Efferent intestinal lymph protein responses in nematode-resistant, -resilient and -susceptible lambs under challenge with Trichostrongylus colubriformis

The mechanisms underlying resistance to challenge by gastrointestinal nematode parasites in sheep are complex. Using DIGE, we profiled ovine lymph proteins in lambs with host resistance (R), resilience (Ri) or susceptibility (S) to a daily trickle challenge with the nematode Trichostrongylus colubriformis. Efferent intestinal lymph was collected prior to infection (day 1) and on days 5 and 10 post-infection. Eight proteins identified by LC-MS/MS, showed differences relating to host genotype. Of these, Serpin A3-3 and Serpin A3-7 have not been reported previously in the lymph proteome. Three acute phase proteins showed significant differences relating to interactions between breeding line and parasite challenge, including complement C3β, C3α and haptoglobin (Hp) β. In the R lambs C3α was significantly up regulated (P<0.05) on day 10, while in the Ri lambs Hp β was significantly down regulated (P<0.05). In the S lambs, levels of C3β were up regulated and levels of Hp β down regulated (both P<0.05) on day 10. Hence we demonstrate that acute phase inflammation proteins contribute to differences in the innate immune response of sheep to challenge by T. colubriformis. The findings may lead to the development of new approaches to combat nematode infestations in sheep production systems.

BIOLOGICAL SIGNIFICANCE

Breeding lines of sheep with resistance (R), resilience (Ri) or susceptibility (S) to nematode infections provide an experimental model to examine the biological mechanisms underlying the ability of some sheep to expel worms and remain healthy without the use of an anthelmintic. Using proteomics we identified differences in the expression of acute phase lymph proteins in the R, Ri and S lambs. The results will assist the development of alternative control strategies to manage nematode infections in livestock.

C6, a new monoclonal antibody, reacts with the follicle-associated epithelium of calf ileal Peyer's patches

The follicle-associated epithelium (FAE) of Peyer's patches (PPs) contains M cells that are important for reducing mucosal immune responses by transporting antigens into the underlying lymphoid tissue. We generated a monoclonal antibody (C6) that reacted with the FAE of calf ileal PPs, and analyzed the characteristics of C6 using immunohistochemistry and Western blotting. FAE of the ileal PP was stained with C6 during both late fetal developmental and postnatal stages. Neither the villous epithelial cell nor intestinal crypt basal cells were stained at any developmental stage. During the prenatal stages, FAE of the jejunal PP was C6-negative. However, a few C6-positive cells were distributed diffusely in some FAE of the jejunal PPs during the postnatal stages. The protein molecular weight of the antigen recognized by C6 was approximately 45 kDa. These data show that C6 is useful for identifying the FAE in ileal PPs and further suggest that differentiation of the FAE in these areas is independent of external antigens.

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.

Immune mechanisms of resistance to gastrointestinal nematode infections in sheep

Infections with gastrointestinal nematode parasites are a major problem for the sheep industry in Australia and New Zealand and have been the subject of intensive research to define mechanisms of resistance. The ability to take continuous biopsy samples of infected organs and cannulate both afferent and efferent lymphatics of draining lymph nodes has been particularly useful in illuminating the kinetics of immune responses at the site of infection. Distinct localized immune responses were shown to occur within and between sheep breeds at different sensitization regimes, as well as at different developmental stages of the parasite within the host. Using localized antibodies derived from mucus and lymph nodes, two major antigens have been identified on the infective L3 stage, which may be responsible for inducing protection and have potential as vaccine targets. Recent advances in sheep genomics also offer the potential of gaining further insight into the underlying genetics of resistance to nematode infections.

Intraspecific epitopic variation in a carbohydrate antigen exposed on the surface of Trichostrongylus colubriformis infective L3 larvae

The carbohydrate larval antigen, CarLA, is present on the exposed surface of all strongylid nematode infective L3 larvae tested, and antibodies against CarLA can promote rapid immune rejection of incoming Trichostrongylus colubriformis larvae in sheep. A library of ovine recombinant single chain Fv (scFv) antibody fragments, displayed on phage, was prepared from B cell mRNA of field-immune sheep. Phage displaying scFvs that bind to the surface of living exsheathed T. colubriformis L3 larvae were identified, and the majority of worm-binding scFvs recognized CarLA. Characterization of greater than 500 worm surface binding phage resulted in the identification of nine different anti-CarLA scFvs that recognized three distinct T. colubriformis CarLA epitopes based on blocking and additive ELISA. All anti-CarLA scFvs were specific to the T. colubriformis species of nematode. Each of the three scFv epitope classes displayed identical Western blot recognition patterns and recognized the exposed surface of living T. colubriformis exsheathed L3 larvae. Surprisingly, each of the anti-CarLA scFvs was able to bind to only a subset of worms. Double-labelling indirect immunofluorescence revealed that the three classes of anti-CarLA scFvs recognize distinct, non-overlapping, T. colubriformis sub-populations. These results demonstrate that individual T. colubriformis L3 larvae display only one of at least three distinct antigenic forms of CarLA on their surface at any given time, and suggest that antigenic variation within CarLA is likely a mechanism of immune evasion in strongylid nematodes.

Strongylid nematode worm parasites currently infect hundreds of millions of people, and most farmed animals, causing enormous morbidity and economic loss. These parasites commonly produce chronic gastrointestinal infections that are highly refractory to immune clearance mechanisms. Mucosal antibodies against a carbohydrate surface antigen (CarLA) can cause rapid expulsion of incoming larval nematodes. Sheep develop strong anti-strongylid immunity following long-term grazing on contaminated pasture. From these sheep, we identified and characterized recombinant antibodies that recognize CarLA on living L3 stage infective larvae of the strongylid parasite, Trichostrongylus colubriformis. The selected antibodies are specific only to larvae of the T. colubriformis species and, surprisingly, recognize only a subset of these worms. Three different anti-CarLA antibody classes were found and each recognizes different, non-overlapping subsets of worms which, together, comprise virtually the entire population. These results are the first demonstration of “intraspecific epitopic variation” within strongylid nematodes and suggest that these parasites have a mechanism that permits the surface presentation of at least three different antigenic forms of CarLA to avoid immune clearance.

