Identification of a common T/natural killer cell progenitor in human fetal thymus

Identification of a Common Developmental Pathway for Thymic Natural Killer Cells and Dendritic Cells

Current data support the notion that the thymus is seeded by a yet uncommitted progenitor cell able to generate T cells, B cells, natural killer (NK) cells, and dendritic cells (DCs). We assess in this report the developmental relationship of DCs and NK cells derived from a small subset of CD34+ human postnatal thymocytes that, like the earliest precursors in the fetal thymus, display low CD33 surface expression. Culture of these isolated CD34+CD33lo thymic progenitors with a mixture of cytokines, including interleukin-7 (IL-7), IL-1α, IL-6, granulocyte-macrophage colony-stimulating factor, and stem cell factor, results in predominant generation of DCs. However, the addition of IL-2 to the cytokine mixture leads to the simultaneous development of DCs and NK cells. Both developmental pathways progress through a transient population of CD34+CD44brightCD5lo/−CD33+ large-sized cells, distinct from small-sized T-lineage precursors, that contain bipotential NK/DC progenitors. These data provide evidence of linked pathways of NK cell and DC development from intrathymic precursors and suggest that NK cells and DCs branch off the T lineage through a common intermediate progenitor.

Natural killer activating receptors trigger interferon gamma secretion from T cells and natural killer cells

Proliferation of human CD4+ alphabeta T cells expressing a natural killer cell activating receptor (NKAR) has been shown to be enhanced, particularly in response to low doses of antigen, if the target cells present appropriate human class I major histocompatibility complex (MHC) molecules. Here, we show that NKAR also enhance proliferation and killing of target cells by subsets of CD8+ alphabeta and CD8+ gammadelta T cells, as well as by NK cells. Strikingly, interferon gamma secretion from all of these types of lymphocytes was markedly increased by interaction of the NKAR with their MHC class I ligands, independently of enhancement of proliferation. Thus, the recognition of class I MHC molecules by NKAR on both T cells and NK cells may provide a regulatory mechanism that affects immune responses through the secretion of interferon gamma and possibly other cytokines. It represents a signal for cytokine secretion alternative and/or augmentative to that through the T cell receptor.

FLT-3 Ligand and Marrow Stroma-Derived Factors Promote CD3γ, CD3δ, CD3ζ, and RAG-2 Gene Expression in Primary Human CD34+LIN−DR− Marrow Progenitors

We hypothesize that early lymphoid commitment from primitive hematopoietic marrow progenitors is governed by signals from the marrow microenvironment leading to sequential induction of lineage-specific genes. Using expression of lymphoid genes as markers of differentiation, we characterize a highly purified population (>99.8% by double sorting) of primary human CD34+Lin−DR− progenitors. This population was then used to evaluate the effects of supplemental cytokines (interleukin-2 [IL-2], IL-3, IL-7, c-kit ligand), FLT-3 ligand (FL), and stroma-derived factors on lymphoid differentiation in vitro. CD3, RAG-1, Ikaros, CD10, and TdT transcripts were detected in the starting CD34+Lin−DR− population. By contrast, CD3γ, CD3δ, CD3ζ, and RAG-2 transcripts were not present in any samples tested. The presence of supplemental cytokines alone at culture initiation permitted stimulation of the expression of CD3ζ, but not of CD3γ or CD3δ. However, when FL and stroma-derived factors were added to cytokines, CD3 gene expression was induced in all samples. The predominant CD3 transcripts induced by optimal culture conditions were alternatively spliced isoforms lacking transmembrane sequences (CD3δ and CD3γ) and portions of the intracellular and extracellular domains (CD3γ). The combination of cytokines, FL, and stromal factors also provided a potent stimulus for RAG-2 gene expression. These findings show that FL in combination with stroma-derived factors provide important signals to promote early events required for lymphoid differentiation.

