and high responders were compared. The reduction in establishment of the worms was shown by a lower worm burden and increased percentage of fourth-stage (L4) larvae. Worm length and fecundity were similarly reduced by both priming doses but, the speed by which the effect occurred differed between animals primed with 30 000 or 100 000 L3. The difference in establishment between the responder types demonstrates that the ability of intermediate responders to mount a more effective and faster immune response compared to low responders is sustained after secondary infection.

Expression of acquired immunity to a local isolate of Haemonchus contortus by the Nigerian West African Dwarf goat

The capacity of young Nigerian West African Dwarf (WAD) goats to express good acquired immunity to their native geographic strain of Haemonchus contortus and the correlates of this responsiveness were studied in a laboratory experiment involving forty 7-8 month old kids. A primary immunising infection with 2000 L3 (equivalent to 260-450 L3/kg body weight) with or without challenge on D42 with 2000 L3 resulted in a mild chronic infection with a pre-patent period of 18-20 days and little or no reduction in worm burden between D14 and D56. In contrast, another group (D) of kids, whose immunising infection had been truncated with fenbendazole on D35 and later received similar challenge infection, developed good protection against challenge. Thus, worm burdens were largest in group E (challenge control), larger in group C (primary+challenge) and least in group D. Of the measures of infection used, namely faecal worm egg counts (FECs), circulating eosinophil (EOS) responses, packed cell volume (PCV) and body weight, FEC and EOS responses exhibited marked individual variability, but only FEC (geometric mean of transformed counts) and PCV showed strong correlation with worm burden. There was also a significant negative correlation between FEC and PCV. The size of inoculum used was well tolerated by the kids, as it induced only mild changes in PCV in some goats and no effect at all on body weights. This suggests that the WAD goat may possess a good measure of resistance to the pathogenic effects of its native strain of H. contortus. The wide individual variability in FEC and its strong relationships to worm burden and PCV are pointers to its likely genetic basis. There are, therefore, good prospects for further studies to identify H. contortus resistant genotypes among the WAD goat population.

The immunobiology of gastrointestinal nematode infections in ruminants

The major gastrointestinal nematode parasites of ruminants all belong to the Order Strongylida and the family Trichostrongyloidea. Despite this close evolutionary relationship, distinct differences exist in the microenvironmental niches occupied by the developmental stages of the various parasites, which may account for the variable susceptibility of the different parasite species to the immune effector mechanisms generated by the host. In addition, different manifestations of resistance have been observed against the adult and larval stages of the same parasite species, and even against the same parasite stage. In particular, both rapid and delayed rejection of infective larval stages of gastrointestinal nematode parasites has been documented. This review will give an overview of the various manifestations of resistance to gastrointestinal nematode parasites of ruminants, as well as the immune mechanisms and antigens associated with the generation of immunity by the ruminant hosts to these parasites. In addition, a working model is provided aimed at reconciling most of the present knowledge on the different immune responses generated during infection with the various parasite rejection profiles. Extrapolation of these results to field conditions will need to take into account the variability imposed by seasonal changes and management practices, as well as the individual variability in immune responsiveness present in outbred animal populations.

Studies on the role of mucus and mucosal hypersensitivity reactions during rejection of Trichostrongylus colubriformis from the intestine of immune sheep using an experimental challenge model

Nematode-naive sheep and sheep immunised by truncated infections with Trichostrongylus colubriformis were fitted with intestinal cannulae to allow administration of challenge infection and collection of intestinal fluids. Sheep were slaughtered at various times after challenge and the distribution of larvae along the small intestine was determined. Results showed that immune sheep had significantly fewer larvae in their intestines and that some sheep could expel the challenge infection within 2 h. Mucus samples from immune sheep contained increased parasite-specific antibody, histamine and anti-parasite activity as measured by larval migration inhibition assay. Higher levels of antibody and histamine were seen in intestinal fluids of immune sheep after challenge. Immunisation of sheep by truncated infections stimulated serum IgE and resulted in significantly higher numbers of IgE-positive cells in gut tissue sections before challenge and at 2 h and 24 h after challenge. Immune sheep also had greater numbers of mucosal mast cells and globule leucocytes after challenge, compared with naive sheep. When challenge larvae were mixed with mucus from immune sheep and infused back into naive recipient sheep, there was a distinct displacement of the larval population towards the distal part of the intestine, compared with the profile of larval establishment after infusion with mucus from naive sheep. These results are further evidence for an immediate hypersensitivity reaction in the intestine of immune sheep, where challenge larvae are expelled within 2 h and confirm the direct anti-larval properties of mucus. The cannulated-sheep challenge model described here will be a useful tool to unravel the mechanism of larval rejection from immune sheep and could lead to novel therapies.

