Generation of chimeric CD40 transgenic mice. (A) Schematic representation of the constructs used to generate transgenic mice expressing chimeric CD40 molecules consisted of the extracellular domain of the human CD40 and the transmembrane and varied cytoplasmic domains of murine CD40 under the MHC class II promoter control. Point substitutions are labeled by asterisks. Restriction enzyme site SalI has been inserted to connect 2 CD40 cytoplasmic tail fragments into the pDOI-5 vector. (B) Chimeric CD40 transgenic receptor expression on B220⁺ B cells were confirmed by FACS analysis. Filled histograms represented antihuman CD40 staining of the transgenic receptor; open histograms, the isotype control.