A glycosylation-dependent CD45RB epitope defines previously unacknowledged CD27−IgMhigh B cell subpopulations enriched in young children and after hematopoietic stem cell transplantation

•The B cell compartment is still immature 2 years after pediatric HSCT.•The number CD27−IgMhigh cells that do not express transitional marker is increased.•This population also exists in healthy children and decreases with age.•The CD45RBMEM55 glyco-epitope divides the CD27−IgMhigh B cells into subpopulation.•If combined with the dye Rhodamine 123, novel subpopulations can be defined.

The immune system is dysfunctional for years after hematopoietic stem cell transplantation (HSCT). A potential cause is an intrinsic B cell deficiency. In a cohort of pediatric HSCT patients few CD27+ B cells formed after transplantation with the number of CD27+IgMhigh cells more affected than class-switched ones. A previously unacknowledged population of CD27−IgMhigh cells made up the majority of B cells and this population was also enlarged in healthy children compared to adults. Only a minority of these CD27−IgMhigh B cells expressed markers typical for transitional B cells, and the non-transitional CD27−IgMhigh cells could be further divided into subpopulations based on their ability to extrude the dye Rhodamine 123 and their expression of CD45RBMEM55, a glycosylation-dependent epitope. Thus, we define several novel human CD27−IgMhigh B cell subpopulations in blood, all of which are present in higher frequencies and numbers in young children and after HSCT than in adults.