Clinical Streptococcus pneumoniae isolates induce differing CXCL8 responses from human nasopharyngeal epithelial cells which are reduced by liposomes
© The Author(s). 2016
Received: 11 January 2016
Accepted: 11 July 2016
Published: 19 July 2016
Streptococcus pneumoniae causes several human diseases, including pneumonia and meningitis, in which pathology is associated with an excessive inflammatory response. A major inducer of this response is the cholesterol dependent pneumococcal toxin, pneumolysin. Here, we measured the amount of inflammatory cytokine CXCL8 (interleukin (IL)-8) by ELISA released by human nasopharyngeal epithelial (Detroit 562) cells as inflammatory response to a 24 h exposure to different pneumococcal strains.
We found pneumolysin to be the major factor influencing the CXCL8 response. Cholesterol and sphingomyelin-containing liposomes designed to sequester pneumolysin were highly effective at reducing CXCL8 levels from epithelial cells exposed to different clinical pneumococcal isolates. These liposomes also reduced CXCL8 response from epithelial cells exposed to pneumolysin knock-out mutants of S. pneumoniae indicating that they also reduce the CXCL8-inducing effect of an unidentified pneumococcal virulence factor, in addition to pneumolysin.
The results indicate the potential of liposomes in attenuating excessive inflammation as a future adjunctive treatment of pneumococcal diseases.
KeywordsStreptococcus pneumoniae CXCL8 Liposomes Pneumolysin Capsule
Streptococcus pneumoniae is a major human pathogen responsible for diseases including pneumonia and meningitis which are characterized by significant inflammatory responses . Such responses are associated with the release of cytokines including CXCL8, also known as interleukin (IL)-8 as well as IL-6, IL-1β, granulocyte-macrophage colony stimulating factor (GM_CSF), transforming growth factor (TGF) α and β . Triggering of an inflammatory response may be due to interaction between host cells and several different bacterial factors. Respiratory epithelial cells express pattern-recognition receptors (PRRs) such as Toll-like receptors (TLRs) 2–6 with TLR2 recognizing bacterial factors such as peptidoglycan and lipoteichoic acid . Epithelial cells expressing TLRs may release CXCL8 during respiratory infection which has chemotactic activity for neutrophils and monocytes . Innate immunity is also comprised of antimicrobial effectors including antimicrobial peptides which have been shown to be effective against S. pneumoniae .
Excessive inflammation can have a deleterious effect on the host and therefore treatments are sought which can moderate this response to the bacteria. A major trigger for inflammatory cytokine release from the host is thought to be the cholesterol-dependent toxin, pneumolysin. However, natural variants of pneumolysin exist with different haemolytic activity, particularly a non-haemolytic pneumolysin associated with serotype 1 strains of multi-locus sequence type (MLST) ST306 [6, 7]. A liposomal formulation has been designed with the specific aim of sequestering pore-forming toxins, including pneumolysin, thereby preventing it from inserting into host cell membranes and initiating the inflammatory response . These liposomes have so far been tested on a limited number of pneumococcal strains [8, 9].
As well as the virulence factor pneumolysin, most pneumococci express one of more than 90 different polysaccharide capsules . The presence or absence of polysaccharide capsule can affect CXCL8 release by human respiratory epithelial cells in vitro  but whether the capsule serotype plays a role in CXCL8 induction is unknown. Here we have tested whether clinical isolates of different serotypes induced different levels of CXCL8 release from respiratory epithelial cells and whether this is linked to capsule type using mutants with the same genetic background expressing capsules of different serotypes.
To date, studies on the effectiveness of liposomes have been confined to a limited number of pneumococcal strains. Here we tested the effectiveness of liposomes against a panel of clinical isolates including serotype 1 strains of different haemolytic activity. Pneumolysin mutants were also tested to determine whether these liposomes had an effect on any other CXCL8-inducing bacterial factor.
Fourteen wild type Streptococcus pneumoniae strains with different serotypes and haemolytic activity were used, including strain 202.67, a non-haemolytic serotype 1 strain of ST306. Also used were one non-encapsulated mutant of strain 106.66 (named 106.66 Janus) and 8 capsule switch mutants of strain 106.66, for example mutant 106.66cpsB201.73 refers to strain 106.66 (originally serotype 6B) which has had its capsule operon replaced by that of strain B201.73 resulting in it becoming serotype 19 F .
