RATIONALE: Pseudomonas aeruginosa (PA) is an environmental pathogen that commonly infects individuals with cystic fibrosis (CF) and non-CF bronchiectasis, impacting on morbidity and mortality. To understand the pathobiology of interactions between the bacterium and host adaptive immunity and to inform rational vaccine design, it is important to understand the adaptive immune correlates of disease. OBJECTIVES: We characterized T cell immunity to the PA antigen, outer membrane porin F (OprF), analyzing immunodominant epitopes in relation to infection status. METHODS: Non-CF bronchiectasis patients were stratified by frequency of PA isolation. T cell IFNγ immunity to OprF and its immunodominant epitopes was characterized. Patterns of HLA-restriction of immunodominant epitopes were defined using HLA class II transgenic mice. Immunity was characterized with respect to cytokine and chemokine secretion, antibody response and T cell activation transcripts. MEASUREMENTS AND MAIN RESULTS: Patients were stratified according to whether PA was never, sometimes (<50%) or frequently (>or=50%) isolated from sputum. Patients with frequent PA sputum-positive isolates were more likely to be infected by mucoid PA, and showed a narrow T cell epitope response and a relative reduction in Th1 polarizing transcription factors, but enhanced immunity with respect to antibody production, innate cytokines and chemokines. CONCLUSIONS: We have defined the immunodominant, HLA-restricted, T cell epitopes of OprF. Our observation that chronic infection is associated with a response of narrowed specificity, despite strong innate and antibody immunity, may help to explain susceptibility in these individuals and pave the way for better vaccine design to achieve protective immunity.