A panel of correlates predicts vaccine-induced protection of rats against respiratory challenge with virulent Francisella tularensis

Pascalis, Roberto De; Hahn, Andrew C.; Brook, Helen M.; Rydén, Patrik; Donart, Nathaniel; Mittereder, Lara; Frey, Blake; Wu, Terry; Elkins, Karen L.

doi:10.1371/journal.pone.0198140

Cited by 20 publications

(44 citation statements)

References 38 publications

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“…In contrast to mice, which are highly susceptible to virulent strains of F. tularensis, rats exhibit essentially similar susceptibility as humans and may therefore serve as a better model than the mouse [7]. In support of the utility of using rat cells in the co-culture model, it has been shown that peripheral blood leukocytes from immunized rats were able to control intra-macrophage growth of LVS in a co-culture assay and, as in the mouse model, it was demonstrated that the in vitro hierarchy of intra-macrophage growth control of LVS exhibited a pattern that directly correlated with the efficacy of the investigated vaccines to protect against challenge with a highly virulent strain in vivo [8]. Importantly, a similar co-culture assay was also developed using human-derived cells and this model successfully separated the efficacy of PBMC from LVS-vaccinated persons from non-vaccinated persons to control intracellular growth of the highly virulent strain SCHU S4 [9].…”

Section: Introductionmentioning

confidence: 99%

Vaccine-Mediated Mechanisms Controlling Francisella tularensis SCHU S4 Growth in a Rat Co-Culture System

et al. 2020

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Francisella tularensis causes the severe disease tularemia. In the present study, the aim was to identify correlates of protection in the rat co-culture model by investigating the immune responses using two vaccine candidates conferring distinct degrees of protection in rat and mouse models. The immune responses were characterized by use of splenocytes from naïve or Live vaccine strain- (LVS) or ∆clpB/∆wbtC-immunized Fischer 344 rats as effectors and bone marrow-derived macrophages infected with the highly virulent strain SCHU S4. A complex immune response was elicited, resulting in cytokine secretion, nitric oxide production, and efficient control of the intracellular bacterial growth. Addition of LVS-immune splenocytes elicited a significantly better control of bacterial growth than ∆clpB/∆wbtC splenocytes. This mirrored the efficacy of the vaccine candidates in the rat model. Lower levels of IFN-γ, TNF, fractalkine, IL-2, and nitrite were present in the co-cultures with ∆clpB/∆wbtC splenocytes than in those with splenocytes from LVS-immunized rats. Nitric oxide was found to be a correlate of protection, since the levels inversely correlated to the degree of protection and inhibition of nitric oxide production completely reversed the growth inhibition of SCHU S4. Overall, the results demonstrate that the co-culture assay with rat-derived cells is a suitable model to identify correlates of protection against highly virulent strains of F. tularensis

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Section: Introductionmentioning

confidence: 99%

Vaccine-Mediated Mechanisms Controlling Francisella tularensis SCHU S4 Growth in a Rat Co-Culture System

et al. 2020

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“…The mechanisms of in vitro bacterial growth control depend on CD4 + or CD8 + effector T cell functions, including IFN-γ, TNF-α, and nitric oxide (NO) production, as well as roles for IL-6, T-bet, and IL-12Rβ2; in contrast, B cells, NK cells, and myeloid cells have minimal if any contributions [23][24][25][26][27][28][29] . In mice and rats, this in vitro co-culture system has proved to provide a functional correlate of vaccine-induced protection in vivo [30][31][32][33][34] , supporting its relevance for studies of infection and in vivo mechanisms central to protective immunity. Results indicate that mechanisms identified to date do not account for all bacterial growth control, and thus additional mechanisms by which T cells limit intramacrophage bacterial growth await discovery 24,25,35 .…”

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confidence: 77%

“…The in vitro co-culture approach was designed to mimic in vivo interactions between macrophages and lymphocytes, which are colocalized within the architecture of lymphoid organs such as spleen and lymph nodes. The www.nature.com/scientificreports/ relevance of this tissue culture model to in vivo infection is supported by its demonstrated value in serving as a functional correlate that predicts in vivo efficacy of Francisella and M. tuberculosis vaccines in both mice and rats 20,24,[30][31][32][33][34] . Further, its application has previously uncovered novel mechanisms by which T cells, the dominant cell type responding to in vivo Francisella infection, control infection in vivo, such as IL-6 utilization and engagement of IL12Rβ2 28,29 .…”

Section: Discussionmentioning

confidence: 99%

Immune lymphocytes halt replication of Francisella tularensis LVS within the cytoplasm of infected macrophages