FLT-3 Ligand and Marrow Stroma-Derived Factors Promote CD3γ, CD3δ, CD3ζ, and RAG-2 Gene Expression in Primary Human CD34+LIN−DR− Marrow Progenitors

We hypothesize that early lymphoid commitment from primitive hematopoietic marrow progenitors is governed by signals from the marrow microenvironment leading to sequential induction of lineage-specific genes. Using expression of lymphoid genes as markers of differentiation, we characterize a highly purified population (>99.8% by double sorting) of primary human CD34+Lin−DR− progenitors. This population was then used to evaluate the effects of supplemental cytokines (interleukin-2 [IL-2], IL-3, IL-7, c-kit ligand), FLT-3 ligand (FL), and stroma-derived factors on lymphoid differentiation in vitro. CD3, RAG-1, Ikaros, CD10, and TdT transcripts were detected in the starting CD34+Lin−DR− population. By contrast, CD3γ, CD3δ, CD3ζ, and RAG-2 transcripts were not present in any samples tested. The presence of supplemental cytokines alone at culture initiation permitted stimulation of the expression of CD3ζ, but not of CD3γ or CD3δ. However, when FL and stroma-derived factors were added to cytokines, CD3 gene expression was induced in all samples. The predominant CD3 transcripts induced by optimal culture conditions were alternatively spliced isoforms lacking transmembrane sequences (CD3δ and CD3γ) and portions of the intracellular and extracellular domains (CD3γ). The combination of cytokines, FL, and stromal factors also provided a potent stimulus for RAG-2 gene expression. These findings show that FL in combination with stroma-derived factors provide important signals to promote early events required for lymphoid differentiation.

Acquisition of Ly49 receptor expression by developing natural killer cells

The formation of the repertoire of mouse natural killer (NK) cell receptors for major histocompatibility complex (MHC) class I molecules was investigated by determining the developmental pattern of Ly49 receptor expression. During the first days after birth, few or no splenic NK cells express Ly49A, Ly49C, Ly49G2, or Ly49I receptors. The proportion of Ly49+ splenic NK cells gradually rises to adult levels during the first 6-8 wk of life. The appearance of appreciable numbers of splenic Ly49+ NK cells coincides with the appearance of NK activity at 3-4 wk. After in vivo transfer, NK cells not expressing specific Ly49 receptors can give rise to NK cells that do, and cells expressing one of these four Ly49 receptors can give rise to cells expressing others. Once initiated, expression of a Ly49 receptor is stable for at least 10 d after in vivo transfer. Hence, initiation of Ly49 receptor expression occurs successively. Interestingly, expression of one of the receptors tested, Ly49A, did not occur after in vivo transfer of Ly49A- cells. One possible explanation for these data is that the order of Ly49 receptor expression by NK cells is nonrandom. The results provide a framework for evaluating models of NK cell repertoire formation, and how the repertoire is molded by host class I MHC molecules.

Expression of CD3 epsilon, CD3 zeta, and RAG-1/RAG-2 in decidual CD56+ NK cells

In early pregnancy the uterine mucosa, the decidua, is infiltrated by numerous CD56bright, CD16- natural killer (NK) cells. In this paper we have shown that unlike adult peripheral blood NK cells these uterine NK cells express cytoplasmic CD3 and CD3 zeta, but CD3 gamma and CD3 delta were not found in the majority of cells. In contrast to previous reports, there was no evidence of RAG-1 nor RAG-2 expression in decidual NK cells. These findings have implications for the ontogeny and development of decidual NK cells.

Natural Killer Cell Development and Function Precede αβ T Cell Differentiation in Mouse Fetal Thymic Ontogeny

Natural killer (NK) cells mediate MHC-unrestricted cytolysis of virus-infected cells and tumor cells. In the adult mouse, NK cells are bone marrow-derived lymphocytes that mature predominantly in extrathymic locations but have also been suggested to share a common intrathymic progenitor with T lymphocytes. However, mature NK cells are thought to be absent in mouse fetal ontogeny. We report the existence of thymocytes with a mature NK cell phenotype (NK1.1+/CD117−) as early as day 13 of gestation, approximately 3 days before the appearance of CD4+/CD8+ cells in T lymphocyte development. These mature fetal thymic NK cells express genes associated with NK cell effector function and, when freshly isolated, display MHC-unrestricted cytolytic activity in vitro. Moreover, the capacity of fetal thymic NK cells for sustained growth both in vitro and in vivo, in addition to their close phenotypic resemblance to early precursor thymocytes, confounds previous assessments of NK lineage precursor function. Thus, mature NK cells may have been inadvertently included in previous attempts to identify multipotent and bipotent precursor thymocytes. These results provide the first evidence of functional NK lymphocytes in mouse fetal ontogeny and demonstrate that NK cell maturation precedes αβ T cell development in the fetal thymus.

