IMMUNOLOGICAL RESPONSES AND PROTECTION AGAINST MYCOBACTERIUM-BOVIS INCALVES VACCINATED WITH A LOW-DOSE OF BCG

Groups of calves (20 per group) were vaccinated subcutaneously with a single dose of BCG Pasteur (6 x 10(4) or 6 x 10(6) colony forming unit s) and two months later, 15 calves from each group were challenged int ratracheally with virulent Mycobacterium bovis. Vaccination with eithe r dose of BCG induced significant protection against the development o f tuberculous lesions compared to non-vaccinated controls. Seven month s after BCG vaccination, many of the vaccinated animals which had no l esions and were M. bovis culture-negative at necropsy showed positive reactions for M. bovis in three assays for measuring cellular immune r esponses (comparative intradermal test, interferon-gamma assay and lym phocyte proliferation assay). This effect was most noticeable in the B CG-vaccinated calves which had been challenged with M. bovis rather th an in the nonchallenged animals. Antibody, responses to M. bovis were very low or absent in the calves during the study,. The kinetics of th e interferon-gamma response of peripheral blood lymphocytes cultured i n vitro with bovine PPD showed that BCG vaccination induced a rapid ri se in the response followed by a sharp decline, while infection with v irulent M. bovis resulted in an increase in the interferon-gamma respo nse by four weeks after challenge and this response remained high thro ugh the study.