Natural killer (NK) cells contribute to the first line of defense against viruses and to the control of tumor growth and metastasis spread. The discovery of HLA class I specific inhibitory receptors, primarily of killer Ig-like receptors (KIRs), and of activating receptors has been fundamental to unravel NK cell function and the molecular mechanisms of tumor cell killing. Stemmed from the seminal discoveries in early ‘90s, in which Alessandro Moretta was the major actor, an extraordinary amount of research on KIR specificity, genetics, polymorphism, and repertoire has followed. These basic notions on NK cells and their receptors have been successfully translated to clinical applications, primarily to the haploidentical hematopoietic stem cell transplantation to cure otherwise fatal leukemia in patients with no HLA compatible donors. The finding that NK cells may express the PD-1 inhibitory checkpoint, particularly in cancer patients, may allow understanding how anti-PD-1 therapy could function also in case of HLA class I neg tumors, usually susceptible to NK-mediated killing. This, together with the synergy of therapeutic anti-checkpoint monoclonal antibodies, including those directed against NKG2A or KIRs, emerging in recent or ongoing studies, opened new solid perspectives in cancer therapy.

Trophoblastic cells lack classical HLA class I and class II molecules but express HLA-G1. Although this may prevent allorecognition by maternal T cells, it renders trophoblastic cells potentially susceptible to lysis by natural killer (NK) cells. As shown here, only a fraction of peripheralblood NK cells in pregnant women express the HLA-G1-specific CD94͞NKG2A and͞or LIR-1 receptors. However, all NK cells isolated from maternal decidua during the first trimester expressed either one or both of these receptors. Perhaps more importantly, a fraction of cells expressed p49, an HLA-G1-specific inhibitory receptor, undetectable in peripheral-blood NK cells. p49 was expressed on virtually all NK cells isolated from placenta at term. Functional analyses revealed that the HLA class I-negative 221 lymphoblastoid cell line transfected with HLA-G1 was only partially protected from lysis by peripheral-blood NK cells isolated from pregnant women, whereas it was fully protected from decidual NK cells. As indicated by the addition of specific antibodies to cytolytic tests, all the above receptors contributed to HLA-G1 recognition by decidual NK cells, although p49 would appear to play a predominant role.

A small percentage of human T lymphocytes, predominantly CD8+ T cells, express receptors for HLA class I molecules of natural killer type (NK-R) that are inhibitory for T-cell antigen receptor (TCR)-mediated functions. In the present study, it is demonstrated that the various NK-R molecules typically expressed by NK cells are also expressed on periheral blood T lymphocytes. These CD3+ NK-R+ cells have a cell surface phenotype typical of memory cells as indicated by the expression of CD45RO and CD29 and by the lack of CD28 and CD45RA. Furthermore, by the combined use of anti-TCR Vf3-specific antibodies and a semiquantitative polymerase chain reaction assay, the TCR repertoire in this CD3+ NK-R+ cell subset was found to be skewed; in fact, one or two V8 families were largely represented, and most of the other Vj8s were barely detected. In addition, analysis of recombinant clones of the largely represented V,B families demonstrated that these Vf3s were oligoclonally or monoclonally expanded.

and CD8؉ cells expressed CD94, the simultaneous expression of NKG2A (i.e., the other component of the CD94͞NKG2A inhibitory NKR) was confined to CD8 ؉ cells. Similar data were obtained in T cell populations activated in mixed lymphocyte cultures in the presence of IL-15. The expression of CD94͞NKG2A led to an impairment of allo-specific cytolytic activity by mixed lymphocyte culture-derived T cell populations or clones. Remarkably, cytolysis could be restored by the addition of anti-CD94 mAb, i.e., by masking the inhibitory NKRs.

