Cells from the mononuclear phagocytic system (MPS) and γδ T lymphocytes interact with Mycobacterium avium subspecies paratuberculosis (Map) and with each other to shape the ensuing immune response. The central hypothesis of this thesis was that bovine γδ T lymphocyte subsets (WC1+ and WC1neg) differentially regulate host immunity during early Map infection by interacting with cells of the MPS, the preferred host cells for Map. The main purpose of this research was to understand how bovine γδ T lymphocyte subsets modulate effector function of cells from the MPS during Map infection in vitro. The first study investigated how WC1+ γδ T lymphocytes from heifers mediate autologous macrophage function during Map infection in vitro. We showed that the viability of Map recovered from monocyte-derived macrophages (MDMs) was significantly reduced when WC1+ γδ T lymphocytes were in direct contact with Map-infected MDMs. Both MDMs and WC1+ γδ T lymphocytes generated increased concentrations of IFN-γ and IL-4. Co-cultures of MDMs/WC1+ γδ T lymphocytes were associated with higher expression of MHC-I on MDMs and increased concentration of nitrites. In the second study we investigated how WC1+ or WC1neg γδ T lymphocytes from young calves influence effector functions of autologous MDMs during Map infection in vitro. We again showed that viability of recovered Map was significantly reduced when γδ T lymphocytes were cultured in direct contact with Map-infected MDMs. Our main findings were: 1) increased IFN-γ in co-cultures of Map-infected MDMs with WC1neg γδ T lymphocytes; 2) expression of CD1b on MDMs is significantly increased when co-cultured in direct contact with WC1neg γδ T lymphocytes and 3) expression of CD25 is significantly higher on WC1+ compared to WC1neg γδ T lymphocytes. Finally, in the third study our aim was to understand how both WC1+ and WC1neg γδ T cell subsets of calves influence autologous monocyte differentiation and DC maturation during Map infection in vitro. In this study, significant findings included: 1) reduced viability of Map recovered from monocyte-derived cells differentiated in presence of WC1neg γδ T lymphocytes compared to MDMs; and 2) reduced phagocytosis indicating functional DC maturation in the presence of γδ T lymphocytes.