Variation in immune responsiveness is associated with pregnancy status in beef heifers.

Variation in immune responsiveness among heifers may contribute to differences in reproductive success. Herein, we evaluated whether cell-mediated immune (CMI) responses at early pregnancy (day 62) differ between pregnant and non-pregnant heifers by characterizing responses to viral and mitogen stimulation under standardized in vitro conditions. To this end, a subset of pregnant and non-pregnant heifers (n = 12 per group) was selected and assessed for immune responses using in vitro stimulation with BVDV-1, BVDV-2, and mitogen. No differences were observed in immune cell population frequencies or responses to non-specific mitogen stimulation and in non-stimulated cells (P > 0.05). Non-pregnant heifers exhibited significantly greater antigen-specific IFN-γ response following BVDV-2 stimulation for the CD4+ population (P ≤ 0.05), and a similar trend was also observed for the CD335+ population. While similar results were observed for the BVDV-1 stimulation, these observations were not significant (P > 0.05). These findings indicate that pregnant heifers demonstrate reduced antigen-specific CMI responses while maintaining overall immune competence. Collectively, our results suggest that immune responsiveness was associated with pregnancy status, and further studies are needed to validate these relationships and elucidate underlying mechanisms.