The effect of continuous drug exposure on the immune response to Trichostrongylus colubriformis in sheep

The effect of albendazole (ABZ)-capsule (CRC) administration on parasite establishment and immunity to ABZ-resistant (RES) and -susceptible (SUS) T. colubriformis was measured in Romney lambs. During 12 weeks of twice-weekly dosing with 3000 parasite larvae (L3), eggs were observed in faeces from CRC-treated and untreated lambs given RES L3, but not CRC-treated lambs given SUS L3. Following the period of trickle challenge all lambs were drenched and, 1 week later, dosed with 20000 SUS L3. Resulting worm burdens were higher in control lambs than in those previously treated with CRCs and challenged with SUS, which in turn were higher than those in the CRC-treated or -untreated lambs previously challenged with RES L3. During the period of trickle challenge, the number of peripheral eosinophils and titres of anti-L3 and anti-adult antibody were raised only in those groups given RES L3. There was no effect of CRC administration. Following drench and challenge, antibody titres and eosinophil numbers increased in the control animals but not in those groups which had received previous trickle infection. The results demonstrate that the larval challenge alone resulted in incomplete though substantial protection against subsequent parasite challenge. The use of CRCs may potentially impact on subsequent animal performance and selection for anthelmintic resistance through a reduced level of immunity.

The effect of truncated infections with Ostertagia ostertagi on the development of acquired resistance in calves

The relative contribution of the third (L3), fourth (L4) and adult stages of Ostertagia ostertagi to the development of immunity was assessed in calves which were either continuously infected during 21 weeks or subjected to infections truncated by anthelmintic treatment at the L3 or L4 stage. A fourth group remained uninfected (control group). Faecal samples and blood samples were collected weekly for faecal egg counts and determination of pepsinogen and antibody levels. Only the continuously infected animals showed positive egg counts, which fell towards the end of the primary infection period. Pepsinogen and antibody levels remained high in the continuously infected group until the end of the primary infection period. At that time, they were significantly higher compared to the control calves, with intermediate values in the truncated infection groups. After the 21 weeks primary infection period all animals were dewormed. To evaluate the protection provided by the different immunisation protocols, all animals were challenged 1 week later with 156000 Ostertagia L3, spread over 12 consecutive days. The marked reduction in egg counts following challenge infection indicated a certain degree of immunity in the continuously infected calves, which was confirmed at necropsy by the reduced worm burdens, the high percentage of inhibited early L4 larvae, the reduced size of the adult worms and the higher numbers of mucosal mast cells in this group. Numbers of globule leucocytes and eosinophils were not significantly different from the control group. Infections truncated by anthelmintic treatment elicited poor development of immunity as shown by the egg output after the challenge infection and the percentages of arrested larvae and the lengths of adult worms which were intermediate to those of the continuously infected calves and control animals.

Prospects for the immunological control of ruminant gastrointestinal nematodes: natural immunity, can it be harnessed?

Current understanding of the mucosal immune response to nematode parasites is briefly reviewed with emphasis on the role of the cytokines interleukins-4 and -12 and gamma interferon (IFN gamma) in the development of T-helper responses in rodents. Data from laboratory animals and ruminants indicate that the events associated with a T-helper 2 (Th2) cell response, notably IgE synthesis, eosinophilia and mucosal mastocytosis are protective. Evidence that effector mechanisms may vary for different parasite species is considered. Current gaps in understanding such as the location in the gut and mechanism of antigen processing and presentation as well as the relative contribution of non-immunological effector responses, such as gut motility and mucus secretion, to worm expulsion are discussed.

