Défense de thèse de doctorat en sciences biologiques "Brucella melitensis"

Relationship between Brucella melitensis and the associated immune response after intranasal infection in mice

Jury

Kris HUYGEN (Inst. Pasteur, Bxl), Camille LOCHT (Inst. Pasteur, Lille), Jacques GODFROID (Univ. Tromso, Norway), Benoît MUYLKENS, président (UNamur), Jean-Jacques LETESSON, promoteur (UNamur)

Résumé

 

Brucella melitensis is a facultative intracellular bacterium responsible for one of the most common zoonosis in the world, mainly transmitted by oral ingestion of contaminated food and aerosol. Infected humans have usually an undulant fever that can become a chronic disease if it is not treated. Abortion and sterility are the major symptoms in domestic and wild animals. If classical intraperitoneal (i.p.) model of infection in mice have been useful to discover virulence factors of Brucella and to characterize effectors of the protective host immunity, this approach is not representative of the natural route of infection and much remains unknown regarding immune response and mechanisms developed by Brucella to persist in the host. Consequently, we decided to develop an intranasal (i.n.) model of infection to study the immune response of mice against Brucella melitensis.

 

In the first part of the study, using wild type and genetically deficient C57BL/6 mice, we analyzed the nature and the function of immune cellular populations controlling Brucella infection, including the action of specific actors of the mucosal immunity. We confirmed the crucial role of IFN--producing CD4 Th1 cells in the control of the primary i.n. infection by Brucella melitensis. We also highlighted an early role of gd T lymphocytes and production of IL-17A in the control of the bacterial load in lungs. We showed that B lymphocyte deficiency do not affect the efficiency of the primary or secondary immune response against Brucella.

In the second part of the work, using susceptible IL12p40-deficient BALB/c mice, we characterized the preferential cellular niche for Brucella in spleen during the persistent phase of infection. We confirmed that Brucella melitensis presents a tropism for myeloid lineage in susceptible IL12p40-deficient BALB/c mice. The large majority of infected cells expressed CD11c and CD205 markers, specific of dendritic cells. But they are lipid-rich and they display high level of Arginase1, a typical marker of IL-4-induced alternative activated M2 macrophages. To determine if these cells are M2 macrophages, we analysed the impact of IL-4/STAT6 signaling deficiency on the course of infection and the phenotype of infected cells in IL12p40-deficient mice. We observed no differences between IL-12p40 and IL-12p40/STAT6 deficient mice, demonstrating that infected cells are not dependent on IL-4 and cannot be considered as alternative activated M2 macrophages induced by Th2.

 

In conclusion, this work allows a better understanding of the immune response induced by Brucella melitensis in an i.n. model of infection, and describes, for the first time, the phenotype of a preferential cellular niche of the pathogen in susceptible mice during the chronic phase of infection.

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