Mycoplasma mycoides ssp. mycoides biotype small colony-secreted components induce apoptotic cell death in bovine leucocytes.

Contagious bovine pleuropneumonia, caused by Mycoplasma mycoides ssp. mycoides biotype small colony (MmmSC), is one of the most serious cattle diseases in Africa. Several observations suggested that MmmSC had evolved an efficient way to escape the bovine immune responses by triggering host-cell cytotoxicity. This study was implemented to determine whether the cytotoxic effect was due to apoptotic cell death. To that end, bovine blood cells were cultured for up to 3 days in the presence of viable or heat-killed MmmSC compared to unstimulated cultures. The findings provided evidence for a viable MmmSC-induced, time-dependent apoptosis in bovine blood leucocytes, whereas heat-killed MmmSC had no effect. Morphological and physiological changes (evidenced by TUNEL and annexin V staining) typical of apoptosis were observed in response to viable MmmSC. All the lymphocyte subsets as well as the monocyte/granulocyte subset exhibited extensive apoptosis after exposure to viable MmmSC. Our results demonstrated a potential role for MmmSC-secreted components as pathogenic factors able to induce programmed cell death in bovine blood leucocytes.