Effects of opsonization and gamma interferon on growth of Brucella melitensis 16M in mouse peritoneal macrophages in vitro

Entry of opsonized pathogens into phagocytes may benefit or, paradoxically, harm the host. Opsonization may trigger antimicrobial mechanisms such as r eactive oxygen or nitric oxide (NO) production but may also provide a safe haven for intracellular replication. Brucellae are natural intramacrophage pathogens of rodents, ruminants, dogs, marine mammals, and humans. We evalu ated the role of opsonins in Brucella-macrophage interactions by challengin g cultured murine peritoneal macrophages with Brucella melitensis 16M treat ed with complement- and/or antibody-rich serum. Mouse serum rich in antibod y against Brucella lipopolysaccharide (LPS) (aLPS) and human complement-ric h serum (HCS) each enhanced the macrophage uptake of brucellae. Combination s of suboptimal levels of aLPS (0.01%) and HCS (2%) synergistically enhance d uptake. The intracellular fate of ingested bacteria was evaluated with an optimal concentration of gentamicin (2 mu g/ml) to control extracellular g rowth but not kill intracellular bacteria. Bacteria opsonized with aLPS and /or HCS grew equally well inside macrophages in the absence of gamma interf eron (IFN-gamma). Macrophage activation with IFN-gamma inhibited replicatio n of both opsonized and nonopsonized brucellae but was less effective in in hibiting replication of nonopsonized bacteria. IFN-gamma treatment of macro phages with opsonized or nonopsonized bacteria enhanced NO production, whic h was blocked by N-G-monomethyl L-arginine (MMLA), an NO synthesis inhibito r. MMLA also partially blocked IFN-gamma-mediated bacterial growth inhibiti on. These studies suggest that primary murine macrophages have limited abil ity to control infection with B. melitensis, even when activated by IFN-gam ma in the presence of highly opsonic concentrations of antibody and complem ent. Additional cellular immune responses, e.g., those mediated by cytotoxi c T cells, may play more important roles in the control of murine brucellos is.