Role of Chlamydiae in Chronic Airway Diseases: Humans versus Calves
ACVIM 2008
Petra Reinhold, Dr. med. vet. habil., PhD
Jena, Germany

Role of Chlamydiae in Chronic Airway Diseases in Humans

Chlamydiae are important pathogens of pulmonary diseases in human medicine. On the one hand, psittacosis is a typical example of a pulmonary zoonotic disease caused by Chlamydophila (C.) psittaci. On the other hand, Chlamydophila (formerly Chlamydia) pneumoniae is known to be involved in a variety of diseases related to the respiratory system.1,2

Acute chlamydial infections have been reported to affect upper airways (e.g., pharyngitis, sinusitis, and otitis), central and lower airways (e.g., bronchitis, bronchiolitis), as well as lung tissue (pneumonia). Furthermore, the respiratory pathogen C. pneumoniae is known for its propensity to cause chronic or recurrent infections that often appear clinically inapparent. The highest prevalence of respiratory chlamydial infections has been reported in children with recurrent or chronic bronchitis and pneumonia and in adults with chronic obstructive pulmonary disease (COPD), indicating a higher prevalence from asymptomatic to severe, from upper to lower, and from acute to chronic respiratory tract involvement.3

In the last decade, attention has focused on the pathogenetic role of C. pneumoniae in chronic inflammatory airway diseases in humans, i.e., asthma bronchiale and COPD.4,5 This bacterium is thought to be involved in both 1) the development and 2) the severity of chronic inflammatory airway diseases. Indeed, C. pneumoniae infection may promote the development of persistent airflow limitation leading to a significantly greater annual decline in lung function6, and chronic colonisation with C. pneumoniae was found to be significantly associated with lower FEV1.7 In addition, there are consistent reports of an association between C. pneumoniae infection and acute exacerbations of human asthma and COPD.7-11 If steroid standard treatment of status asthmaticus was extended for a ketolide (antibiotic treatment) a faster resolution of the asthma attack could be documented. Over time, however, more exacerbation results in faster progression of airway obstruction. Even after complete resolution of C. pneumoniae infection, hyperreactivity of the airways persists for months. It has been assumed that chlamydiae may cause long lasting neurogenic inflammation, explaining these observations.

The presence of pulmonary emphysema12,13 and even lung cancer14,15 have been linked to respiratory chlamydial infections, too. Furthermore, there is an ongoing discussion about an association between the intracellular persistence of C. pneumoniae and the manifestation of non-respiratory chronic diseases, i.e., atherosclerosis, coronary artery disease, myocarditis, or depression.16,17 Interestingly, chlamydial infections have been shown to be inversely correlated with allergic inflammation in human patients.18

Another area of debate is the significance of chlamydiae as co-pathogens. In human medicine, C. pneumoniae is known to be frequently involved in mixed or co-infections with other respiratory pathogens.19 For example, the co-existence of Mycoplasma infections and chlamydial infections is a well-known phenomenon in human patients with respiratory diseases.20,21 However, the pathogenetic relevance of such co-infections has yet to be clarified.

There is a lack of serious knowledge about a causative relationship between the presence of Chlamydiaceae in pets or farm animals and chronic diseases or chronic airflow limitation in people at risk dealing with those animals.

Role of Chronic Chlamydial Infections in the Respiratory System of Calves

Based on the knowledge from human medicine, a prospective study was undertaken in order to evaluate functional consequences of subclinical chlamydial infections in the bovine respiratory system. In calves persistently infected with C. abortus and/or C. pecorum, respiratory chlamydial infection was found to be associated with chronic inflammation of lung and airways as follows:22

Non-invasive lung function tests (impulse oscillometry, volumetric capnography) revealed significant peripheral airway obstruction (Figure 1). Despite changes noted on a sub-clinical level (no respiratory distress was seen), pulmonary dysfunctions were already detectable at the age of 2 months and persisted over at least 6 months.22,23

Pulmonary disorders were confirmed histologically by a markedly activated bronchus-associated lymphoid tissue (BALT) in the apical pulmonary lobes causing partial obstruction of bronchiolar lumina. Furthermore, significantly elevated concentrations of total protein and 8-iso-prostane (8-IP), as well as increased activities of matrix metalloproteinase 2 (MMP-2) were measured in broncho-alveolar lavage fluids of calves with respiratory chlamydial infections indicating pulmonary inflammation and the involvement of airway remodeling (Table 1).

