**4. Asthma and** *C. pneumoniae*

In the early 1990s, Hahn and colleagues were the first to suggest a possible link between *C. pneumoniae* and asthma when they observed a connection between Ig levels, wheezing, and adult-onset asthma [49]. To further investigate this association, they followed 10 patients who had acute C. pneumonia infection and *de novo* wheezing for 10 years, collecting clinical and microbiological data [50]. Among the 10 patients, one had pneumonia, while the other nine had bronchitis. Of the nine with bronchitis, four improved without treatment, while the remaining five developed chronic asthma during follow-up.

The investigation of the association between *C. pneumoniae* and asthma is impeded by the lack of standardized, sensitive, and specific detection methods for the pathogen. Nucleic acid amplification tests (NAATs), such as real-time PCR assays, offer accurate and efficient means of diagnosing acute *C. pneumoniae* infections [51]. However, the microimmunofluorescent antibody test is the most sensitive and specific serologic test for acute infection [40], despite its technical challenges and subjective interpretation. *C. pneumoniae* culturing is difficult and should be performed in cell culture. Additionally, there are practical and ethical obstacles to sampling the lower respiratory tract in representative populations of asthma patients and control subjects. Clinical research serological testing methods are limited by the high prevalence of antibodies to *C. pneumoniae* in the general population and the short duration of the initial antibody response (3–5 years), indicating that chronic infection and reinfection are common [52]. Serological methods cannot distinguish between acute, chronic, or reactivated prior infections. Therefore, new molecular diagnostic methods, such as PCR, have been developed to detect the pathogen's DNA. Although PCR testing can detect uncultivable organisms, it cannot differentiate between viable and nonviable organisms when used in antibiotic treatment studies [53]. However, reverse

#### Chlamydia pneumoniae *and Childhood Asthma DOI: http://dx.doi.org/10.5772/intechopen.111711*

transcriptase-PCR can identify metabolic activity by detecting messenger RNA and may overcome this limitation [54].

*Several studies using serological diagnostic techniques have linked C. pneumoniae to stable asthma in both adults and children*. The studies used heat-shock proteins (HSPs) of *C. pneumoniae*, which are overproduced in persistent infections and associated with hypersensitivity and immunopathology [55]. Significant differences in the prevalence of antibodies to these HSPs were observed [56–58]. Falck et al. found that persistently increased levels of *C. pneumoniae* IgA antibodies were associated with pronounced symptoms of chronic respiratory tract disease [56]. A recent study concluded that asthmatics with IgA and IgG against *C. pneumoniae* have more severe disease with increased airway obstruction, higher doses of ICS, more signs of air trapping, and less type-2 inflammation [59].

A dose-response relationship between *C. pneumoniae* HSP60 IgA antibodies and pulmonary function has also been observed, with an inverse association seen between IgG antibodies to *C. pneumoniae* and percent-predicted FEV1 in asthmatics with elevated IgG and/or IgA levels. These elevated levels of IgA antibodies have also been associated with a higher daytime asthma symptom score and the need for high-dose inhaled corticosteroids. In general, higher *C. pneumoniae* antibody titers appear to be linked to several asthma severity markers [60].

A study involving 332 asthmatic patients discovered a significant correlation between asthma and elevated levels of IgG antibodies to *C. pneumoniae*, with the strongest correlation being observed in non-atopic longstanding asthma [58]. However, a population-based study conducted in Italy found a significant correlation between *C. pneumoniae* seropositivity and atopy among young adults [59].

Regarding children with reactive airway disease, Emre et al. discovered a correlation between *C. pneumoniae* infection and wheezing, with 85.7% of the 14 wheezing asthmatic patients testing positive for *C. pneumoniae* [61]. Immunoblotting detected anti-*C. pneumoniae* IgE, while anti-*C. pneumoniae* IgG and IgM were not detected by microimmunofluorescence. This suggests that the production of specific IgE may be a mechanism underlying reactive airway disease in some patients with *C. pneumoniae* infection. A subsequent study of asthmatic children found *C. pneumoniae*-specific IgE antibodies even in the absence of acute airway infection (negative PCR), suggesting that *C. pneumoniae* can stimulate allergic responses [61].

