Vaccination against Haemonchus contortus: performance of native parasite gut membrane glycoproteins in Merino lambs grazing contaminated pasture.

In a replicated trial, parasitological and antibody responses of grazing weaner Merino sheep were assessed following vaccination with gut membrane proteins prepared from adult worms of the gastrointestinal nematode, Haemonchus contortus. Each vaccinated animal received 100 microg native H11 and 100 microg native H-gal-GP combined together in 5mg Quil A administered intramuscularly on days 0, 34, 80 and 127. Control animals received 5mg Quil A alone on the same days. Vaccinated and unvaccinated control animals grazed pastures contaminated with the parasite from day 34 of the trial, and levels of parasitism were monitored by worm-egg counts (WECs) in faeces and packed cell volumes (PCVs) in blood. The level of larval contamination on pasture was estimated from the worm counts of tracer sheep introduced monthly to the paddocks. WECs and anaemia were significantly reduced in vaccinated animals, and, in contrast to vaccinates, all control sheep required salvage treatment with anthelmintic. By the last 2 months of the trial, pastures grazed by vaccinated animals had significantly lower contamination with H. contortus larvae. Vaccinated animals had high levels of vaccine antigen-specific IgG1 and IgG2 antibodies in plasma, whereas those responses in the control sheep were very low. IgG1 titres in the vaccinated group, but not IgG2 titres, were inversely correlated with worm-egg counts. The levels of systemic IgA and IgE remained low but increased in both groups towards the end of the experiment most probably from exposure to the natural infection from pasture. The results showed that H11 and H-gal-GP behaved like "hidden" antigens producing high levels of protection that were probably mediated through mechanisms involving antibodies, and in particular, IgG1. It was concluded that if similar protective effects could be obtained with recombinant versions of the proteins present in either H11 or H-gal-GP, then the prospects for a commercial Haemonchus vaccine were real.

Failure of low level gamma-irradiation of infective larvae to influence establishment or expulsion of Trichostrongylus colubriformis in guinea pigs.

Trichostrongylus colubriformis infective larvae were subjected to various levels of gamma-irradiation and administered to guinea pigs. The worms surviving were subsequently counted. Irradiation with 12.9 C kg-1 inhibited worm establishment but lower doses neither influenced worm establishment nor survival.

Cross-immunity between Haemonchus contortus and Trichostrongylus colubriformis in sheep.

Cross-protective immunity between the nematode parasites, Haemonchus contortus and Trichostrongylus colubriformis, was examined in sheep vaccinated with irradiated larvae of either species. Secondary immunological responsiveness stimulated in this manner protected only against challenge infection with the species used for vaccination. Significant cross-protective immunity was not observed. Titres of serum antibody to an extract of adult but not infective larval T. colubriformis reflected the specificity for protective immunity. Immediate hypersensitivity skin reactions to nematode extracts did not reflect the antigen-specificity for protective immunity.

Eosinophil responses in sheep selected for high and low responsiveness to Trichostrongylus colubriformis.

A breeding programme, based on selection for faecal egg counts, has produced lines of sheep which demonstrate either increased resistance (high responder) or susceptibility (low responder) to challenge infection with T. colubriformis after vaccination with irradiated larvae. Circulating blood eosinophilia, a hallmark of helminth infections, was examined in third generation lambs from two separate selective matings and random bred control lambs. Numbers of eosinophils were higher in high responder lambs when compared to low responders after vaccination and challenge infections. Analysis of eosinophil counts confirmed a strong line effect and there was no evidence of a sex effect. Random bred lambs showed wide individual variations in eosinophil numbers and their response to infection. It was concluded that peripheral eosinophilia was more a measure of host responsiveness to infection than an indicator of helminthiasis. As such the eosinophil may serve as an indicator of the hosts ability to respond to T. colubriformis vaccination and infection.

Population dynamics of Trichostrongylus colubriformis and Ostertagia circumcincta in single and concurrent infections.

Twenty-one-week-old worm-free pen-reared lambs were infected weekly with either 10,000 T. colubriformis larvae, 5000 O. circumcincta larvae, or with both species (15,000 larvae per week). Larval establishment and total worm burdens were estimated after 4, 7, 10 and 13 weeks of infection. Faecal egg counts and lamb bodyweights were measured weekly, and numbers of eosinophils in blood were estimated before infection and at weeks 5, 8 and 14. For both species of worms, the dynamics of infection (establishment, worm burdens, egg counts) were not affected by concurrent or pre-existing infection with the other species. Infection with T. colubriformis alone did not protect against O. circumcincta, but infection with O. circumcincta alone provided slight protection against the T. colubriformis larvae. Blood eosinophils increased between 5 and 8 weeks of infection and were similar for the three infections. This corresponded to the reduction in establishment for both species.