Bradford

Elkins

2020

Sci Rep

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Francisella tularensis is a highly infectious intracellular bacterium that causes tularemia by invading and replicating in mammalian myeloid cells. Francisella primarily invades host macrophages, where it escapes phagosomes within a few hours and replicates in the cytoplasm. Less is known about how Francisella traffics within macrophages or exits into the extracellular environment for further infection. immune t lymphocytes control the replication of Francisella within macrophages in vitro by a variety of mechanisms, but nothing is known about intracellular bacterial trafficking in the face of such immune pressure. Here we used a murine model of infection with a Francisella attenuated live vaccine strain (LVS), which is under study as a human vaccine, to evaluate the hypothesis that immune T cells control intramacrophage bacterial growth by redirecting bacteria into toxic intracellular compartments of infected macrophages. We visualized the interactions of lymphocytes and LVS-infected macrophages using confocal microscopy and characterized LVS intramacrophage trafficking when co-cultured with immune lymphocytes. We focused on the late stages of infection after bacteria escape from phagosomes, through bacterial replication and the death of macrophages. We found that the majority of LVS remained cytosolic in the absence of immune pressure, eventually resulting in macrophage death. In contrast, co-culture of LVS-infected macrophages with LVS-immune lymphocytes halted LVS replication and inhibited the spread of LVS infection between macrophages, but bacteria did not return to vacuoles such as lysosomes or autophagosomes and macrophages did not die. Therefore, immune lymphocytes directly limit intracellular bacterial replication within the cytoplasm of infected macrophages. Francisella tularensis is a gram-negative, facultative intracellular bacterium that replicates in macrophages and causes tularemia in humans 1-3. Francisella can infect people via multiple routes, but respiratory infection leads to the most severe form of the disease and can be fatal if not treated. F. tularensis is found throughout North America and is endemic in Europe, especially in Scandinavia, and in Asia. Tularemia is not a large public health concern in developed countries. However, Francisella was investigated as a bioweapon in the mid-1900s by both the United States and Soviet Union; the bacterium is therefore is currently categorized as a Select Agent in the United States 3. The intracellular lifecycle of Francisella has been visualized in vitro with fixed and live-cell microscopy. Francisella enters macrophages via phagocytosis and escapes from the resulting phagosome within 1 to 4 h, avoiding lysosomal fusion 4-13. Cytosolic Francisella replicates to high numbers over the next day 9. To date, in-depth characterization has focused on the first 24 h after infection of host myeloid cells. One set of reports has described clustering of LVS about 20 h after infection of murine macrophages into "Francisella-containing vacuoles" (FCVs), which ...

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“…Because vaccination with Schu S4ΔaroD induced both responses in mice, this vaccine will serve as a valuable tool in future studies to discriminate the specific protective mechanisms. It is noteworthy that vaccination with Schu S4 aroD induced a functional T cell response as determined in the splenocyte co-culture assay [24,26,46]. This assay perhaps is the best "correlate" of immunity thus far identified because it relies on antigen-specific T cells to produce IFN-γ leading to the activation of macrophages that control bacterial replication and survival by inducing anti-microbial and inflammatory genes.…”

Section: Discussionmentioning

confidence: 99%

Characterization of Schu S4 aro mutants as live attenuated tularemia vaccine candidates

et al. 2020

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There is a need for development of an effective vaccine against Francisella tularensis, as this potential bioweapon has a high mortality rate and low infectious dose when delivered via the aerosol route. Moreover, this Tier 1 agent has a history of weaponization. We engineered targeted mutations in the Type A strain F. tularensis subspecies tularensis Schu S4 in aro genes encoding critical enzymes in aromatic amino acid biosynthesis. F. tularensis Schu S4ΔaroC, Schu S4ΔaroD, and Schu S4ΔaroCΔaroD mutant strains were attenuated for intracellular growth in vitro and for virulence in vivo and, conferred protection against pulmonary wild-type (WT) F. tularensis Schu S4 challenge in the C57BL/6 mouse model. F. tularensis Schu S4ΔaroD was identified as the most promising vaccine candidate, demonstrating protection against high-dose intranasal challenge; it protected against 1,000 CFU Schu S4, the highest level of protection tested to date. It also provided complete protection against challenge with 92 CFU of a F. tularensis subspecies holarctica strain (Type B). Mice responded to vaccination with Schu S4ΔaroD with systemic IgM and IgG2c, as well as the production of a functional T cell response as measured in the splenocytemacrophage co-culture assay. This vaccine was further characterized for dissemination, histopathology, and cytokine/chemokine gene induction at defined time points following intranasal vaccination which confirmed its attenuation compared to WT Schu S4. Cytokine, chemokine, and antibody induction patterns compared to wild-type Schu S4 distinguish protective vs. pathogenic responses to F. tularensis and elucidate correlates of protection associated with vaccination against this agent.

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A panel of correlates predicts vaccine-induced protection of rats against respiratory challenge with virulent Francisella tularensis

Cited by 20 publications

References 38 publications

Vaccine-Mediated Mechanisms Controlling Francisella tularensis SCHU S4 Growth in a Rat Co-Culture System

Vaccine-Mediated Mechanisms Controlling Francisella tularensis SCHU S4 Growth in a Rat Co-Culture System

Immune lymphocytes halt replication of Francisella tularensis LVS within the cytoplasm of infected macrophages

Characterization of Schu S4 aro mutants as live attenuated tularemia vaccine candidates

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