Identification of Immune Effectors Essential to the Control of Primary and Secondary Intranasal Infection with Brucella melitensis in Mice.

The mucosal immune system represents the first line of defense against Brucella infection in nature. We used genetically deficient mice to identify the lymphocytes and signaling pathways implicated in the control of primary and secondary intranasal infection with B. melitensis Our analysis of primary infection demonstrated that the effectors implicated differ at the early and late stages and are dependent on the organ. TCR-δ, TAP1, and IL-17RA deficiency specifically affects early control of Brucella in the lungs, whereas MHC class II (MHCII) and IFN-γR deficiency impairs late control in the lungs, spleen, and liver. Interestingly, IL-12p35(-/-) mice display enhanced Brucella growth in the spleen but not in the lungs or liver. Secondary intranasal infections are efficiently contained in the lung. In contrast to an i.p. infectious model, in which IL-12p35, MHCII, and B cells are strictly required for the control of secondary infection, we observed that only TCR-ß deficiency or simultaneous neutralization of IL-12p35- and IL-17A-dependent pathways impairs the memory protective response against a secondary intranasal infection. Protection is not affected by TCR-δ, MHCII, TAP1, B cell, IL-17RA, or IL-12p35 deficiency, suggesting that CD4(+) and CD8(+) α/ß(+) T cells are sufficient to mount a protective immune response and that an IL-17A-mediated response can compensate for the partial deficiency of an IFN-γ-mediated response to control a Brucella challenge. These findings demonstrate that the nature of the protective memory response depends closely on the route of infection and highlights the role of IFN-γ-and IL-17RA-mediated responses in the control of mucosal infection by Brucella.

Humoral immunity and CD4+ Th1 cells are both necessary for a fully protective immune response upon secondary infection with Brucella melitensis.

Brucella spp are intracellular bacteria that cause brucellosis, one of the most common zoonoses in the world. Given the serious medical consequences of this disease, a safe and effective human vaccine is urgently needed. Efforts to develop this vaccine have been hampered by our lack of understanding of what constitutes a protective memory response against Brucella. In this study, we characterize the cells and signaling pathways implicated in the generation of a protective immune memory response following priming by the injection of heat-killed or live Brucella melitensis 16M. Using a panel of gene-deficient mice, we demonstrated that during a secondary recall response, both the Brucella-specific humoral response and CD4+ Th1 cells must act together to confer protective immunity in the spleen to B. melitensis infection. Humoral protective immunity is induced by the inoculation of both heat-killed and live bacteria, and its development does not require T cells, MyD88/IL-12p35 signaling pathways, or an activation-induced deaminase-mediated isotype switch. In striking contrast, the presence of memory IFN-γ-producing CD4+ Th1 cells requires the administration of live bacteria and functional MyD88/IL-12p35 pathways. In summary, our work identifies several immune markers closely associated with protective immune memory and could help to define a rational strategy to obtain an effective human vaccine against brucellosis.

Brucella melitensis invades murine erythrocytes during infection.

Brucella spp. are facultative intracellular Gram-negative coccobacilli responsible for brucellosis, a worldwide zoonosis. We observed that Brucella melitensis is able to persist for several weeks in the blood of intraperitoneally infected mice and that transferred blood at any time point tested is able to induce infection in naive recipient mice. Bacterial persistence in the blood is dramatically impaired by specific antibodies induced following Brucella vaccination. In contrast to Bartonella, the type IV secretion system and flagellar expression are not critically required for the persistence of Brucella in blood. ImageStream analysis of blood cells showed that following a brief extracellular phase, Brucella is associated mainly with the erythrocytes. Examination by confocal microscopy and transmission electron microscopy formally demonstrated that B. melitensis is able to invade erythrocytes in vivo. The bacteria do not seem to multiply in erythrocytes and are found free in the cytoplasm. Our results open up new areas for investigation and should serve in the development of novel strategies for the treatment or prophylaxis of brucellosis. Invasion of erythrocytes could potentially protect the bacterial cells from the host's immune response and hamper antibiotic treatment and suggests possible Brucella transmission by bloodsucking insects in nature.

In situ microscopy analysis reveals local innate immune response developed around Brucella infected cells in resistant and susceptible mice.

Brucella are facultative intracellular bacteria that chronically infect humans and animals causing brucellosis. Brucella are able to invade and replicate in a broad range of cell lines in vitro, however the cells supporting bacterial growth in vivo are largely unknown. In order to identify these, we used a Brucella melitensis strain stably expressing mCherry fluorescent protein to determine the phenotype of infected cells in spleen and liver, two major sites of B. melitensis growth in mice. In both tissues, the majority of primary infected cells expressed the F4/80 myeloid marker. The peak of infection correlated with granuloma development. These structures were mainly composed of CD11b⁺ F4/80⁺ MHC-II⁺ cells expressing iNOS/NOS2 enzyme. A fraction of these cells also expressed CD11c marker and appeared similar to inflammatory dendritic cells (DCs). Analysis of genetically deficient mice revealed that differentiation of iNOS⁺ inflammatory DC, granuloma formation and control of bacterial growth were deeply affected by the absence of MyD88, IL-12p35 and IFN-γ molecules. During chronic phase of infection in susceptible mice, we identified a particular subset of DC expressing both CD11c and CD205, serving as a reservoir for the bacteria. Taken together, our results describe the cellular nature of immune effectors involved during Brucella infection and reveal a previously unappreciated role for DC subsets, both as effectors and reservoir cells, in the pathogenesis of brucellosis.

