BACKGROUND: Although epidemiological studies have found an association between Chlamydia pneumoniae infection and severe asthma, the causality and underlying mechanism are largely unknown. We hypothesized that C. pneumoniae infection increases the proliferation and enhances the survival of immune and inflammatory cells, resulting in reduced responsiveness to corticosteroids and suggesting that the underlying mechanism is related to a TNF-alpha-dependent pathway. METHODS: Human peripheral blood mononuclear cells (PBMCs) were cultured in vitro in the presence or absence of C. pneumoniae infection. Responsiveness to corticosteroids was assayed by adding dexamethasone, and the underlying mechanism was investigated by treating cells with infliximab that is a chimeric anti-TNF-alpha monoclonal antibody. Cellular proliferation and apoptosis was assessed by thymidine uptake and counting apoptotic cells using flow cytometry. RESULTS: Cellular proliferation was significantly higher in C. pneumoniae-infected PBMCs than in uninfected PBMCs, which is more prominent in Th2-dominant microenvironment. The anti-proliferative and pro-apoptotic effect of corticosteroid were significantly reduced in C. pneumoniae-infected PBMCs compared with uninfected PBMCs. The proliferative effect of C. pneumoniae infection and the reduced response to corticosteroid were effectively reversed by blocking the TNF-alpha pathway at least partially. CONCLUSION: C. pneumoniae infection enhanced the proliferation and survival of immune and inflammatory cells, resulting in steroid resistance. The reversal of these phenomena by the TNF-alpha inhibitor suggests that TNF-alpha may play an important role in the induction of steroid dependence or resistance. A TNF-alpha inhibitor may therefore be a candidate agent for managing steroid-dependent or -resistant severe asthma.