Host Immune response profiles of calves following vaccination with live BCG and inactivated Mycobacterium bovis vaccine candidates

Bovine tuberculosis (BTB) caused by Mycobacterium bovis (M. bovis) occurs endemically in livestock and wildlife in parts of southern Africa and is detrimental to socio-economics and conservation efforts. Test-and- slaughter schemes to control BTB have proved inefficient in many developing countries with a human-wildlife- livestock interface and limited surveillance of animal diseases. Vaccination is therefore considered an alternative control strategy. The safety of the use of live BCG in cattle in an area of high HIV prevalence, however, is not known. As such vaccination with inactivated vaccine candidates may provide a safer alternative.The aim of this study was to investigate cellular and humoral immune responses of cattle vaccinated with live M. bovis BCG Danish compared to two inactivated M. bovis vaccine candidates. Twenty-four calves, aged 4-6 months, were randomly divided into 4 groups and vaccinated sub-cutaneously with either live M. bovis BCG; formalin-inactivated BCG; heat-killed M. bovis or PBS/Montanide (control). Animals receiving inactivated vaccines were boosted at 3 weeks post-priming. Interferon-gamma responsiveness and antibody production were measured prior to vaccination and at weekly intervals up to 9 weeks after vaccination using the BOVIGAM assay to measure interferon-gamma and the IDEXX TB serum ELISA, respectively. At week 9 the animals were subjected to a skin test with bovine and avian tuberculins (PPD-B and PPD-A).The animals in the heat-killed M. bovis group showed significantly higher responses in both assays compared to those in the control group as well as the groups that received live or inactivated BCG. Cell-mediated immunity in the heat-killed M. bovis group peaked at week 4 post-priming and showed significantly elevated OD readouts (OD-bovine PPD minus OD-avian PPD) up to 3,3 fold until week 9 (p<0.05). Significant humoral responsiveness in the heat-killed M. bovis group was observed at week 3 post-priming, and antibody levels steadily increased up to 99.4 fold compared to the control group at week 9. Treatment groups vaccinated with live and inactivated BCG did not show significantly raised levels of cell-mediated or humoral immune responses compared to the control group in the course of 9 weeks. In the tuberculin skin test, the live BCG and the heat- killed M. bovis vaccinated animals showed significant responses (PPD-B minus PPD-A).In conclusion, the heat-killed M. bovis vaccine was shown to elicit strong and sustained cell-mediated, both interferon-gamma and skin test readings, as well as humoral immune responses in calves and warrants further investigation in experimental and/or field challenge studies.