Phenotypic markers for selection of nematode-resistant sheep.

Nematode-resistance of an animal can be defined as an enhanced natural ability, relative to its peers, to both prevent establishment of larval nematodes and evict any that do establish. These parameters are not measurable in a practical sense and consequently nematode-resistance has usually been defined in terms of low faecal nematode egg counts (FEC). Studies in New Zealand and Australia have demonstrated that nematode-resistance, as measured by FEC, has a heritability of about 0.3 in Romney and Merino sheep. However as a selection trait FEC has practical limitations and its use may incur production penalties through withholding drench treatment for prolonged periods or from a need for artificial challenge. FEC is influenced by the level and composition of a natural nematode challenge and especially the expression of the immune response. Thus immunological parameters which reflect the underlying genetic resistance could potentially be used as phenotypic markers. Ideally, a useful phenotypic marker would be easy to sample and its assay would be inexpensive and able to be automated, in addition to being strongly correlated with nematode-resistance. Results from several New Zealand trials have indicated that antibody levels (particularly IgG1) to excretory/secretory antigens of L3 nematodes such as Trichostrongylus colubriformis may meet these criteria. Levels of antibody against L3 antigens are also independent of on-farm drenching strategies. Blood eosinophil count has also been considered for use as a selection parameter but a high degree of sample variability reduces its potential. Other immunological parameters associated with nematode-resistance which have potential as phenotypic markers include serum nematode-specific IgE and products of mucosal mast cells such as proteinases. It is likely that as the critical immune responses of sheep to nematodes become more clearly defined, new immunological parameters with potential for use as phenotypic markers will be found. The definition of these immune responses will also assist in the identification and characterization of genetic markers.

Relationship between eosinophilia and responsiveness to infection with Trichostrongylus colubriformis in sheep.

Lambs with genetically determined increased immunological responsiveness to Trichostrongylus colubriformis (high responders) had more eosinophils in cutaneous reactions to the mitogen phytohaemagglutinin (PHA) both before and during infection compared with those bred for susceptibility (low responders). In contrast, eosinophil numbers in both blood and cutaneous reactions elicited by third-stage T. colubriformis larval antigen were similar in high and low responders before infection. Following vaccination and challenge, high responders had elevated eosinophil numbers in blood and antigen-stimulated skin. In unselected sheep, although eosinophil numbers in cutaneous reactions to PHA were related to responsiveness to a challenge infection with T. colubriformis, there was a closer relationship between blood eosinophil numbers and responsiveness. Infection with T. colubriformis increased eosinophil numbers in cutaneous reactions to PHA and appeared to augment the difference between eosinophil counts in high and low responder sheep. Measurement of the ability to produce eosinophil activating factors, or for eosinophils to respond to such factors might therefore be useful in identifying individual sheep with increased responsiveness to T. colubriformis infection.

The value of circulating eosinophil count as a selection criteria for resistance of sheep to trichostrongyle parasites.

In merino sheep bred for either increased or decreased resistance to Haemonchus contortus, faecal worm egg counts (FEC) were lower in the resistant line (6,831 vs 17,645 epg, P < 0.01), and circulating eosinophils (EOS) were higher, but not significantly so (3.40 x 10(4) ml(-1) vs 1.40 x 10(4) ml(-1), P = 0.1 1). Another flock was artificially infected with Trichostrongylus colubriformis and significant genetic variation was found in both FEC (heritability 0.40 +/- 0.11) and EOS (0.19 +/- 0.08). In a third flock comprising tropical sheep breeds, a natural challenge with T. colubriformis resulted in significant sire effects on FEC (heritability 0.20 +/- 0.10) but not EOS (heritability inestimable). We conclude that EOS offers no advantage over FEC as it selection criterion for resistance.

Genetic control of resistance to helminths in sheep.

