Background
Mycobacterial infections are still a major cause of morbidity and mortality worldwide. In 2013 (WHO) an estimated number of 9.0 million people developed Tuberculosis (TB) and 1.5 million people died of this chronic inflammatory disease [
1]. It is currently not clear whether the comprehensive strategies developed by the WHO will lead to the elimination of this infectious disease by 2050. Independently, nontuberculous mycobacterial (NTM) infections mainly caused by
M. avium, M. intracellulare, M. abscessus, and
M. chelonae are increasing [
2]. NTM are ubiquitous in the environment and can cause a wide range of infections in humans, as well as in animals [
3]. Many different animal models have been developed in mycobacterial research since Robert Koch’s period. However, existing models have so far failed to mimic human disease. Major disadvantages include significant differences in mycobacteria-induced pathology and relative resistance (mice and rats), high costs (non-human primates), or different immunological capacities compared to humans (guinea pigs and rabbits) [
4‐
8]. Thus the lack of appropriate models for basic research in mycobacterial infection of the human host hampers new insights into disease mechanisms and scientific progress with regard to successful measures to accomplish that goal [
9].
Various in vitro models with human cells have been established. However the results obtained by different research groups are often hard to compare [
10] due to the use of different bacterial strains and infection doses [
11,
12] and the large differences in study design including different (i) cell types (monocytes, macrophages, neutrophils, and microglia [
13‐
16]), (ii) cell lines (macrophage-like cells and non-phagocytic cell lines [
17‐
21]), (iii) host cell sources (human, healthy or patients, and animals [
22‐
24]), and (iv) last but not least incubation media (containing supplements or not [
25,
26]). In addition, most of the gathered information indicates that it is extremely difficult to induce mycobactericidal activity in purified populations of phagocytes. Therefore, some more complex models have been developed, e. g. co-culture of immune cells [
13], whole blood assays [
27], or microenvironments comprising epithelial and endothelial cells [
28], as well as the use of distinct stimuli (e.g. cytokines, vitamins, lipids, and nucleotides [
29‐
32]). Nonetheless, these large variations of results do not allow definite conclusions.
From a host perspective it needs to be mentioned that 50 % of individuals exposed to
M. tuberculosis (Mtb) never become tuberculin skin test positive, which may indicate that the mycobacterium is removed by the innate immunity [
33]. Likewise, there are several lines of epidemiological evidence supporting a protective role for innate immunity in tuberculosis. The successful elimination of pathogenic mycobacteria early on during infection by the innate immune-system is still controversially discussed and very likely underestimated due to the lack of human studies.
In order to study early innate effector mechanisms upon mycobacterial infection and on the current lack of complex human-relevant models, an ex vivo tissue culture model, referred as STST (Short-Term Stimulation of Tissues), was developed. The main advantage of this lung tissue model is the maintenance of the intact lung microenvironment with its native cell population, orientation, and structural integrity. The STST model of human lung tissue has been successfully used to obtain valuable information about early steps in the pathogenesis of several infectious lung diseases, including infections with
Legionella pneumophila, Pseudomonas aeruginosa, Streptococcus pneumonia, Chlamydia pneumoniae, and
Haemophilus influenzae [
34‐
38].
Methods
Ethical statement and collection of samples
Human lung tissue specimens were acquired from surgical material of 65 patients, who underwent pneumonectomy or lobectomy due to cancer at the LungenClinic Grosshansdorf, Germany. The study was performed with permission of the local ethical committee at the University of Lübeck, written informed consent was obtained (Approval number: 07–157).
Bacterial strains and culture
Following strains were used for the study at different colony forming units (CFU)/ml (104–107), which were cultivated in Löwenstein–Jensen medium (LJ): M. abscessus 9547/00 (type strain, =AB1), M. abscessus 8562/11 (clinical isolate, =AB2), M. avium 3725/07 (strain 104, =AV2), M. avium 3439/10 (clinical isolate, =AV1), M. tuberculosis 9679/00 (type strain H37Rv, =TB2), and M. tuberculosis 1616/12 (clinical isolate from a German patient, =TB1). In order to precipitate mycobacterial clumps, suspensions were centrifuged at low speed (100 × g) for 5 min. BBL™ MGIT™ PANTA™ antibiotic mixture (BD diagnostics, USA) was added to the suspensions to prevent other bacterial growths. The concentrations of viable mycobacteria (CFU/ml) in the stock suspensions were controlled three times during the study. Basically, the stock solutions were serially diluted (1:10 each) until 100 CFU/ml. From 100 to 103 CFU/ml 0.3 ml were cultured in petri dishes containing LJ medium. Cultivation time for M. abscessus was 1–2 weeks and for M. avium, as well as M. tuberculosis 4–6 weeks, respectively. The mycobacterial colonies were counted visually and the CFU/ml were determined. For infection of the lung tissue specimens, 2 ml of suspension from each strain were used.