Development of Natural Killer Cells, B Lymphocytes, Macrophages, and Mast Cells From Single Hematopoietic Progenitors in Culture of Murine Fetal Liver Cells

We have recently established a clonal culture system that supports the growth of immature natural killer (NK) cells from murine fetal thymocytes. We now describe a culture system for mixed NK cell colony formation from single lymphohematopoietic progenitors. When Sca-1+c-kit+ fetal liver cells were cultured in methylcellulose media with interleukin (IL)-2, IL-7, IL-11, and steel factor (SF ), we found mixed colonies consisting of diffuse small round cells characteristic of immature NK cells and other types of cells. The single cell origin of the mixed colonies was established by micromanipulation. Individual mixed colonies derived from single cells were characterized by flow cytometric analysis and May-Grünwald Giemsa staining. All mixed colonies contained Thy-1+B220− cells, which can differentiate to mature NK cells in fetal thymus organ culture. Most of the colonies contained B220+ B-lineage cells and macrophages, and some contained mast cells. IL-1α and IL-3, which have previously been shown to inhibit the T- and B-cell potentials of blast colonies, suppressed the formation of mixed NK cell colonies. The clonal culture assay presented here may be useful in analysis of the developmental pathway and commitment of NK cells from multipotential progenitors.

Generation of lytic natural killer 1.1+, Ly-49- cells from multipotential murine bone marrow progenitors in a stroma-free culture: definition of cytokine requirements and developmental intermediates

We have developed a stroma-free culture system in which mouse marrow or thymus cells, known to be enriched for lymphoid progenitors, can be driven to generate natural killer (NK) cells. Culture of lineage marker (Lin)-, c-kit+, Sca2+, interleukin (IL)-2/15Rbeta (CD122)- marrow cells in IL-6, IL-7, stem cell factor (SCF), and flt3 ligand (flt3-L) for 5-6 d followed by IL-15 alone for an additional 4-5 d expanded the starting population 30-40-fold and gave rise to a virtually pure population of NK1.1+, CD3- cells. Preculture in IL-6, IL-7, SCF, and flt3-L was necessary for inducing IL-15 responsiveness in the progenitors because the cells failed to significantly expand when cultured in IL-15 alone from the outset. Although culture of the sorted progenitors in IL-6, IL-7, SCF, and flt3-L for the entire 9-11-d culture period caused significant expansion, no lytic NK1.1+ cells were generated if IL-15 was not added, demonstrating a critical role for IL-15 in NK differentiation. Thus, two distinct populations of NK progenitors, IL-15 unresponsive and IL-15 responsive, have been defined. Similar results were obtained with Lin-, CD44+, CD25-, c-kit+ lymphoid progenitors obtained from adult thymus. The NK cells generated by this protocol lysed the NK-sensitive target YAC-1 and expressed markers of mature NK cells with the notable absence of Ly-49 major histocompatibility complex (MHC) receptors. However, despite the apparent lack of these inhibitory MHC receptors, the NK cells generated could distinguish MHC class I+ from class I- syngeneic targets, suggesting the existence of novel class I receptors.

Inhibition of T cell and promotion of natural killer cell development by the dominant negative helix loop helix factor Id3

Bipotential T/natural killer (NK) progenitor cells are present in the human thymus. Despite their bipotential capacity, these progenitors develop predominantly to T cells in the thymus. The mechanisms controlling this developmental choice are unknown. Here we present evidence that a member(s) of the family of basic helix loop helix (bHLH) transcription factors determines lineage specification of NK/T cell progenitors. The natural dominant negative HLH factor Id3, which blocks transcriptional activity of a number of known bHLH factors, was expressed in CD34+ progenitor cells by retrovirus-mediated gene transfer. Constitutive expression of Id3 completely blocks development of CD34+ cells into T cells in a fetal thymic organ culture (FTOC). In contrast, development into NK cells in an FTOC is enhanced. Thus, the activity of a bHLH transcription factor is necessary for T lineage differentiation of bipotential precursors, in the absence of which a default pathway leading to NK cell development is chosen. Our results identify a molecular switch for lineage specification in early lymphoid precursors of humans.

The immunoregulatory effects of NK cells: the role of TGF-beta and implications for autoimmunity

The cytotoxic activities of natural killer (NK) cells--important in innate immunity--have received considerable attention, but NK cells also regulate T- and B-cell functions as well as hematopoiesis. Here, David Horwitz and colleagues focus on the capacity of NK cells to regulate antibody production positively and negatively, and in particular on the role of NK-cell transforming growth factor beta (TGF-beta) in downregulation of B-cell activity.

