CD40 LIGAND AND APPROPRIATE CYTOKINES INDUCE SWITCHING TO IGG, IGA, AND IGE AND COORDINATED GERMINAL CENTER AND PLASMACYTOID PHENOTYPIC DIFFERENTIATION IN A HUMAN MONOCLONAL IGM(-CELL LINE()IGD(+) B)

B lymphocytes are induced to undergo Ig class switching and a complex phenotypic differentiation by the milieu of the germinal center. Partl y as a result of the lack of a suitable in vitro B cell model, the rel ationship between these processes in the humans has never been formall y established in vitro. We have identified a human monoclonal B cell l ine, CL-01, that expresses surface IgM and IgD and, upon induction wit h CD40 ligand, IL-4, and IL-10, switches to all seven downstream isoty pes, showing typical DNA switch recombination preceded by germline;tra nscription of targeted C-H regions. In CL-01 cells, switch-inducing st imuli trigger concomitant changes in expression of surface IgD, CD23, CD38, and CD77 that parallel those reported in ex vivo isolated tonsil lar centroblasts, centrocytes, and memory B cells. Eventually, in the presence of IL-6, CL-01 cells express CD56 and accumulate cytoplasmic IgG and IgA, both traits of plasmacytoid differentiation. Analysis of transcription and recombination of the Ig H locus in sorted CL-01 cell s suggest that Ig class switching begins in centroblasts, it extends t o all isotypes in centrocytes, and it is extinct in memory B cells. Th us, we have induced coordinated Ig class switching, progression throug h germinal center phenotypic stages, and differentiation to memory B c ells and plasma cells at the level of a single B clonotype. Our data s uggest that these processes are likely regulated by a common maturatio n program, the activation of which may require CD40 ligand, IL-4, IL-1 0, and IL-6 only.