Revaccination of neonatal calves with Mycobacterium bovis BCG reduces the level of protection against bovine tuberculosis induced by a single vaccination

Abstract

Cattle may provide a suitable model for testing ways of improving tuberculosis vaccine efficacy in human infants. A vaccination and challenge study was undertaken in calves to determine the optimal time to vaccinate neonatal animals with Mycobacterium bovis bacillus Calmette-Guérin (BCG) for protection against tuberculosis and to determine whether revaccination with BCG was beneficial. Calves (10 per group) were vaccinated with BCG within 8 h of birth or at 6 weeks of age, when immune responses to antigens of environmental mycobacteria were detectable, or vaccinated at birth and revaccinated at 6 weeks. A control group was not vaccinated. BCG vaccination at birth induced strong antigen-specific gamma interferon (IFN-gamma) and interleukin-2 (IL-2) responses and antigen-specific activation in CD4(+), CD8(+), and WC1(+) gammadelta T-cell subsets from blood. The proportions of animals per group with macroscopic tuberculous lesions after challenge were 0/10 for BCG at birth, 1/9 for BCG at 6 weeks, 4/10 for the revaccinated group, and 10/10 for the nonvaccinated group. There was no significant difference in the levels of protection between groups vaccinated at birth or at 6 weeks, while animals vaccinated both at birth and at 6 weeks had significantly less protection than those vaccinated only at birth. The revaccinated calves that subsequently developed tuberculous lesions had significantly stronger IFN-gamma and IL-2 responses to bovine purified protein derivative after the BCG booster than those in the same group that did not develop lesions. The results indicated that BCG vaccination at birth induced a high level of immunity and that the sensitization of very young animals to antigens of environmental mycobacteria by 6 weeks of age did not affect the effectiveness of BCG. However, BCG revaccination of these young animals was contraindicated.

FIG. 1.

Levels of IFN-γ (A) and IL-2 (B) released from bovine PPD-stimulated whole blood cultures from animals vaccinated with BCG at birth (▴), at 6 weeks of age (□), and at birth and 6 weeks of age (○) and from nonvaccinated animals (⧫). IFN-γ levels are presented as mean concentrations in plasma (nanograms per milliliter), and IL-2 levels are presented as mean SIs. The bars above the data points represent SEMs. V represents BCG vaccination dates (0 and 6 weeks), and C represents the challenge with M. bovis, which was undertaken when the calves were 14 to 17 weeks of age.

FIG. 2.

Comparison of IFN-γ (A) and IL-2 (B) responses after vaccination for the animals from the group vaccinated at birth and at 6 weeks of age which subsequently developed tuberculous lesions after challenge (•) and those without lesions (○). IFN-γ and IL-2 were released from whole blood cultures stimulated with bovine PPD. IFN-γ levels are presented as mean concentrations in plasma (ng/ml), and IL-2 levels are presented as mean SIs. The bars above the data points represent SEMs. An asterisk (*) indicates an IFN-γ or IL-2 level significantly different from that for the nonvaccinated group (P < 0.05).

FIG. 3.

Mean percentages of CD4⁺, CD8⁺, and WC1⁺ γδ T cells from PBMCs of nonvaccinated (NV) and BCG-vaccinated (BCG) calves expressing CD25 (IL-2 receptor) following culture with (black column) or without (white column) bovine PPD. The bars above the columns represent SEMs.

FIG. 4.

Effects of vaccination and M. bovis challenge on mean skin test responses to bovine PPD at 11 weeks after the first vaccination (A) or 15 weeks after challenge (B). Data are expressed as the increase in skin thickness (mm) between the time of inoculation and 72 h later. The bars above the columns represent SEMs.