Uncertainties over the continued effectiveness of currently available anthelmintics and the massive costs associated with development of new drugs have provided an impetus to search for alternative measures to control gastrointestinal nematodes in sheep. One option is to exploit the genetically determined variability in resistance existing within host populations. A number of selection experiments, comprising divergent and control lines, have been initiated to investigate the nature of this genetic regulation. It was found that the heritability of worm-egg counts in faeces after infection ranges from 0.2 to 0.4, indicating that worthwhile genetic gains can be achieved in commercial breeding programmes. Immune responses directed against parasites are under genetic control and appear to be the major factor responsible for the interline differences. Consequently, selection for increased resistance to gastrointestinal nematodes has resulted in an enhanced reactivity across a broad range of immunological functions (humoral, cellular and effector responses). These mechanistic studies have relevance to the development of vaccines and vaccination strategies, as well as for the application of phenotypic and genetic markers to measure resistance more accurately or to identify genetically resistant animals independently of infection.

Induction of tissue eosinophilia by platelet-activating factor in Merino sheep.

A study was undertaken on the capacity of platelet-activating factor (PAF) to induce eosinophil accumulation in the mammary glands of non-lactating sheep. Platelet-activating factor induced dose-dependent accumulation of eosinophils in mammary exudates 24 h after infusion. Infection, by intraruminal injection of 20,000 infective Trichostrongylus colubriformis larvae, did not modify the responsiveness of outbred sheep to intramammary infusion of PAF. Mature ewes from high and low responder lines of a flock of sheep, selected on the basis of their responses to vaccination and experimental challenge with T. colubriformis as lambs, did not differ in the magnitude of the eosinophil responses to doses of PAF from 5 x 10(-13) to 5 x 10(-7) mol per gland. Intramammary infusion of an extract from third stage larvae of Haemonchus contortus elicited inflammatory exudates containing five- to ten-fold more eosinophils than that elicited by the highest dose of PAF tested. The experiments indicate that the eosinophil chemotactic agonist PAF can induce tissue eosinophilia in sheep and thus may play a role in directing the accumulation of eosinophils in tissues during disease states such as gastrointestinal parasitism.

Molecular analysis and nematode resistance association of a polymorphism at the 5' end of the sheep IgE gene.

Previous work using Southern analysis of genomic DNA detected a polymorphism at the 5' end of the sheep and cattle IgE gene. Identical length differences found between fragments following digestion with restriction enzymes indicated that the basis for the polymorphism was an insertion/deletion event. To characterise the polymorphism, the entire cattle and sheep Cvarepsilon genes were sequenced including 668bp of 5' untranslated DNA. Sequence comparison revealed a high degree of similarity between the ovine and bovine genes at both the nucleotide and amino acid level. A feature of the 5' untranslated DNA was the presence of an 87bp repeat starting at -365 upstream of the Cvarepsilon start site. PCR primers were designed to span most of the 5' untranslated sequence, including the repeat unit, and used to amplify genomic DNA from a panel of 40 sheep. Three alleles were found with frequencies of 0.7, 0.29, 0.01 which were identical to the Southern analysis results. Sequencing of the two commonest alleles revealed the basis for the polymorphism was a 36bp deletion from the 87bp repeat. Association studies in a sheep selection flock phenotypically assessed for parasite resistance found a highly significant association between one of the IgE alleles and resistance to the intestinal nematode parasite Trichostrongylus colubriformis (P=0.005). Attempts to confirm this finding in two other flocks using linkage analysis and genotype association failed to find any significant associations between the IgE polymorphism and resistance to either T. colubriformis or Haemonchus contortus.

Development of a new monoclonal antibody to ovine chimeric IgE and its detection of systemic and local IgE antibody responses to the intestinal nematode Trichostrongylus colubriformis.

