Scheme of CD28i and semiquantitative measurement of CD28i mRNA levels.

(A) Schematic representation of human CD28 and CD28i. Exon 1, exon 2, exon 3, and exon 4 are shaded in different tones. The transmembrane domains in exon 3 are indicated. (B) Semiquantitative measurement of CD28i. The cytoplasmic RNAs from PBL-T cells, thymocytes, and Jurkat T cells were examined by RT-PCR for CD28i messages using a specific primer. EtBr-stained PCR products were estimated semiquantitatively after 21, 23, 25, 27, 29, and 31 cycles to ensure near-linear amplification. The percent values indicate the proportional amplification compared with the similarly amplified CD28 message that scores 100%. The data represent the 3 experiments with similar results.