Early-life dietary spray-dried plasma influences immunological and intestinal injury responses to later-life Salmonella typhimurium challenge

The gastrointestinal (GI) tract continues to undergo significant developmental changes in postnatal life. Environmental influences during this critical developmental period, including diet, stress and mucosal injury, have been shown to induce long-term changes in intestinal physiology and disease susceptibility in animal models(1 – 4). Similarly, in human subjects, increasing epidemiological evidence supports the concept that adverse early-life environmental factors, such as stress, are associated with subsequent GI diseases such as irritable bowel syndrome(5 – 9). In the case of pigs, early (age ,21 d) weaning of piglets is a significant, early-life stress that has been shown to have deleterious impacts on GI function, including increased intestinal permeability(10,11), inflammation(12), hypersecretion(10), reductions in the activity of brush-border digestive enzymes(13), altered nutrient transport mechanisms (14,15), and marked changes in villus and crypt morphology (reduced villus surface area and increased crypt depth)(16). The mechanisms and factors associated with weaning stress (e.g. maternal and littermate separation, dietary changes, and transport stress) are not completely understood yet; however, it was demonstrated by Moeser et al.(10) that activation of the corticotropin-releasing factor (CRF) receptor system in the intestine, and subsequent activation of mast cells, were responsible for increased intestinal permeability and hypersecretion, demonstrating the role of stress signalling pathways in the intestine of the weaned pig. It is now evident that the deleterious effects of early weaning stress on the intestinal tract of the pig are seen well beyond the immediate postweaning (PW) period. Smith et al.(17) demonstrated that early-weaned pigs (weaned between 15 and 21 d of age) exhibited greater intestinal permeability at 9 weeks PW, compared with late-weaned pigs (weaned between 23 and 28 d of age). In addition, McLamb et al.(1) showed that early-weaned pigs exhibited heightened clinical disease (increased severity of diarrhoea and reduced growth rate) and intestinal injury (increased intestinal permeability), in response to an enterotoxigenic Escherichia coli (ETEC) challenge at approximately 3 weeks PW. On the whole, results from the aforementioned studies provide strong evidence that PW intestinal injury can have lasting deleterious impacts on intestinal function. Therefore, therapeutic approaches to ameliorate GI injury during the PW period could have a positive impact on long-term barrier function and defence against subsequent pathogenic challenges.

Dietary inclusion of spray-dried plasma (SDP) proteins in nursery pig diets has proven to have a beneficial effect on PW gastrointestinal health and performance in young pigs(18,19). Previous studies demonstrated that SDP not only promotes growth responses in young pigs, but also confers protective effects in GI infectious challenge models. Van Dijk et al.(20) demonstrated that weaned pigs challenged with K88 ETEC, and fed a nursery diet containing 8% SDP, exhibited reduced diarrhoea and increased average daily gain and average daily feed intake, compared with pigs fed control diets containing whey protein. In another experiment, weaned pigs fed with diets containing 6% SDP exhibited reduced cytokine responses and intestinal inflammatory cell infiltrates, following a challenge with ETEC(21). Similarly, diarrhoeal disease induced by an experimental rotavirus challenge has been observed to be reduced in neonatal piglets fed a diet containing 15% SDP compared with control diets containing soya protein isolate(22). Peace et al.(18) confirmed that inclusion of SDP at dietary levels of 2·5 and 5% for 2 weeks PW reduced intestinal permeability, intestinal inflammatory cytokines and diarrhea in early-weaned pigs. However, in the aforementioned previous experiments, growth responses and intestinal protective effects of SDP described were measured while SDP was in the diet; whether inclusion of SDP in early-life pig diets retains beneficial effects after its removal from the diet has not been investigated. Given that early-weaning stress induces short- and long-term deleterious changes in intestinal function and disease susceptibility, and that SDP has proven beneficial in reducing early changes in intestinal permeability and inflammatory responses in weaned pigs, we hypothesized that inclusion of SDP in PW pig diets would have sustained beneficial effects on intestinal responses to a later-life pathogenic challenge, even after the removal of SDP from the diet. The specific objective of the present study was to determine whether the inclusion of SDP during the first 2 weeks PW influenced intestinal epithelial barrier function, immune responses and clinical disease in response to a later-life challenge with Salmonella typhimurium.

All procedures were approved by the North Carolina State University Institutional Animal Care and Use Committee (protocol no. 12-051-A).

A total of thirty-two Yorkshire–Large White piglets weaned from 16 to 17 d of age with a similar body weight of 5·49 (SEM 0·1) kg were used in the present experiment. The weaned piglets were housed in four nursery pens (eight pigs per pen; 1·09m2/pig) and were offered ad libitum access to water and one of three experimental nursery diets containing either 0% SDP (fed to two pens, n 16 pigs), 2·5% SDP (fed for 7 d PW; n 8 pigs) or 5% SDP (fed for 14 d PW; n 8 pigs) (Fig. 1). Sex and litter origin were distributed equally across the experimental groups. The variable dietary levels of 2·5 and 5% SDP along with feeding duration PW (7 v. 14 d PW) were selected to mimic the range of dietary level and feeding duration of SDP commonly utilised in commercial swine feeding. Diets were supplied in mashed form, and were formulated to contain identical levels of metabolisable energy and digestible lysine to meet nutrients requirements of the NRC (1998)(11). At 7d PW, pigs fed with the 2·5%SDP treatment were switched to control (0% SDP) diets. At 14 d PW, all pigs were fed the same diet (0% SDP), and maintained in the nursery for an additional 21 d.