The J558L cell line, previously transfected with the ovine Cepsilon gene, was induced to secrete a chimeric IgE protein composed of the ovine heavy chain and a mouse light chain with MW of approximately 80 and 26 kDa, respectively. After purification, the chimeric protein was used to immunise BALB-c mice and monoclonal antibodies (mAbs) were generated. The mAb 2F1, which had greatest anti-IgE activity in preliminary screens, was chosen for further characterisation and an examination of systemic and local IgE responses to the intestinal nematode, Trichostrongylus colubriformis. The chimeric IgE protein was not recognised in enzyme linked immunosorbent assay (ELISA) by mAbs raised against ovine IgG1, IgG2, IgA or IgM. However, 2F1 was highly specific to the chimeric IgE protein, and did not cross-react with ovine IgG1, IgG2 or IgA. Western blot analysis also showed that 2F1 and secretory IgA (sIgA) did not cross-react, and that 2F1 and the anti-IgA mAb identified different MW bands from colostrum (approximately 200 and 400 kDa, respectively). 2F1 bound to mucosal mast cells (MMC) isolated from the intestines of lambs infected with T. colubriformis, but cultured bone marrow-derived mast cells (BMMC) required prior incubation with the chimeric IgE protein for this binding to occur. Distinctive staining of plasma cells and putative mast cells were observed using 2F1 on immunohistological sections of mesenteric lymph node and jejunum.ELISA incorporating 2F1 was able to detect >0.4 ng chimeric protein. Total IgE in ovine colostrum and intestinal homogenates was quantified using a capture ELISA, with known amounts of chimeric protein used to produce a standard curve. Colostrum from outbred Merino ewes had 0.55-11.05 ng ml(-1) total IgE, and their lambs, at necropsy after infection with a total of 18,000 T. colubriformis infective larvae over a 9-week period, had 45-620 ng g(-1) total IgE in intestinal tissue. Compared to genetically susceptible lambs, antigen-specific levels of IgE were significantly higher in genetically resistant lambs after infection with 4500 T. colubriformis infective larvae (TcL3) per week for 9 weeks (161.4 versus 44.8 geometric mean titres; P=0.043). In western blots, distinctive bands (19-21 and 27 kDa) from T. colubriformis larval antigen were differentially recognised by IgE, as identified by 2F1, in intestinal homogenates from genetically resistant animals. These results have demonstrated the value of 2F1 for quantification of IgE responses in samples derived from ovine fluids and tissues using ELISA, western blots and immunohistology. In this respect, it recognises native ovine IgE and does not require pre-treatment of the sample with denaturing agents or ammonium sulphate.

Effects of dietary protein intake on responses of young sheep to infection with Trichostrongylus colubriformis.

The effects of protein supplementation and infection with Trichostrongylus colubriformis on production responses and immune function in young sheep and on nematode population dynamics were assessed. Eighty-four 3-month-old Merino wether sheep were housed in individual pens and fed ad libitum chopped oaten hay containing 0.5% urea, together with 50 g day-1 lucerne meal. Fish meal (FM) was given as a source of protected protein at 0, 50 or 100 g day-1 (FM0, FM50, FM100; from Days --28 to 140). From Days 1 to 140, 0 or 1000 T. colubriformis infective larvae were given on Mondays, Wednesdays and Fridays. Infected sheep were slaughtered after 35, 70, 105, or 140 days of infection. Live-weight gain was reduced significantly by infection with T. colubriformis in sheep given FM0, but not in sheep given FM50 or FM100. Greasy wool production and fibre diameter were increased by FM, whereas the effects of infection with T. colubriformis on wool measurements depended on the level of FM given. Worm egg concentrations in faeces were significantly lower for sheep given FM100 than for those given FM0 or FM50 during the last 28 days of infection. Similarly, the apparent rate of worm expulsion was considerably higher in sheep given FM than in those not given FM. The rate of expulsion of T. colubriformis correlated with levels of circulating eosinophils as well as with the concentration of intestinal sheep mast cell proteases. Levels of parasite-specific and non-specific circulating antibodies were either unaffected or reduced as a result of supplementation with FM, although lymphocyte stimulation in vitro in response to T. colubriformis third stage larval antigen was enhanced significantly in infected animals given FM100. It was concluded that supplementary feeding with FM substantially reduced the production losses attributable to infection with T. colubriformis and was associated with enhanced expulsion of the parasite burden.

Some cutaneous responses to mitogen injection in sheep, with special reference to eosinophil leucocytes.

Research in laboratory animals suggests that eosinophil numbers in some cutaneous reactions may reveal inherent differences in resistance to parasite infection. An intradermal injection with 10 microgram of the mitogen phytohaemagglutinin, followed by counting eosinophils in the deep dermis of skin biopsies removed 24 hours later, appears to be a practical and reproducible way of comparing the ability of individual sheep to marshal eosinophils in a tissue. This procedure will allow study of the relationship between eosinophil response and resistance to parasite infection in sheep.

Basophil leucocytes in responses to parasitic infection and some other stimuli in sheep.

Basophil leucocytes are a significant component of the infiltrating cells in a variety of tissue reactions in guinea pigs. However, little is known about the participation of basophils in similar reactions in most other animal species. The circulating blood, skin and small intestinal mucosa of sheep were examined after they had received stimuli known to elicit basophil-rich responses in guinea pigs but relatively few basophils were found.