Antibodies to surface epitopes of the carbohydrate larval antigen CarLA are associated with passive protection in strongylid nematode challenge infections

Sheep were immunized by multiple truncated infections with the gastrointestinal nematodes Trichostrongylus colubriformis, Haemonchus contortus and Teladorsagia circumcincta. Three infections with T. colubriformis of 14 days plus five booster doses of L3 stimulated highly effective protection against challenge (99%). Three infections of 14 days plus three booster doses with H. contortus also resulted in significant protection against challenge infection (87%), but the same procedure was not effective for T. circumcincta. Antibodies derived from gastrointestinal mucus of these immunized sheep were tested for their ability to reduce worm burden following injection of antibody-coated exsheathed larvae into the abomasum (H. contortus and T. circumcincta) or duodenum (T. colubriformis) of nematode-naïve sheep in a passive immunity test. The IgG fraction from the mucus of immunized sheep reduced worm burdens by 62%, 76% and 91% in three tests with T. colubriformis but was not effective for either of the abomasal dwelling nematodes H. contortus and T. circumcincta. Antibodies in immune mucus predominantly recognized two larval surface antigens on immunoblots of L3 extract, a high MW surface glycoprotein and the carbohydrate larval antigen (CarLA). Antibodies raised against purified T. colubriformis glycoprotein Tc-120 and CarLA were tested in the passive immunity model and it was found that only the antibody against CarLA resulted in a significant reduction of infection (87%). The protective anti-CarLA antibodies strongly recognized the surface of living T. colubriformis L3. Antibodies from abomasal mucus of sheep immunized by H. contortus and T. circumcincta infections reacted weakly with CarLA and the larval surface and did not reduce worm counts in a passive immunity test. The results provide further evidence that the larval surface carbohydrate antigen CarLA has potential as a mucosal immunogen for a strongylid nematode vaccine.

Fibrinogen is localized on dark zone follicular dendritic cells in vivo and enhances the proliferation and survival of a centroblastic cell line in vitro

Follicular dendritic cells (FDC) in the germinal centers (GC) of secondary lymphoid organs increase the survival and proliferation of antigen-stimulated B cells and are pivotal for the affinity maturation of an antibody response and for maintenance of B cell immunological memory. The dark zone (DZ) and the light zone (LZ) constitute distinct areas of the GC containing different subtypes of FDC as identified by their morphology and phenotype. Until now, most available FDC-specific reagents identify LZ FDC, and there are no reagents recognizing DZ FDC specifically. Here, we report a new mAb, D46, which stains FDC specifically in the DZ of bovine and ovine GC within the secondary follicles. We identify its ligand as bovine fibrinogen, and using commercially available anti-human fibrinogen antibodies, show that this inflammatory protein is also present on DZ FDC of human GC within palatine tonsils. In vitro, the addition of exogenous fibrinogen stimulates the proliferation and survival of BCR-stimulated L3055 cells, which constitute a clonal population of centroblastic cells and retain important features of normal GC B cells. Together, our results suggest that fibrinogen localized on DZ FDC could support the extensive proliferation and survival of GC B cells within the DZ in vivo.

Cytokine and antibody subclass responses in the intestinal lymph of sheep during repeated experimental infections with the nematode parasite Trichostrongylus colubriformis

The expression of interleukin (IL)-4, IL-5, IL-10, IL-13, TNF-alpha and IFN-gamma genes, and parasite-specific IgM, IgG1, IgG2, IgA and total IgE levels, were monitored daily in intestinal lymph of sheep infected repeatedly with the nematode parasite Trichostrongylus colubriformis. Host genotype had a significant influence on IL-13 gene activity, with resistant-line (R) sheep consistently expressing higher levels of mRNA than susceptible-line (S) sheep. Mean gene expression of IL-13, IL-4 and IFN-gamma did not differ significantly between the first and second nematode challenge. Field-primed R and S as well as field-primed R and naïve S sheep had lower mean gene expression of IL-5 and IL-10, respectively, during the second when compared to primary challenge. Genes for IL-13 and IL-5 were transiently and strongly up-regulated after nematode infection, particularly in animals with previous exposure to nematodes. Genes for TNF-alpha and IFN-gamma were also transiently up-regulated, but to a lesser extent and more typically after primary challenge. Naïve sheep of both genotypes produced relatively little antibody response after primary challenge. A second nematode challenge resulted in large increases in the lymphatic levels of all antibody sub-classes which were significant for adult antigen-specific IgA and larval antigen-specific IgG1. In naïve S line sheep, the larval-specific IgA and IgG2 response appeared delayed when compared to the R line animals. Field-primed R and S line sheep had relatively high lymphatic IgG1 levels prior to experimental infection and these did not change significantly afterwards. These results demonstrate that during nematode infections, the intestinal micro-environment of sheep is transiently skewed towards Th2 cytokine dominance, although IFN-gamma gene expression continues. This response is accompanied by increases of nematode-specific IgG1, IgA, IgG2 and IgM, as well as of total IgE in lymph plasma.

Vaccines against veterinary helminths

The majority of attempts to develop commercial vaccines for veterinary helminths have focussed on identifying protein antigens, which could be formulated as protective vaccines. Notable successes have been achieved for some cestode parasites, where recombinant proteins have been developed into highly effective vaccines. Although effective protection can also be obtained using some nematode proteins in their native forms, it has not yet been possible to formulate commercially successful vaccines for other helminth parasites of veterinary significance. Increasing evidence suggests that parasite glycan moieties may provide an alternative source of vaccine antigens, and increased attention is now being given to this class of compounds. In addition to identifying candidate protective antigen(s), an increased research effort is needed to develop appropriate strategies for the formulation and delivery of helminth vaccines.

