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Resident alveolar macrophages are susceptible to and permissive of Coxiella burnetii infection.

Calverley M, Erickson S, Read AJ, Harmsen AG - PLoS ONE (2012)

Bottom Line: Coxiella burnetii, the causative agent of Q fever, is a zoonotic disease with potentially life-threatening complications in humans.We have found that lower doses of both phase I and phase II Nine Mile C. burnetii multiply and are less readily cleared from the lungs of mice compared to higher infectious doses.The low rate of phase I and II Nine Mile C. burnetii growth in murine lungs may be a direct result of the limited size of the susceptible resident AM cell population.

View Article: PubMed Central - PubMed

Affiliation: Department of Immunology and Infectious Diseases, Montana State University, Bozeman, MT, USA.

ABSTRACT
Coxiella burnetii, the causative agent of Q fever, is a zoonotic disease with potentially life-threatening complications in humans. Inhalation of low doses of Coxiella bacteria can result in infection of the host alveolar macrophage (AM). However, it is not known whether a subset of AMs within the heterogeneous population of macrophages in the infected lung is particularly susceptible to infection. We have found that lower doses of both phase I and phase II Nine Mile C. burnetii multiply and are less readily cleared from the lungs of mice compared to higher infectious doses. We have additionally identified AM resident within the lung prior to and shortly following infection, opposed to newly recruited monocytes entering the lung during infection, as being most susceptible to infection. These resident cells remain infected up to twelve days after the onset of infection, serving as a permissive niche for the maintenance of bacterial infection. A subset of infected resident AMs undergo a distinguishing phenotypic change during the progression of infection exhibiting an increase in surface integrin CD11b expression and continued expression of the surface integrin CD11c. The low rate of phase I and II Nine Mile C. burnetii growth in murine lungs may be a direct result of the limited size of the susceptible resident AM cell population.

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Inflammation is not sufficient to account for decreased relative bacterial numbers in higher dose infection.A) C57BL/6 mice or CCR2 KO mice receiving Gr1-depleting antibody were infected with either 103 or 107 NMII Coxiella. Bacterial burdens in lung tissue were assessed 9 days PI by quantitative RT-PCR. Data is expressed as Log10 change in total genome copies per lung. Groups were comprised of 5 mice with Mean and SEM plotted for each group. Low dose infection of both wild-type mice and KO mice receiving depleting antibody treatment resulted in greater bacterial replication relative to either wild-type mice or KO mice receiving depleting antibody treatment and high dose infection. B) Either low or high dose infection of KO mice receiving depleting antibody treatment exhibited measurable bacterial burdens in spleen tissue 9 days PI; whereas, wild-type mice at either infectious dose did not exhibit bacterial burdens in the spleen at the same time-point. C) Gr1-depletion resulted in a significant reduction in the number of neutrophils recruited to the lung at day 9 PI in high dose infection. D) CCR2 KO animals exhibited a significant decrease in the number of recruited Ly6c+/CD11b+/CD11c+ monocytes at day 9 PI in high dose infection. (p<0.05 ANOVA).
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pone-0051941-g005: Inflammation is not sufficient to account for decreased relative bacterial numbers in higher dose infection.A) C57BL/6 mice or CCR2 KO mice receiving Gr1-depleting antibody were infected with either 103 or 107 NMII Coxiella. Bacterial burdens in lung tissue were assessed 9 days PI by quantitative RT-PCR. Data is expressed as Log10 change in total genome copies per lung. Groups were comprised of 5 mice with Mean and SEM plotted for each group. Low dose infection of both wild-type mice and KO mice receiving depleting antibody treatment resulted in greater bacterial replication relative to either wild-type mice or KO mice receiving depleting antibody treatment and high dose infection. B) Either low or high dose infection of KO mice receiving depleting antibody treatment exhibited measurable bacterial burdens in spleen tissue 9 days PI; whereas, wild-type mice at either infectious dose did not exhibit bacterial burdens in the spleen at the same time-point. C) Gr1-depletion resulted in a significant reduction in the number of neutrophils recruited to the lung at day 9 PI in high dose infection. D) CCR2 KO animals exhibited a significant decrease in the number of recruited Ly6c+/CD11b+/CD11c+ monocytes at day 9 PI in high dose infection. (p<0.05 ANOVA).