Selective breeding for the control of nematodiasis in sheep.

Genetic manipulation of sheep by selective breeding offers a means to reduce the current reliance on chemotherapy for the control of gastro-intestinal nematodes. Simulated epidemiological studies support this view as, compared to lambs of 'normal' susceptibility, those 'selected' for resistance to Trichostrongylus colubriformis have lower worm burdens and reduced production losses. Considerable genetic variation exists both between and within breeds of sheep, and a number of breeding programmes have demonstrated that selection for animals with heightened levels of resistance to nematodes is feasible. Animals from these selection experiments are currently being used to investigate the nature of this genetic regulation and the economic benefits that can be achieved. An understanding of the mechanisms of resistance, facilitated by having animals with defined extremes of responsiveness, is crucial for studies into the specificity of selection, identification of predictive markers with resistance, and determination of suitable vaccines and vaccination strategies in unselected populations. Immunity plays a major role in host resistance to parasites, and from studies with selected animals, it appears that a broad range of immune responses are under genetic control. Genetic diversity within the parasite population may manifest itself in adaptation to withstand host resistance mechanisms. Such an occurrence could limit the effectiveness of the genetic approach.

The influence of reproductive physiology and nutrient supply on the periparturient relaxation of immunity to the gastrointestinal nematode Trichostrongylus colubriformis in Merino ewes.

A pen experiment was conducted to investigate the interaction of early-weaning and nutrient supply on the periparturient relaxation of immunity to the gastrointestinal nematode (GIN) Trichostrongylus colubriformis in Merino ewes. Mixed-age pregnant and non-pregnant (dry) ewes were infected with 8,000 T. colubriformis L(3)/week, and fed either a high or low quality diet. Following parturition, lambs were either removed from their mothers at 2 days of age or allowed to continue suckling. Systemic immunity began to wane during late pregnancy with circulating eosinophils and plasma total antibody (Ab) levels declining from day -37 (relative to the midpoint of lambing) and day -24, respectively. Pregnant ewes fed the low quality diet exhibited an increasing faecal worm egg count (WEC) from day -24 and had higher intestinal worm burdens on day 13, whereas ewes fed the high quality diet had a delayed transient rise in WEC of lower magnitude. Dry and early-weaned ewes remained highly resistant to T. colubriformis at all times. In the post-lambing/lactation period, ewes fed the high quality diet had higher levels of local total Ab and numbers of goblet cells (GC) in the small intestine on days 13 and 41. Lactating/suckled ewes had a lower anti-parasite local immune response as indicated by reduced titres of total Ab, IgG(1), IgM and IgA and lower numbers of mucosal mast cells (MMC), globule leukocytes (GL) and GC in small intestinal tissue compared to their dry and early-weaned counterparts. Early-weaning resulted in rapid recovery of blood eosinophils and total Ab. On day 13 post-lambing, titres of total Ab, IgG(1), IgM, IgA and IgE, and numbers of MMC and GL were greater than those measured in dry and suckled ewes. When fed the high quality diet, ewes had a higher dry matter (DM) intake, maternal weight, fat score, greater fat depth and eye muscle depth, birthed heavier lambs that had higher growth rates, and produced more milk. The physiological status of pregnancy resulted in a higher DM intake but lower measures of fat depth and eye muscle depth, and suckling led to an increase in DM intake but a reduction in body weight and fat score through mobilisation of fat and muscle reserves. Despite the marked effect of diet quality on production traits, some inconsistencies were observed between body composition and apparent parasite resistance, measured by WEC and worm counts, suggesting that the nutritional influence was not necessarily always mediated through changes in body composition. Although reproductive status affected blood leptin levels, diet had no effect within suckled ewes and therefore it was concluded that leptin has no causative role in maintaining the periparturient relaxation of immunity to T. colubriformis.

The periparturient relaxation of immunity in Merino ewes infected with Trichostrongylus colubriformis: endocrine and body compositional responses.