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.

Total and nematode-specific IgE responses in intestinal lymph of genetically resistant and susceptible sheep during infection with Trichostrongylus colubriformis

Total and antigen-specific IgE responses in afferent (AIL) and efferent (EIL) intestinal lymph of sheep with a nematode resistant (R) or susceptible (S) genotype during challenge infection with the intestinal nematode parasite Trichostrongylus colubriformis were examined. Within each sheep line, lambs with a nematode naive or nematode field-primed pre-challenge status were used. Total IgE level in AIL and EIL was dependent on nematode infection and was further influenced by genotype or the immune phenotype (nematode immune mean FEC+/-SDM=77+/-179 or non-immune mean FEC+/-SDM=4016+/-4318) of the animal. During T. colubriformis challenge immune animals had higher levels of total IgE in lymph than non-immune sheep, R line sheep had higher concentrations of total IgE than S line sheep, and field-primed animals had higher total IgE levels than nematode naive animals. Concentrations of total IgE were consistently higher in AIL than EIL or serum and were higher in lymph draining the proximal than the distal jejunum demonstrating that polyclonal IgE in AIL was largely derived from the intestinal mucosa of the anatomical compartment where the nematodes reside. The consistently higher concentration of total IgE in AIL was dependent on phenotype or genotype and in S genotype sheep also on the pre-challenge status. Concentrations of nematode specific IgE were significantly higher in EIL than AIL indicating a preference for the production of IgE reacting with excretory secretory products of the infective T. colubriformis larvae in the regional lymph node.

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.

Identification and characterisation of an aspartyl protease inhibitor homologue as a major allergen of Trichostrongylus colubriformis

Allergens were identified from the gastrointestinal nematode of sheep, Trichostrongylus colubriformis, by probing Western blots of infective larvae (third stage) somatic antigen with IgE purified from the serum of sheep grazed on worm contaminated pasture. A 31 kDa allergen was frequently recognised by sera from immune sheep, particularly those deriving from a line that has been genetically selected over 23 years for parasite resistance. Using a proteomic approach, the 31 kDa allergen was identified as an aspartyl protease inhibitor homologue. The entire coding sequence of T. colubriformis aspartyl protease inhibitor (Tco-api-1) was obtained and the mature protein expressed in Escherichia coli. Anti-Tco-API-1 antibodies revealed that a commonly observed 21 kDa T. colubriformis allergen species is a truncated form of Tco-API-1. Specific IgE responses to T. colubriformis aspartyl protease inhibitor were significantly correlated with the degree of resistance to nematode infection as measured by faecal egg count in sheep. Surprisingly, IgE responses to Tco-API-1 were not correlated with breech soiling (dag score), which is thought to be caused, in part, by allergic hypersensitivity to worms. Therefore, a specific IgE response to this allergen may be a suitable marker for identifying lambs at an early age that will develop strong immunity to gastrointestinal nematodes.

Characterization of a 35-kDa carbohydrate larval antigen (CarLA) from Trichostrongylus colubriformis; a potential target for host immunity

In an accompanying paper we show that antibodies in intestinal mucus that recognize a 35-kDa antigen from the surface of the L3 stage of the sheep intestinal nematode parasite, Trichostrongylus colubriformis, are strongly associated with immune rejection of L3 in a truncated infection model of immunity in sheep. Monoclonal antibody PAB-1 was used to immunopurify this antigen from T. colubriformis L3. The antigen is resistant to digestion with a range of proteases including proteinase K and does not stain on gels or blots treated with protein-detecting reagents but does stain with carbohydrate-detecting reagents. The antigen is also resistant to degradation by the action of lipases and is not soluble in organic solvents, suggesting that lipid components are not present or not accessible. Treatment with glycosidases does not affect the antigen, indicating either that sialic acid and N-linked or O-linked sugars are not present or that they are not accessible to enzyme attack. The antigen is not destroyed by harsh alkaline degradation with up to 8 m NaOH with or without borohydride reducing agent, or by extensive hydrazinolysis. Strong acid hydrolysis with trifluoroacetic acid or boiling in hydrochloric acid for 20 min does destroy the antigen. The antigen migrates as a poorly defined high molecular weight complex on native electrophoresis gels, but is detected as a major band at 35 kDa on SDS PAGE either by carbohydrate staining or by immunoblotting with antibody from immune sheep intestinal mucus and with mAb PAB-1. Proteinase K digestion and alkaline degradation of extracts from L3 of 10 other parasitic nematode species revealed that L3 of each species contained a carbohydrate staining molecule which can be detected by mAb PAB-1 and by mucus antibody from immune sheep. Because antibodies in intestinal mucus are directed against these antigens and may be responsible for protective immunity, carbohydrate larval antigens (CarLA) could represent a new family of molecules with potential as targets for stimulating host immunity.

A road less travelled: large animal models in immunological research

The main advances in immunology have been forged or underpinned by animal experiments. However, animal research now focuses excessively on one laboratory species, and there is too much redundant repetition and too few transfers from basic discovery to successful clinical application. These features can be improved markedly by placing more emphasis on biological relevance when evaluating animal models and by taking greater advantage of the unique experimental opportunities that are offered by large animals.