Infection of the lung tissue specimens
Under gross morphologic examination, only parts of the surgical materials without inflammatory consolidations, pleura, neoplasia, or anthracosis, were selected and dissected with a size of 0.5–1 cm3 (~30 mg) for the investigation. In order to optimize the viability of the tissues in the ex vivo system, as well as to determine the best suitable amount of tissues and volume of culture medium, MTT assays [(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] have initially been performed (data not shown). According to these, each piece of tissue was infected with 2 ml of the suspensions. To distribute the mycobacteria, medium was gently re-suspended by pipetting. Infection and cultivation of tissues were performed for 16 h in RPMI1640 (1×) GlutaMAX™-I (Invitrogen, Darmstadt, Germany) supplemented with 10 % FCS (PPA, Pasching, Austria), 0.02 M HEPES buffer solution (Life technologies, Invitrogen, Carlsbad, California, USA), and 0.01 mM sodium pyruvate (Biochrom, Berlin, Germany) at 37 °C overnight in 24-well, flat bottom Corning® Costar ® tissue culture plates (Sigma-Aldrich Co., St. Louis, USA). Titration experiments were initially carried out to select optimal infection dose of mycobacteria, where the highest number of infected cells was obtained. Thus a CFU/ml of 107 was used for all experiments. Additionally, two kinds of control tissues were included, one was freshly fixed without cultivation, and another was incubated under the same culture conditions without mycobacterial infection (medium control).
Tissue processing and histopathological analysis
HOPE® fixation technique (HEPES-glutamic acid buffer mediated Organic solvent Protection Effect) (DCS Diagnostics, Hamburg, Germany) was used as previously published [
39,
40]. There were no washing steps for the ex vivo infected tissues before fixation. Briefly, specimens were fixed in HOPE® solution at 4 °C overnight. Dehydration procedure was performed with 100 % acetone at 4 °C and repeated 4 times. Tissue samples were directly embedded in low-melting paraffin (DCS
labLine, Germany). Tissue blocks were cut on a microtome, sections were deparaffinized with isopropanol (2×10 min at 60 °C) and subsequently visualized by histological stainings depending on purposes: 1) Hematoxylin and Eosin (H&E) staining, allowing a general morphological inspection after ex vivo incubation (Merck, Darmstadt, Germany), 2) Gram staining for the detection of gram-positive bacterial contamination (Merck, Darmstadt, Germany), 3) cold Ziel Neelsen (ZN) technique/Kinyoun staining for the detection of mycobacteria (bioMérieux SA, Craponne, France).
IHC and fluorescence double staining
Double-staining was performed comprising IHC (to identify cell types) with Auramine-Rhodamine fluorescence staining (to detect mycobacteria) (Waldeck GmbH & Co. KG, Münster, Germany). For immunohistochemical staining, following antibodies were used depending on the purpose. Herein: Anti-Human CD 68 for macrophages (diluted 1:200, clone: PG-M1, monoclonal, mouse, Dako Cytomation, Glostrup, Denmark), Anti-Human Neutrophil Elastase (NE) for neutrophils (diluted 1:200, clone: NP57, monoclonal, mouse, Dako Cytomation, Glostrup, Denmark), Anti-Surfactant Protein-C (SP-C) for pneumocytes-II (diluted 1:300, clone: FL-197, polyclonal, rabbit, Santa Cruz Biotechnology Inc., Dallas, Texas, U.S.A.), for lymphoid cells Anti-Human T cell, CD8 (diluted 1:50, clone: C8/144B, monoclonal, mouse, Dako Cytomation, Glostrup, Denmark), Anti-Human CD20cy, B cell (diluted 1:100, clone: L26, monoclonal, mouse, Dako Cytomation, Glostrup, Denmark), Anti-human CD30, (diluted 1:50, clone: Ber-H2, monoclonal, mouse, Dako Cytomation, Glostrup, Denmark), Anti-Human T cell, CD4 (diluted 1:100, clone: MT310, monoclonal, mouse, Dako Cytomation, Glostrup, Denmark), and Anti-Human CD79a (diluted 1:200, clone: SP18, monoclonal, rabbit, DCS Innovative Diagnostik-System GmbH & Co, Hamburg, Germany). For detection, the ZytoChemPlus (HRP) Polymer kit (Zytomed Systems, Berlin, Germany) was applied with above mentioned antibodies and Diaminobenzidine (DAB) as substrate. After IHC staining, the slides were subsequently transferred to Auramine-Rhodamine dye (15 min), HCl- Alcohol 70 % (2 min) and potassium permanganate (KMnO4) 0.5 % (2 min 30 s) in the dark. Examination and capture of images were carried out by a fluorescence microscope (Eclipse 80i, Nikon). In order to provide simultaneous detection of double staining, the same area of fluorescent photomicrographs of the fluorochrome (Rhodamine/TRITC, spectral characteristics: 550–580) and color inverted brightfield-images of the IHC detection were overlaid (FixFoto).
