5.1.1. Correlation between Hsp60 and C. pneumoniae antibodies (I, II)

When serum samples from asthma patients and healthy controls were analysed for Hsp60 antibodies, a correlation (r = 0.50, P < 0.001) was found between serum IgA antibodies to C. pneumoniae Hsp60 and human Hsp60, and it remained similar (r = 0.49, P < 0.001) when only the participants demonstrating immune response to C. pneumoniae (IgG MIF titre ≥ 1:32) were analysed (I). The corresponding IgG antibodies did not correlate with each other (I).

In a comparison between serum and sputum samples for Hsp60 antibodies, a weak correlation (r = 0.25, P = 0.04) was found between serum and sputum IgA antibodies to C. pneumoniae Hsp60 (II). Only a weak correlation emerged between serum antibodies to whole C. pneumoniae and Hsp60 of C. pneumoniae. When serum antibodies to C. pneumoniae were measured by MIF, the correlation was 0.25 (P = 0.02) for IgA antibodies, while IgG antibodies did not correlate with each other (I). When serum antibodies to C. pneumoniae were measured by EIA, the correlation was 0.36 (P < 0.001) for IgA antibodies and 0.32 (P < 0.001) for IgG antibodies (II). Sputum IgA antibodies to C. pneumoniae Hsp60 did not correlate with serum IgA or IgG antibodies to whole C. pneumoniae (II).

5.1.2. Association between C. pneumoniae infection and asthma (I, II)

The MIF results, which were reported in study I, showed that the frequency of serum IgA antibodies to C. pneumoniae (titre ≥ 1:10) was significantly higher among asthma patients (72%) compared with asymptomatic controls (44%, P = 0.04). The seropositivity among the controls with acute bronchitis (88%) was much higher than the seropositivity of asymptomatic controls (P = 0.002), but did not differ from the seropositivity of the asthma patients (P = 0.18). Thus, IgA antibodies against C. pneumoniae were associated with both asthma and acute bronchitis (I).

In study II, EIA was used to measure serum IgA antibodies to C. pneumoniae, and no difference was found between asthma patients and healthy controls with respect to serum IgA antibodies to C. pneumoniae (II).

In contrast to IgA antibodies, serum IgG antibodies to C. pneumoniae (titre ≥ 1:16) were common in all study groups (from 84% to 92%; study I), and regardless of the method or the study population, the asthma patients did not differ from the controls (I, II).

5.1.3. Association between Hsp60 antibodies and asthma (I, II)

In study I, the Hsp60 antibody results were divided into tertiles. Serum IgA antibodies against C. pneumoniae Hsp60, but not against human Hsp60, were associated with asthma (Fig. 6). Asthma patients differed significantly from asymptomatic controls (P = 0.02), but not from bronchitis controls (P = 0.12) regarding C. pneumoniae Hsp60 IgA antibodies. However, when the two control groups were combined, the asthma group differed from this combined control group (P = 0.02), as the bronchitis controls did not differ from the asymptomatic controls (P = 0.65). Thus, IgA antibodies to C. pneumoniae Hsp60 were associated only with asthma, not with acute bronchitis (I).

The association found between C. pneumoniae Hsp60 IgA antibodies and asthma remained significant in a comparison of geometric means adjusted for age, sex and (ever) smoking (I). The geometric EIU (optical density multiplied by 1000) means for C. pneumoniae Hsp60 IgA antibodies were 184 EIU for asthma patients, 143 EIU for bronchitis controls and 139 EIU for asymptomatic controls. The differences were statistically significant when asthma patients were compared to asymptomatic controls (P = 0.03) or to all controls (P = 0.01). In the latter case, the association remained significant (P = 0.02) even after C. pneumoniae IgA MIF seropositivity (titre ≥ 1:10) was added as an additional covariate (I).

In study II, it was found that the median EIU levels of both serum and sputum IgA antibodies to C. pneumoniae Hsp60 were higher in the groups with moderate and mild asthma compared with the control group. The EIU (optical density multiplied by 1000) medians (IQR) for C. pneumoniae Hsp60 serum IgA antibodies were 168 (154–265) EIU for moderate asthma, 176 (147–228) EIU for mild asthma and 156 (136–182) EIU for the control group. A comparison of the levels showed a statistically significant difference between the patients with moderate asthma and the controls (P = 0.04), and a difference of borderline significance between the patients with mild asthma and the controls (P = 0.10). The EIU (optical density multiplied by 1000) medians (IQR) for C. pneumoniae Hsp60 sputum IgA antibodies were 56 (0–225) EIU for moderate asthma, 73 (9–230) EIU for mild asthma and 19 (0–98) EIU for the control group. A difference of borderline significance was found only when the patients with mild asthma were compared to the healthy controls (P = 0.06) (II).

In contrast to IgA antibodies, the levels of serum IgG antibodies to C. pneumoniae Hsp60 or human Hsp60 did not differ between the asthma patients and the controls (I, II).

5.1.4. Hsp60 antibodies and pulmonary function (I)

Serum IgA antibodies to C. pneumoniae Hsp60 were inversely correlated with pulmonary function, as measured by FEV₁, in the study group as a whole, i.e. including asthma patients, bronchitis controls and healthy controls (r = –0.23, P = 0.04), suggesting an association with the severity of pulmonary obstruction (I). When asthma patients were analysed separately, the correlation was similar (r = –0.21) but not statistically significant (I).

5.1.5. Role of C-reactive protein (II)

Serum CRP concentrations were significantly higher in the asthma patients suffering from either moderate or mild asthma compared to the healthy controls (II). The median CRP levels were 2.53 mg/l for moderate asthma, 1.79 mg/l for mild asthma and 0.85 mg/l for the control group. A comparison of the CRP levels showed a statistically significant difference between the patients with moderate asthma and the controls (P < 0.001) as well as between the patients with mild asthma and the controls (P = 0.009). A comparison of the CRP levels of only the two asthma groups showed a difference of borderline significance (P = 0.051).

CRP concentration above the 2 mg/l cut-off level was found in 13% of the healthy controls, in 45% of the patients with mild asthma (P = 0.003) and in 58% of the patients with moderate asthma (P < 0.001) (II).

The asthma patients and healthy controls were divided into two groups on the basis of CRP concentrations. The participants with a CRP level 2 mg/l had significantly higher levels of serum IgA antibodies to whole C. pneumoniae and Hsp60 of C. pneumoniae than the participants with a CRP level < 2 mg/l (Table 9). The levels of other antibodies did not differ between these two groups.