Immune rejection of Trichostrongylus colubriformis in sheep; a possible role for intestinal mucus antibody against an L3-specific surface antigen

Sheep that have been immunized by multiple truncated infections with the parasitic nematode Trichostrongylus colubriformis contain anti-larval activity in their intestinal mucus and high-speed mucus supernatants. This activity induces T. colubriformis L3 to clump in vitro and causes a significant reduction in larval establishment in naive sheep after infusion of larvae and mucus into the intestinal lumen via a duodenal cannula. In this report, we provide evidence that one factor contributing to the anti-larval activity of immune mucus is antibody against a 35-kDa L3-specific cuticular antigen. The anti-larval activity in mucus is > 100 kDa by membrane filtration, is heat labile and sensitive to either protease digestion or reduction with DTT. Immunoblotting showed that mucus and supernatants of ultracentrifuged mucus from immune sheep contained IgG1 and IgA antibodies that recognized predominantly a larval antigen with an estimated molecular weight of 35 kDa on SDS-PAGE. Antibodies eluted from the surface of washed larvae that had been incubated in immune mucus also reacted specifically with the 35 kDa antigen on blots of larval homogenate. Immunofluorescence and immunogold electron microscopy showed that the 35 kDa antigen is present on the epicuticle of L3 and is shed during the moult to L4. The antigen is not present in eggs, L1, L2, L4 or adult worms and is found only in extracts of sheaths and L3 before infection and up to 4 days after infection. We hypothesize that the binding of antibody to the larval surface prevents larvae from establishing at their preferred site, causing them to be eliminated from the intestine. Monoclonal antibody PAB-1 recognizes the 35 kDa T. colubriformis larval antigen and also cross-reacts with antigens of similar molecular weight on blots of L3 extracts of the parasitic nematodes Haemonchus contortus and Ostertagia circumcincta; and with a 22-kDa antigen on blots of L3 extracts from Cooperia curticei and Nematodirus spathiger. This indicates that an antigenically related surface antigen with immunizing potential is present on several nematode species and can be identified by mAb PAB-1. The 35 kDa T. colubriformis larval antigen and related molecules in other nematodes are potential novel targets for stimulating host-protective immunity against nematode infections.

Phenotypic diversity of antigen-presenting cells in ovine-afferent intestinal lymph

The phenotypic and functional repertoire of antigen-presenting cells (APCs) remains incompletely characterized, particularly during the migratory phase of their life history, when these cells leave peripheral tissues and travel via afferent lymphatic vessels to regional lymph nodes. Lymphatic cannulation procedures were used to collect ovine APCs as they migrated from the mucosa of the small intestine to regional lymph nodes. A panel of 19 new monoclonal antibodies (mAbs) was produced to characterize surface molecules expressed on APCs by means of two-color flow cytometry and microscopy. Two broad patterns of mAb reactivity were evident. Twelve mAbs reacted almost exclusively with cells in the APC-gated region, because all these mAbs stained < 3% of cells in the lymphocyte-gated region. Within this group, some mAbs identified distinct subsets of dendritic cells (DCs). The second group of seven mAbs displayed high-intensity staining on cells in the APC-gated region but also reacted with variable numbers (4-26%) of cells in the lymphocyte-gated region. This indicates that molecules recognized by these mAbs are highly expressed on APCs but also occur on other lineages. When new mAbs were analyzed by two-color flow cytometry of cells in afferent intestinal lymph, a wide range of differences in reactivity were observed, especially on CD11b(+), CD11c(+), CD4(+), MHCII(+), and gammasigmaTCR(+) cells. Although molecular specificities of mAbs reported here remain undefined, marked heterogeneity of staining patterns indicates considerable phenotypic, and probably functional, diversity within APC population in afferent intestinal lymph. MAbs reported here will provide useful tools to explore these features further.

Intestinal infusion of soluble larval antigen stimulates rejection of heterologous nematode larvae by immune sheep

Sheep immunised by multiple truncated infections with Trichostronglyus colubriformis were highly resistant to subsequent challenge with homologous exsheathed larvae, administered via a surgically implanted duodenal cannula. The duration of immunity after truncated infections was 12-14 weeks against challenge with T. colubriformis or Cooperia curticei, but there was little cross-protection against challenge with Nematodirus spathiger. When immune sheep were given booster doses of T. colubriformis larvae before challenge with N. spathiger, there were 97% fewer N. spathiger larvae in the first 5 m of small intestine, and an overall 79% reduction of N. spathiger larvae in immunised sheep, compared with naive controls. Boosting T. colubriformis immune sheep with killed T. colubriformis larvae plus soluble T. colubriformis L3 antigen, or with soluble antigen alone, also caused significant displacement of N. spathiger challenge larvae (98% and 100% respectively), indicating a non-specific expulsion process. These results indicate that N. spathiger can be used as an indicator species in T. colubriformis immune sheep, to quantify the effects of stimulating mucosal immunity with specific antigens, which may lead to identification of the antigens required for immunisation against nematodes.

DNA-containing extracellular 50-nm particles in the ileal Peyer's patch of sheep

Follicles of the ileal Peyer's patch are sites of B cell proliferation and of diversification of the primary immunoglobulin repertoire in ruminants. We demonstrate here that 50-nm carbonic anhydrase-reactive particles released in the intercellular space in the follicle-associated epithelium of the ileal Peyer's patch of lambs contain DNA protected with a detergent-resistant membrane. We named these particles DiCAPs (DNA in carbonic anhydrase particles). DiCAPs can be purified from a suspension collected from ileal Peyer's patch follicles by sedimentation in a sucrose gradient. The DiCAP membrane is resistant to several ionic and non-ionic detergents alone, but can be disrupted by a combination of Triton X-100 and proteinase K. Differential nuclease treatment of purified DiCAPs indicates that they contain DNA. Digestion of DiCAP DNA with six-base pair restriction enzymes produces smears, suggesting that individual DiCAPs contain unique sequences. Nonetheless, the size of DiCAP DNA is smaller (approximately 16 kb) than that of lamb genomic DNA. Polymerase chain reaction and sequence analysis of DiCAP DNA reveals the presence of light and heavy chain variable genes as well as housekeeping genes. The data demonstrate the presence of DNA in these extracellular particles, and suggest a role of DiCAPs in transfer of DNA between cells within the ileal Peyer's patch. This raises the possibility of a novel form of communication between cells mediated by nucleic acids.

