Marine Brucella species have been isolated from marine mammals worldwide; however, the pathogenicity of these marine species has not been characterized. Lesions attributed to marine Brucella include osteomyelitis, neurobrucellosis, and abortion, though Brucella has also been isolated from animals with no apparent lesions. Marine Brucella species are zoonotic as evidenced by four documented human cases. Terrestrial Brucella cause significant morbidity in host-adapted species, while non-host adapted species may either become chronically infected or clear the infection, depending in part on the lipopolysaccharide structure of the Brucella. Infection of mononuclear phagocytes occurs through lipid raft-mediated entry or phagocytosis, and the Brucella replicates intracellularly after attenuating the host cell response. We hypothesize that marine Brucella have a predilection for, and are phagocytosed preferentially by, marine mammal mononuclear phagocytic cells, and diminish host cell immune response by causing a reduction of respiratory burst activity. We further propose that human mononuclear phagocytes, as non-host adapted cells, will display a more robust response to infection than marine mammal mononuclear phagocytes.
Phagocytosis and respiratory burst activity of beluga and human monocytes and granulocytes were evaluated using flow cytometry. Whole blood was incubated with dichlorodihydrofluorescein diacetate (DCFDA), a marker of reactive oxygen species, and challenged with either Staphylococcus aureus or marine Brucella labeled with propidium iodide. Cellular response was evaluated over a 2 hour time course post-infection.
Beluga monocytes and granulocytes both phagocytosed large numbers of S. aureus. As is the case with terrestrial Brucella species, marine Brucella was phagocytosed primarily by monocytes, with minimal granulocyte response in comparison to S. aureus infection. As expected, respiratory burst activity was highest in beluga granulocytes infected with S. aureus, though Brucella-infected granulocytes also displayed a low level of respiratory burst activity. Monocyte respiratory burst response to Brucella was low, consistent with the proposed mechanism of reduced intracellular response to this pathogen. However monocyte respiratory burst response to S. aureus was also unexpectedly low, indicating either reduced intracellular response or an inability to properly detect the maximal host cell response.
Human monocytes and granulocytes both phagocytosed large amounts of Brucella as well as S. aureus, unlike beluga granulocytes which displayed a minimal phagocytic response to Brucella infection. Respiratory burst activity was again highest in human granulocytes infected with S. aureus. Human monocytes infected with S. aureus and granulocytes and monocytes infected with Brucella displayed minimal respiratory burst activity.
The above results support the hypothesis that beluga mononuclear phagocytic cells have a predilection for the phagocytosis of marine Brucella in comparison to granulocytes, though this response is not apparent for human granulocytes and monocytes. The differential respiratory burst response between monocytes and granulocytes in response to infection with S. aureus may be attributed to monocyte production of reactive oxygen species which may not be adequately measured by the fluorescent probe (DCFDA) used in these experiments. Studies are underway to evaluate cytokine production by human and beluga mononuclear phagocytes to further characterize the intracellular response to this pathogen.