Fig. 1. Experimental design. Piglets (n 8 per treatment) were weaned from their sows and offered nursery diets containing either 0% spray-dried plasma (SDP), 2·5% SDP for 7 d post-weaning (PW) or 5% SDP for 14 d PW. SDP was removed from the experimental diets at indicated times and fed identical diets to the control pigs. At 34 d PW, pigs were challenged with Salmonella typhimurium. At 2 d post challenge, tissues were harvested for analysis. P-1, phase 1 diet; P-2, phase 2 diet. (A colour version of this figure can be found online at http://www.journals.cambridge.org/bjn

At 34 d PW, all pigs were transferred from the nursery to isolation rooms located in a nearby North Carolina State University research facility at the College of Veterinary Medicine campus. Upon arrival, the pigs were housed, by treatment, with eight pigs/pen (0·3m2/pig). The pens were equipped with tenderfoot flooring, and pigs were allowed ad libitum access to feed and water. On the following day, eight pigs from each experimental group were inoculated orally with 3 £ 109 colony-forming units of S. typhimurium in 4ml of culture media as described previously(23). A non-challenged control group was housed in a separate, identical room within the facility, and was administered similarly with 4ml sterile media. The S. typhimurium DT104 strain used in the present study exhibited antimicrobial resistance to ampicillin, chloramphenicol, sulfisoxazole, streptomycin and tetracycline.

Salmonella cultures were grown overnight at 378C on Luria broth agar, and then added to a sterile 0·7% saline solution to obtain a final concentration of 7·5 £ 108 colony-forming units per ml, and verified using a NanoDrop 2000c nanospectrometer (Thermo Fisher Scientific, Inc.). In the present study, we chose S. typhimurium as the challenging agent because it has dual relevance to human and swine diseases(24).

Body weight (BW) was recorded at days 0 and 14 during the PW nursery phases, and on days 0 and 2 of the S. typhimurium challenge study, and average daily gain was calculated. Given the short (2 d) challenge period, growth data were presented as the percentage of BW loss. Pen feed intakes were recorded during the PW and S. typhimurium challenge periods, and estimated feed intake per pig was calculated for each pen. Faecal scores were analysed by persons, who were blinded to the experimental treatments, according to a previously published scoring system by our group(1) using a scale from 1 (no diarrhoea) to 4 (severe profuse diarrhoea).

On day 2 post-challenge, pigs were sedated with a TKX cocktail containing Telazol (500 mg), Ketamine (250 mg) and Xylazine (250 mg) that were administered intramuscularly at a dose of 0·025 ml/kg BW. Euthanasia was followed by the administration of an overdose (86 mg/kg BW) of sodium pentobarbital solution (Fatal-Plus; Virbac Animal Health) via a catheterised ear vein. The distal small intestine (ileum) was harvested from each pig immediately after euthanasia, and opened along the anti-mesenteric border. Intestinal mucosa was stripped from the seromuscular layer in oxygenated (95% O2 and 5% CO2) Ringer solution (in mmol/l: 154 Naþ, 6·3 Kþ, 137 Cl2, 0·3 H2PO4, 1·2 Ca2þ, 0·7 Mg2þ and 24 HCO3 2; pH 7·4), and mounted in 1·13 cm2 aperture Ussing chambers (World Precision Instruments, Inc.). Ileal mucosa was bathed on the serosal and mucosal sides with 10 ml Ringer solution. The serosal bathing solution contained 10mM-glucose, which was osmotically balanced on the mucosal side with 10mM-mannitol. Bathing solutions were oxygenated (95% O2 and 5% CO2) and circulated in water jacketed reservoirs maintained at 378C. The spontaneous potential difference (PD) was measured using Ringer-agar bridges connected to calomel electrodes, and the PD was short-circuited through Ag–AgCl electrodes using a voltage clamp that corrected for fluid resistance. Tissues were maintained in the short-circuited state, except for brief intervals to record the open-circuit PD. Transepithelial electrical resistance (TER, measured as V·cm2) was calculated from the spontaneous PD and short-circuit current (Isc), as described previously(25). After a 30 min equilibration period on Ussing chambers, TER and Isc were recorded at 15 min intervals over a 1 h period, and then averaged to derive the basal TER and Isc values for a given animal.

After a 30 min equilibration period on Ussing chambers, 4 kDa fluorescein isothiocyanate dextran (FD4, 100 mg/ml; Sigma) was added to the mucosal bathing reservoir of Ussing chambers. Standards were taken from the serosal side of each chamber 15 min after the addition of FD4, and a 60 min flux period was established by taking 0·5ml samples in triplicate from the mucosal compartment. The quantity of FD4 was established by measuring fluorescence in mucosal reservoir fluid samples in a fluorescence plate reader at 540 nm. Data are presented as the rate of FD4 flux in mg FD4 flux/min per cm2.

Ileum was fixed in 10% neutral-buffered formalin and processed for paraffin embedding. Paraffin blocks were sectioned (5 mm thick) and stained with haematoxylin and eosin for histological analysis. A histological scoring system was applied to the tissue sections, and was performed by a board-certified veterinary pathologist (L. B. B.), who was blinded to the experimental treatments. The intestinal scoring system used was based on villus morphology and blunting (villus height and crypt depth), villus fusion, reduced lymphoid recruitment and neutrophil numbers. The detailed scoring criteria were designated as follows: villus blunting–0¼crypt:tip ratio of at least 1:4, 1 ¼ crypt:tip ratio of 1:3, 2 ¼ 1:2, 3 ¼ 1:1 and 4 ¼ complete tip loss; lymphoid depletion and villus fusion – 0 ¼ normal, 1 ¼ mild, 2 ¼ moderate and 3 ¼ severe; neutrophils – 0 ¼ none to 10 neutrophils/40 £ field, 1 ¼ 11–20 neutrophils/40 £ field, 2 ¼ 21–30 neutrophils/40 £ field and 3 ¼ 31–40 neutrophils/ 40 £ field. Neutrophils were identified based on nuclear and cytoplasmic morphology(26). Measurements for crypt depth and villus height were taken utilising the calibrated measurement caliper option, and villus measurements were taken from three well-oriented villi in five different fields/slide, such that fifteen villi/slide per pig were measured. Villi chosen for measurements were based on the criteria that (1) the entire crypt and villi be captured in the cross-section, and (2) the central lacteal be present. Villi overlying the gut-associated lymphoid tissue were excluded from the measurement. Photomicrographs were acquired with 20 £ and 40 £ magnifications at a resolution using imaging software (Infinity Analyze Software), running a high-resolution digital camera (Lumenera) equipped to a clinical light microscope (Model OMFL400; Meiji Microscope Solutions).