Natural killer (NK) cells express surface receptors for defined groups of HLA class I alleles. The specific interaction between these receptors and HLA class I molecules expressed on target cells results in inhibition of NK-mediated target cell lysis. In this report, we analyzed whether similar mechanisms were operating in cytolytic T lymphocytes (CTLs) capable of lysing NK-sensitive target cells. T cell clones were screened for their ability to lyse K562 target cells. The selected clones expressed either gamma delta or alpha beta TCR. The majority of these clones failed to lyse the HLA class I+ R8/15375 cell line; however, upon addition of the previously described A6-136 (IgM) or 6A4 F(ab')2 anti-HLA class I mAbs, target cells were efficiently lysed. Lysis of autologous phytohemagglutinin blasts in the presence of anti-HLA class I mAbs occurred primarily with TCR gamma delta+ CTLs. Recognition of HLA class I molecules on target cells implies the expression of NK-related specific receptors in CTL clones. Indeed, phenotypic analysis of > 300 CTL clones with NK-like activity revealed that 28% expressed p58 molecules (specific for HLA-C alleles) while 30% expressed CD94 molecules (specific for the Bw6 specificity). These receptor molecules were found to function as inhibitory receptors, as revealed by the effect of anti-p58 or anti-CD94 mAbs (of IgG isotype) on the lysis of the Fc gamma R+ K562 target cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Natural killer (NK) cells are the main lymphoid population in the maternal decidua during the first trimester of pregnancy. Decidual NK (dNK) cells display a unique functional profile and play a key role in promoting tissue remodeling, neoangiogenesis, and immune modulation. However, little information exists on their origin and development. Here we discovered CD34(+) hematopoietic precursors in human decidua (dCD34(+)). We show that dCD34(+) cells differ from cord blood- or peripheral blood-derived CD34(+) precursors. The expression of IL-15/IL-2 receptor common β-chain (CD122), IL-7 receptor α-chain (CD127), and mRNA for E4BP4 and ID2 transcription factors suggested that dCD34(+) cells are committed to the NK cell lineage. Moreover, they could undergo in vitro differentiation into functional (i.e., IL-8- and IL-22-producing) CD56(bright)CD16(-)KIR(+/-) NK cells in the presence of growth factors or even upon coculture with decidual stromal cells. Their NK cell commitment was further supported by the failure to undergo myeloid differentiation in the presence of GM-CSF. Our findings strongly suggest that decidual NK cells may directly derive from CD34(+) cell precursors present in the decidua upon specific cellular interactions with components of the decidual microenvironment.

Different HLA class I-specific killer inhibitory receptors (KIR) are expressed in vivo by a fraction of activated T cells, predominantly CD8+ , in which they may inhibit TCR-mediated cell functions. In an attempt to identify mechanisms leading to KIR expression in T cells, we analyzed the effect of transforming growth factor-g (TGF-g ) in T cells responding to bacterial superantigens in vitro. We show that TGF-g induces the expression of CD94/NKG2A in cells responding to toxic shock syndrome toxin 1 or to other staphylococcal superantigens. Remarkably, maximal CD94 expression occurred at (low) TGF-g concentrations which have no substantial effect on lymphocyte proliferation. Maximal CD94 expression occurred when TGF-g was added shortly after the cells were placed in culture. No expression could be induced in CD94/NKG2A-negative T cell clones. Although both CD4 + and CD8 + expressed CD94, the simultaneous expression of NKG2A was mostly confined to CD8 + cells. Monoclonal antibody-mediated cross-linking of CD94/NKG2A led to an impairment of T cell triggering via CD3, as determined in a redirected killing assay using the Fc + receptor-positive P815 murine target cells.

P75͞AIRM1 is a recently identified surface molecule that belongs to the sialoadhesin family and displays homology with the myeloid cell antigen CD33. In lymphoid cells, p75͞AIRM1 is confined to natural killer cells and mediates inhibition of their cytolytic activity. In this study, we show that p75͞AIRM1 is also expressed by cells of the myelomonocytic cell lineage, in which it appears at a later stage as compared with CD33. In vitro proliferation and differentiation of cord blood-derived CD34 ؉ cells (induced by stem cell factor and granulocyte-macrophage colony-stimulating factor) were consistently inhibited by the addition of anti-p75͞AIRM1 mAb. Engagement of CD33 led to inhibition in some experiments. A sharp decrease of cell proliferation͞survival was detected in all three p75͞AIRM1؉ chronic myeloid leukemias analyzed when cultured in the presence of either anti-p75͞AIRM1 or anti-CD33 mAbs. Thus, the present study suggests that p75͞AIRM1 and CD33 may play a regulatory role in normal myelopoiesis and may be viewed as suitable target molecules to counteract the proliferation͞survival of chronic myeloid leukemias.