Effect of age and pregnancy status on adaptive immune responses of Canadian Holstein replacement heifers.

Selection for production traits with little or no emphasis on health traits has led to an increase in the incidence of disease in Canadian dairy herds. We describe here a patented protocol for estimating the breeding value for immune responsiveness in heifers that combines measures of both cell-mediated (CM) and antibody-mediated (AM) immune responses (IR). The ability of putative type 1 and type 2 antigens used to induce CMIR and AMIR, respectively, was assessed in replacement Holstein heifers, and the effects of age and pregnancy on type 1 and type 2 IR bias were estimated. Results demonstrated that the type 1 and type 2 antigens induced polarized type 1 and type 2 responses in heifers regardless of age and pregnancy status, and can therefore be used to identify animals with superior overall immune responsiveness. However, age and pregnancy status had significant effects on adaptive IR profiles, highlighting the need for appropriate statistical modeling of such effects when ranking animals on their ability to mount CM and AMIR. Responses became increasingly type 1 biased as heifers approached 12 mo of age, from which point, responses then became increasingly type 2 biased with age and length of gestation. Knowledge of how age and pregnancy influence the dynamics of type 1 and type 2 IR bias is expected to improve our ability to select animals with enhanced immune responsiveness and aid in the development of effective vaccines through strategic targeting of vaccine components to recipients.