Future technologies for control of nematodes of sheep

New approaches to nematode control in sheep are urgently needed as anthelmintic drench resistance becomes ever more widespread among worm populations. Here we briefly describe and assess a number of new technologies which will become increasingly important in anti-nematode control programmes in the future. These include vaccines, immunomodulants, strategic grazing practices, the use of biological agents to destroy nematode larvae, biological anthelmintics and targeted silencing of genes regulating nematode development.

Splenectomy selectively affects the distribution and mobility of the recirculating lymphocyte pool

The spleen plays a major role in immune surveillance, but the impact that splenectomy exerts on the immune competence of an individual is not fully resolved. Here we show that neonatal splenectomy in sheep does not abrogate the development of a large, nonrecirculating pool of lymphocytes and that it has no effect on the acquisition of a normal blood lymphocyte profile. Splenectomy did, however, result in a significant decrease in blood residency time of recirculating lymphocytes and in an enhanced accumulation of recirculating lymphocytes in lymph nodes. Furthermore, nonrecirculating peripheral blood lymphocytes were less likely to migrate to the lung, possibly because of saturation of the marginal pool by recirculating lymphocytes. Although splenectomy has little effect on the development or distribution of lymphocyte subsets in blood and lymph, it has marked effects on the rate of recirculation of lymphocytes, which may have significant implications for peripheral immune surveillance in patients who undergo splenectomy.

Structure and expression of ovine complement receptor type 2

The structure of sheep complement receptor type 2 (CR2) was characterised by cDNA cloning, protein sequencing and immunoprecipitation. The primary structure of sheep CR2 is similar to known mammalian homologues but the higher-order structure is unusual. Two distinct CR2 isoforms occur, one of which is ubiquitinated in the cytoplasmic domain, and the two molecular forms are expressed at the cell surface as non-covalently associated dimers. The percentage of sheep B-cells that express CR2 changes during development and varies between different body compartments. CR2+ and CR2 B-cell subsets also differ in the expression of other surface markers and in functional properties. Differential expression of CR2 may, therefore, delineate B-cells that arose by alternative developmental pathways, or it could be a marker for B-cells at different phases of antigen exposure.

Diversification of sheep immunoglobulins

The sheep antibody repertoire was assessed by sequence analysis of Vlambda Vkappa and V(H) cDNA clones. In fetuses, all immunoglobulin chains were essentially in germline configuration, although limited diversity occurred at the junctional regions of Vkappa chains. Fetal B cells expressed 12 distinct Vlambda and six Vkappa gene segments. V(H) gene segments were also in germline configuration in fetuses but became extensively mutated with advancing post natal age. Two distinct types of mutations, point mutations and block mutations, were detected in V(H) transcripts and these localised differently along the gene. This raises a question as to whether a novel mutational mechanism may be involved in V(H) diversification in sheep.

Diversity of Ig light chain variable region gene expression in fetal lambs

The usage of lambda and kappa light chain variable region genes in peripheral fetal lamb B cells was assessed at intervals from 61 to 146 days of gestation (term = 150 days). Transcripts of 12 distinct Vlambda genes were identified, eight of which belonged to the previously described families I or II, while four segments defined a new Vlambda family, provisionally called family VI. A total of six different Vkappa gene segments were identified and grouped into four families, while two Jkappa gene segments were expressed. Southern blot analysis indicated that the sheep Igkappa locus probably contains approximately 10 Vkappa genes and suggests that the V gene cluster has been duplicated. Throughout fetal ontogeny, Vkappa transcripts contained more diverse CDR3 sequences than did Vlambda transcripts, where the V-J junctions were nearly always invariant and canonical. There was also a shift in usage of Vkappa genes and in the pattern of Vkappa-Jkappa gene recombination during fetal ontogeny.

Ubiquitination and Dimerization of Complement Receptor Type 2 on Sheep B Cells

Complement receptor type 2 (CR2) is a membrane-anchored glycoprotein that specifically binds C3d, as well as other ligands, and plays diverse roles in regulating immunity. Here we show that two distinct isoforms of CR2 are expressed on the surface of sheep B lymphocytes. One (CR2no 150 kDa) is structurally similar to known mammalian homologues while the other (CR2ub 190 kDa) has been modified by the covalent attachment of ubiquitin to the cytoplasmic domain and is identified for the first time. CR2no and CR2ub are expressed on the surface of sheep B cells as noncovalently associated dimers and the external topography of the two isoforms differs in some respect. The basis for these unusual higher-order structural properties may lie in the primary sequence of sheep CR2, since the transmembrane domain contains a region resembling a rare 7-amino acid dimerization motif, and two lysine residues in the cytoplasmic domain provide potential sites for posttranslational ubiquitination. The primary structures of sheep ubiquitin and C3d ligand are extensively conserved. In conjunction with the results of separate in vivo studies, these findings suggest that selective ubiquitination plays a role in modulating the higher-order structure and/or expression of CR2 during B cell development.

Ubiquitination and dimerization of complement receptor type 2 on sheep B cells

Complement receptor type 2 (CR2) is a membrane-anchored glycoprotein that specifically binds C3d, as well as other ligands, and plays diverse roles in regulating immunity. Here we show that two distinct isoforms of CR2 are expressed on the surface of sheep B lymphocytes. One (CR2no 150 kDa) is structurally similar to known mammalian homologues while the other (CR2ub 190 kDa) has been modified by the covalent attachment of ubiquitin to the cytoplasmic domain and is identified for the first time. CR2no and CR2ub are expressed on the surface of sheep B cells as noncovalently associated dimers and the external topography of the two isoforms differs in some respect. The basis for these unusual higher-order structural properties may lie in the primary sequence of sheep CR2, since the transmembrane domain contains a region resembling a rare 7-amino acid dimerization motif, and two lysine residues in the cytoplasmic domain provide potential sites for posttranslational ubiquitination. The primary structures of sheep ubiquitin and C3d ligand are extensively conserved. In conjunction with the results of separate in vivo studies, these findings suggest that selective ubiquitination plays a role in modulating the higher-order structure and/or expression of CR2 during B cell development.

