Effect of pregnancy on the immune response of cattle to a Brucella vaccine.

An experiment was performed to determine whether humoral- or cell-mediated immune responses of cattle to a Brucella abortus vaccine were influenced by the stage of gestation. Heifers were vaccinated 2 mth before and 2 mth after breeding with cell envelopes of B. abortus in an oil adjuvant containing trehalose dimycolate and muramyl dipeptide. Control groups received adjuvant alone or no vaccine. Following breeding, vaccinated animals were divided into pregnant and nonpregnant subgroups. Immune responses to two outer membrane proteins were measured at monthly intervals by ELISA and lymphocyte blastogenesis tests. Skin tests were performed during the ninth month of gestation. Vaccination induced sustained immune responses, but few differences were detected between pregnant and non-pregnant animals. The relative increase in IgA antibodies to group 3 protein in nonpregnant heifers exceeded that in pregnant heifers during months 4 and 6 of gestation (P less than 0.05). Dermal hypersensitivity, measured by changes in double skin thickness, was significantly greater in nonpregnant heifers to porin (P less than 0.01) and group 3 (P less than 0.05) antigens at 24 h post-injection, but no significant differences in skin thicknesses or in the nature of the lesions were observed at 48 h. Animals which received adjuvant alone demonstrated negligible responses. Pregnancy had no significant effect on the responses of lymphocytes to phytohemagglutinin (PHA) or Concanavalin A (Con A). However, plasmas from nonvaccinated pregnant heifers taken during the sixth and seventh (but not eight or ninth) months of pregnancy decreased responses of normal donor cells to PHA and Con A when compared with those in autologous plasma (P less than 0.05).