Interaction of Rhodococcus equi with phagocytic cells from R. equi-exposed and non-exposed foals. 
The interaction of Rhodococcus equi with alveolar macrophages from adult horses, foals experimentally exposed to R. equi (sensitized foals) and non-exposed foals was studied using in vitro bactericidal assays, cytochemical staining and transmission electron microscopy.  It was demonstrated that R. equi is a facultative intracellular parasite, able to survive and multiply within the alveolar macrophages of the host by interfering with phagosome-lysosome fusion.  Opsonization of R. equi with antibody against capsular components was associated with increased phagosome-lysosome fusion and significantly enhanced (P less than 0.05) killing of the organism by alveolar macrophages from non-exposed foals.  Macrophages from non-exposed foals were able to ingest the non-opsonized organism, but unable to kill greater than 65% of the infective dose by 6 h post-exposure.  Alveolar macrophages from sensitized foals behaved as adult macrophages, able to kill greater than 95% of the infective dose by 6 h. Lymphocyte factors, derived by in vitro incubation of sensitized peripheral blood lymphocytes with R. equi surface antigens, enhanced macrophage bactericidal activity.  Macrophages from non-exposed foals incubated in the presence of the lymphocyte factors had a 50% increase in killing of R. equi, while sensitized macrophages incubated with lymphocyte factors had a greater than 100% increase in killing capacity.