Intestinal alkaline phosphatase (IAP) is a brush border enzyme secreted by enterocytes, playing a crucial role in maintaining gut mucosal defense. This study investigated the expression dynamics of IAP in the small intestine of pigs challenged with Escherichia coli (E. coli) K88, compared to healthy controls. Five-week-old pigs (n = 8) were orally administered E. coli K88 at a concentration of 2 × 10⁸ CFU/mL, with a dose of 2 mL per pig at 0 and 24 h. Five days post-challenge, tissue samples from the duodenum, jejunum, and ileum were collected for mucosal morphometric analysis and evaluation of IAP expression via immunohistochemistry, Western blotting, and real-time PCR. The results revealed the presence of IAP on the apical surface of villi throughout the small intestine, along with significantly upregulated IAP expression in E. coli-challenged pigs compared to controls. These findings suggest that Gram-negative bacteria such as E. coli can induce IAP expression, likely through lipopolysaccharide (LPS) stimulation, thereby enhancing its enzymatic activity as part of the intestinal defense mechanism. This study provides insight into the protective role of IAP and highlights its potential as a biomarker for assessing gut health and diagnosing enteric infections in animals.

Background/Objectives: Porcine epidemic diarrhea (PED) is a highly contagious enteric disease caused by porcine epidemic diarrhea virus (PEDV) and is associated with severe clinical signs and high mortality in neonatal piglets. Vaccination is an important strategy for PED control through the induction of humoral immunity. This study aimed to compare immune responses induced by inactivated and live-attenuated PEDV vaccines and to evaluate a heterologous prime-boost vaccination strategy in PEDV-naïve replacement gilts. Methods: Twenty-four PEDV-naïve replacement gilts were randomly assigned to four groups: unvaccinated control, inactivated vaccine administered twice (K/K), live-attenuated vaccine administered twice (L/L), and live-attenuated priming followed by an inactivated booster (L/K). Pigs received two intramuscular vaccinations at 16 weeks of age and two weeks later. Serum samples collected up to 42 days post-vaccination were analyzed for PEDV-specific IgG and IgA antibodies by ELISA, and serum-neutralizing antibody titers were determined using a serum neutralization test. Results: The L/K regimen induced the highest PEDV-specific IgG responses, with peak levels at day 28 post-vaccination that were significantly higher than those in the K/K and control groups. Serum-neutralizing antibody titers were significantly higher in the L/K and L/L groups than in the K/K and control groups. Serum IgA responses were low and transient across all vaccination groups. Conclusions: A heterologous prime-boost vaccination strategy using a live-attenuated PEDV vaccine followed by an inactivated booster induces strong systemic humoral immune responses in replacement gilts and represents a promising approach for PEDV vaccination programs.