The detection of long lasting lung function disorders and pulmonary inflammation has quantitatively demonstrated the persisting effects of subclinical naturally acquired chlamydial infections on the pulmonary system. It should be taken into account that these pathophysiological features might interfere with postnatal lung development. In the bovine species, the period of postnatal lung growth and development is determined by further morphological differentiation and functional maturation of the lung, and functional maturity of the respiratory system is not reached before one year of age or a body weight of 300 kg.24 During this period, respiratory infections are likely involved in growth retardation that may cause impaired lung function for a long time and or even life long.

Click on the image to see a larger view.

Figure 1.
Figure 1.

Respiratory impedance during expiration (expressed as respiratory resistance (R ex) and respiratory reactance (X ex) per kg. body weight) in calves aged 2 months (A) and 7 months (B).22

Chl-: Calves without chlamydial infections (n = 12); Chl+: calves with clinically latent chlamydial infections (n = 13).

 

Already at the age of two months, expiratory resistance in calves with chlamydial infections (Chl+) was markedly higher than in Chl- calves, and this difference between groups remained statistically significant during the course of the study. Consequently, even at the age of seven months, calves of the Chl+ Group showed a much higher expiratory resistance per kg b.w. than calves of the Chl- Group.

Table 1. Markers of inflammation measured in broncho-alveolar lavage fluids (BALF) of 25 clinically normal calves aged 7 months without (Chl-) and with (Chl+) chlamydial infections22.

 

Unit

Chl-

Chl+

W test

n

Median

Min.

Max.

n

Median

Min.

Max.

Protein

µg/mL

12

340.0

148.0

705.0

13

610.0

391.0

2205.0

P < 0.01

8-IP

pg/mL

12

104.2

26.1

647.0

13

340.0

64.7

908.0

P < 0.05

MMP-2

AU

12

403.0

177.0

954.0

12*

836.5

453.0

1591.0

P < 0.001

8-IP: 8-iso-prostane; MMP-2: matrix metalloproteases 2; AU: arbitrary units ; *one BALF sample was not possible to be analyzed

Pathogenetic Features of Chlamydial Infections & Relationships to Chronicity of a Disease

Being intracellular pathogens, chlamydiae are entirely dependent on the host for their replication within cells, where they form characteristic vacuole-like inclusions and replicate within an endosome. All chlamydial species have the potential to cause both acute and chronic infection, and the remarkable variety of clinical manifestations of chlamydial diseases should, at least in part, be a consequence of the distinctive features of the causative agents, particularly their unique biphasic developmental cycle.

In the human lung, primary host cells of C. pneumoniae are epithelial cells and macrophages. Chlamydiae enter the cell as infectious elementary bodies (EBs). In the course of a replication cycle, these infectious, but metabolically inactive elementary bodies evolve into non-infectious, but metabolically active reticulate bodies (RBs). The latter reside in a vacuole-like inclusion of the host cell and undergo binary fission before transforming back into elementary bodies to start a fresh cycle. This life cycle enables the pathogen to pursue distinctive survival strategies in the host, which are giving rise to the evasion of host defense, as well as chronic and persistent courses of infection.