The temporal association between the periparturient rise in worm egg count of grazing Merino ewes to infection with Trichostrongylus colubriformis and the underlying causal mechanisms was investigated in an experiment that incorporated two levels of pregnancy (pregnant or unmated), two levels of infection (infected with 6000 T. colubriformis L(3)/week or uninfected) and, following lambing, three levels of lactation (non-lactating/"dry", early-weaned 2 days after parturition, or suckled). In addition to parasitological and host immune responses reported in a previous paper (Beasley et al., 2010), a range of endocrine and body compositional changes were monitored from day -50 to day 42, relative to the midpoint of lambing (day 0). By day -19, pregnant ewes had begun to mobilise fat and eye muscle, and after 42 days of suckling had lost 31% and 8%, respectively, of their existing depots. In comparison to non-pregnant (dry) ewes, the endocrine profiles of late-pregnant ewes were characterised by low levels of cortisol and prolactin and high levels of progesterone and oestradiol. Lactating ewes had lower levels of cortisol and leptin and higher levels of prolactin compared to both dry and early-weaned ewes. The mobilisation of fat and protein reserves throughout lactation in suckled ewes was closely associated with leptin and cortisol profiles, and provided strong evidence of an underlying nutritional basis for the periparturient relaxation of immunity. Both leptin and cortisol concentrations were also associated with both parasite burden and the immune status of the ewe. It is suggested that lower blood cortisol levels in suckled ewes contribute to a Th1 biased immune response that leads to an increased susceptibility to gastrointestinal nematodes. The results provide a detailed characterisation of the physiology underlying the periparturient relaxation of immunity to T. colubriformis, from which further investigations will aim to expose potential causal factors.

The periparturient relaxation of immunity in Merino ewes infected with Trichostrongylus colubriformis: parasitological and immunological responses.

The temporal association between the periparturient rise (PPR) in worm egg count (WEC) of grazing Merino ewes to infection with Trichostrongylus colubriformis and the underlying causal mechanisms was investigated in an experiment that incorporated two levels of pregnancy (pregnant or unmated), two levels of infection (infected with 6000 T. colubriformis L(3)/week or uninfected) and, following lambing, three levels of lactation (non-lactating/"dry", early-weaned 2 days after parturition, or suckled). The 128 ewes in the experiment were monitored for a range of parasitological and host systemic and local immune responses from days -50 to 42, relative to the midpoint of lambing (day 0). Unmated/non-lactating (dry) ewes remained largely resistant to T. colubriformis infection with WEC remaining below 200 eggs per gram (epg) throughout the experiment. For infected ewes, the PPR in WEC commenced during late pregnancy and peaked at 1552epg on day 42. There was evidence of a relaxation of immunity that preceded the rise, as demonstrated by a reduction in circulating eosinophils and plasma total antibody (Tab) and lower titres of Tab and IgG(1) in the small intestine in pregnant ewes. Circulating eosinophils and plasma Tab remained low in suckled ewes, and at the local level, lactating ewes also had lower titres of Tab, IgG(1), IgM, IgA and IgE as well as fewer mast cells (MC), globule leucocytes (GL) and goblet cells (GC) in intestinal tissue. These effects were associated with higher worm burdens. Weaning at 2 days after lambing resulted in a rapid amelioration of immunological responsiveness and a reduction in WEC, worm burden and parasite establishment rate. A rapid increase in circulating eosinophils and Tab titre followed by an improvement in components of the local immune response was also observed. Over the 6-week period following lambing, weaned ewes exhibited higher titres of Tab, IgG(1), IgM, IgA and IgE as well as higher numbers of MC, GL and GC present in the small intestinal tissue, compared to lactating ewes. The results provide a detailed characterisation of the periparturient relaxation of immunity to T. colubriformis, and highlight possible causative factors that deserve further investigation.

Selective transport of IgE into ovine mammary secretions.

Immunoglobulin (Ig) E is actively transported into ovine colostrum. Here we examine the degree of selectivity and the mechanism by which this transfer occurs in sheep. Results indicate that during colostrogenesis in sheep, transfer of immunoglobulins was most selective for IgG1 and IgA followed by IgE, IgM and IgG2. In milk, selectivity was greatest for IgA, followed by IgM, IgE, IgG1 and IgG2. The neonatal Fc receptor (FcRn) and poly immunoglobulin receptor (pIgR) mediate the transport of IgG1 and IgA across the ovine mammary epithelium respectively. In primates and rodents, the low-affinity IgE receptor, Fc epsilonRII, functions to transport IgE across the intestinal epithelium. We therefore investigated the expression of the low-affinity IgE receptor (CD23), pIgR and FcRn transcripts in the ovine mammary gland. The expression profiles of FcRn, pIgR and CD23 mRNA reflected concentrations of their Ig ligands in mammary secretions. These findings suggest a role for CD23 in transport of IgE across the mammary epithelium of sheep.