Collaboration of bovine T lymphocytes and macrophages in T-lymphocyte response to Brucella abortus.

Brucella abortus-induced bovine macrophage-T-lymphocyte collaboration was studied as a prerequisite for the eventual clearance of this infectious organism. Esterase-positive, peripheral blood monocytes functioned as an adherent antigen-presenting cell population. A dual requirement for expression of bacterial antigens in combination with self major histocompatibility complex class II products was required by adherent cells for the activation of T lymphocytes. Comparison of antigen-presenting cell populations that were either trypsinized or nontrypsinized following B. abortus ingestion substantiated the need for phagocytosis and antigen processing. A monoclonal antibody (H4) directed against major histocompatibility complex class II determinants was able to block or, with complement, to abrogate T-lymphocyte responses. Measurement of both proliferation and interleukin 2 production via [3H]thymidine incorporation confirmed specific activation of an enriched T-lymphocyte population. These results indicate that in vivo-primed T lymphocytes of peripheral blood origin recognize phagocytized bacterial components of the facultative intracellular bacterium B. abortus and may contribute to the removal of the bacteria. Furthermore, bovine peripheral blood-adherent cells function as classic antigen-presenting cells, which suggests that macrophages are capable of processing this bacteria. Therefore, any lymphocyte-mediated dysfunction attributable to B. abortus most likely occurs at some point in the cascade of immune events following initial macrophage-T-lymphocyte collaboration.