Studies comparing the T helper responses in *C. pneumoniae*-infected peripheral blood mononuclear cells (PBMC) of asthmatic patients to those of non-asthmatic control subjects revealed that *C. pneumoniae* infection can induce allergic responses in asthmatic PBMC, as indicated by an increase in the production of Th2-type cytokines (such as IL-4) and induction of IgE responses [62]. Recent studies with similar findings have suggested that *C. pneumoniae* infection may trigger IgE-specific responses in both asthmatic children and adult asthma patients [63, 64].

Research has indicated that *C. pneumoniae* infections can lead to the development of organism-specific IgE chemical mediators, which can cause airway inflammation and consequent wheezing. Furthermore, CP-specific IgE has been linked to severe persistent asthma, indicating that persistent infection may be causing asthma symptoms. Therefore, treating the underlying *C. pneumoniae* infection may help to lessen or even abolish symptoms [65].

Teig et al. conducted a study involving 38 children with stable chronic lung disease and 42 healthy controls. They found that 24% of the children with lung disease tested positive for *C. pneumoniae* using PCR, while none of the controls tested positive [66]. In a similar study, Cunningham et al. detected *C. pneumoniae* DNA in nasal specimens from 28% of stable asthmatic children, and the PCR result remained positive for a few months [67]. Biscione et al. utilized reverse transcriptase PCR to detect RNA of the major outer membrane protein (MOMP) from *C. pneumoniae*, which is only created during productive infection. This method was found to distinguish colonization from productive infection. They reported an increase in the detection of this organism in asthmatic patients compared to nonatopic spouses of asthmatic patients who served as controls [68].

*There is evidence that C. pneumoniae infection may be related to asthma exacerbations*. Acute asthma exacerbations are a common cause of hospitalization and visits to the Emergency Department (ED) in children, and they account for a significant proportion of asthma-related issues. Respiratory infections have been strongly linked to exacerbations, making them potential targets for treatment. Evidence suggests that *C. pneumoniae* is associated with asthma attacks, particularly in cases of severe attacks in children [69, 70]. Furthermore, atypical bacterial infections have been shown to cause attacks that are associated with persistent symptoms and a slower rate of recovery after 3 weeks [71].

*Mounting evidence suggests that C. pneumoniae could play a role in the pathogenesis of asthma*. Components of *C. pneumoniae*, such as transcription factors, have been found to activate components in bronchial tissue, leading to increased cytokine release and airway remodeling [72]. Furthermore, studies have shown that patients with *C. pneumoniae*specific antibodies are more likely to experience severe airway inflammation than those without [73]. These findings suggest that *C. pneumoniae* reactivation could be a potential trigger for neutrophilic airway inflammation in people with asthma.

*C. pneumonia infections might be worsening asthma*. Webley found that 33% of asthma patients had C. pneumonia present in their bronchoalveolar lavage (BAL) samples by culture, and 67% were PCR-positive [74]. In a study of a heterogeneous group of children with asthma and recurrent bronchial obstructions, Schmidt et al. reported a 52% PCR-positivity rate for *C. pneumonia* in bronchoalveolar lavage specimens [75]. This suggests that C. pneumonia infections are more common in asthmatic patients than previously thought. It is worth considering whether *C. pneumonia* infection or colonization has a worsening effect on chronic respiratory diseases, as these invasive procedures such as bronchoscopy are only performed in treatment-resistant patients.

*C. pneumoniae* infection has been linked to an increase in the number and longevity of immune and inflammatory cells, which can lead to a reduced response to steroid treatment and increase the likelihood of treatment resistance [76].

