THE CD40 LIGAND EXPRESSED BY HUMAN B-CELLS COSTIMULATES B-CELL RESPONSES

The possibility that activated B cells might express a ligand for CD40 that was of functional importance for B cell responses was examined b y using highly purified human peripheral blood B cells, as well as a v ariety of B lymphoblastoid cell lines and hybridomas. Following stimul ation with the combination of a calcium ionophore and a phorbol ester, human B cells bound a soluble fusion protein containing the extracell ular portion of CD40 and the Fc region of IgG1 (CD40.Ig). A variety of B cell lines and hybridomas also bound CD40.Ig, either constitutively or after activation. In addition, CD40.Ig specifically immunoprecipit ated a 33-kDa glycoprotein from surface I-125-labeled activated B cell s. The nucleotide sequence of the coding region of the CD40 ligand mRN A amplified by RT-PCR from activated T cells and B cell lines was iden tical. The CD40 ligand expressed on human B cells was important functi onally because homotypic aggregation of CD40 ligand-expressing B cells was inhibited by the CD40.Ig construct. Additionally, RNA and DNA syn thesis as well as Ig production by polyclonally activated, highly puri fied peripheral B cells and a variety of B cell lines were inhibited s ignificantly by the CD40.Ig construct. Finally, B cell lines expressin g the CD40 ligand induced Ig production from resting normal B cells in a CD40-dependent manner. These results indicate that human B cells ex press a ligand for CD40 that is identical with that expressed by activ ated T cells and that the B cell-expressed CD40 ligand plays an import ant role in facilitating responses of activated B cells.