Host Defense Peptides in Porcine Immunity: Porcine Host Defense Peptides and SCFAs in Antimicrobial Defense

Pig production provides nearly one-third of the world’s meat supply but faces major challenges from infectious diseases and the heavy use of antibiotics. Frequent antibiotic use contributes to antimicrobial resistance, making its reduction a priority for both animal and public health. One promising strategy is to strengthen the pig’s innate immune system through host defense peptides, small molecules with antimicrobial and immunomodulatory functions. The aim of this thesis was to investigate whether dietary compounds can enhance the expression of HDPs in pigs and to examine the antimicrobial and antiviral functions of specific peptides. To study the nutritional regulation of HDPs, the effects of short-chain fatty acids were examined. Butyrate supports intestinal health, but its rapid absorption in the upper gut limits its availability in the lower intestine, where mucosal defenses are most important. Therefore, the esterified derivatives monobutyrin and monovalerin were tested in porcine intestinal epithelial cells and in piglets. Both compounds significantly increased PMAP-37 expression in vitro and in the jejunum of piglets without causing toxic effects and were associated with enhanced antibacterial activity. These findings suggest that SCFA derivatives can strengthen intestinal immunity and may serve as promising feed additives to support animal health. The functional properties of PMAP-37 were further investigated. The peptide showed broad antibacterial activity against both Gram-positive and Gram-negative bacteria by disrupting bacterial membranes. In addition to these antimicrobial effects, PMAP-37 also exhibited immunomodulatory activity by neutralizing bacterial endotoxins such as lipopolysaccharide, Lipid A, and lipoteichoic acid, which reduced nitric oxide production and the release of pro-inflammatory cytokines in macrophages. PMAP-37 also inhibited the entry of LPS into host cells, further limiting inflammatory responses. Moreover, the peptide displayed antiviral activity against porcine epidemic diarrhea virus (PEDV) by directly interacting with viral particles and reducing their ability to infect host cells. Together, these findings demonstrate that PMAP-37 has antibacterial, antiviral, and immunomodulatory properties. To place these results in a broader perspective, the antiviral activity of cathelicidins from different species was compared. Several porcine peptides, including PMAP-23, PMAP-36, and PR-39, showed little or no antiviral activity against PEDV. In contrast, the human peptide LL-37 and the chicken peptide CATH-B1 strongly inhibited PEDV infection at non-toxic concentrations by directly interacting with viral particles and disrupting their structure. These findings indicate that antiviral activity is not common to all cathelicidins and depends on specific structural features of the peptides. Finally, the antimicrobial activity of porcine RegIIIγ was investigated. Recombinant RegIIIγ was successfully expressed and purified, and showed modest antibacterial activity against Listeria monocytogenes. However, no activity was observed against other bacteria or PEDV, suggesting a limited antimicrobial role under the tested conditions. Overall, the findings of this thesis support the development of alternative strategies to reduce antibiotic use in pig production. Nutritional approaches that stimulate the expression of endogenous host defense peptides represent a promising way to strengthen intestinal immunity and improve animal health. Future studies should focus on validating these strategies in long-term animal trials and further exploring the mechanisms that regulate HDP activity.