IgA-driven antibacterial activity against Streptococcus pneumoniae by mouse lung lymphocytes

To investigate the role of lung lymphocytes (LL) in the local defense mechanisms, we studied the natural antibacterial (NA) activity of mouse LL with an in vitro assay using S. pneumoniae type 3 as target. In parallel, natural killer (NK) activity against YAC-1 tumor cells was investigated. Lung cells obtained by enzymatic digestion were found to exert detectable NA and NK activities, which were further increased after purification of LL (greater than 90\% lymphocytes) by carbonyl iron and magnet treatment. Depletion experiments with antibodies and complement indicated that the effector cell of NA activity was a Thy 1.2+, L3T4+, aGM1+ lymphocyte, whereas the effector of NK activity was found to have a Thy 1.2-, aGM1+ phenotype. Preincubation of LL with anti-IgA antibodies, but not with anti-IgG, completely inhibited NA activity, suggesting that it was mediated by preexisting IgA bound to the LL surface. Furthermore, purified IgA from S107 plasmacytoma with specificity for phosphorylcholine, a component of the outer wall of S. pneumoniae, was able to enhance the antibacterial activity of LL and to restore their activity after treatment with anti-IgA. In addition, S107 antibodies were found to specifically induce antibacterial activity against S. pneumoniae in resident alveolar macrophages (AM) and peritoneal exudate cells, which did not express NA activity. We conclude that mouse LL include a subset of IgA-bearing lymphocytes with the phenotype of helper-T cells, which are able to exert NA activity against pneumococcus through an IgA-driven mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)