Age-dependent variation in innate immune responses to porcine epidemic diarrhea virus infection in suckling versus weaned pigs

Porcine epidemic diarrhea (PED) is an enteric coronaviral infection that causes severe morbidity and mortality in suckling pigs, but less severe disease in older pigs. Consequently, it causes significant economic losses to the pork industry. There are limited studies on the innate immune responses to PED virus (PEDV) in pigs. The aims of our study were to investigate differences in innate immune responses to PEDV infection in suckling and weaned pigs and to examine if disease severity coincides with reduced innate immune responses. Weaned 26-day-old pigs (n = 20) and 9-day-old nursing pigs (n = 20) were assigned to PEDV inoculated or uninoculated control groups. The pigs were observed daily for clinical signs, virus shedding and were euthanized at post-inoculation days (PIDs) 1 and 5 to assay immune responses. Blood samples were collected at PIDs 1, 3 and 5. The natural killer (NK) cell frequencies, NK cell activities (lysis of target K562 tumor cells in vitro), CD3+CD4+ T cell and CD3+CD8+ T cell frequencies were measured in blood and ileum at PIDs 1 and 5. The PEDV infected suckling pigs showed severe diarrhea and vomiting at PID 1, whereas the PEDV infected weaned pigs showed milder clinical signs starting at PID 3. PEDV infected suckling pigs had significantly higher diarrhea scores, earlier fecal PEDV RNA shedding and significantly higher viremia (viral RNA in serum) compared to weaned pigs. There was no mortality in either infected suckling or infected weaned pigs. The control pigs not inoculated with PEDV did not show any clinical signs and no detectable fecal or serum PEDV RNA. Strikingly, PEDV infected suckling pigs had significantly lower NK cell frequencies, undetectable NK cell activity and lower IFNγ producing NK cells in blood and ileum compared to PEDV infected weaned pigs. Pro-inflammatory cytokine profiles of PEDV infected suckling pigs differed from those of PEDV infected weaned pigs and coincided with onset of fecal PEDV RNA shedding and serum PEDV RNA titers. The infected suckling pigs have higher and earlier increases in serum IFNα, but lower serum IL-8 and TNFα levels compared to infected weaned pigs. CD3+CD4+ T cell frequencies were significantly higher in ileum of suckling pigs than in weaned pigs, whereas there was no difference in CD3+CD8+ T cell frequencies. In conclusion, the observations of impaired lytic activity and IFN-γ production by NK cells in suckling pigs coincided with the increased severity of PEDV infection in the suckling pigs compared with the weaned pigs.