Characterisation of the lymph node immune response following Mycoplasma mycoides subsp. Mycoides SC infection in cattle

Contagious bovine pleuropneumonia (CBPP), caused by Mycoplasma mycoides subsp. mycoides biotype Small Colony (MmmSC), is still a major cattle disease in Africa. Development of long-term protective vaccines, the only relevant strategy to achieve CBPP eradication, requires the characterisation of the protective immune mechanism. To this aim, the present study investigated the cellular immune response persisting in the lymph nodes of cattle infected naturally and experimentally by contact, one year post exposure. The lymph node cell composition, MmmSC responsiveness and phenotype of the MmmSC-responding lymphocytes were compared between animals according to the different outcomes of the infection. To unravel the protective mechanism, the study focussed on the MmmSC-specific memory immune response generated in recovered cattle, known to develop long-term immunity and to be resistant to reinfection. An MmmSC-specific immune response, mediated by IFNgamma-secreting CD4 T-cells, was detected in the lymph nodes of all recovered cattle. Furthermore, the magnitude of this immune response was significantly higher in animals with complete recovery than in recovered animals presenting lung sequestra. The findings suggest that, in recovered cattle, a subset of MmmSC-primed IFNgamma-secreting CD4 T-cells homed to the regional lymph nodes as MmmSC-specific memory T-cells, likely responsible for the protective anamnestic response. Induction and expansion of this subset of MmmSC-specific CD4 memory T-cells might be a major goal to develop efficient long term protective vaccines against CBPP.