Lymphocyte subset proliferative responses of Mycobacterium bovis-infected cattle to purified protein derivative

Despite highly successful eradication efforts in several countries, Mycobacterium bovis infection of cattle remains a significant health concern worldwide. Immune mechanisms of resistance to and/or clearance of M. bovis infection of cattle, however, are unclear. Recent studies have provided evidence supporting a role for CD4+, CD8+, and γδ TCR+ T cells in the response of cattle to M. bovis. In the present study, we utilized a flow cytometric-based proliferation assay to determine the relative contribution of individual lymphocyte subsets in the response to M. bovis infection and/or sensitization with mycobacterial purified protein derivative (PPD). Peripheral blood mononuclear cells (PBMC) from M. bovis-infected cattle proliferated in response to in vitro stimulation with M. bovis PPD. CD4+ T cells and γδ TCR+ cells were the predominate subsets of lymphocytes responding to PPD. γδ TCR+ cells also proliferated in non-stimulated cultures; however, the γδ TCR+ cell proliferative response of infected cattle was significantly (p<0.05) greater in PPD-stimulated cultures as compared to non-stimulated cultures. Intradermal injection of PPD for comparative cervical testing (CCT) induced a boost in the in vitro proliferative response of CD4+ but not γδ TCR+ cells of infected cattle. Administration of PPD for CCT also boosted interferon-γ (IFN-γ) production by PBMC of infected cattle following in vitro stimulation with M. bovis PPD. Injection of PPD for CCT did not, however, elicit a proliferative or IFN-γ response in cells isolated from non-infected cattle. These data indicate that CD4+ and γδ TCR+ cells of M. bovis-infected cattle proliferate in a recall response to M. bovis PPD and that the CD4+ cell response is boosted by intradermal injection with PPD for CCT.