Interaction of bovine peripheral blood polymorphonuclear cells and Leptospira species; innate responses in the natural bovine reservoir host.

Authors: Wilson-Welder, Jennifer; Frank, Ami; Hornsby, Richard; Olsen, Steven; Alt, David

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2016
Publication Date: 7/19/2016
Citation: Wilson-Welder, J.H., Frank, A.T., Hornsby, R.L., Olsen, S.C., Alt, D.P. 2016. Interaction of bovine peripheral blood polymorphonuclear cells and Leptospira species; innate responses in the natural bovine reservoir host. Frontiers in Microbiology. 7:1110.

Interpretive Summary: Neutrophils are one of the most numerous leukocytes and circulate in an active state ready to eliminate pathogens. Studies have evaluated the interaction between neutrophils, the first line of immune defense, with Leptospira, a bacteria that infects humans and animals. Leptospirosis is a leading cause of reproductive failure in cattle and infected cattle can transmit disease to other cattle and humans. Because of observed difference between host species in initial immune responses to infection, we wanted to evaluate how cattle cells interact with different Leptospira strains. When exposed to Leptospira bacteria, cattle neutrophils activated to produce neutrophil traps or NETS, produced small amounts of reactive oxygen and nitrogen compounds used to kill pathogens, and upregulated expression of cytokine genes associated with inflammatory responses. No differences were seen between cattle-adapted strains, human pathogenic strains and non-pathogenic or even killed Leptospira. Furthermore, this activation did not result in the killing or reduction in viable Leptospira numbers. We believe these differences between animals in early immune responses may be explained by differences in receptors on cells, and could contribute to the chronic carrier state in cattle. Our study demonstrates the importance of evaluating pathogens in the host species of interest and that alternative models may not be sufficient to understand disease progression. This work will be of interest to other scientists working in infectious disease and will also be of interest to cattle producers as they try to develop strategies to eliminate leptospirosis from their herds.

Technical Abstract: Cattle are the reservoir hosts of Leptospira borgpetersenii serovar Hardjo, and also be reservoir hosts of other Leptospira species such as L. kirschneri, and L. interrogans. As a reservoir host, cattle shed Leptospira, infecting other animals, including humans. Previous studies with human and murine neutrophils have shown activation of neutrophil extracellular trap or NET formation, and upregulation of inflammatory mediators by neutrophils in the presence of Leptospira. Humans, companion animals and most widely studied models of Leptospirosis are of acute infection, hallmarked by systemic inflammatory response, neutrophilia and septicemia. In contrast, cattle exhibit chronic infection with few outward clinical signs aside from reproductive failure. Taking into consideration that there is host species variation in innate immunity, especially in pathogen recognition and response, the interaction of bovine peripheral blood polymorphonuclear cells (PMNs) and several Leptospira strains was evaluated. Studies including bovine-adapted strains, human pathogen strains, a saprophyte and inactivated organisms. Incubation of PMNs with Leptospira did induce slight activation of neutrophil NETs. Very low levels of reactive oxygen peroxides were produced, but significant levels of reactive nitrogen intermediaries were measured. PMNs incubated with Leptospira induced RNA transcripts of IL-1ß, IL-8, MIP-1a, and TNF-a, with greater amounts induced by live organisms when compared to heat-inactivated leptospires. However, incubation of Leptospira strains with bovine PMNs did not affect Leptospira viability as measured by limiting dilution culture. This is in contrast to previously reported results of innate inflammatory activation by Leptospira in human and other animal models, or the activation and interaction of bovine PMNs with Escherichia coli and other bacterial pathogens. While it could be hypothesized that variations in innate receptor recognition, specifically variance in toll-like receptor 2, could underlie the observed reduction of activation in bovine PMNs, additional studies would be needed to explore this possibility. Reduction in neutrophil responses may help to establish nearly asymptomatic chronic Leptospira infection of cattle. This study emphasizes the importance of studying host-pathogen relationships in the appropriate species as extrapolation from other animal models may be incorrect and confounded by differences in the host responses.