Distinct recirculating and non-recirculating B-lymphocyte pools in the peripheral blood are defined by coordinated expression of CD21 and L-selectin

The continual recirculation of lymphocytes between the blood, tissues, and lymph is essential for the coordination and dissemination of immune responses. We have compared the functional and phenotypic properties of lymphocytes isolated from blood and lymph, the two major migratory populations. Lymph-borne lymphocytes migrated readily into the lymphatic recirculation pathway, but greater than one third of all peripheral blood lymphocytes (PBLs) were excluded from the lymphatic circuit and showed an enhanced migration to the spleen. Phenotypic analysis showed that most non-recirculating PBLs were B cells. The migration competence of B cells correlated with the surface expression of CD21 and L-selectin; recirculating B cells expressed both of these molecules, whereas non-recirculating B cells lacked both antigens. These results establish that blood contains distinct pools of lymphocytes that differ in their recirculation competence. Clearly, blood sampling is not an efficient method to directly measure the status of the recirculating immune system, and implies important constraints and restrictions in the interpretation of experimental or clinical data that include phenotypic and quantitative analyses of blood lymphocytes.

A single predominantly expressed polymorphic immunoglobulin VH gene family, related to mammalian group, I, clan, II, is identified in cattle

In order to understand the generation of antibody diversity in cattle, seven cDNAs, from heterohybridomas secreting bovine IgM and IgG1 antibodies, were cloned and structurally analyzed for rearranged bovine VDJ genes. All of the seven bovine VH genes, together with four available bovine VH gene sequences, shared a high nucleotide sequence homology (84.2-93.5%). Based upon the criteria of nucleic acid homology > or =80%, all of the bovine VH gene sequences isolated from the expressed antibody repertoire constitute a single VH gene family, which we have designated as bovine VH1 (Bov VH1). An analysis of 44 bovine IgM-secreting mouse x cattle heterohybridomas, originating from polyclonally-activated PBLs from bovine leukemia virus-infected cattle, revealed that all of these expressed Bov VH1 (100%) based upon DNA sequencing and Northern dot blot. The bovine VH genes showed highest DNA sequence similarity, ranging between 81.5 and 87.6%, with a single sheep VH gene family (related to human VH4) and are, thus, closest to the VH genes from another ruminant species. The Bov VH1 gene family is most homologous to the murine VH Q-52 (71.8-78%) and human VH4 (67.4-69.8%) gene families, which belong to mammalian group, I, clan, II, VH genes. The CDR3 length of rearranged bovine VDJ genes is characteristically long (15-23 amino acids). The bovine JH gene segments were most homologous to human JH4 (82.1-87.2%) and JH5 (84.6-89.7%) genes, suggesting the existence of at least two JH gene segments. An analysis of CDRs provides evidence that somatic hypermutations contribute significantly to the generation of antibody diversity in cattle. Southern blot analysis of BamH I, EcoR I and Hind III digested genomic DNA from four cattle breeds (Holstein, Jersey, Hereford and Charolais) revealed three RFLP patterns; the genomic complexity of Bov VH1 ranged between 13 and 15 genes. These observations provide evidence for polymorphism at the bovine Ig-VH locus, similar to that seen in mice and humans.

TCR gamma delta+ cells are prominent in normal bovine skin and express a diverse repertoire of antigen receptors

More than 80% of T cells in bovine skin localized in the superficial 0.5 mm of the dermis. Only 3% occurred within the epidermis or made contact with the stratum basale while the remainder occupied deeper dermal sites. The gamma delta-T-cell receptor (TCR) was expressed by 44% of T cells in skin and 39% and 35% expressed, respectively, the CD4 and CD8 markers. Some cells co-expressed CD8 and the gamma delta TCR. A highly diverse repertoire of gamma delta TCR was expressed in skin due mainly to the usage of multiple V delta segments and to extensive sequence variation at the junctions of both TCR gamma and TCR delta chains. However, a single receptor isotype was used. Transcripts encoding several new components of the bovine gamma delta TCR were identified, including three new V gamma segments, the C gamma 5 region and 13 new functional V delta segments. Taken together with earlier findings, these results emphasize that ruminant gamma delta T cells express exceptionally diverse antigen receptors and suggest they may have a more elaborate recognitive capacity than do their counterparts in other species.

Hypermutation generating the sheep immunoglobulin repertoire is an antigen-independent process

Somatic hypermutation of light chain V genes during development of B cells in sheep ileal Peyer's patches was studied in three experimental conditions: in sterile fragments of the ileum surgically isolated from the gut during fetal life, in germ-free sheep, and in animals thymectomized during early fetal life. The somatic mutation pattern was found identical to control tissues in all three experiments. The same age-dependent amount of mutations, a higher than theoretical R/S ratio in complementarity-determining regions (CDRs), and a similar clustering of mutations in CDRs were observed. The mechanism, as estimated from the silent mutation pattern, appears to target mutations to CDRs; moreover, the major V lambda genes have a specific codon usage with a high purine content at the first two bases of the codons and a low content at the third position, which, together with a specific targeting of mutations to purines, favors replacement mutations in CDRs.

