Allergic bronchopulmonary aspergillosis (ABPA) is a hypersensitivity lung disease characterized by Aspergillus fumigatus (Af) colonization, IgE and IgG anti-Af antibodies, pulmonary infiltrates, bronchiectasis, and pulmonary fibrosis. Little is known regarding T cell responses and their role in the pathogenesis of ABPA. To examine T cell reactivity to Af antigens, T cell clones (TCC) specific to the Asp f 1 antigen, an 18-kD protein of Af, were established from the peripheral blood of three ABPA patients. The majority of TCC isolated from ABPA patients, and specific for the Asp f 1 allergen of Af, are IL-4 producing CD4+ cells of the Th2 phenotype. Further analysis in this study revealed that the majority of TCC reacted to mainly two epitopes of Asp f 1, while the remaining TCC reacted to three additional "minor" epitopes. Blocking studies using monoclonal antibodies specific for class II HLA-D region gene products showed that most TCC, 19/21, were restricted by HLA-DR molecules, and the remaining two clones by HLA-DP molecules. The use of a panel of HLA-matched and mismatched EBV-transformed B cells as antigen presenting cells revealed that the HLA-DR restriction was mediated exclusively by either the HLA-DR2 or HLA-DR5 alleles. Genotyping of DRB1 gene products showed that class II presentation for most clones was not restricted to a single allele, representing DRB1 gene products of either HLA-DR2 or DR5. These studies offer insight into the cellular and molecular determinants which contribute to the immunopathophysiology of ABPA.