Genetically coupled receptor-ligand pair NKp80-AICL enables autonomous control of human NK cell responses.

NKp80 is a C-type lectin-like receptor broadly expressed on human natural killer (NK) cells, triggering cytotoxicity via an atypical cytoplasmic hemi-immunoreceptor tyrosine-based activation motif. As with other lectin-like NK receptors, NKp80 is encoded in the natural killer gene complex, but unlike most of these, adjacent to its ligand, ie, activation-induced C-type lectin (AICL). The reasons for the tight genetic linkage of this receptor-ligand pair remain elusive. Previous studies showed that NKp80 augments NK cell responses toward malignant and nonmalignant myeloid cells. Here, we report that resting human NK cells not only express NKp80 but also contain intracellular stores of AICL colocalizing with the Golgi complex. Domain-swapping experiments revealed that intracellular localization of AICL is determined by its C-type lectin-like ectodomain. Exposure of NK cells to monokines associated with conversion into memorylike cells induces substantial AICL cell surface expression, whereas NKp80 is downregulated, and NK cells become refractory to NKp80-mediated stimulation. AICL on monokine-exposed NK cells elicits NKp80-dependent effector responses by autologous NK cells and, hence, renders monokine-activated NK cells susceptible to NKp80-mediated cytolysis. Altogether, our data report a previously unrecognized regulatory circuit enabling autonomous control of human NK cell responses via the NKp80-AICL axis.