Multiparameter analysis of human B lymphocytes identifies heterogeneous CD19⁺ CD21^lo subsets

Autoreactive B cell subsets have been described in a variety of settings, using multiple classification schemes and cell surface markers also found on healthy cells. CD19⁺ CD21^lo B cells have been identified as an autoreactive-prone subset of B cells, although the downregulation of CD21 has been observed on a variety of B cell subsets in health and disease. This variation has led to confusion regarding the meaning and applicability of the loss or reduction of CD21 in peripheral B cells. To better understand the relationships between commonly used B cell markers and their associated characteristics, we analyzed human B cells from healthy participants using multiparameter flow cytometry and the visualization algorithm, tSNE. This approach revealed significant phenotypic overlap amongst five previously described autoimmune-prone B cell subsets, including CD19⁺ CD10^- CD27^- CD21^lo B cells. Interestingly, 12 different subpopulations of CD19⁺ CD21^lo B cells were identified, some of which mapped to previously described autoreactive populations, while others were consistent with healthy B cells. This suggests that CD21 is downregulated in a variety of circumstances involving B cell activation, all of which are present in low numbers even in healthy individuals. These findings describe the utility of unbiased multiparameter analysis using a relatively limited panel of flow cytometry markers to analyze autoreactive-prone and normal activated B cells.