Ileal mucosa was homogenised in PBS containing protease inhibitors, and the supernatant was collected and analysed for protein content using a BCA assay(18). Samples were then diluted 1:10 in PBS and assayed for TNF, IL-8 and IL-6, using commercial porcine ELISA assays (R&D Systems). Concentrations of each cytokine are expressed on a per mg protein basis.

The distal ileum was obtained from each pig, opened lengthwise, and rinsed in cold Ringer solution. The epithelium and lamina propria were scraped from the seromuscular layers over ice using a glass slide, and then frozen in liquid N2 and stored at 2808C. The ileal mucosal scrapings were thawed and homogenised in 0·5% hexadecyltrimethylammonium bromide buffer (50mM-phosphate buffer, pH 6), to release myeloperoxidase (MPO) from the primary granules of neutrophils. The homogenate was subjected to three cycles of freezing at 2808C, thawed, and sonicated on ice. Samples were centrifuged at 21 000 g at 48C for 15 min, and the supernatant assayed for MPO activity. An aliquot of the supernatant was allowed to react with a solution of tetramethylbenzidine in N-dimethylformamide and H2O2. Absorbance (655 nm) readings were taken at 30 s intervals over 15 min. MPO activity was determined based on a MPO standard curve, and is expressed as units per g (wet weight) mucosa (for ileum) or per ml of plasma(27).

Ileal mucosal protein was extracted from mucosal scrapes using Mammalian Protein Extraction Reagent containing protease and phosphatase inhibitors (Fisher Scientific). Samples were sonicated and centrifuged at 14 000 rpm for 15 min at 48C. Protein concentration was determined using the Pierce BCA Protein Assay Kit (Fisher Scientific). Total protein was resolved by SDS–PAGE, and transferred to polyvinylidene difluoride membranes. The membranes were blocked with 5% (w/v) non-fat milk in Tris-buffered saline with 0·1% Tween-20 (TBS-T) for 1 h at room temperature, washed in TBS-T, and incubated with CRF-RI/II antibody that detects both receptors (Santa Cruz Biotechnology). Subsequently, the membranes were washed and incubated with an appropriate secondary antibody for 1 h at room temperature, followed by washing with TBS-T and incubation with the SuperSignal West Pico Chemiluminescent Substrate (Fisher Scientific). As an internal loading control, the antibody was stripped from the membranes with Restoree Western blot stripping buffer (Thermo Fisher Scientific, Inc.), and the membranes were reprobed with a b-actin antibody (Cell Signaling Technology). Bands were visualised with ChemiDoce MP Imaging System (Bio-Rad), densitometric analysis was performed using the Bio-Rad Image Lab software (version 4.1), and the CRF receptor band intensities were normalised to b-actin.

Data are presented as means with their standard errors based on the experimental sample number (n). With the exception of histological and faecal score data, all other data were analysed using a standard one-way ANOVA (SigmaStat; Jandel Scientific). A post hoc Tukey’s test was used to determine the differences between treatments following ANOVA. Statistical significance was set at a level of P,0·05. Histological and faecal scores were analysed using the non-parametric Kruskal–Wallis test (GraphPad Prism) with Dunn’s post-test.

In the first 2 weeks PW, estimated feed intake for pigs fed diets containing 0, 2·5% SDP (for 7 d) and 5% SDP (for 14 d) was 0·221, 0·231 and 0·238 kg/d, respectively. The average daily gain (kg/d) during the first 2 weeks PW for pigs fed the 0, 2·5 and 5% SDP diets was 0·121 (SEM 0·015), 0·119 (SEM 0·021) and 0·142 (SEM 0·025), respectively. All pigs remained clinically normal throughout the nursery phase. During the 2 d S. typhimurium challenge study, at 34 d PW, the control (non-challenged, 0%SDP) pigs gained 5%of theirBW, whereas growth responses in pigs challenged with S. typhimurium were significantly reduced and ranged between 0·5 and 21% BW gain (Fig. 2(A)). Compared with the non-challenged control pigs, the estimated feed intake of S. typhimurium-challenged pigs over the 2 d challenge period was 1·54, 1·20, 1·22 and 1·17 kg in pens from the control, 0% SDP-, 2·5% SDP- and 5% SDP-challenged groups, respectively. All pigs challenged with S. typhimurium exhibited diarrhoea as indicated by higher (P,0·05) faecal scores, compared with the non-challenged control pigs (Fig. 2(C)). Rectal body temperatures were also significantly elevated in challenged pigs compared with the control ones (P,0·05; Fig. 2(B)). Dietary inclusion of SDP (2·5 or 5% SDP) during the PW period had no significant effect on BW loss, faecal score or body temperature in response to S. typhimurium challenge in the present study.

