PSIII-18 Immune response variation to foreign antigen in a crossbred Angus-Brahman beef cattle population.

Abstract Genetic selection for improved health in cattle breeding programs has recently gained attention in dairy cattle but is still underexplored in beef cattle. However, selective breeding for enhanced immune competence offers a potential strategy to increase disease resistance. We characterized keyhole limpet hemocyanin (KLH)-induced humoral and cellular immune responses in 165 UF Multibreed Angus-Brahman (MAB) heifers to assess breed effects and identify genetically superior sires. MAB heifers were categorized into five breed groups (BG) based on Brahman breed percentage, with BG1 ranging from 0-19%, BG2 from 20-39%, BG3 from 40-59%, BG4 from 60-79%, and BG5 from 80-100%. All cattle received an intramuscular KLH vaccination on day 0 with a booster on day 14. Plasma samples collected on days 0, 14, and 28 were analyzed for KLH-specific IgG concentration via indirect ELISA. Additionally, cellular immune responses were evaluated on day 28 using a delayed-type hypersensitivity test. For this assessment, an intradermal injection of KLH was administered at two designated test sites on the animal’s neck, while a PBS injection served as a negative control at a separate site. Skinfold thickness was measured immediately prior to injection (0 hours) and again 24 hours later as a proxy for cellular response. To ensure normally distributed residuals, IgG concentrations and skinfold thickness ratios were log₁₀-transformed prior to analysis. Separate linear mixed models were fit to assess the effects of breed group and sire on immune responses across timepoints and specifically on day 28. KLH-specific log₁₀-IgG levels increased significantly from day 0 to day 28 for all breed groups (P < 2.2 × 10⁻¹⁶), confirming the successful elicitation of antibody immune responses. By day 28, MAB log₁₀-IgG concentrations ranged from 3.01 ± 0.06 relative units per mL (RU/mL) in BG1 to 2.74 ± 0.07 RU/mL in BG5. Notably, higher Brahman percentage was significantly associated with reduced antibody immune response when compared to high Angus percentage (P = 0.048). Substantial variability observed within breed groups indicates that additional genetic factors, other than breed, influence cattle immunity. Furthermore, the sire effect was significant across and on day 28 humoral responses, explaining roughly 14% and 4% of total phenotypic variability, respectively. Cellular immune response showed a similar numerical trend, with the log₁₀-skinfold thickness ratio ranging from 0.09 ± 0.01 mm in BG1 to 0.04 ± 0.02 mm in BG5. Although a higher Brahman percentage suggests an association with decreased T-cell mediated response, this difference was not statistically significant. Immune responses varied significantly between groups and widely within them, indicating genetic influences beyond breed composition. These findings advance understanding of genetic influences on bovine immune function and may inform breeding strategies to leverage genetic diversity for enhanced herd immunity and disease resistance.