Complement receptor distinguishes between two subsets of large granular lymphocytes with different natural killer activity and cytochemical and ultrastructural features.

Human peripheral blood large granular lymphocytes (LGL)--that is, cells with intracytoplasmic azurophilic (electron-dense) granules, with a positivity for the cytochemical localization of certain acid hydrolases, and with avid surface receptors for the Fc portion of IgG--have been purified on Percoll density gradients. Approximately 30% of these cells expressed receptors for the third complement component (C3R). They were separated into C3R-positive and C3R-negative cells. C3R+ cells had a significantly greater natural killer (NK) activity against K562 target cells than C3R+ cells. This difference was unrelated to the presence in the C3R+ cells of a contaminant cell type incapable of NK activity, since cytochemical and ultrastructural analysis revealed that C3R+ and C3R- fractions contained comparable LGL numbers. Agarose cytotoxicity assays at the single-cell level demonstrated that C3R+ LGL contained a large number of cells that bound to but did not lyse the target. The remaining fully cytotoxic C3R+ LGL had, however, the same killing and recycling properties as the cells from the C3R fraction. Electron microscopy and cytochemical studies showed that C3R+ cells had fewer electron-dense granules than C3R cells and stained more faintly for the localization of alpha-naphtyl acetate esterase. In contrast to C3R cells, C3R+ LGL displayed morphological features suggesting that an active process of granule formation was taking place. Taken together, the data indicate that C3R+ cells represent a discrete subset or a maturational stage of LGL.