Wild type and mutant S. pneumoniae strains used
Wild types (clinical isolates) 
Wild type 
Capsule operon replaced by a Janus cassette 
Capsule switch mutants 
Wild type, non-haemolytic (current study)
Wild type, poorly haemolytic (current study)
Wild type, haemolytic (current study)
Wild type, non-haemolytic (current study)
Wild type 
Mutant lacking pneumolysin 
Mutant lacking capsule 
Mutant lacking pneumolysin and capsule 
Bacterial stocks were stored at −80 °C using Protect bacterial preservers (Technical Service Consultants, Heywood, U.K.). The bacteria were plated out on Columbia sheep blood agar (CSBA) plates and incubated overnight at 37 °C at 5 % CO2. Three to ten colonies were used to inoculate 5 ml Brain Heart Infusion (BHI) broth (Becton Dickinson and Company, le Pont de Claix, France) for overnight culture in a waterbath at 37 °C. 1 ml of the overnight culture was added to 9 ml BHI and incubated in a waterbath at 37 °C until reaching mid-log phase, OD600nm = 0.4. The bacterial cells were collected by centrifugation of 10 ml culture and were washed with Minimum Essential Media (MEM, Gibco, Life Technologies, Switzerland). The pellets were re-suspended in 10 ml MEM.
Detroit cell culture
The human pharyngeal epithelial cell line Detroit 562 (ATCC CCL-138) was cultured submerged in complete medium consisting of Minimum Essential Media (MEM) with 10 % heat-inactivated fetal calf serum (FCS), 2 mM of L-glutamine, 0.075 % sodium bicarbonate, 1x MEM non-essential amino acid solution, 1 mM sodium pyruvate, 100 μg/ml streptomycin and 100 U/ml penicillin (all from Gibco, Life Technologies, Switzerland) at 37 °C at 5 % CO2. Cells were harvested using 0.05 % Trypsin-EDTA (Gibco, Switzerland) when the cells reached 70–90 % confluence.
CXCL8 (IL-8) cytokine assay
3 × 105 Detroit cells in 1 ml MEM without antibiotics, was added to each well of a 24-well plate (TPP tissue culture plates, Sigma-Aldrich,). The plate was incubated overnight at 37 °C at 5 % CO2 then integrity of the monolayer checked by microscopy. The medium was aspirated and 0.5 ml MEM without FCS or antibiotics added per well.
A suspension of bacteria of approximately 6 × 106 CFU/ml was made (to give an estimated MOI of 10). Serial dilutions of the suspension were plated out for accurate quantification of CFU/ml, and therefore MOI.
Liposomes (CAL02) were provided by LASCCO (Geneva, Switzerland) and have previously been shown to be neither bactericidal nor toxic to epithelial cells  (Additional file 1: Figure S2). The liposome concentrations utilized in these experiments were chosen based preliminary experiments using liposome concentration ranging from 50ug to 1 mg (data not shown). Therefore, 1 mg or 100 μg of liposomes were added per well followed by 0.5 ml MEM containing the bacteria (6 × 106 CFU/ml) in the subsequent experiments The plate was centrifuged at 120 x g for 3 min at 25 °C and then incubated at 37 °C at 5 % CO2. For experiments involving lysis of the P21 strain of bacteria by antibiotic, 10 μl / well of a 10 mg/ml solution of ceftriaxone (Rocephine®, Roche Pharma, Basel, Switzerland) was added after 3 h of incubation. After incubating for a total of 24 h at 37 °C at 5 % CO2 the supernatant was collected in 1.5 ml tubes, spun down at 20 000 x g for 3 min at room temperature and the supernatant stored at −80 °C. CXCL8 concentrations were measured by ELISA (R&D systems ELISA kits, Abingdon, United Kingdom). Experiments were performed in triplicate on three different days. Mean CXCL8 concentration for Detroit cells plus 1 mg liposomes, but no bacteria, for all experiments involving liposomes was 412 pg/ml, indicating that the liposomes do not have a cytotoxic effect on eukaryotic cells as reported previously .
The bacteria were grown overnight on CSBA plates at 37 °C at 5 % CO2 and then cultured overnight in 5 ml BHI containing 5 % FCS. 1 ml of overnight culture was added to 5 ml BHI + FCS and subcultured until OD600nm 0.4. The bacteria were then centrifuged at 5000 x g for 10 min at room temperature and resuspended in 100 μl PBS. To release haemolysins from the bacteria, they were sonicated for 5 min on ice. In a round-bottomed 96-well plate (Sarstedt, NC, USA), 50 μl PBS (pH 7.4) was added per well along with 50 μl of bacterial sonicate or 50 μl pneumolysin (2 mg/ml, as a positive control) and doubling dilutions made across the plate. One row of wells was used as negative control (PBS only). 50 μl of 2 % sheep red blood cell suspension in PBS was added per well and incubated for 30 min at 37 °C and lysis monitored.
To assess the significance of the results ANOVA (with Tukey’s post hoc test) or student t test was used in GraphPad Prism as indicated. A p value < 0.05 was considered statistically significant.