There is increasing evidence that chlamydiae are capable of persistence and therefore may reactivate a present infection and so causing recurrent infections.25-27 Intracellular persistence is characterized by the development of inclusions containing atypical RBs and only few EBs. Research on the interaction of C. pneumoniae with its host cells has demonstrated that the response of infected cells may contribute to the initiation and maintenance of inflammation as well as to tissue remodeling and fibrosis. Establishment of chlamydial persistence is accompanied by diverse molecular mechanisms which are not yet fully understood. Nevertheless, there is little doubt that the persistence stage plays an important role in the chlamydial development and may be considered as the third phase of the developmental cycle. The hypothesis that chlamydiae may promote inflammatory processes in chronic diseases is based on the theoretical ability of this pathogen to survive in its host cells in a persistent state. This hypothesis, however, has yet to be proven in vivo.

Chlamydiae are capable to either induce or inhibit host cell apoptosis, which is an important feature for their reproduction within the host cell. As persistent chlamydiae establish a long-term relationship with the host cell and persistent aberrant chlamydial bodies are not released at the end of the normal acute developmental cycle, enduring inhibition of apoptosis during the persistent state is conceivable. Inhibition of host cell apoptosis may facilitate colonization of the organism and seems to play a role in long-term survival of persistent chlamydiae within the host cell. Indeed, there is evidence from in-vitro data that chlamydiae are able to inhibit apoptosis of infected cells.28 The resistance of infected cells against apoptotic stimuli may be essential for chlamydiae to complete their growth cycle but it may also promote intracellular persistence. In cultures of HEp-2 epithelial cells persistently infected with C. pneumoniae IL-6 production is continuously up-regulated suggesting that a persistent infection may cause chronic inflammation.29

Furthermore, chlamydial infections are linked to mechanisms that might be involved in irreversible structural changes of tissues called 'tissue remodeling'. In chronic asthma and COPD the structural remodeling of the airway wall leads to irreversible airflow obstruction. These remodeling processes are characterized by smooth muscle cell hyperplasia and subepithelial fibrosis. C. pneumoniae stimulates the expression of connective tissue growth factor (CTGF) in epithelial cells and the production of basic fibroblast growth factor (bFGF) by bronchial smooth muscle cells.30,31 Because both CTGF and bFGF mediates smooth muscle cell proliferation, these data provide a mechanism by which chlamydial infection may promote airway remodeling. Matrix metalloproteinases (MMPs) are also discussed with respect to tissue remodeling, and MMPs do play a significant role in the pathogenesis of asthma and COPD.32 C. pneumoniae infection of smooth muscle cells increased the production of MMP-1 and MMP-3, infected macrophages secreted MMP-1 and MMP-9.33,34 As mentioned above, an elevated activity of MMP-2 (an indicator of fibroblastic activity suggestive of remodeling) was observed in broncho-alveolar lavage fluid of chlamydia-infected calves which paralleled subclinical and chronic inflammatory obstructive changes of airways.22

Conclusions

The hypothesis that chlamydiae may be involved in the pathogenesis of human asthma and/or COPD is supported by recent findings in bovines. In asymptomatic calves, naturally acquired respiratory infections with chlamydiae have been shown to be associated with 1) persisting peripheral airway obstruction, 2) chronic pulmonary inflammation, and 3) the involvement of bronchial remodeling. Taking these findings together, there is profound evidence that there are morphologic and functional consequences for the organ system where chlamydiae reside; even in the absence of clinical signs.

The pathogenesis of chlamydia-associated chronic airway diseases is not yet fully understood. Both, 1) the persistence of chlamydiae in the host cell and 2) the modification of host cell apoptosis by chlamydiae might be key players in the molecular pathogenesis. However, pathogenetic mechanisms that are known from in vitro studies have yet to be studied for their relevance in vivo. Because no animal model is available to examine long-term effects of chronic respiratory chlamydial infections, naturally infected calves include a potential for modeling chlamydia-associated chronic airway diseases in a natural host of the chlamydial agent, rather than in rodent models which are more different in lung anatomy compared to humans and poorly suited for functional testing.

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Speaker Information
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Petra Reinhold, DMVH, PhD
Fed. Research Inst. for Animal Health
Jena, Germany


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