Fig. 2. Body weight, body temperature and faecal scores in pigs challenged with Salmonella typhimurium. (A) Body-weight loss was calculated from body weights recorded on day 0 and day 2 post-challenge. (B) Rectal temperature and (C) faecal scores were recorded on day 2 post-challenge. Values are means, with their standard errors represented by vertical bars (n 8). a,b Mean values with unlike letters were significantly different (P,0·05; one-way ANOVA). SDP, spray-dried plasma. , Control; , S. typhimurium; , 2·5% SDP þ S. typhimurium; , 5% SDP þ S. typhimurium

Compared with the non-challenged control group, S. typhimurium-challenged pigs exhibited elevated ileal histological injury scores (Fig. 3(A)). Histological scores from pigs fed with the 5% SDP-14 d diet during the nursery period were lower compared with those from the challenged control pigs. Marked lymphoid depletion, an index of an overwhelming immune response, was observed in all S. typhimurium-challenged pigs, but was less severe in pigs fed with the 2·5 %-7 d and 5% SDP-14 d PW diets. Extensive villus blunting (Fig. 3(A) and (B)) and fusion (adhesion) was observed in all pigs challenged with S. typhimurium; however, there were no effects of PW SDP treatments on theseparameters. Crypt depth was found to be increased (P,0·05) in the ileum from S. typhimurium-challenged pigs compared with the control ones (Fig. 3(C)). Ileum from pigs fed with the 5% SDP-14 PW diet had increased (P,0·05) crypt depth compared with all the other treatments. Increased numbers of ileal neutrophils (Fig. 3(A)) were observed in response to S. typhimurium challenge, which corresponded with higher activity of ileal MPO, a marker of neutrophil activation (Fig. 4(A)). MPO and neutrophil numbers were lower in the ileum from pigs fed with 5% SDP-14d diet in the PWperiod.

Fig. 3. Impact of early-life dietary spray-dried plasma (SDP) on histological damage caused by subsequent challenge with Salmonella typhimurium. (A) Histological scores, (B) villus height, (C) crypt length and (D) histological appearance from pig ileal tissues, at 2 d post-S. typhimurium challenge. Representative histological sections were taken at 4 £ and 20 £ magnification. Values are means, with their standard errors represented by vertical bars (n 8). a,b,c Mean values with unlike letters were significantly different (P,0·05; one-way ANOVA). , Control; , S. typhimurium; , 2·5% SDP þ S. typhimurium; , 5% SDP þ S. typhimurium. (A colour version of this figure can be found online at http://www.journals.cambridge.org/bjn

TNF concentrations were found to be elevated in the ileum from all S. typhimurium-challenged groups compared with the non-challenged control group (Fig. 4(C)). There was a trend (P¼0·06) for elevated TNF levels in response to S. typhimurium challenge in pigs fed the 5% SDP-14 d diet, compared with the other challenged treatments. Ileal IL-8 levels were increased in the S. typhimurium-challenged group (Fig. 4(B)); however, IL-8 levels were lower in the ileum from the challenged groups fed the 2·5 %-7 d or 5% SDP-14 d diet in the PW period (Fig. 4(B)).

To assess the effects of S. typhimurium infection and SDP nursery feeding on systemic inflammatory responses, plasma levels of MPO (Fig. 4(D)), TNF (Fig. 4(E)) and cortisol (Fig. 4(F)) were assessed 2 d post-challenge. Plasma TNF was elevated in all pigs challenged with S. typhimurium. In line with responses observed in the ileum, pigs fed with the 5% SDP-14 d nursery diet exhibited the greatest levels of plasma TNF in response to S. typhimurium challenge. Plasma cortisol levels tended to be elevated (P¼0·09) in the S. typhimurium-challenged control group, but were not different from the other experimental treatment groups.

Fig. 4. Impact of early-life dietary spray-dried plasma (SDP) on ileal and plasma immunological responses to subsequent Salmonella typhimurium challenge. (A) Ileal myeloperoxidase (MPO), (B) ileal IL-8, (C) ileal TNF, (D) plasma MPO, (E) plasma TNF and (F) plasma cortisol were measured at 2 d post-S. typhimurium challenge. Values are means, with their standard errors represented by vertical bars (n 8). a,b,c Mean values with unlike letters were significantly different (P,0·05; one-way ANOVA). * At trend for ileal TNF between 5% SDP and the other challenged treatments (P¼0·07) and for plasma cortisol between S. typhimurium control and control (P¼0·09). , Control; , S. typhimurium; , 2·5% SDP þ S. typhimurium; , 5% SDP þ S. typhimurium.

Utilising an antibody that recognises both CRF receptor subtypes (CRF1 and CRF2), we found that the CRF1/2 antibody recognised three major protein bands in porcine ileal protein extracts at approximately 55, 37 and 28 kDa (Fig. 5). These protein bands are consistent with the unprocessed form (55 kDa), the deglycosylated form (37 kDa) and the soluble CRF receptor forms (28 kDa)(28), and have been reported previously in studies on the rodent intestine(29,30). Based upon densitometric analysis, intestinal CRF1/2 receptor proteins (50 and 28 kDa bands) were markedly up-regulated (P,0·05) in response to S. typhimurium challenge. However, the nursery SDP treatment did not appear to influence the level of ileal CRF receptor expression in the challenged pigs.

Fig. 5. (A) Western blot and (B) densitometric analysis of corticotropin-releasing factor (CRF1/2) receptors (CRFR) in porcine ileal mucosal scrapes from control and Salmonella typhimurium-challenged pigs. (B–D) Densitometry values for each protein band normalised to b-actin are shown. Values are means, with their standard errors represented by vertical bars (n 3). a,b Mean values with unlike letters were significantly different (P,0·05; one-way ANOVA). SDP, spray-dried plasma. , Control; , S. typhimurium; , 2·5% SDP þ S. typhimurium; , 5% SDP þ S. typhimurium

At 2 d post-challenge, FD4 flux rates, an index of paracellular permeability, were elevated (P,0·05) in the ileum from pigs challenged with S. typhimurium (P,0·05) (Fig. 6(A)). Pigs fed with the 5% SDP-14 d diet during the PW period exhibited lower FD4 flux rates at 2 d post-challenge, compared with the other challenged treatment groups. Ileal TER was higher in the challenged pigs compared with the non-challenged control pigs (P,0·05; Fig. 6(B)). There was a trend (P¼0·06) for increased ileal TER in pigs fed the 2·5% SDP-7 d and 5% SDP-14 d diets during the PW period compared with the challenged control pigs. Transepithelial PD and short-circuit current (Isc) were reduced in S. typhimurium-challenged pigs compared with the non-challenged control pigs (Fig. 6(C) and (D), respectively). Pigs fed with the 5% SDP-14 d diet exhibited greater ileal PD, compared with the challenged control pigs (P,0·05). Ileal Isc was greater in pigs fed with the 2·5% SDP-7 d and 5% SDP-14 d diets compared with non-challenged control pigs.