Mentions: To address the possibility that high dose infection was altering the immune status of the resident AM and therefore resulting in the observed reduction in bacterial replication compared to low dose infection, an experiment to block inflammatory cellular influx into the lung during high dose infection was performed. Groups of 5 C57BL/6 mice or CCR2 KO mice having received Gr1-depleting antibody were infected with either 103or 107 NMII Coxiella. The CCR2 KO mice that received Gr1-depleting antibody were anticipated to lack the ability to recruit either inflammatory monocytes or neutrophils to the site of infection. At day 9 PI, bacterial burdens in lung tissue were assessed by quantitative RT-PCR. As we found previously (Figure 1), in the WT mice not depleted of Gr-1+ cells, the low dose NMII resulted in more bacterial proliferation relative to the inocula given than did high dose infection. In the Gr-1+ cell-depleted CCR2 KO mice, the results were similar, with the low dose resulting in relatively more growth of NMII, compared to the high dose. In low dose infection, no significant difference in the change in bacterial burdens was observed between wild-type and Gr-1+ cell-depleted CCR2 KO mice. Both groups exhibited greater than a 2-log increase in bacterial burdens over the inoculum number (Figure 5A). This similarity in results, regardless of blockade of inflammatory cell influx, could be due to the minimal overall influx of inflammatory cells during low dose infection (Figure 5C and Figure 5D). In the high dose infection, there was a significant decrease in both recruited neutrophils (Figure 5C) and recruited monocytes (Figure 5D) in the Gr-1+ cell-depleted CCR2 KO mice. This blockade of inflammatory cell influx did result in less of an overall reduction in bacterial burdens from the inoculum number in the Gr-1+ cell-depleted CCR2 KO mice, compared to wild-type mice. Importantly, any increase in bacterial replication due to the blockade of inflammatory cell influx was insufficient to increase final bacterial burdens in these animals beyond the initial inoculum number. Thus, differences in inflammation between high dose and low dose infection could not account for all of the differences in growth of Coxiella observed between the low and high dose infections. In addition, the results indicate that in the mouse, the influx of new macrophages into the lungs does not supply a source of cells susceptible to NMII infection, further supporting that resident AM are the susceptible macrophage phenotype (Figure 5A). Both low and high dose infection of KO mice receiving depleting antibody treatment resulted in measurable bacterial burdens in spleen tissue 9 days PI; whereas, wild-type mice at either infectious dose did not exhibit bacterial burdens in the spleen at the same time-point (Figure 5B). This indicates that inflammation plays a critical role in limiting the dissemination of NMII from the lungs to the spleen. Gr1-depletion did indeed result in a significant reduction in the number of neutrophils recruited to the lung at day 9 PI in high dose infection (Figure 5C). Likewise, CCR2 KO animals did exhibit a significant decrease in the number of recruited Ly6c+/CD11b+/CD11c+ monocytes at day 9 PI in high dose infection (Figure 5D).


Resident alveolar macrophages are susceptible to and permissive of Coxiella burnetii infection.

Calverley M, Erickson S, Read AJ, Harmsen AG - PLoS ONE (2012)

Inflammation is not sufficient to account for decreased relative bacterial numbers in higher dose infection.A) C57BL/6 mice or CCR2 KO mice receiving Gr1-depleting antibody were infected with either 103 or 107 NMII Coxiella. Bacterial burdens in lung tissue were assessed 9 days PI by quantitative RT-PCR. Data is expressed as Log10 change in total genome copies per lung. Groups were comprised of 5 mice with Mean and SEM plotted for each group. Low dose infection of both wild-type mice and KO mice receiving depleting antibody treatment resulted in greater bacterial replication relative to either wild-type mice or KO mice receiving depleting antibody treatment and high dose infection. B) Either low or high dose infection of KO mice receiving depleting antibody treatment exhibited measurable bacterial burdens in spleen tissue 9 days PI; whereas, wild-type mice at either infectious dose did not exhibit bacterial burdens in the spleen at the same time-point. C) Gr1-depletion resulted in a significant reduction in the number of neutrophils recruited to the lung at day 9 PI in high dose infection. D) CCR2 KO animals exhibited a significant decrease in the number of recruited Ly6c+/CD11b+/CD11c+ monocytes at day 9 PI in high dose infection. (p<0.05 ANOVA).
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Related In: Results  -  Collection

