Ingestion of Soy Isoflavones Alters the Immune Response of Pigs during a Respiratory Viral Challenge

Porcine reproductive and respiratory syndrome virus (PRRSV) is the most prevalent disease facing the swine industry globally. Infection of weanling pigs with PRRSV leads to a complex immune response resulting in impaired growth performance and economic losses. Building upon recent evidence linking ingestion of soybean products and immunity, the objective of this experiment was to identify a mode of action for soy isoflavones to influence the immunological response of PRRSV-infected weanling pigs. Four dietary treatments differed by soy source [soy protein concentrate (SPC) vs. HP 300^®, a value-added soy product; Hamlet Protein, Findlay, OH] and isoflavone supplementation (none vs. 1,500 mg total isoflavones/kg diet from Novasoy 400^®; ADM, Decatur, IL); the control diet (CON) contained SPC and no supplemental isoflavones. All diets were isonitrogenous, isocaloric, and contained identical standardized ileal digestible AA concentrations. Weanling pigs (60 barrows, 21 d of age, 5.71 ± 0.44 kg) were individually housed in disease containment chambers and provided ad libitum access to experimental diets for 7 d before receiving either a sham inoculation (sterile PBS) or a 9.28 x 10³ 50% tissue culture infective dose of PRRSV at 0 d post-inoculation (DPI). A total of 5 experimental treatments included an uninfected group receiving the CON diet, plus four infected groups each receiving different diets. Growth performance and rectal temperature data were recorded weekly, and blood was collected at 0, 3, 6, 8, 12, and 14 DPI for immunological analyses. Data were analyzed as a 2-way ANOVA for all treatments involving PRRSV-infected pigs, and a t-test was used to compare uninfected and infected groups receiving the CON diet. A successful infection was confirmed through reduced (P < 0.05) growth performance and increased (P < 0.05) serum PRRSV concentrations when comparing uninfected and infected pigs fed the CON diet. No dietary treatment effects were observed within infected groups for growth performance or rectal temperatures. However, serum PRRSV load was increased in pigs receiving the isoflavone-supplemented HP 300 diet at 3 and 6 DPI compared with all other groups (interaction, P = 0.003). In general, no change in T-cell (TC) numbers was observed, but PRRSV infection increased (P < 0.05) the proportion of circulating cytotoxic TC. Moreover, isoflavone supplementation increased (main effect, P = 0.01) the proportion of helper TC by 30%, thereby suggesting a benefit to the adaptive immune response. In conclusion, ingestion of supplemental soy isoflavones may alter the immune response of PRRSV-infected weanling pigs.