The effects of four neuropeptides on the degranulation of mucosal mast cells from sheep

Isolated mucosal mast cells (MMC) were used to examine the ability of four neuropeptides, substance P, vasoactive intestinal peptide, beta-endorphin and somatostatin, to release mediators in the presence or absence of parasite antigen. None of the neuropeptides induced the release of sheep mast cell protease (SMCP) or histamine from MMC of helminth-immune sheep in the absence of parasite antigen. Incubation of immune MMC with 100 and 1.0 microgram/mL parasite antigen induced 32.1 and 15.5% specific SMCP release, respectively. While the neuropeptides did not augment SMCP release at 100 micrograms/mL parasite antigen, significant enhancement (40-98%) of SMCP release at 1 microgram/mL antigen was obtained by each neuropeptide at concentrations from 10(-8) to 10(-12) mol/L. The results provide additional support for modulation of MMC degranulation by neural activity in sheep and, to our knowledge, this is the first demonstration that the threshold antigen concentration for allergic responses may also be lowered by neuropeptides to render the reaction more sensitive to antigen.

Drug-abbreviated infections and development of immunity against Trichostrongylus colubriformis in sheep

A protective immune response without liveweight loss can be induced in sheep against T. colubriformis but results depend on the anthelmintic used and duration of immunizing infections. More than 90% protection was achieved in sheep immunized by three 15- or 7-day oxfendazole abbreviated infections or three 21-day nonabbreviated infections. Only 41% protection was induced by 3-day oxfendazole abbreviated infections. Significantly higher worm burden and faecal egg counts were present after challenge in sheep immunized by 7-day levamizole abbreviated infections compared to 7-day oxfendazole abbreviated infection. Liveweight gains of sheep immunized by 15- and 7-day abbreviated infections were not significantly different than non infected controls. Liveweight loss seemed to be associated with high activity of mucus peroxidase and high numbers of eosinophils in the intestinal lumen. High parasite numbers seemed to be associated with low activity of alkaline phosphatase in mucus. Mucus peroxidase, arylsulphatase, larval migration inhibition of mucus, mucus or serum antibody against L3 excretory/secretory antigen or somatic L3, L4 and adult antigen were not associated with protection.

Changes in inflammatory mediators and larval inhibitory activity in intestinal contents and mucus during primary and challenge infections of sheep with Trichostrongylus colubriformis

Helminthologically naive merino sheep were given either a single infection of 30,000 or a trickle infection of 6000 Trichostrongylus colubriformis infective larvae (TcL3) per week. Faecal egg counts started to fall after 8 weeks in the single infection and after 11 weeks in the trickle infection. Small intestinal contents were collected from indwelling intestinal fistulae over the next 14 weeks. Concentrations of sheep mast cell protease (SMCP) in these contents increased to highest levels 9-11 weeks and 6-10 weeks after the single infection and from the start of the trickle infection, respectively. Similarly, peptidyl leukotriene (PLT) concentrations were highest at 6 weeks and at 6-9 and 13 weeks, respectively. Histamine concentrations increased slightly after both infections to peak values at 7 weeks and 9 weeks, respectively. Inhibition of migration of larvae in vitro was increased in contents sampled at 8 weeks after the single infection and after 6-10 weeks of the trickle infection. Another 2 groups of sheep were immunised by repeated infections with TcL3. Gut contents from 1 group sampled immediately before and after challenge with 30,000 TcL3 at 0 and 18 days had increased levels of larval migration inhibitory (LMI) activity throughout the 35 day period, especially 7-14 days after challenge (DAC). The mediators SMCP increased significantly 5-7 DAC while PLT increased 7-14 DAC. In the second group of immunised sheep, levels of SMCP and PLT increased rapidly within 1 DAC and further increased 3-14 DAC.(ABSTRACT TRUNCATED AT 250 WORDS)

Rejection of heterologous nematodes by sheep immunized with larval or adult Trichostrongylus colubriformis

Sheep immunized by truncated larval infections or by the adoptive transfer of adult Trichostrongylus colubriformis were subsequently challenged with single infections of T. colubriformis, Nematodirus spathiger, Haemonchus contortus or Ostertagia circumcincta or combinations of the parasites. Sheep vaccinated with larval infections were > 90% protected by 4 days after challenge (DAC) against T. colubriformis L3 given in a single or combined infection, whereas no significant protection was exhibited against a single-species infection with the unrelated nematodes N. spathiger or O. circumcincta. Sheep challenged with T. colubriformis together with N. spathiger or O. circumcincta were equally protected against both intestinal nematodes, but O. circumcincta was not affected. Sheep immunized with adult T. colubriformis and challenged with a combined infection of T. colubriformis, N. spathiger and H. contortus expelled around 40 and 80% of the intestinal parasites by 4 and 11 DAC, respectively, but showed no protection against the abomasal parasite, H. contortus. The results confirm the previous findings on 'self-cure' and the non-specific rejection of unrelated parasites living in the same or downstream niches in the gut when the nematode used to induce immunity is included in the challenge infection. The results also indicate that even though L3 antigens effectively elicited the non-specific rejection, antigens produced by L4 and later stages could also induce rejection of unrelated worms in the second week after infection.

