A novel digital PCR-based method to quantify (switched) B cells reveals the extent of allelic involvement in different recombination processes in the IGH locus

B cells fulfill an important role in the adaptive immunity. Upon activation and immunoglobulin (IG) class switching, these cells function in the humoral immunity compartment as plasma cells. For clinical applications, it can be important to quantify (switched) B cells accurately in a variety of body fluids and tissues of benign, inflammatory and malignant origin. For decades, flow cytometry and immunohistochemistry (IHC) have been the preferred methods for quantification. Although these methods are widely used, both depend on the accessibility of B cell epitopes and therefore require intact (fixed) cells. Whenever samples are low in quantity and/or quality, accurate quantification can be difficult. By shifting the focus from epitopes to DNA markers, quantification of B cells remains achievable. During differentiation and maturation, B cells are subjected to programmed genetic recombination processes like VDJ rearrangements and class switch recombination (CSR), which result in deletion of specific sequences of the IGH locus. These cell type-specific DNA "scars" (loss of sequences) in IG genes can be exploited as B cell markers in digital PCR (dPCR) based quantification methods. Here, we describe a novel, specific and sensitive digital PCR-based method to quantify mature and switched B cells in DNA specimens of benign and (copy number unstable) malignant origin. We compared this novel way of B cell quantitation with flow cytometric and immunohistochemical methods. Through cross-validation with flow cytometric sorted B cell subpopulations, we gained quantitative insights into allelic involvement in different recombination processes in the IGH locus. Our newly developed method is accurate and independent of the cellular context, offering new possibilities for quantification, even for (limited) small samples like liquid biopsies.