Dynamic changes in circulating and antigen‐responsive T‐cell subpopulations post‐Mycobacterium bovis infection in cattle

Bovine tuberculosis is a threat to animal and human health in several countries. Greater understanding of the immunology of the disease is required to develop improved tests and vaccines. This study has used a model of bovine tuberculosis, established in the natural host, to investigate the dynamic changes that occur in the circulating T‐cell subpopulations after infection. When the phenotypic composition of the peripheral blood lymphocytes was determined pre‐ and post‐experimental infection, the response to disease comprised three phases. Firstly, the WC1/γδ T cells decreased and then increased, suggesting localization to developing lesions and clonal expansion. Secondly, the CD4 : CD8 ratio increased. Thirdly, the CD4 : CD8 ratio decreased to less than pre‐infection measurements. The latter changes suggested sequential involvement of CD4 and then CD8 T cells. The proportion of cells expressing interleukin‐2 receptor (IL‐2R) also increased. Panels of T‐cell clones were established at various stages post‐infection and all clones that exhibited antigen responsiveness were phenotyped. T‐cell clones from early infection were WC1/γδ and CD4 in phenotype, while CD8 clones appeared later in infection, eventually becoming dominant. Therefore, from in vivo and in vitro evidence, it was suggested that there is a dynamic progression in the T‐cell subpopulations involved dominantly in responses to mycobacteria.