Primary transcripts for all Ig heavy chain isotypes are alternatively processed to encode either secreted or membrane forms of the same antibody and, in plasma cells, a shift towards the secreted form occurs. In principle, measuring the relative quantities of secreted and membrane forms for a particular isotype could monitor B-cell plasmacytoid differentiation. Ratios of α heavy chain mRNA secreted (αs) to membrane (αm) form were assessed by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR; TaqMan) using an IgA plasma cell line (NCI-H929), a surface IgA + line (Dakiki) and human tonsillar B cells. While NCI-H929 cells showed the highest αs:αm ratio as expected, αs mRNA predominated for all unstimulated B cells and Dakiki cells. Treatment of B cells and Dakiki cells with IL-2 and IL-10 resulted in a further progression towards the αs form, correlating with increased human plasma cell antigen-1 (HPC1) mRNA levels. However,α mRNA processing and HPC1 expression were independently regulated, as IFN-γ treatment suppressed HPC1 levels while increasing αs:αm ratios. Cytokine-mediated increases in the αs:αm ratio resulted from strongly enhanced levels of αs with relatively constant αm values. Differentiation-related changes in mRNA processing can thus be tracked by automated quantitative PCR.
- α Heavy chain
- B cell
- Quantitative polymerase chain reaction