Complement Receptor 3, but not Fc gamma Receptor III, abolishes the intracellular survival advantage of virulent Rhodococcus equi in murine macrophages

Abstract Complement receptor 3 (CR3) and Fc gamma receptor III (FcγRIII) are proposed to be essential for mediating Rhodococcus equi entry and killing in macrophages, respectively. Our objective was to investigate the role of each of these receptors in phagocytosis and intracellular replication of R. equi by macrophages. Wild-type (WT), CR3−/−, and FcγRIII−/− J774A.1 macrophages were infected with either virulent or avirulent opsonized R. equi. Quantitative culture was performed immediately post-infection (T0) and after 48 h (T48). Data were analyzed using mixed-effects modeling with the following outcomes: 1) ratio of inoculum:T0 (phagocytosis); and, 2) ratio of T48:T0 (intracellular survival). Additionally, WT, CR3−/−, and FcγRIII−/− macrophages were infected with virulent or avirulent GFP+ R. equi to confirm intracellular infection by confocal microscopy. Our results demonstrate that phagocytosis of either virulent or avirulent R. equi was unaffected by knocking out CR3 or FcγRIII, and imaging revealed that CR3−/− and FcγRIII−/− internalized both R. equi strains similarly to WT macrophages. As expected, intracellular survival in WT was significantly higher for virulent than avirulent R. equi. In CR3−/−, however, no significant difference in survival between virulent and avirulent strains was observed. Knocking out FcγRIII did not alter R. equi intracellular replication. Our findings demonstrate that CR3 is not necessary for R. equi infection and show that CR3 deletion eliminated the intracellular survival advantage of virulent strains. Last, FcγRIII-mediated phagocytosis does not improve intracellular killing as previously suggested.