STAT6 Mediates Footpad Immunopathology in the Absence of IL-12p40 Following Infection of Susceptible BALB/c Mice With Leishmania major.

Leishmania major (L. major) parasites are intracellular parasites belong to the Trypanosomatidae family and are the causative agent of cutaneous leishmaniasis. This disease affects approximately 1.5 million per year worldwide and there is currently no prophylactic vaccine available. L. major is transmitted by the bite of an infected sandfly and has been considered for decades now as a mouse model of choice to identify the factors implicated in T helper (Th)1 and Th2 polarization due to the natural resistance and susceptibility to infection of C57BL/6 and BALB/c mice, respectively. In this study, we refine the role of IL-12p40 cytokine, which is implicated the development of a protective Th1 response, and STAT6, a transcription factor involved in the signaling via detrimental interleukin (IL)-4 and IL-13 associated Th2 cytokines during L. major infection in the BALB/c model. In the absence of STAT6 and IL-12p40 signaling, double knockout (DKO) susceptible BALB/c mice displayed reduced footpad swelling and ulcerative lesion compared to IL-12p40-/- mice upon L. major infection. Hence, they expressed slower upregulation of keratinocyte markers implicated in the inhibition of wound healing, such as keratin 6a (Krt6a) and Krt16. This coincides with the presence of neutrophils displaying an altered phenotype characterized by a lower expression of surface markers Ly6C, CD11b, and Ly6G. These neutrophils exhibited very lower levels of apoptosis similarly to neutrophils present in resistant STAT6-/- mice. Interestingly, the reduced footpad swelling in DKO mice is associated with a high footpad parasite level similar to susceptible IL-12p40-/- mice. In conclusion, this study demonstrate that in the absence of both STAT6 and IL-12p40 signaling, L. major-infected mice display smaller and less ulcerated lesions, which does, however, not correlate with reduced parasite load. In addition, the presence of neutrophils with an altered phenotype is associated with reduced apoptosis and delayed immunopathologies, demonstrating the detrimental role of STAT6 in infected susceptible BALB/c mice.

Imported human brucellosis in Belgium: Bio and molecular typing of bacterial isolates, 1996-2015.

OBJECTIVES: The aim of this study was to characterize by classical biotyping and Multi-Locus variable number tandem repeats (VNTR) Analysis (MLVA) all Brucella spp. derived from human cases in Belgium from 1996 to 2015. Final goals were to determine the species and biovar, to trace-back on genetic grounds the origin of each strain when patient history and risk factors were missing, and to survey for particular trends at the national level. METHODS: A total of 37 Brucella strains, isolated from 37 patients in Belgium, were analyzed by both classical biotyping and MLVA, and the genetic patterns compared to those of human strains isolated worldwide. RESULTS: Classical biotyping revealed that isolates were mainly Brucella melitensis. Most of them belonged to biovar 3, the most abundant biovar in the Mediterranean region. MLVA confirmed that Brucella melitensis is too diverse in VNTRs to be able to make clusters associated to each biovar, but it allowed retrieving precious epidemiological information. The analysis highlighted the imported nature of the strains from all over the world with a dominant part from the Mediterranean countries. Findings of the MLVA11 testing were in line with the travel history of patients coming from Italy, Turkey, Lebanon and Peru. The analysis was particularly useful because it suggested the geographical origin of the infection for 12/16 patients for whom no case history was available. CONCLUSION: Classical biotyping and MLVA analysis are not exclusive but remain complementary tools for Brucella melitensis strain surveillance. MLVA11 is sufficient for Brucella-free countries such as Belgium to trace the geographical origin of infection, but complete MLVA16 is needed to search for links with endemic areas.

In Situ Characterization of Splenic Brucella melitensis Reservoir Cells during the Chronic Phase of Infection in Susceptible Mice.

Brucella are facultative intracellular Gram-negative coccobacilli that chronically infect humans as well as domestic and wild-type mammals, and cause brucellosis. Alternatively activated macrophages (M2a) induced by IL-4/IL-13 via STAT6 signaling pathways have been frequently described as a favorable niche for long-term persistence of intracellular pathogens. Based on the observation that M2a-like macrophages are induced in the spleen during the chronic phase of B. abortus infection in mice and are strongly infected in vitro, it has been suggested that M2a macrophages could be a potential in vivo niche for Brucella. In order to test this hypothesis, we used a model in which infected cells can be observed directly in situ and where the differentiation of M2a macrophages is favored by the absence of an IL-12-dependent Th1 response. We performed an in situ analysis by fluorescent microscopy of the phenotype of B. melitensis infected spleen cells from intranasally infected IL-12p40-/- BALB/c mice and the impact of STAT6 deficiency on this phenotype. Most of the infected spleen cells contained high levels of lipids and expressed CD11c and CD205 dendritic cell markers and Arginase1, but were negative for the M2a markers Fizz1 or CD301. Furthermore, STAT6 deficiency had no effect on bacterial growth or the reservoir cell phenotype in vivo, leading us to conclude that, in our model, the infected cells were not Th2-induced M2a macrophages. This characterization of B. melitensis reservoir cells could provide a better understanding of Brucella persistence in the host and lead to the design of more efficient therapeutic strategies.