Structural and functional evolution of the extracellular regions of T cell receptors

The primary structure of T cell receptor (TcR) heterodimers has been elucidated to varying degrees in 11 vertebrate species and the availability of these sequences provides a broader perspective for reassessing TcR evolution. Many features of the alpha beta TcR have been well conserved between divergent taxa whereas the structure and diversity of the gamma delta TcR is more variable. An alternative evolutionary scheme is presented whereby the gamma delta TcR and Ig molecules represent the earliest separation between specific cell mediated and humoral immunity with the alpha beta TcR emerging later. It is proposed that a set of ligands expressed on epithelia mediated the earliest selection of gamma delta T cells and B cells and was used as an antigen presentation substrate by gamma delta T cells. A subset of the earliest selection ligands became expert at binding small peptide fragments and the emergent 'classical' MHC molecules then co-evolved with the alpha beta TcR as obligate ligand-receptor pairs. Progression between the major stages in this sequence followed from adaptations in the selecting and presenting ligands and can be characterized as MHC 'capture'.

A novel multi-gene family of sheep gamma delta T cells

The WC1 protein is a cell surface constituent of bovine gamma delta T cells and is absent from most or all CD4+, CD8+ T cells and from B cells. It is a single polypeptide chain of 1413 amino acids consisting of 11 non-identical repeats of a 110 amino acid consensus sequence, homologous to the macrophage scavenger receptor cysteine rich (SRCR) domain. A 1059 nucleotide segment of the bovine WC1 cDNA sequence was used as a probe to molecularly clone homologous DNA segments from a sheep genomic library in which the presence of numerous positive plaques was documented. The high representation of such recombinants (1-2/1000 clones) within the library suggested the existence of multiple genes for WC1 (called T19 in sheep) and supported Southern blotting data which revealed an unexpectedly high number of WC1/T19 restriction fragments in sheep genomic DNA. Restriction digests of 27 samples of T19 genomic recombinants were examined by electrophoresis and Southern blotting. All but two pairs of recombinants exhibited non-overlapping restriction digest patterns. Four recombinant DNA samples were partially sequenced and in all cases putative exons were identified and exhibited high homology to appropriate segments of the WC1 cDNA at the levels of both nucleotide and amino acid sequence. Furthermore, multiple nucleotide and amino acid differences occurred between all sequences compared, establishing the existence of a repertoire of non-identical T19 genes, each with the potential to encode a different protein.

Lymph node homing cells biologically enriched for gamma delta T cells express multiple genes from the T19 repertoire

Sheep gamma delta T cells have been shown serologically to express T19, a membrane protein of 180-200 kDa which is a member of the scavenger receptor superfamily. Previous work from this laboratory resulted in the detection of a multigene family of T19-like genes in the sheep genome. In this study nucleotide sequences from several T19 genes were determined and are reported along with the corresponding segments of a number of expressed mRNA molecules. A segment of a single sheep T19-like gene was sequenced and these data, along with the corresponding sequences from cloned T19-like cDNA molecules from sheep and cow, were used to design an oligonucleotide primer system suitable for amplification of corresponding segments of many T19 genes and their cDNAs. Between 30 and 40% of cloned T19 genes were amenable to amplification using the selected primers, and sequence analysis of cloned PCR products confirmed that different T19 genes encode unique amino acid sequences. The expression of multiple T19 genes was established using cDNA molecules obtained from a single sample of sheep lymphocyte mRNA. The possible role of the T19 family of genes is discussed.

A highly conserved phenylalanine in the alpha, beta-T cell receptor (TCR) constant region determines the integrity of TCR/CD3 complexes

In the present study, we have investigated the importance of a phenylalanine (phe195) in the Tcr-C alpha region on Tcr-alpha,beta/CD3 membrane expression. An exchange of phe195 with a tyrosine residue does not affect Tcr/CD3 membrane expression; however, exchange with aspartic acid, histidine or valine prohibit completely Tcr/CD3 membrane expression. This seems to be due to a lack of interaction between mutated Tcr-alpha,beta/CD3-gamma epsilon,delta epsilon complexes and zeta 2 homodimers. The Tcr-C alpha region around phe195 seems together with the same region in the Tcr-C beta region to constitute an interaction site for zeta 2 homodimers. The presence of phe195 on both Tcr-C alpha and Tcr-C beta causes high avidity interaction with zeta 2 homodimers, whereas his195 in both Tcr-C gamma and Tcr-C delta results in an apparently lower avidity interaction with zeta 2 homodimers. It is suggested that the phe195 region (on beta-strand F) and eventually adjacent aromatic amino acid residues on beta-strand B region may play an important role in Tcr-alpha,beta/CD3 membrane expression, in Tcr-alpha,beta/CD3 competition with Tcr-gamma,delta/CD3 complexes for zeta 2 homodimers and in the control of formation of 'mixed' Tcr heterodimers.

Ontogeny of leucocyte populations in the spleen of fetal lambs with emphasis on the early prominence of B cells

The presence and distribution of B cells and other early leucocyte populations are described in the spleen of fetal lambs from 40 to 134 days of gestation (length of gestation 150 days). Computer-assisted morphometric analysis and flow cytometry were used to quantify the early predominance of B cells in mid-gestation. B cells appeared at about 48 days and increased in number to occupy over 20% of the spleen area at 77 days. All spleens were collected on their respective livers and at no stage did the livers contain more than a few IgM-positive (+) cells, which were usually close to blood vessels. Two-colour flow cytometry demonstrated that only 1-2% of IgM+ cells expressed CD5 at 81 days. Beyond 77 days, with the expanding presence of T cells, the percentage of area occupied by IgM+ cells declined to stabilize at about 7% during late gestation. The conventional organization of the splenic white pulp was observed from 90 days along with 5' nucleotidase-positive primary follicles. Double staining technique using immunohistochemical methods demonstrated that IgM+ cells were proliferating in the spleen from as early as 51 days and that clusters of proliferating IgM+ cells were prominent between 60 and 77 days. The results of the present study suggest that during the ontogeny of fetal lambs the spleen is a site of B-cell development or expansion before colonization of the ileal Peyer's patch and the subsequent generation of the preimmune antibody repertoire.

