In vitro determination of phagocytosis and intracellular killing of Brucella melitensis by goat's polymorphonuclear phagocytes

The Role of Neutrophils in Brucellosis

Brucellosis is a bacterial disease of domestic animals and humans. The pathogenic ability of Brucella organisms relies on their stealthy strategy and their capacity to replicate within host cells and to induce long-lasting infections. Brucella organisms barely induce neutrophil activation and survive within these leukocytes by resisting microbicidal mechanisms. Very few Brucella-infected neutrophils are found in the target organs, except for the bone marrow, early in infection. Still, Brucella induces a mild reactive oxygen species formation and, through its lipopolysaccharide, promotes the premature death of neutrophils, which release chemokines and express "eat me" signals. This effect drives the phagocytosis of infected neutrophils by mononuclear cells that become thoroughly susceptible to Brucella replication and vehicles for bacterial dispersion. The premature death of the infected neutrophils proceeds without NETosis, necrosis/oncosis, or classical apoptosis morphology. In the absence of neutrophils, the Th1 response exacerbates and promotes bacterial removal, indicating that Brucella-infected neutrophils dampen adaptive immunity. This modulatory effect opens a window for bacterial dispersion in host tissues before adaptive immunity becomes fully activated. However, the hyperactivation of immunity is not without a price, since neutropenic Brucella-infected animals develop cachexia in the early phases of the disease. The delay in the immunological response seems a sine qua non requirement for the development of long-lasting brucellosis. This property may be shared with other pathogenic alphaproteobacteria closely related to Brucella We propose a model in which Brucella-infected polymorphonuclear neutrophils (PMNs) function as "Trojan horse" vehicles for bacterial dispersal and as modulators of the Th1 adaptive immunity in infection.

Phagocytic cell function in active brucellosis

In this study, we analyzed phagocytic cell function in 51 patients with active brucellosis and its relationship with different clinical, serological, and evolutionary variables. A control group was made up of 30 blood donors of similar geographic extraction, age, and sex, with no previous history of brucellosis or known exposure ot the infection or specific antibodies. The investigations were carried out at the time of diagnosis, at the conclusion of treatment, and after 6 months of follow-up. Polymorphonuclear leukocyte adherence and nitroblue tetrazolium reduction in response to Brucella antigen were significantly increased in the patients at the time of diagnosis with respect to the control group. In contrast, chemotaxis in response to Brucella antigen and phagocytosis were significantly reduced in the patients with respect to the control group. The alterations in phagocytic cell function were greater in patients with bacteremia, with focal forms of the disease, or with a longer diagnostic delay. Most of these initial alterations tended to normalize with treatment, indicating their transient character.

Phagocytosis, killing, and oxidant production by bovine monocyte-derived macrophages upon exposure to Brucella abortus strain 2308

Phagocytosis and intracellular survival of Brucella abortus, and oxidant production by monocyte-derived macrophages from ten B. abortus-naive cows were studied. Phagocytosis of bacteria opsonized with naive-autologous sera or reactor serum was significantly less than phagocytosis of bacteria opsonized with fetal bovine serum. After phagocytosis, intracellular survival of bacteria opsonized with naive-autologous or reactor sera was significantly less than survival of bacteria opsonized with fetal bovine serum. Production of oxidant by macrophages stimulated with B. abortus opsonized with naive-autologous, reactor, or fetal bovine sera was not significantly different. Although macrophages from one animal showed significantly less phagocytic activity, intracellular killing and oxidant production by macrophages from the ten individual cows toward B. abortus opsonized with naive-autologous, reactor, and fetal calf sera were homogeneous. The abilities of the macrophages to phagocytize and to kill B. abortus were not associated with each other or with oxidant production. Innate resistance or sensitivity to B. abortus was not identified in the cows based on macrophage function.

Effect of Brucella abortus lipopolysaccharide on oxidative metabolism and lysozyme release by human neutrophils

Both Brucella abortus lipopolysaccharide (LPS) and lipid A were low activators of nitroblue tetrazolium reduction and lysozyme release in human neutrophils. The stimulation was dose dependent and was higher in the presence of autologous plasma than in its absence. The comparison between Brucella LPS and lipid A versus Salmonella LPS revealed that at least 100 times more LPS and 1,000 times more lipid A of the former genus were required to induce significant nitroblue tetrazolium reduction and a corresponding lysozyme release in neutrophils. Low Brucella LPS-mediated superoxide and lysozyme production might contribute to the survival of these facultative intracellular bacteria in phagocytic cells.