Cellular Mechanisms Controlling Surfacing of AICL Glycoproteins, Cognate Ligands of the Activating NK Receptor NKp80.

AICL glycoproteins are cognate activation-induced ligands of the C-type lectin-like receptor NKp80, which is expressed on virtually all mature human NK cells, and NKp80-AICL interaction stimulates NK cell effector functions such as cytotoxicity and cytokine secretion. Notably, AICL and NKp80 are encoded by adjacent genes in the NK gene complex and are coexpressed by human NK cells. Whereas AICL is intracellularly retained in resting NK cells, exposure of NK cells to proinflammatory cytokines results in AICL surfacing and susceptibility to NKp80-mediated NK fratricide. In this study, we characterize molecular determinants of AICL glycoproteins that cause intracellular retention, thereby controlling AICL surface expression. Cys87 residing within the C-type lectin-like domain not only ensures stable homodimerization of AICL glycoproteins by disulfide bonding, but Cys87 is also required for efficient cell surface expression of AICL homodimers and essential for AICL-NKp80 interaction. In contrast, cytoplasmic lysines act as negative regulators targeting AICL for proteasomal degradation. One atypical and three conventional N-linked glycosylation sites in the AICL C-type lectin-like domain critically impact maturation and surfacing of AICL, which is strictly dependent on glycosylation of at least one conventional glycosylation site. However, although the extent of conventional N-linked glycosylation positively correlates with AICL surface expression, the atypical glycosylation site impairs AICL surfacing. Stringent control of AICL surface expression by glycosylation is reflected by the pronounced interaction of AICL with calnexin and the impaired AICL expression in calnexin-deficient cells. Collectively, our data demonstrate that AICL expression and surfacing are tightly controlled by several independent cellular posttranslational mechanisms.

Modulation of NK cell function by genetically coupled C-type lectin-like receptor/ligand pairs encoded in the human natural killer gene complex.

Functional responses of natural killer (NK) cells including eradication of "harmful" cells and modulation of immune responses are regulated by a broad variety of activating and inhibitory NK receptors. Whereas the leukocyte receptor complex (LRC) encodes for NK receptors of the immunoglobulin superfamily, genes of C-type lectin-like NK receptors are clustered in the mammalian natural killer gene complex (NKC). Besides the thoroughly studied C-type lectin-like receptors NKG2D, CD94/NKG2x, and members of the murine Ly49 subfamily, the NKC also encodes for NK receptors of the less characterized NKRP1 subfamily. The prototypic mouse NKRP1 receptor is Nkrp1c (also known as NK1.1), while human members of the NKRP1 subfamily are NKRP1A, NKp80, and NKp65. The latter are not straight homologs of mouse NKRP1 receptors, but share distinct subfamily-specific traits classifying them as members of the NKRP1 subfamily. Ligands of the human NKPR1 receptors are likewise C-type lectin-like glycoproteins belonging to the CLEC2 subfamily (i.e., LLT1, AICL, and KACL), and are encoded in the NKC in tight genetic linkage to their respective receptors. Similarly, certain members of the mouse NKRP1 subfamily interact with genetically coupled CLEC2 glycoproteins, while the reasons for this intriguing tight genetic linkage remain unknown. Recent studies provided new and unique insights into the expression, interaction, and signaling of NKRP1 receptors and their ligands, thereby substantially advancing our understanding of their function and biology. Here, we review our current knowledge on NKRP1 receptors and their genetically linked CLEC2 ligands with an emphasis on the human receptor/ligand pairs NKRP1A-LLT1, NKp80-AICL, and NKp65-KACL.

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.

Interference with RUNX1/ETO leukemogenic function by cell-penetrating peptides targeting the NHR2 oligomerization domain.

The leukemia-associated fusion protein RUNX1/ETO is generated by the chromosomal translocation t(8;21) which appears in about 12% of all de novo acute myeloid leukemias (AMLs). Essential for the oncogenic potential of RUNX1/ETO is the oligomerization of the chimeric fusion protein through the nervy homology region 2 (NHR2) within ETO. In previous studies, we have shown that the intracellular expression of peptides containing the NHR2 domain inhibits RUNX1/ETO oligomerization, thereby preventing cell proliferation and inducing differentiation of RUNX1/ETO transformed cells. Here, we show that introduction of a recombinant TAT-NHR2 fusion polypeptide into the RUNX1/ETO growth-dependent myeloid cell line Kasumi-1 results in decreased cell proliferation and increased numbers of apoptotic cells. This effect was highly specific and mediated by binding the TAT-NHR2 peptide to ETO sequences, as TAT-polypeptides containing the oligomerization domain of BCR did not affect cell proliferation or apoptosis in Kasumi-1 cells. Thus, the selective interference with NHR2-mediated oligomerization by peptides represents a challenging but promising strategy for the inhibition of the leukemogenic potential of RUNX1/ETO in t(8;21)-positive leukemia.