Identification of two subpopulations of purified human blood B cells, CD27- CD23+ and CD27high CD80+, that strongly express cell surface Toll-like receptor 9 and secrete high levels of interleukin-6.

B-cell expression of certain Toll-like receptors (TLRs) is important in linking innate and adaptive immune responses in normal and pathological conditions. The expression of TLR9 plays a role in the recognition of conserved pathogen motifs in a manner that is dependent on B-cell localization, deduced from B-cell phenotype. The nature of TLR9 function is unclear. A first step in unravelling the function of this pattern recognition receptor is to discover the precise nature of the cell types that express TLR9. This study used three-colour flow cytometry to characterize the B lymphocytes from human peripheral blood mononuclear cells (PBMCs) that express TLR9 on the surface. We sorted TLR9-positive B and non-B cells from the PBMC population and detected TLR9 expression on naïve and memory B cells. Moreover, we identified two discrete subpopulations of B cells: CD19(+) CD27(-) CD23(+) cells and CD19(+) CD27(high) CD80(+) cells. These subpopulations expressed high levels of membrane TLR9 and exhibited a strong in vitro response to binding a relevant CpG motif by secreting high levels of interleukin-6 (compared to controls). Our finding that this pattern recognition receptor is expressed on a variety of cell subsets adds to the current understanding of the functional complexity of B-cell membrane TLR9.