Clinical pneumococcal isolates varied in their induction of CXCL8 from epithelial cells
Liposomes reduced CXCL8 from epithelial cells exposed to different clinical pneumococcal isolates
Lytic antibiotic is required to induce CXCL8 response to serotype 3 strain P21, which was reduced by liposomes
Reduction of CXCL8 response by liposomes depended on haemolytic activity of serotype 1 pneumococcal strains
Liposomes acted on pneumolysin and another virulence factor
An excessive inflammatory response can cause damage to the host during pneumococcal diseases which may be exacerbated by treatment with lytic antibiotics as they cause release of inflammatory factors. The cholesterol-dependent toxin pneumolysin plays an important role in induction of inflammation but the presence of capsule has also been shown to have an influence [11, 14, 15]. Here we determined whether the inflammatory response, as measured by release of the inflammatory cytokine CXCL8 from human nasopharyngeal epithelial cells in vitro, was affected by the pneumococcal serotype. The pneumococcal clinical isolates tested induced different amounts of the inflammatory cytokine CXCL8 (Fig. 1a). By using capsule switch mutants we have shown that capsule serotype had no significant effect on CXCL8 levels, under the in vitro conditions used (Fig. 1b). Deletion of capsule in strain D39 resulted in an increase in release of CXCL8 from the epithelial cells but when capsule was deleted in strain 106.66 no significant difference in CXCL8 was observed, suggesting a strain-specific effect. Cytokine networks involved in pneumococcal infections are incompletely understood but IL-1β has been shown to regulate CXCL8 release from epithelial cells in reponse to S. pneumoniae  and would be an interesting target for future study. Furthermore, it has also been shown, in vivo, that pneumococcal capsule could impair recognition by the innate immune system, in particular the Toll-like receptor mediated pathways . However, the most evident difference was between wild types and pneumolysin mutants indicating that pneumolysin is the predominant CXCL8 inducer.
Liposomes designed to sequester cholesterol-dependent toxins such as pneumolysin  caused a marked, and dose dependent, decrease in CXCL8 release from the epithelial cells for all the serotypes of clinical isolates tested (Fig. 2). This indicates the potential of this liposome treatment to be effective against pneumococcal diseases caused by different pneumococcal strains in patients.
In pneumococcal meningitis mortality and morbidity are exacerbated by an excessive inflammatory response. Treatment with lytic antibiotic, although effective at killing the bacteria, causes release of pneumolysin triggering an even greater inflammatory response. In an infant rat model of pneumococcal meningitis a serotype 3 strain (P21) is often used to induce the disease  and here we have shown that in vitro, following antibiotic lysis, which would release the pneumolysin, liposome treatment greatly reduced CXCL8 levels from the epithelial cells (Fig. 3). This raises the possibility of a future anti-inflammatory treatment for pneumococcal meningitis including this liposomal formulation. Serotype 3 strains are also associated with severe forms of pneumonia and with septic shock [19, 20] and so the effectiveness of the liposome treatment against this serotype, as least in vitro, is encouraging.
In nature several different alleles of pneumolysin exist with differing haemolytic activity [6, 21]. Serotype 1 has particularly been noted to include non-haemolytic and poorly-haemolytic variants. We found liposomes to be effective at reducing the CXCL8 response to haemolytic and poorly-haemolytic strains (Fig. 4). Non-haemolytic strains did not trigger CXCL8 levels above the baseline secretion by epithelial cells so liposomes did not reduce the level further. Overall, the results indicate that liposomes may be useful in counteracting the inflammatory effect of a wide range of clinical pneumococcal strains of different haemolytic activity as well as different serotypes.
Using pneumolysin and capsule mutants of strain D39 (Fig. 5), we showed for the first time that the liposomes act not only on pneumolysin, but also significantly reduce the CXCL8 response to another factor which remains to be identified.
We found that clinical pneumococcal isolates varied in their capacity to induce CXCL8 production by respiratory epithelial cells. Capsule type had little effect on this variation compared with pneumolysin which had the predominant effect. Liposomes designed to sequester pneumolysin were extremely effective in vitro in reducing CXCL8 levels from epithelial cells exposed to a range of clinical pneumococcal isolates. Liposomes also reduced CXCL8 release induced by mutant pneumococci lacking pneumolysin indicating that they also act on another factor.
This work was supported by the Institute for Infectious Diseases. Liposomes (CAL02) were provided by LASCCO (Geneva, Switzerland). D.B. was supported by a grant from the Swiss-European Mobility Programme. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.
Availability of data and material
The datasets supporting this article have been uploaded as Additional file 1.
DB and SA carried out the lab work. DG, SLL, AD, EB and LJH participated in design of the study and all authors were involved in data analysis. LJH conceived the study and drafted the manuscript. All authors read and approved of the final manuscript.
We have no competing interests.
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