A further link between innate and adaptive immunity: C3 deposition on antigen-presenting cells enhances the proliferation of antigen-specific T cells

Murine cells of the a lymphoblastoid line A20 and concanavalin A-elicited p eritoneal macrophages are shown to activate and fix C3 fragments covalently when incubated in fresh, autologous serum under conditions allowing the in itiation of the alternative complement pathway. For the detection of cell-b ound C3, cytofluorimetry was performed using FITC-labeled F(ab')(2) fragmen ts of anti-mouse C3, Cell-bound C3 fragments are not internalized or shed b y the cells under culture conditions for at least two hours, When the antig en-presenting capacity of serum-treated cells was tested using various anti gens and experimental systems, augmentation of the proliferation of antigen -specific T cells was found, This enhancing effect was particularly pronoun ced at suboptimal antigen doses. The elevation of T cell proliferation indu ced by C3-opsonized antigen-presenting cells (APC) could be abrogated by F( ab')(2) fragments of goat anti-mouse C3, suggesting the involvement of C3 r eceptors expressed by T cells in the process, Using the 7G6 mAb recognizing murine CR1/CR2, the presence of these complement receptors on activated T cells is demonstrated by cytofluorimetry and immunoprecipitation, as well, These results point to the role of C3 bound to acceptor sites on APC in the facilitation of antigen presentation, providing a further link between inn ate and adaptive immunity.