Fig. 6. Impact of early-life dietary spray-dried plasma (SDP) on intestinal permeability and transepithelial short-circuit current (Isc) following subsequent Salmonella typhimurium challenge in pigs. (A) Fluorescein isothiocyanate dextran (FD4) flux rates, (B) ileal transepithelial electrical resistance (TER), (C) ileal transepithelial potential difference (PD), and (D) Isc were measured using the ileum mounted on Ussing chambers on 2 d post-S. typhimurium challenge. Values are means, with their standard errors represented by vertical bars (n 8). a,b,c Mean values with unlike letters were significantly different (P,0·05; one-way ANOVA). , Control; , S. typhimurium; , 2·5% SDP þ S. typhimurium; , 5% SDP þ S. typhimurium

Inclusion of SDP into animal diets has been shown to promote growth responses(31 – 34), and lessen inflammatory processes and clinical disease in pathogen challenge models(20 – 22,35). Previously, we demonstrated that dietary SDP, at 2·5 and 5% of weaned pig diets, was beneficial in reducing intestinal permeability and inflammation induced in early-weaned pigs(18). While the aforementioned studies on rodents and pigs demonstrated the beneficial effects of SDP on growth and intestinal inflammatory responses to stress and pathogenic challenges, the response variables were measured when SDP was currently being included in the diet. Whether or not SDP confers beneficial effects on the intestine after its removal from the diet has not been investigated. Here, we showed that dietary inclusion of SDP during the first 2 weeks PW can modify subsequent intestinal and immunological responses to a pathogenic challenge with S. typhimurium in pigs.

In the present study, pigs fed diets containing SDP during the first 2 weeks PW exhibited differential intestinal and systemic immune responses following an S. typhimurium challenge at 50 d of age; compared with the challenged control pigs, those fed the 2·5% SDP-7 d and 5% SDP-14 d diets during the PW period exhibited reduced ileal IL-8 levels. IL-8 is a major inflammatory cytokine produced by intestinal epithelial cells during S. typhimurium infection, and it acts as a chemoattractant for the recruitment of circulating neutrophils into the intestine, resulting in classic intestinal inflammatory lesions associated with S. typhimurium enteritis(36,37). In line with this role, pigs fed with the 5% SDP-14 d nursery dietary treatment had reduced neutrophil infiltration, MPO levels, and histological injury in response to S. typhimurium challenge. Bosi et al.(21) demonstrated earlier, in agreement with our findings, that piglets fed with 6%SDP exhibited reduced ileal IL-8 concentrations, induced by ETEC challenge; however, unlike the present study, IL-8 levels were measured while SDP was included in the diet at the time of the challenge.

Interestingly, despite the dampened histopathological and inflammatory responses exhibited by the challenged pigs fed with the 5% SDP-14 d nursery diet, ileal and plasma TNF levels were higher compared with the challenged control pigs. As mentioned above, S. typhimurium induces an intestinal inflammatory response mediated via the production of pro-inflammatory cytokines, including TNF and IL-8 and subsequent neutrophil recruitment and activation(38). TNF is best recognised as a pro-inflammatory cytokine that is central to the pathogenesis of a number of inflammatory disorders, and to stress-induced intestinal permeability(39,40).

Also, TNF is recognised as a critical and beneficial modulator of immune function and pathogen defence. For example, Nauciel & Espinasse-Maes(41) demonstrated that administration of anti-TNF antibodies  to mice exacerbated bacterial proliferation and mortality, following a sub-lethal dose of S. typhimurium. In similar studies carried out by Gulig et al.(42) and Tite et al.(43), anti-TNF antibodies increased the numbers of splenic colony-forming units of S. typhimurium following challenge.

Collectively, these studies have suggested that elevated TNF responses are critical for the control of infections. The present study is consistent with the study by Touchette et al.(44) showing that early-weaned pigs fed with a diet containing 7% SDP for 7 d PW exhibited a 2-fold higher increase in serum TNF levels in response to systemic LPS challenge, compared with pigs that did not receive SDP. The authors also demonstrated a marked 110-fold increase in (interferon-a) in pigs, fed with the SDP-enriched diet, in contrast to a 16-fold increase in pigs fed a diet without SDP. Taken as a whole, the present study, along with the previous investigations, demonstrated that dietary SDP can modulate local and systemic immune responses to weaning and pathogen challenges. However, the present study provides, for the first time, evidence that the effects of SDP on immune responses can be retained even after the removal of SDP from the diet.

In addition to investigating the effects of early dietary SDP and subsequent S. typhimurium challenge on inflammatory signals, we also investigated stress signalling pathways. Specifically, we showed that plasma cortisol and intestinal expression of CRF receptors were increased 2 d postchallenge; however, there were no differences between pigs fed SDP. While elevations in plasma cortisol, following S. typhimurium challenge, have been shown previously in studies on pigs(23), our findings on the marked up-regulation of intestinal CRF1/2 receptors during an acute challenge with S. typhimurium are novel. Given the increasingly recognized role of the intestinal CRF system in inflammatory and stressinduced functional GI disorders, in human subjects and laboratory research animal models(17,45,46), these findings warrant further investigation into the role of CRF in infectious inflammatory diseases.

In the present study, S. typhimurium challenge induced impairment in intestinal barrier function, which was indicated by increased ileal permeability to the paracellular probe FD4. The increase in ileal permeability in S. typhimuriumchallenged pigs was attenuated in pigs fed with the 5% SDP-14 d nursery diet, suggesting either an protective or reparative influence of early SDP dietary inclusion on intestinal barrier function.

