Tracking membrane and secretory immunoglobulin alpha heavy chain mRNA variation during B-cell differentiation by...

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 alpha heavy chain mRNA secreted (alphas) to membrane (alpham) 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 alphas: alpham ratio as expected, alphas 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 alphas form, correlating with increased human plasma cell antigen-1 (HPC1) mRNA levels. However, alpha mRNA processing and HPC1 expression were independently regulated, as IFN-gamma treatment suppressed HPC1 levels while increasing alphas: alpham ratios. Cytokine-mediated increases in the alphas: alpham ratio resulted from strongly enhanced levels of alphas with relatively constant alpham values. Differentiation-related changes in mRNA processing can thus be tracked by automated quantitative PCR.