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pone-0051941-g005: Inflammation is not sufficient to account for decreased relative bacterial numbers in higher dose infection.A) C57BL/6 mice or CCR2 KO mice receiving Gr1-depleting antibody were infected with either 103 or 107 NMII Coxiella. Bacterial burdens in lung tissue were assessed 9 days PI by quantitative RT-PCR. Data is expressed as Log10 change in total genome copies per lung. Groups were comprised of 5 mice with Mean and SEM plotted for each group. Low dose infection of both wild-type mice and KO mice receiving depleting antibody treatment resulted in greater bacterial replication relative to either wild-type mice or KO mice receiving depleting antibody treatment and high dose infection. B) Either low or high dose infection of KO mice receiving depleting antibody treatment exhibited measurable bacterial burdens in spleen tissue 9 days PI; whereas, wild-type mice at either infectious dose did not exhibit bacterial burdens in the spleen at the same time-point. C) Gr1-depletion resulted in a significant reduction in the number of neutrophils recruited to the lung at day 9 PI in high dose infection. D) CCR2 KO animals exhibited a significant decrease in the number of recruited Ly6c+/CD11b+/CD11c+ monocytes at day 9 PI in high dose infection. (p<0.05 ANOVA).
Mentions: To address the possibility that high dose infection was altering the immune status of the resident AM and therefore resulting in the observed reduction in bacterial replication compared to low dose infection, an experiment to block inflammatory cellular influx into the lung during high dose infection was performed. Groups of 5 C57BL/6 mice or CCR2 KO mice having received Gr1-depleting antibody were infected with either 103or 107 NMII Coxiella. The CCR2 KO mice that received Gr1-depleting antibody were anticipated to lack the ability to recruit either inflammatory monocytes or neutrophils to the site of infection. At day 9 PI, bacterial burdens in lung tissue were assessed by quantitative RT-PCR. As we found previously (Figure 1), in the WT mice not depleted of Gr-1+ cells, the low dose NMII resulted in more bacterial proliferation relative to the inocula given than did high dose infection. In the Gr-1+ cell-depleted CCR2 KO mice, the results were similar, with the low dose resulting in relatively more growth of NMII, compared to the high dose. In low dose infection, no significant difference in the change in bacterial burdens was observed between wild-type and Gr-1+ cell-depleted CCR2 KO mice. Both groups exhibited greater than a 2-log increase in bacterial burdens over the inoculum number (Figure 5A). This similarity in results, regardless of blockade of inflammatory cell influx, could be due to the minimal overall influx of inflammatory cells during low dose infection (Figure 5C and Figure 5D). In the high dose infection, there was a significant decrease in both recruited neutrophils (Figure 5C) and recruited monocytes (Figure 5D) in the Gr-1+ cell-depleted CCR2 KO mice. This blockade of inflammatory cell influx did result in less of an overall reduction in bacterial burdens from the inoculum number in the Gr-1+ cell-depleted CCR2 KO mice, compared to wild-type mice. Importantly, any increase in bacterial replication due to the blockade of inflammatory cell influx was insufficient to increase final bacterial burdens in these animals beyond the initial inoculum number. Thus, differences in inflammation between high dose and low dose infection could not account for all of the differences in growth of Coxiella observed between the low and high dose infections. In addition, the results indicate that in the mouse, the influx of new macrophages into the lungs does not supply a source of cells susceptible to NMII infection, further supporting that resident AM are the susceptible macrophage phenotype (Figure 5A). Both low and high dose infection of KO mice receiving depleting antibody treatment resulted in measurable bacterial burdens in spleen tissue 9 days PI; whereas, wild-type mice at either infectious dose did not exhibit bacterial burdens in the spleen at the same time-point (Figure 5B). This indicates that inflammation plays a critical role in limiting the dissemination of NMII from the lungs to the spleen. Gr1-depletion did indeed result in a significant reduction in the number of neutrophils recruited to the lung at day 9 PI in high dose infection (Figure 5C). Likewise, CCR2 KO animals did exhibit a significant decrease in the number of recruited Ly6c+/CD11b+/CD11c+ monocytes at day 9 PI in high dose infection (Figure 5D).

Bottom Line: Coxiella burnetii, the causative agent of Q fever, is a zoonotic disease with potentially life-threatening complications in humans.We have found that lower doses of both phase I and phase II Nine Mile C. burnetii multiply and are less readily cleared from the lungs of mice compared to higher infectious doses.The low rate of phase I and II Nine Mile C. burnetii growth in murine lungs may be a direct result of the limited size of the susceptible resident AM cell population.

View Article: PubMed Central - PubMed

Affiliation: Department of Immunology and Infectious Diseases, Montana State University, Bozeman, MT, USA.

ABSTRACT
Coxiella burnetii, the causative agent of Q fever, is a zoonotic disease with potentially life-threatening complications in humans. Inhalation of low doses of Coxiella bacteria can result in infection of the host alveolar macrophage (AM). However, it is not known whether a subset of AMs within the heterogeneous population of macrophages in the infected lung is particularly susceptible to infection. We have found that lower doses of both phase I and phase II Nine Mile C. burnetii multiply and are less readily cleared from the lungs of mice compared to higher infectious doses. We have additionally identified AM resident within the lung prior to and shortly following infection, opposed to newly recruited monocytes entering the lung during infection, as being most susceptible to infection. These resident cells remain infected up to twelve days after the onset of infection, serving as a permissive niche for the maintenance of bacterial infection. A subset of infected resident AMs undergo a distinguishing phenotypic change during the progression of infection exhibiting an increase in surface integrin CD11b expression and continued expression of the surface integrin CD11c. The low rate of phase I and II Nine Mile C. burnetii growth in murine lungs may be a direct result of the limited size of the susceptible resident AM cell population.

Show MeSH
Related in: MedlinePlus