CD7+ and CD56+ Myeloid/Natural Killer Cell Precursor Acute Leukemia: A Distinct Hematolymphoid Disease Entity

The disease spectrum of natural killer (NK) cell leukemias and lymphomas has recently been expanding with the continuing evolution in diagnostic concepts. We describe here seven cases of acute leukemia of conceivable myeloid and NK cell precursor phenotype in six men and one woman varying from 19 to 59 years of age (median, 46 years). Striking extramedullary involvement was evident at initial presentation, with peripheral lymphadenopathy and/or mediastinal masses. Two lacked any leukemic cells in the bone marrow at diagnosis. Using cytochemical myeloperoxidase staining, less than 3% of the leukemic cells showed positive reactivity. However, expression of CD7, CD33, CD34, CD56, and frequently HLA-DR, but not other NK, T-cell, and B-cell markers was observed. Cytoplasmic CD3 was detected in three of the cases by flow cytometry and in six by Northern blotting, suggesting an origin from common progenitors between the NK cell and myeloid lineages. All but one presented germline configurations of the T-cell receptor β and γ chain genes and Ig heavy chain gene. With regard to morphology, the cells were generally L2-shaped, with variation in cell size, round to moderately irregular nuclei and prominent nucleoli, pale cytoplasm, and a lack of azurophilic granules. Histopathologic examination of biopsied specimens of extramedullary tumors showed a lymphoblast-like morphology, implying the differential diagnostic problem from lymphoblastic lymphomas, especially in cases lacking bone marrow involvement. Three patients were successfully treated with chemotherapy for acute myeloid leukemia (AML), whereas three other patients proved refractory to chemotherapeutic regimens for lymphoid malignancies, although two responded to subsequent AML chemotherapy. However, despite intensive chemotherapy, including allogeneic bone marrow transplantation, most persued fatal courses within 41 months. These data suggested that the CD7+ and CD56+ myeloid/NK cell precursor acute leukemia might constitute a distinct biologic and clinical disease entity. Its recognition appears to be particularly important for the clinicopathologic evaluation of CD56+ hematolymphoid malignancies and the development of therapeutic approaches to such disease.

Cord blood CD16+56- cells with low lytic activity are possible precursors of mature natural killer cells

Human natural killer (NK) cells are defined as being membrane CD3-, CD16+, and/or CD56+ lymphocytes; however, little is known about the ontogenic development and maturational pathways of human NK cells. The functional, phenotypic, and maturational characteristics of human umbilical cord blood (CB) NK cell subsets were studied to gain insight into the ontogenic and maturational pathways of human NK cells. We have previously shown that there is a novel subset of CD16+ CD56- NK cells present in CB. Here we further demonstrate differences in the expression of the NK-associated molecules CD2, CD7, CD8, and CD25 between CB and peripheral blood (PB) NK cells and between CB NK cell subsets. Although CB NK cell subsets were deficient in or had less lytic activity against K562 cells compared to PB NK cells, CB NK cells did possess the lytic molecules perforin and granzyme B and when artificially stimulated to secrete their granules during lytic assays, were capable of lytic activity equivalent to that of PB NK cells. Regardless of differences in phenotype and function of CB NK cell subsets, short-term and long-term incubation with cytokines induced functional (adult-like NK activity) and phenotypic (adult-like CD16+56+ or CD16-56+ surface antigen phenotype) maturation, respectively. Interleukin-2 (IL-2), IL-12, and IL-15, but not IL-7, interferon-gamma (IFN-gamma) nor tumor necrosis factor-alpha (TNF-alpha) induced functional and phenotypic maturation of CB NK cell subsets. Interestingly, culture of CB NK cell subsets with IL-2 or IL-15 led to acquisition of predominantly a CD16+56+ phenotype, while culture with IL-12 led to acquisition of both CD16+56+ and CD16-56+ phenotypes. Both functional and phenotypic maturation were not dependent upon proliferation. Studies using neutralizing anti-IFN-gamma and anti-TNF-alpha antibodies showed that survival and phenotypic maturation upon cytokine stimulation is influenced by endogenous production of TNF-alpha but not IFN-gamma. These results demonstrate that CB NK cell subsets are functionally and phenotypically immature but are capable of maturation. Additionally, CD16+56- NK cells are implicated as possible precursors of mature CD16+56+ and CD16-56+ NK cells.

