THE SIGNALING ACTIVITY OF MURINE CD19 IS REGULATED DURING B-CELL DEVELOPMENT

CD19, a B cell-specific transmembrane protein, is essential for murine B-1 cell development and T cell-dependent B cell immune responses. Wh ereas signaling by the human B cell Ag receptor can be modulated by CD 19, less is known about the biochemical properties of murine CD19. We have used a novel rat mAb specific for murine CD19 to study the bioche mical properties of the murine protein. We demonstrate that murine CD1 9 shares with human CD19 an association with complement receptor CD21 and CD81, tyrosine phosphorylation, binding of phosphatidylinositol-3 kinase, and synergistic signaling with membrane IgM. Murine CD19 is sh own also to enhance signaling through the mu-surrogate light chain com plex of primary pre-B cells. We found that although expressed in the e arliest B cell precursors, CD19 ligation does not activate Ca2+ mobili zation until the pre-B cell stage of development. in mature B cells, C D19 cross-linking activates Ca2+ flux in B-2 cells but not in B-1 cell s, although it can synergize with surface IgM in both B-1 and B-2 cell s. These biochemical properties of CD19 will be important for understa nding its function in B cell development and the humoral immune respon se.