A subpopulation of 5–25% of asthmatics, typically those with more severe disease and uncontrolled symptoms despite high doses of steroids, are labeled as having severe, steroid-resistant asthma. Respiratory infections are being implicated in the pathogenesis of severe, steroid-resistant asthma, and neutrophil-dominated endotypes of disease. Neutrophilic asthma is found to be associated with increased bacterial burden and interleukin 8 levels [34]. It has been suggested that neutrophilic asthma is less responsive than eosinophilic asthma to anti-inflammatory therapies, including corticosteroids. A study of children with asthma found that those who were PCR-positive for *C. pneumoniae* had higher concentrations of IL-8 and neutrophils in their bronchoalveolar lavage fluid than those who were PCR-positive for *C. trachomatis* or mycoplasma organisms but PCR-negative for *C. pneumoniae* [34]. This suggests that undiagnosed *C. pneumoniae* infections in children may contribute to inadequately controlled asthma by inducing IL-8.

Several studies have suggested that chronic *C. pneumoniae* infection is associated with a decline in respiratory function and more severe disease in both children and adults [68, 77, 78], and these associations are supported by biologically plausible

#### Chlamydia pneumoniae *and Childhood Asthma DOI: http://dx.doi.org/10.5772/intechopen.111711*

mechanisms [79]. Cigarette smoke exposure is a known risk factor for steroid resistance in asthma [80]. Similarly, *C. pneumoniae* (CP) is known to induce ciliostasis of the pulmonary bronchial epithelium [81] and can infect alveolar macrophages and lung monocytes, resulting in increased production of TNF-, IL-1, IL-6, and IL-8, as well as human bronchial smooth muscle cells, leading to the production of IL-6 and basic fibroblast growth factor (with potential effects on bronchial hyperreactivity and lung remodeling) and chronic infection exposes tissues to cHSP60 and LPS, which have been linked to increased inflammation and asthma [82].

*Numerous clinical studies have found associations between C. pneumoniae infection and the onset of childhood asthma*. However, the relationship between *C. pneumoniae* infection and late-onset asthma in adult studies has yielded contradictory findings. A cross-sectional study conducted on patients with severe, late-onset, nonatopic asthma showed intriguing results suggesting a possible link between *C. pneumoniae* infection and fixed airway obstruction in adults [83].

A case-control study conducted in Italy found that children aged 2–14 years who presented to the pediatric emergency department with an acute episode of wheezing had a significantly higher incidence (15.5%) of acute *C. pneumoniae* infection compared to healthy controls [83]. Follow-up revealed that those who were not treated with antibiotics were more likely to experience recurrent wheezing than those without the infection. A study conducted in Japan also demonstrated similar findings, with higher CP-IgM levels present in hospitalized wheezing infants than in controls and a higher incidence of asthma in those with *C. pneumoniae* infection than in those without [84]. A larger follow-up study conducted 2 years later revealed that *C. pneumoniae* infection, a family history of allergic diseases, the number of eosinophils, and the serum IgE concentration at the initial examination were risk factors for asthma progression [85].

The use of mouse models has enabled researchers to determine the mechanisms by which Chlamydia respiratory infections in early life may be associated with the emergence and increased severity of allergic airway disease (AAD) later in life. Infections at all ages (neonatal, infant, and adult) were found to induce inflammation. However, it was observed that chlamydial infection during early life, but not in adulthood, was associated with the development of asthmatic characteristics in allergen-induced AAD. In particular, neonatal and infant infections were found to result in mixed type 1/type 2 immunity with increased levels of interleukin-13 (IL-13) and interferon (IFN), which, in turn, was associated with increased mucus-secreting cells and airway hyperreactivity (AHR) in AAD later in life, when compared to age-matched uninfected controls [86, 87]. Jupelli et al. later confirmed the effects of infant infection on the structure and function of the respiratory system [88]. Interestingly, it was found that infant infection increased the number of airway eosinophils [84, 85, 89]. Further investigation revealed that inflammation and AHR can lead to steroid resistance [75].