Phenotype and function of stromal cells cloned from the ileal Peyer's patch of sheep

The ileal Peyer's patch (PP) is the major site of B cell production and immunoglobulin diversification in lambs, but the factors which regulate these processes are poorly understood. As a first step toward identifying possible regulatory mechanisms, stable long-term cultures of ileal PP stromal cells were established at the clonal level. Four distinct cell types were identified by their phenotype and growth requirements. Immunohistochemical staining confirmed that all clones were mesenchymal (vimentin+; cytokeratin-) in origin and were negative for T cell, B cell, and macrophage markers. Three cell lines were negative for major histocompatibility complex (MHC) I and II molecules, but one cell line, SCN, expressed MHC I, MHC II and CD44 molecules, and a subpopulation of SCN cells expressed BAQ44A, a B cell differentiation molecule. The four cell lines produced different types and amounts of extracellular matrix proteins, and their growth was not influenced by exogenous human interleukin 1 (IL-1), IL-2, transforming growth factor beta 1 (TGF beta 1), or bovine fibroblast growth factor (FGF) but was influenced by serum. When tested for their capacity to support lymphocyte growth, all clones produced a soluble factor(s) that was mitogenic for ileal and jejunal PP cells and thymocytes. Similar growth promoting activity was observed with culture supernatants of murine, human and bovine fibroblasts but could not be reproduced using recombinant human cytokines. Furthermore, coculture of stromal cells with ileal PP follicular B cells elicited a proliferative response unique to each stromal cell line. Coculture with increasing numbers of SCN cells inhibited B cell proliferative responses, whereas coculture with SCG2 and SCF32 cells enhanced B cell proliferative response at both low and high stromal cell densities. Ileal PP follicular B cells rapidly bound to the surface of all stromal cell clones, and this interaction was specific when compared with thymocytes or peripheral blood lymphocytes. These results suggest that ileal PP stromal cells are a phenotypically and functionally heterogeneous population that may enhance or inhibit B lymphopoiesis in the ileal PP.

Divergent evolution of T cell repertoires: extensive diversity and developmentally regulated expression of the sheep gamma delta T cell receptor

Sheep gamma delta T cells express an unprecedented repertoire of antigen receptors contributed by increased diversity in both variable and constant region gene segments. Variable region diversity results mainly from the utilization of a large family of duplicated V delta genes that have retained two distinct hypervariable segments comparable with the complementarity determining regions present in other antigen receptor V genes. This implies that sheep V delta chains have been intensely selected during evolution, probably at sites involved in ligand recognition. The sheep gamma delta heterodimer occurs in at least five isotypic variants formed by the association of a single C delta segment with one of five functional C gamma segments, each with distinctive hinge regions. Our analysis also shows that the establishment of a normal peripheral repertoire is both developmentally regulated and dependent on the continual presence of a functional thymus during ontogeny. The existence of an expanded V gene repertoire and multiple receptor isotypes together with the prominence of gamma delta T cells in the sheep immune system argues that this lineage of T cells has a more elaborate functional role in this evolutionary pathway.

Tissue-specific migration pathways by phenotypically distinct subpopulations of memory T cells

A proportion of T cells recirculate in a tissue-selective manner. Recent studies which showed that the skin-tropic subset of T cells was of memory/activated type, led us to examine whether the preferential homing of T cells to the gut also involved memory T cells, and if so whether these memory T cells were phenotypically distinct from other memory T cells. Lymphocytes migrating through the gut and the skin of sheep was collected by cannulating the lymphatic ducts draining these tissues. Both naive and memory T cells were found to recirculate through the gut, although only memory T cells migrated through the skin. However, when T cells from the gut were labeled with fluorescein isothiocyanate and assessed for their migration back to the gut, it was the memory population which showed a tropism for the gut. Gut-tropic memory T cells migrated poorly through the skin, indicating that these cells were distinct from skin-tropic memory T cells. This was confirmed by phenotypic analysis. Gut memory T cells expressed very low levels of the alpha 6 and beta 1 integrins, in contrast to skin memory T cells which expressed high levels. There was no evidence for heterogeneity within the naive T cell population, which migrated preferentially to lymph nodes. This migration pattern could be explained in part by the high expression of the L-selectin (lymph node homing receptor, LAM-1) on naive T cells, in contrast to memory T cells from gut or skin which were mostly L-selectin negative. These results in sheep indicate that subsets of alpha/beta memory T cells show tissue-selective migration patterns, which probably develop in a particular environment following encounter with antigen.

Mixed micelles as a proliposomal, lymphotropic drug carrier

Four lipophilic, low molecular weight drugs solubilized in phosphatidylcholine-bile salt mixed micelles were injected s.c. into the hind legs of sheep and their cumulative recoveries in lymph draining from the site of application were determined. Surprisingly, the cumulative recoveries (percentage of dose) varied between less than 1 and 60%. We found that there is a correlation between the lipophilicity of the drug (log P octanol/water approximately Rm degrees value) and the proportion of the dose absorbed by the lymphatic route. Drugs with Rm degrees values greater than 10 are absorbed preferentially by the lymphatics (greater than 50% of dose), whereas compounds with Rm degrees values less than 4 are hardly absorbed at all by the lymphatics (less than 10% of dose). By applying the prodrug principle we demonstrated that it is also possible to target drugs with Rm degrees values less than 4 to the lymphatics. Furthermore, the analysis of the collected lymph samples by gel filtration, quasi-elastic light scattering, and electron microscopy revealed that, following s.c. administration, mixed micelles are converted into homogeneous, unilamellar vesicles. In conclusion, these results suggest that mixed micelles may represent a suitable delivery system for low molecular weight drugs whose targets are lymphoid cells. In addition, for drugs where liposomal application leads to a therapeutic advantage, the thermodynamically stable mixed micelle could be a good alternative to the liposome. However, for both applications a high drug lipophilicity is a prerequisite.