Measurements of cellular components
The following parameters were measured: 1) Cytoplasmic and nuclear diameters, 2) Light densities (LDs) of cytoplasm and nucleus. Then, N/C ratio (a ratio of the nuclear size to the cytoplasmic size) and arithmetic mean of nuclear and cytoplasmic LDs were estimated. All measurements were performed with the Infinity analyze software after background correction. To keep measuring light densities of cells constant camera control options were kept at a magnification of 400×, exposure time = 200 ms, gain = 2, gamma = 1, color correction matrix/light source = daylight.
Viability test
TUNEL (Terminal deoxynucleotidyl transferase dUTP nick end labeling) assay was performed in HOPE-preserved tissue sections. The slides were initially incubated for 15–30 min at 37 °C with proteinase K and rinsed twice with PBS buffer. After adding TUNEL reaction mixtures, the slides were incubated at 37 °C in humidified atmosphere for 1 h and again rinsed 3 times with PBS buffer. All procedures were carried out in a dark environment. Freshly HOPE-fixed lung tissues served as positive control (alive), whereas tissues treated by DNase I served as negative control. For counter staining Vectashield mounting medium with 4′–6-diamidino-2-phenylindole (DAPI) was used (Vector Laboratories, Burlingame, USA). Examination and capture of images were performed by a fluorescence microscope (Eclipse 80i, Nikon). Cell counts were acquired using Infinity Analyze software after adjustment of linear contrast. With a purpose of analyzing viability, integrated measurements of all ex vivo incubated samples were compared with positive (untreated lung specimens) and negative controls (DNase I treated lung specimen).
Software and statistics
FixFoto (version 3.20, Joachim Koopmann Software, Wrestedt/Stederdorf, Germany), Image J (version 1.48b, Wayne Rasband, Maryland, USA), Infinity analyze (version 6.0.02, Lumen
era corporation, Ontario, Canada) were used for optimizing micrographs. IBM SPSS (version 20.0.0, IBM Corporation, New York, United States) and Graph Pad Prism 6 (version 6.04, GraphPad Software, Inc. California, USA) were used for statistical analysis. Statistical tests were chosen as indicated in the figure legends. A p-value <0.05 was considered statistically significant. Correspondence analysis was performed using MATLAB (MathWorks, Inc., Natick, MA, USA) as described [
41].
Conclusion
In this study we demonstrate that the STST model can be successfully employed in the analysis of mycobacterial infection of human intact lung parenchyma. We observed at the cellular level, that different cell types (macrophages, monocytes, neutrophils, and pneumocytes-II) were infected. In these infected cells, characteristic morphologic changes (cell shrinkage and nuclear alterations) were observed, most likely reflecting cell death. The infection frequency of a given cell type and the infection dependent extent of morphologic changes were significantly associated with mycobacterial species. This adds to the current understanding of primary infection of mycobacteria. Especially, the involvement of several cell types, their cooperation, cell death dependent and cell death independent effector functions do contribute to the overall capacity of a human individual to efficiently limit the growth of mycobacteria upon exposure. Thus, the STST model provides a valuable novel tool to address the complex mechanisms of innate effector faction in the very early onset of TB disease. Further studies are ongoing in order to characterize the interaction of pathogenic mycobacteria with the human host at the subcellular, cellular and supracellular organ level.
Acknowledgements
The authors gratefully acknowledge A. Witt and K. Ott for help with stimulating tissues at the division of National Reference Center (NRC) for Mycobacteria, as well as J. Tiebach, M. Lammers, S. Fox, K. Wiczkowski, and A. Schiller for their excellent technical assistance. We wish to extend special thanks to Dr. D. Lang, Dr. S. Marwitz, and Dr. I. Watermann for their advice. The BioMaterialBank (BMB) North is funded in part by the Airway Research Center North (ARCN), Member of the German Center for Lung Research (DZL) and is member of popgen 2.0 network (P2N) which is supported by a grant from the German Ministry for Education and Research (01EY1103). The publication of this article was funded by the Open Access fund of the Leibniz Association.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
DG carried out the study and drafted the manuscript with SS. ER was responsible for mycobacterial cultures and took part in the design of the study. EV was responsible for the histopathological parts. NR was involved in finalizing the manuscript and took part in the design of the study. KF was responsible for the bioinformatics. KIG and ChK provided the aspects of thoracic surgery and biobanking. TG conceived the study and was responsible for the design. All authors were involved in finalizing the manuscript and read and approved the submitted version.