It is not understood how early SDP feeding resulted in lasting protective effects on intestinal barrier during S. typhimurium infection in the present study. However, it is known that intestinal neutrophil infiltration in response to S. typhimurium infection is a central process, contributing to the breakdown of intestinal barrier function. Neutrophilmediated disruption of intestinal barrier function involves a multi-step mechanism, including increased myosin light chain phosphorylation and myosin light chain kinase, up-regulation of tight junction phosphotyrosine and phopshoserine residues(47), and activation of epithelial protease-activated receptors(48). Given that the 5% SDP-14 d inclusion in nursery diets resulted in reduced neutrophil infiltration in response to S. typhimurium infection, it is plausible that this may represent an important mechanism, by which SDP led to a protective effect on the intestinal barrier in the present study.

In addition, we observed unexpected results with regard to ileal TER in the present study. Despite the elevated FD4 permeability in the ileum at 2 d post-challenge, ileal TER was significantly elevated in all the challenged groups. FD4 flux rates and TER measure two different aspects of intestinal epithelial barrier function: TER reflects changes in ion (predominantly Naþ) permeability across the tight junction pores, while FD4 flux reflects large molecule fluxes across the leaky tight junctions. Another difference between the two measurements is that TER is calculated, on the basis of measured values of transepithelial voltage (PD) and current (Isc), according to Ohm’s law (V ¼ IR), and expressed on the basis of the surface area of the tissue chamber aperture. Therefore, significant alterations in either PD or Isc could have a significant impact on calculated TER values. Further analysis of PD across S. typhimurium-infected ileum tissues revealed a significant reduction in PD, which indicates a compromised ability of the intestinal epithelium to resist ion flow through the paracellular space, and thus is in line with the elevated FD4 flux. However, in contrast to the FD4 flux data, PD was not significantly influenced by the early nursery 5% SDP-14 d dietary treatment. S. typhimurium challenge also resulted in significant reductions in ileal Isc, which, in turn, might have contributed to the increased calculated TER values. Furthermore, Isc was greater in pigs fed the 2·5 %-7 d and 5% SDP-14 d nursery diets, which accounted for the increased TER in pigs fed the SDP treatments. The basis for increased Isc observed in the challenged pigs fed SDP in the nursery is not clear. However, the suppressive influence of S. typhimurium on Isc has been demonstrated in previous investigations on pigs and mice(49,50). The mechanisms for reduced Isc in the ileum from S. typhimurium-challenged pigs could be due to reduced anion (Cl2 or HCO3 2) secretion or electrogenic cation (e.g. Naþ) absorption. In a recent study, it has been demonstrated that mice challenged with S. typhimurium exhibited reduced basal- and adenosine 30,50-cyclic monophosphate-mediated electrogenic Isc, an effect associated with reduced expression and/or localization of colonic epithelial ion transporters including the Cl2/HCO32 exchanger down-regulated in adenoma and the cystic fibrosis transmembrane regulator(51). These suppressive effects were, in part, mediated by secreted S. typhimurium effector proteins. Therefore, in the light of these findings, it is plausible that the influence of early SDP on subsequent Isc responses to S. typhimurium challenge could be directly related to the effects of SDP treatments on subsequent S. typhimurium pathogenicity in the porcine intestine. The precise host intestinal pathways modulated by early SDP feeding in pigs that contribute to the Isc response remain to be elucidated.

Despite marked changes in immune and epithelial barrier responses, SDP had little influence on S. typhimuriuminduced morphology of intestinal villi (villus blunting or villus fusion) and epithelium (denuded epithelium). Interestingly, increased crypt depths were observed in pig fed the 5% SDP-14 d nursery diet. Increased crypt depth (crypt expansion) is a hallmark of intestinal injury, but at the same time is an index of epithelial repair processes, as increased proliferation of immature crypt enterocytes will migrate to the villus tip to replace damaged or denuded villus epithelial cells. Therefore, the increased crypt depths in pigs fed the 5% SDP nursery diet could indicate increased epithelial renewal, which might potentially prove beneficial in later stages of recovery from S. typhimurium.

As mentioned previously, there are a number of studies in the literature that describe the beneficial impact of dietary SDP on growth responses. However, in the present study, there were no significant differences in growth and/or clinical responses in pigs observed either during the PW period or during the subsequent S. typhimurium post-challenge period. Despite the lack of measurable growth response to SDP in the present study, significant effects on immunological and intestinal responses were demonstrated.

There are several reasons that could explain the lack of SDP growth responses in the present study. First, the primary objective was not to measure growth performance, but to determine whether early dietary SDP influenced subsequent immunological and intestinal physiological responses to a later-life S. typhimurium challenge. Therefore, sufficient animal sample size needed to achieve the statistical power required to appropriately evaluate growth responses was not included in the experimental design.

Second, the experimental environment in which the pigs were raised might not have been ideal to demonstrate a SDPdependent growth response. It has been shown previously that the effects of SDP on pig growth were observed in a commercial farm environment, but not in an experimental university research setting(34). A third reason for the lack of SDP growth response, specifically observed in the postchallenge phase of the experiment, is the short time period (2 d) in which BW changes were measured, which might have been inadequate to assess post-challenge growth responses during the peak challenge response. Given the beneficial effects of early SDP observed on intestinal physiological and immunological responses following S. typhimurium challenge, growth measurements over a longer post-challenge period (e.g. 7–14 d) could have a significant influence on the effects of SDP on growth responses in challenged pigs.

In conclusion, data from the present study demonstrate that early dietary inclusion of SDP have an impact on intestinal immunological and epithelial pathophysiological responses to S. typhimurium challenge, even after SDP has been removed from the diet. Given that stress and diet are increasingly recognised as key early-life factors that determine long-term health outcomes in human subjects and animals, a more fundamental understanding of biological mechanisms and optimal nutritional intervention strategies, such as dietary SDP, have potential to exert a positive impact on long-term intestinal health.

The authors thank the North Carolina State University Laboratory Animal Resources for their excellent technical assistance and advice during the entire period of the study.

The present study was funded by the American Proteins Company (APC), Inc. (to A. J. M.) and The National Institutes of Health (to A. J. M., K08 DK084313 and R01 HD072968).

