MABF2223-25UG Sigma-AldrichAnti-NKp80 Antibody, clone 5D12

NKp65 is an activating human C-type lectin-like receptor (CTLR) triggering cellular cytotoxicity and cytokine secretion upon high-affinity interaction with the cognate CTLR keratinocyte-associated C-type lectin (KACL) selectively expressed by human keratinocytes. Previously, we demonstrated that NKp65-mediated cellular cytotoxicity depends on tyrosine 7, located in a cytoplasmic sequence motif of NKp65 resembling a hemi-immunoreceptor tyrosine-based activation motif (hemITAM). HemITAMs have been reported for a few activating myeloid-specific CTLRs, including Dectin-1 and CLEC-2, and consist of a single tyrosine signaling unit preceded by a triacidic motif. Upon receptor engagement, the hemITAM undergoes phosphotyrosinylation and specifically recruits spleen tyrosine kinase (Syk), initiating cellular activation. In this study, we addressed the functionality of the putative hemITAM of NKp65. We show that NKp65 forms homodimers and is phosphorylated at the hemITAM-embedded tyrosine 7 upon engagement by antibodies or KACL homodimers. HemITAM phosphotyrosinylation initiates a signaling pathway involving and depending on Syk, leading to cellular activation and natural killer (NK) cell degranulation. However, although NKp65 utilizes Syk for NK cell activation, a physical association of Syk with the NKp65 hemITAM could not be detected, unlike shown previously for the hemITAM of myeloid CTLR. Failure of NKp65 to recruit Syk is not due to an alteration of the triacidic motif, which rather affects the efficiency of hemITAM phosphotyrosinylation. In summary, NKp65 utilizes a hemITAM-like motif for cellular activation that requires Syk, although Syk appears not to be recruited to NKp65.

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.

Cellular cytotoxicity is the hallmark of NK cells mediating both elimination of virus-infected or malignant cells, and modulation of immune responses. NK cytotoxicity is triggered upon ligation of various activating NK cell receptors. Among these is the C-type lectin-like receptor NKp80 which is encoded in the human Natural Killer Gene Complex (NKC) adjacent to its ligand, activation-induced C-type lectin (AICL). NKp80-AICL interaction promotes cytolysis of malignant myeloid cells, but also stimulates the mutual crosstalk between NK cells and monocytes. While many activating NK cell receptors pair with ITAM-bearing adaptors, we recently reported that NKp80 signals via a hemITAM-like sequence in its cytoplasmic domain. Here we molecularly dissect the NKp80 hemITAM and demonstrate that two non-consensus amino acids, in particular arginine 6, critically impair both hemITAM phosphorylation and Syk recruitment. Impaired Syk recruitment results in a substantial attenuation of cytotoxic responses upon NKp80 ligation. Reconstituting the hemITAM consensus or Syk overexpression resulted in robust NKp80-mediated responsiveness. Collectively, our data provide a molecular rationale for the restrained activation potential of NKp80 and illustrate how subtle alterations in signaling motifs determine subsequent cellular responses. They also suggest that non-consensus alterations in the NKp80 hemITAM, as commonly present among mammalian NKp80 sequences, may have evolved to dampen NKp80-mediated cytotoxic responses toward AICL-expressing cells.

Receptors encoded by the natural killer (NK) cell gene complex (such as NKG2D) govern the reactivity of NK cells. However, the function and ligand(s) of the NK cell gene complex-encoded human NK cell receptor NKp80 remain elusive. Here we demonstrate that NKp80 binds to the genetically linked 'orphan' receptor AICL, which, like NKp80, is absent from rodents. We defined AICL as a myeloid-specific activating receptor that is upregulated by Toll-like receptor stimulation. AICL-NKp80 interactions promoted NK cell-mediated cytolysis of malignant myeloid cells. In addition, during crosstalk between NK cells and monocytes, NKp80 stimulated the release of proinflammatory cytokines from both cell types. Thus, by specifically bridging NK cells and myeloid cells, NKp80-AICL interactions may contribute to the initiation and maintenance of immune responses at sites of inflammation.