Background
Fractalkine/CX(3)CL1, a surface chemokine, binds to CX(3)CR1 expressed by different lymphocyte subsets. Since CX(3)CL1 has been detected in the germinal centres of secondary lymphoid tissue, in this study we have investigated CX(3)CR1 expression and function in human naïve, germinal centre and memory B cells isolated from tonsil or peripheral blood. Methodology/principal findings: We demonstrate unambiguously that highly purified human B cells from tonsil and peripheral blood expressed CX(3)CR1 at mRNA and protein levels as assessed by quantitative PCR, flow cytometry and competition binding assays. In particular, naïve, germinal centre and memory B cells expressed CX(3)CR1 but only germinal centre B cells were attracted by soluble CX(3)CL1 in a transwell assay. CX(3)CL1 signalling in germinal centre B cells involved PI3K, Erk1/2, p38, and Src phosphorylation, as assessed by Western blot experiments. CX(3)CR1(+) germinal centre B cells were devoid of centroblasts and enriched for centrocytes that migrated to soluble CX(3)CL1. ELISA assay showed that soluble CX(3)CL1 was secreted constitutively by follicular dendritic cells and T follicular helper cells, two cell populations homing in the germinal centre light zone as centrocytes. At variance with that observed in humans, soluble CX(3)CL1 did not attract spleen B cells from wild type mice. OVA immunized CX(3)CR1(-/-) or CX(3)CL1(-/-) mice showed significantly decreased specific IgG production compared to wild type mice.
Significance
We propose a model whereby human follicular dendritic cells and T follicular helper cells release in the light zone of germinal centre soluble CX(3)CL1 that attracts centrocytes. The functional implications of these results warrant further investigation.