Cellular origin of the soluble FcμR. (A) RT-PCR analysis of FCMR transcripts. The CD5⁺/CD19⁺ CLL B (lanes 1 and 4), CD5⁻/CD19⁺non-CLL B (lanes 2 and 5), and CD5⁺/CD19⁻ T cells (lanes 3 and 6) in patients with MT-CLL (lanes 1-3) and UM-CLL (lanes 4-6) were enriched from PBMC by cell sorting. Total RNAs were extracted from the sorted cells and converted to first-strand cDNA before PCR amplification using a set of primers corresponding to the FCMR or GAPDH cDNA. An aliquot of the amplified FCMR (top panel) and GAPDH (bottom panel) products was electrophoresed on 1% agarose and stained with ethidium bromide. Lane 7 is a PCR control without a first-strand cDNA template. (B) Western blot analysis of soluble FcμR in culture supernatants. The above-sorted cells were cultured at 10⁶ cells/mL for 7 days, and the culture supernatants from CLL B (lanes 1 and 2), non-CLL B (lanes 3 and 4), and T cells (lanes 5 and 6) were analyzed by Western blot as described in “Methods.”