Production of vaccines against gastrointestinal nematodes of livestock

Three international collaborations involving Australian research scientists are currently developing vaccines against Haemonchus, Trichostrongylus and Ostertagia parasites using recombinant DNA technology. The variety of protective antigens identified can be classified as 'conventional' (stimulate naturally acquired immunity) or 'novel'/'convert'/'concealed' (protective once immunity is induced by vaccination). To date, the most gratifying progress has resulted in 60-90% protection against Haemonchus and other blood-sucking parasites (e.g. ticks) using novel antigens, where high titres of serum antibody ingested by feeding worms leads to their demise. A great deal of research effort is unravelling the complexity of naturally acquired immunity so that conventional antigens, which may be the principal means of removing 'mucosal-browsing' parasites, may be formulated and delivered to achieve optimal efficacy. This work reveals that to remove early stages of parasites before they take up residence, deliberate induction of hypersensitivity responses akin to asthma, may be a desirable goal for vaccines and that the two models have much in common.

The presence of an early L4 larvae population in relation to the immune response of calves against Ostertagia ostertagi

The influence of different levels of infection with Ostertagia ostertagi on the development of a protective immune response in calves was investigated. Four groups of calves were infected with either 5000 (Group A), 10,000 (Group B), 20,000 (Group C) or 40,000 (Group D) infective larvae (O. ostertagi L3) weekly until treatment began. Group E functioned as controls. All animals were treated with oxfendazole (9 mg ml-1) at Week 17 (Groups A, B and E) or Week 18 (Groups C and D). Sixteen days post-treatment all calves received a challenge infection of 150,000 O. ostertagi L3 spread over 10 consecutive days. Faeces and blood were collected weekly for egg counts and to assess levels of pepsinogen, gastrin and IgG1 and IgG2 Ostertagia antibodies. All calves were necropsied 31 days post-challenge for worm counts. Egg counts and pepsinogen levels were proportional to the infection level during the first few weeks of the experiment. Only in the high-dosed Group D was a gastrin response evoked. Ostertagia IgG1 antibodies increased between Day 25 and Day 95, and in the non-infected control group an antibody rise was observed from Day 67 onwards. All measured parameters except Ostertagia antibodies showed a gradual decrease from Day 70 until the day of treatment. At necropsy there was no significant difference between the groups in the total worm populations. Only the composition of the worm populations differed, with 35% early L4 (EL4) larvae in the previously infected Groups A, B, C and D and only 5% in the control Group E. The results indicate a slow immune response against O. ostertagi in cattle and question the possible role of the EL4 stage in developing immunity.

Studies of stage-specific immunity against Trichostrongylus colubriformis in sheep: immunization with adult parasites

Merino sheep immunized by the adoptive transfer of adult T. colubriformis for 8 weeks were significantly protected against a challenge infection of 20,000 larvae. Two additional groups of sheep received a primary infection of 9000 adult worms which were allowed to persist for 14 weeks before one group was drenched. When both groups were challenged 10 days later with 30,000 larvae, serial necropsies of these and naive sheep revealed that worm rejection did not occur until 7-10 days after challenge. By comparison with the rapid rejection of larval challenges from sheep immunized with normal primary infections, the results suggest that the antigens which elicited rejection in these experiments are stage-specific and were only present or synthesized in sufficient quantities when parasites had developed for 1 week.

A serial study of rejection of Trichostrongylus colubriformis by immune sheep

Host responses and the rejection of worms were measured at intervals following challenge of immune and susceptible sheep with T. colubriformis infective larvae. Immune sheep rejected most of their larvae within the first day after infection. This early rejection was associated with local appearance of globule leucocytes and increased concentration of T. colubriformis-specific IgG1 and IgG2 in intestinal mucus. Rejection of the remaining worms occurred between 3 and 14 days after infection and was associated with increased T. colubriformis-specific IgA and IgG2 in intestinal mucus, local T cell infiltration, activation, differentiation and epithelial necrosis. Local T cell changes included expansion of the T19- gamma delta+ populations in the villous lamina propria and epithelium.