KLRG1+ NK Cells Include Two Distinct Populations Lined by CX3CR1 Expression

Background: Expression of chemotactic molecules in the bone marrow (BM) microenvironment may deeply influence NK cell maturation by controlling their positioning into specific niches. In the present study, we functionally characterized CX3CR1 expressing NK cell subsets. Methods: CX3CR1 expression was assessed by FACS analysis and the functions of NK cells by migration, IFN-gamma intracellular staining and citotoxicity assays. Results: Using CX3CR1+/GFP mice, we showed that CX3CR1 is prevalently expressed by the KLRG1+ NK cell subset that is considered a quiescent and terminally differentiated NK cell population with reduced effector functions. Within KLRG1+ NK cells, CX3CR1+ cells evidenced reduced CXCR4 expression and function that was associated with different positioning within BM as compared to the CX3CR1- NK cells. In addition, CX3CR1+ NK cells displayed impaired capability to produce IFN-gamma and to lyse YAC-1 target cells. By means of adoptive transfer experiments we observed that CX3CR1 expression can be acquired by KLRG1+/CX3CR1- NK cells, while KLRG1+/CX3CR1+ NK cells stably expressed CX3CR1. We also analyzed the role of CX3CR1 during NK cell maturation, comparing NK cell tissue distribution in heterozygous CX3CR1+/GFP and in CX3CR1-deficient CX3CR1GFP/GFP mice. We observed tissue accumulation of GFP+ NK cells in CX3CR1GFP/GFP mice that was associated to anincreased number of BM NK cells during steady state and to a defective BM NK cell mobilization during POLY I:C –induced inflammation. Conclusions: All together, our findings show that CX3CR1 is a marker of a KLRG1+ NK cell subset with impaired effector functions that can irreversibly differentiate from the CX3CR1- cells under steady state conditions.