The authors’ contributions are as follows: J. M. C., J. D. C., J. P. and A. J. M. designed the research; A. J. M., L. L. E., P. E. B., E. S., S. T., M. M., S. D. and L. B. B. conducted the research; A. J. M., L. L. E., S. D. and L. B. B. analysed the data; P. E. B., S. D. and A. J. M. wrote the paper; A. J. M. had primary responsibility for the final content. All authors read and approved the final manuscript.

None of the authors has any conflict of interest to declare.

1. McLamb BL, Gibson AJ, Overman EL, et al. (2013) Early weaning stress in pigs impairs innate mucosal immune responses to enterotoxigenic E. coli challenge and exacerbates intestinal injury and clinical disease. PLOS ONE 8, e59838.

2. Lennon EM, Maharshak N, Elloumi H, et al. (2013) Early life stress triggers persistent colonic barrier dysfunction and exacerbates colitis in adult IL-10-/- mice. Inflamm Bowel Dis 19, 712–719.

3. Gareau MG, Jury J, Yang PC, et al. (2006) Neonatal maternal separation causes colonic dysfunction in rat pups including impaired host resistance. Pediatr Res 59, 83–88.

4. Boudry G, Douard V, Mourot J, et al. (2009) Linseed oil in the maternal diet during gestation and lactation modifies fatty acid composition, mucosal architecture, and mast cell regulation of the ileal barrier in piglets. J Nutr 139, 1110–1117.

5. Agostini A, Rizzello F, Ravegnani G, et al. (2010) Adult attachment and early parental experiences in patients with Crohn’s disease. Psychosomatics 51, 208–215.

6. Chitkara DK, van Tilburg MA, Blois-Martin N, et al. (2008) Early life risk factors that contribute to irritable bowel syndrome in adults: a systematic review. Am J Gastroenterol 103, 765–774.

7. Bradford K, Shih W, Videlock EJ, et al. (2012) Association between early adverse life events and irritable bowel syndrome. Clin Gastroenterol Hepatol 10, 385–390.

8. Howell S, Poulton R & Talley NJ (2005) The natural history of childhood abdominal pain and its association with adult irritable bowel syndrome: birth-cohort study. Am J Gastroenterol 100, 2071–2078.

9. Apley J & Hale B (1973) Children with recurrent abdominal pain: how do they grow up? Br Med J 3, 7–9.

10. Moeser AJ, Klok CV, Ryan KA, et al. (2007) Stress signaling pathways activated by weaning mediate intestinal dysfunction in the pig. Am J Physiol Gastrointest Liver Physiol 292, G173–G181.

11. Moeser AJ, Ryan KA, Nighot PK, et al. (2007) Gastrointestinal dysfunction induced by early weaning is attenuated by delayed weaning and mast cell blockade in pigs. Am J Physiol Gastrointest Liver Physiol 293, G413–G421.

12. Pie SLJ, Blazy F, Laffitte J, et al. (2004) Weaning is associated with an upregulation of expression of inflammatory cytokines in the intestine of piglets. J Nutr 134, 641–647.

13. Makkink CA, Negulescu GP, Qin G, et al. (1994) Effect of dietary protein source on feed intake, growth, pancreatic enzyme activities and jejunal morphology in newly-weaned piglets. Br J Nutr 72, 353–368.

14. Boudry G, Lalles JP, Malbert CH, et al. (2002) Diet-related adaptation of the small intestine atweaning inpigs is functional rather than structural. J Pediatr Gastroenterol Nutr 34, 180–187.

15. Boudry G, Peron V, Le Huerou-Luron I, et al. (2004) Weaning induces both transient and long-lasting modifications of absorptive, secretory, and barrier properties of piglet intestine. J Nutr 134, 2256–2262.

16. Nabuurs MJ, Hoogendoorn A, van der Molen EJ, et al. (1993) Villus height and crypt depth in weaned and unweaned pigs, reared under various circumstances in The Netherlands. Res Vet Sci 55, 78–84.

17. Smith F, Clark JE, Overman BL, et al. (2010) Early weaning stress impairs development of mucosal barrier function in the porcine intestine. Am J Physiol Gastrointest Liver Physiol 298, G352–G363.

18. Peace RM, Campbell J, Polo J, et al. (2011) Spray-dried porcine plasma influences intestinal barrier function, inflammation, and diarrhea in weaned pigs. J Nutr 141, 1312–1317.

19. Van Dijk AJ, Margry RJ, Van Der Lee AG, et al. (2002) Growth performance and health status in weanling piglets fed spraydried porcine plasma under typical Northern European conditions. J Anim Physiol Anim Nutr (Berl) 86, 17–25.

20. Van Dijk AJ, Enthoven PM, Van den Hoven SG, et al. (2002) The effect of dietary spray-dried porcine plasma on clinical response in weaned piglets challenged with a pathogenic Escherichia coli. Vet Microbiol 84, 207–218.

21. Bosi P, Casini L, Finamore A, et al. (2004) Spray-dried plasma improves growth performance and reduces inflammatory status of weaned pigs challenged with enterotoxigenic Escherichia coli K88. J Anim Sci 82, 1764–1772.

22. Corl BA, Harrell RJ, Moon HK, et al. (2007) Effect of animal plasma proteins on intestinal damage and recovery of neonatal pigs infected with rotavirus. J Nutr Biochem 18, 778–784.

23. Balaji R, Wright KJ, Hill CM, et al. (2000) Acute phase responses of pigs challenged orally with Salmonella typhimurium. J Anim Sci 78, 1885–1891.

24. Bahnson PB, Kim JY, Weigel RM, et al. (2005) Associations between on-farm and slaughter plant detection of Salmonella in market-weight pigs. J Food Potect 68, 246–250.

25. Argenzio RA & Liacos JA (1990) Endogenous prostanoids control ion transport across neonatal porcine ileum in vitro. Am J Vet Res 51, 747–751.

