Human M2 macrophages limit NK cell effector functions through secretion of TGF-b and engagement of CD85j

Abstract

NK cells play important roles during immunosurveillance against tumors and viruses as they trigger cytotoxicity against susceptible cells and secrete pro-inflammatory cytokines such as IFN-g. In addition, upon activation, macrophages can become pro-inflammatory (M1) or anti inflammatory (M2) cells. Although the consequences of the crosstalk between M1 and NK cells are known, the outcome of the crosstalk between M2 and NK cells remains ill-defined. Therefore, in the current work, we investigated the outcome and the underlying mechanisms of the interaction between resting or stimulated human NK cells with M1 or M2. We observed a lower percentage of activated NK cells that produced less IFN-g upon co-culture with M2. Also, CD56dim NK cells co-cultured with M2 displayed lower degranulation and cytotoxic activity than NK cells co-cultured with M1. Soluble TGF-b and M2-driven up-regulation of CD85j (ILT-2)on NK cells accounted for the diminished IFN-g production by CD56bright NK cells, while M2-driven up-regulation of CD85j on NK cells accounted for the generation of hyporesponsive CD56dim NK cells with limited degranulation and cytotoxic capacity. Accordingly, M2 expressed higher amounts of HLA-G, the main ligand for CD85j, than M1. Hyporesponsiveness to degranulation in NK cells was not restored at least for several hours upon removal of M2. Therefore, alternatively-activated macrophages restrain NK cell activation and effector functions through different mechanisms, leading to NK cells that display diminished IFN-g production and at least a transiently impaired degranulation ability. These results unravel an inhibitory circuit of possible relevance in pathological situations.