Survival of different Mycobacterium avium subsp. paratuberculosis strains in bovine monocyte-derived macrophages

In this study the ability of different Mycobacterium avium subsp.
paratuberculosis (M. paratuberculosis) strains to survive in bovine
monocyte-derived macrophages (MDMs) was investigated. The following
hypotheses were tested: 1) Infection status of a cow does not
affect the ability of its macrophages to kill M. paratuberculosis
2) Killing capacity of bovine macrophages is not dependent on the
M. paratuberculosis strain MDMs for conducted experiments were
obtained from Johne’s disease-positive (n = 3) and age and stage of
lactation matched Johne’s disease-negative (n = 3) multiparious
cows. Animals were kept on the same dairy operation, thus
non-infected cows had been exposed to M. paratuberculosis strains
present on the farm premises. Peripheral blood mononuclear cells
(PBMC) were isolated from whole blood by density gradient
centrifugation. After a five-day differentiation period in cell
culture using Teflon jars MDMs were obtained and subsequently
challenged in vitro with four M. paratuberculosis strains of
different host specificity (bovine, ovine). MDMs were harvested at
2 hours, 2 days, 4 days and 7 days following infection. For each
time point ingestion and intracellular survival of M.
paratuberculosis strains were determined by fluorescence
microscopy. There was no effect of the origin of MDMs (Johne’s
disease-positive or control animals) on phagocytosis, survival of
bacteria, or macrophage survival. In contrast, important strain
differences were observed. These findings suggest that some M.
paratuberculosis strains interfere more successfully than others
with the ability of macrophages to kill intracellular pathogens
which may make it important to include strain typing when designing
control programs.