26. Wickramasinghe SN (2006) Bone marrow. In Histology for Pathologists, 3rd ed., pp. 799–836 [SE Mills, editor]. Philadelphia: Lippincott Williams & Wilkins.

27. Zadrozny LM, Stauffer SH, Armstrong MU, et al. (2006) Neutrophils do not mediate the pathophysiological sequelae of Cryptosporidium parvum infection in neonatal piglets. Infect Immun 74, 5497–5505.

28. Slominski A, Zbytek B, Pisarchik A, et al. (2006) CRH functions as a growth factor/cytokine in the skin. J Cell Physiol 206, 780–791.

29. Lakshmanan J, Magee TR, Richard JD, et al. (2008) Localization and gestation-dependent pattern of corticotrophinreleasing factor receptor subtypes in ovine fetal distal colon. Neurogastroenterol Motil 20, 1328–1339.

30. O’Malley D, Dinan TG & Cryan JF (2010) Alterations in colonic corticotropin-releasing factor receptors in the maternally separated rat model of irritable bowel syndrome: differential effects of acute psychological and physical stressors. Peptides 31, 662–670.

31. Pierce JL, Cromwell GL, Lindemann MD, et al. (2005) Effects of spray-dried animal plasma and immunoglobulins on performance of early weaned pigs. J Anim Sci 83, 2876–2885.

32. van Dijk AJ (2002) Spray-dried plasma in diets for weaned piglets: influence on growth and underlying mechanisms. Tijdschr Diergeneeskd 127, 520–523.

33. Zhao J, Harper AF, Estienne MJ, et al. (2007) Growth performance and intestinal morphology responses in early weaned pigs to supplementation of antibiotic-free diets with an organic copper complex and spray-dried plasma protein in sanitary and nonsanitary environments. J Anim Sci 85, 1302–1310.

34. Coffey RD & Cromwell GL (1995) The impact of environment and antimicrobial agents on the growth response of early-weaned pigs to spray-dried porcine plasma. J Anim Sci 73, 2532–2539.

35. Perez-Bosque A, Amat C, Polo J, et al. (2006) Spray-dried animal plasma prevents the effects of Staphylococcus aureus enterotoxin B on intestinal barrier function in weaned rats. J Nutr 11, 2838–2843.

36. McCormick BA, Colgan SP, Delp-Archer C, et al. (1993) Salmonella typhimurium attachment to human intestinal epithelial monolayers: transcellular signalling to subepithelial neutrophils. The J Cell Biol 123, 895–907.

37. McCormick BA, Parkos CA, Colgan SP, et al. (1998) Apical secretion of a pathogen-elicited epithelial chemoattractant activity in response to surface colonization of intestinal epithelia by Salmonella typhimurium. J Immunol 160, 455–466.

38. Moeser AJ & Blikslager AT (2007) Mechanisms of porcine diarrheal disease. J Am Vet Med Assoc 231, 56–67.

39. Soderholm JD, Streutker C, Yang PC, et al. (2004) Increased epithelial uptake of protein antigens in the ileum of Crohn’s disease mediated by tumour necrosis factor alpha. Gut 53, 1817–1824.

40. Overman EL, Rivier JE & Moeser AJ (2012) CRF induces intestinal epithelial barrier injury via the release of mast cell proteases and TNF-alpha. PLOS ONE 7, e39935.

41. Nauciel C&Espinasse-Maes F (1992) Role of gamma interferon and tumor necrosis factor alpha in resistance to Salmonella typhimurium infection. Infect Immun 60, 450–454.

42. Gulig PA, Doyle TJ, Clare-Salzler MJ, et al. (1997) Systemic infection of mice by wild-type but not Spv- Salmonella typhimurium is enhanced by neutralization of gamma interferon and tumor necrosis factor alpha. Infect Immun 65, 5191–5197

43. Tite JP, Dougan G & Chatfield SN (1991) The involvement of tumor necrosis factor in immunity to Salmonella infection. J Immunol 147, 3161–3164.

44. Touchette KJ, Carroll JA, Allee GL, et al. (2002) Effect of spray-dried plasma and lipopolysaccharide exposure on weaned pigs: I. Effects on the immune axis of weaned pigs. J Anim Sci 80, 494–501.

45. Gourcerol G, Wu SV, Yuan PQ, et al. (2011) Activation of corticotropin-releasing factor receptor 2 mediates the colonic motor coping response to acute stress in rodents. Gastroenterology 140, 1586–1596.

46. Chatzaki E, Anton PA, Million M, et al. (2013) Corticotropinreleasing factor receptor subtype 2 in human colonic mucosa: down-regulation in ulcerative colitis. World J Gastroenterol 19, 1416–1423.

47. Edens HA, Parkos CA, Liang TW, et al. (1999) Nonserum- dependent chemotactic factors produced by Candida albicans stimulate chemotaxis by binding to the formyl peptide receptor on neutrophils and to an unknown receptor on macrophages. Infect Immun 67, 1063–1071.

48. Chin AC, Lee WY, Nusrat A, et al. (2008) Neutrophilmediated activation of epithelial protease-activated receptors-1 and -2 regulates barrier function and transepithelial migration. J Immunol 181, 5702–5710.

49. Walsh MC, Rostagno MH, Gardiner GE, et al. (2012) Controlling Salmonella infection in weanling pigs through water delivery of direct-fed microbials or organic acids: Part II. Effects on intestinal histology and active nutrient transport. J Anim Sci 90, 2599–2608.

50. Aschenbach JR, Ahrens F, Schwelberger HG, et al. (2007) Functional characteristics of the porcine colonic epithelium following transportation stress and Salmonella infection. Scand J Gastroenterol 42, 708–716.

51. Marchelletta RR, Gareau MG, McCole DF, et al. (2013) Altered expression and localization of ion transporters contribute to diarrhea in mice with Salmonella-induced  enteritis. Gastroenterology 145, 1358–1368.