Immune responses of growing beef steers treated with estrogen/progesterone implants or insulin injections

Five Angus x Hereford steers were assigned to each of three 77-d treatments, controls (daily placebo injections), daily insulin injections (0.45 mg kg-1 BW.75), or single Synovex-S ear implant (20 mg 17 beta-estradiol benzoate, 200 mg progesterone), to determine whether these hormonal manipulations were immunomodulatory. All steers were immunized with ovalbumin (OVA) on days 18 and 46 of the treatment period. Immune responses monitored were in vitro blastogenic responses of peripheral blood mononuclear cells (PBMC) to mitogens (con A and PHA) and antigen (OVA), in vivo antibody responses to OVA, flow cytometric analysis of percentages of CD2+, CD4+, and CD8+ PBMC, and blood leukocyte counts. Insulin treatment caused elevations (P < .05) in PHA-stimulated responses of PBMC, in the magnitude of peak blastogenic response to OVA immunization, and in total leukocyte counts and the ratio of CD4+/CD8+ cells in peripheral blood. However, insulin treatment was also associated with slightly decreased (P < .05) anti-OVA antibody responses. Synovex-S increased (P < .05) spontaneous blastogenic activity of cultured PBMC, but lacked consistent effects on mitogen-stimulated activity, on leukocyte counts, and on the ratio of CD4+/CD8+ cells in peripheral blood. Augmenting (P < .05) effects of Synovex-S on OVA-specific blastogenesis and on antibody responses were, however, consistent. Percentages of CD2+ and CD4+ PBMC, and responsiveness of PBMC to con A were not affected by treatments. In summary, this study showed statistically-significant specific immunomodulatory effects of insulin and Synovex-S in growing beef steers, but mechanisms of action and practical implications of these effects with respect to disease resistance required clarification.