Effects of asthma on cell components in peripheral blood among smokers and non-smokers

Background: Eosinophils play a central role in asthma, but the interplay of the effects of smoking, eosinophils and asthma remains unclear. Objective: The primary objective of our study was to investigate the extent to which smoking modifies the effect of asthma on circulating eosinophils, CD4 ⁺ and CD8⁺ T cell counts. Methods: Data were collected semiannually between 1987 and 1994 from HIV-negative participants in the Multicenter AIDS Cohort Study. Asthma was defined by a questionnaire at baseline as a self-report of diagnosed asthma. A total of 1420 blood samples from 197 asthmatics and 15822 from 1997 non-asthmatics were collected. Results: Eosinophil levels were higher in asthmatics (28% of asthmatics had eosinophils ≥4% and 16% of non-asthmatics) regardless of smoking history, but smoking modified the association between eosinophils and asthma. Namely, the odds ratios for eosinophils being ≥4% in asthmatics to non-asthmatics decreased from 2.7 (95% CI: 2.0, 3.6) in never, to 2.1 (1.4, 3.1) in former, and to 1.5 (0.9, 2.3) in current smokers. Cross-sectional and longitudinal analyses coherently showed that smoking increased eosinophils in non-asthmatics, but the converse was true for asthmatics. In contrast, no differences in peripheral blood T cell counts between asthmatics and non-asthmatics were observed. Conclusion: Under the established link between increased eosinophils and asthma, these data indicate that smoking modified this relationship. This finding suggests that smoking plays a different immunological role in asthmatics and non-asthmatics.