Harmonisation of switched memory B cell analysis for improved CVID diagnosis and classification

To the Editor, Common variable immunodeficiency (CVID) is an inborn error of immunity (IEI) characterized by impaired immunoglobulin (Ig) production, in association with an increased susceptibility to infections and a diversity of non-infectious clinical manifestations, such as autoimmunity and organomegalies (1). Most CVID patients also show abnormalities in B-cell maturation patterns, mostly at the plasma cell (PC) and memory B-cell (MBC) levels, as confirmed by flow cytometric (FCM)-based immunophenotyping (2,3). Since 1998, human MBC have been immunophenotypically defined as CD27 + (i.e., classical MBC) (4,5). This cell surface receptor protein is a member of the TNF-receptor family and was considered the preferred marker to distinguish MBC from antigen-inexperienced (naïve) B cells (IgD + ) based on the established correlation between its expression and the somatic hypermutation status (SHM) of Ig genes (2,6,7). Today, it is recognized that approximately 1-5% of human peripheral blood memory Bcells, in healthy adults, lack CD27 expression on their surface membrane (6,7). Some studies described increased frequencies of double-negative IgD -/CD27 -(DN) B cells in peripheral blood of healthy controls aged > 60 years, compared to younger adults (8), although this age-related increase in DN B cells was not consistently confirmed across studies (9) and their functional role remains to be fully elucidated. Published data suggest that DN B cells are possibly heterogeneous as some authors regarded them as exhausted cells due to repeated antigenic stimulation, whereas, while others consider them as T-cell independent or primary less experienced germinal center (GC) memory B cells (8,10). Those IgD -/CD27 -DN B cells share several characteristics with the IgD -/CD27 + switched memory B cells (sMBC), including similar telomere length and morphology. However, the level of SHM of classswitched Ig genes of IgD -/CD27 -DN B cells is somewhat lower than that of IgD -/CD27 + sMBC (6,7,8,10). Thus, the definition of sMBC has progressed over time, currently recognizing sMBC as heterogeneous for CD27. This is of particular importance when sMBC are part of the diagnostic/prognostic criteria of a disease, as is the case in CVID. Indeed, the current European Society for Immunodeficiencies (ESID) working definitions for the clinical diagnosis of CVID (published in November 2019) include low sMBC (