CD7+ and CD56+ Myeloid/Natural Killer Cell Precursor Acute Leukemia: A Distinct Hematolymphoid Disease Entity

The disease spectrum of natural killer (NK) cell leukemias and lymphomas has recently been expanding with the continuing evolution in diagnostic concepts. We describe here seven cases of acute leukemia of conceivable myeloid and NK cell precursor phenotype in six men and one woman varying from 19 to 59 years of age (median, 46 years). Striking extramedullary involvement was evident at initial presentation, with peripheral lymphadenopathy and/or mediastinal masses. Two lacked any leukemic cells in the bone marrow at diagnosis. Using cytochemical myeloperoxidase staining, less than 3% of the leukemic cells showed positive reactivity. However, expression of CD7, CD33, CD34, CD56, and frequently HLA-DR, but not other NK, T-cell, and B-cell markers was observed. Cytoplasmic CD3 was detected in three of the cases by flow cytometry and in six by Northern blotting, suggesting an origin from common progenitors between the NK cell and myeloid lineages. All but one presented germline configurations of the T-cell receptor β and γ chain genes and Ig heavy chain gene. With regard to morphology, the cells were generally L2-shaped, with variation in cell size, round to moderately irregular nuclei and prominent nucleoli, pale cytoplasm, and a lack of azurophilic granules. Histopathologic examination of biopsied specimens of extramedullary tumors showed a lymphoblast-like morphology, implying the differential diagnostic problem from lymphoblastic lymphomas, especially in cases lacking bone marrow involvement. Three patients were successfully treated with chemotherapy for acute myeloid leukemia (AML), whereas three other patients proved refractory to chemotherapeutic regimens for lymphoid malignancies, although two responded to subsequent AML chemotherapy. However, despite intensive chemotherapy, including allogeneic bone marrow transplantation, most persued fatal courses within 41 months. These data suggested that the CD7+ and CD56+ myeloid/NK cell precursor acute leukemia might constitute a distinct biologic and clinical disease entity. Its recognition appears to be particularly important for the clinicopathologic evaluation of CD56+ hematolymphoid malignancies and the development of therapeutic approaches to such disease.

Cytokine-induced cytotoxic function expressed by lymphocytes of the innate immune system: distinguishing characteristics of NK and LAK based on functional and molecular markers

Several molecular events are now identifiable during the activation, recognition, and killing by natural killer (NK) cells that are distinct from those differentiated in response to cytokines during the generation of lymphokine-activated killer (LAK) cells or during lymphocyte proliferation. Because LAK and MHC-unrestricted killing activities also include the prototypic NK targets as part of their broad recognition spectra, accurate identification of the complete function being studied is critical. In many publications, past and present, only NK-sensitive target cells were used (K562, Molt-4, others), and, therefore, the results do not necessarily indicate whether the effectors are NK or have differentiated into LAK cells. Such a consideration becomes critical when the effectors are grown in interleukin-2 (IL-2), and an attempt is made to define receptor recognition, signal transduction pathways, and specificity at the molecular level. In some instances, effector cells are likely to have stopped, therefore merely expressing NK activity, and have also acquired LAK function. The identified receptors may not have been unique to NK cells or NK function. Not until the targets employed are also confirmed to be NK sensitive, and the effectors do not kill NK-resistant targets can the results of molecular studies be proposed to represent aspects unique to NK. Reports of the use of IL-2-expanded NK clones are most likely providing data concerning the biology of LAK and not of classic NK. The classic NK activity surveying our blood apparently performs an important function, including the ability to respond rapidly to certain cytokines and to acquire additional functions and receptors for use in destroying a vast array of target cells. It is critical for scientists to appreciate the functional distinctions and to identify the molecules and pathways unique to each of these curious cytolytic systems.

Identification of a novel developmental stage marking lineage commitment of progenitor thymocytes

Bipotent progenitors for T and natural killer (NK) lymphocytes are thought to exist among early precursor thymocytes. The identification and functional properties of such a progenitor population remain undefined. We report the identification of a novel developmental stage during fetal thymic ontogeny that delineates a population of T/NK-committed progenitors (NK1. 1(+)/CD117(+)/CD44(+)/CD25(-)). Thymocytes at this stage in development are phenotypically and functionally distinguishable from the pool of multipotent lymphoid-restricted (B, T, and NK) precursor thymocytes. Exposure of multipotent precursor thymocytes or fetal liver- derived hematopoietic progenitors to thymic stroma induces differentiation to the bipotent developmental stage. Continued exposure to a thymic microenvironment results in predominant commitment to the T cell lineage, whereas coculture with a bone marrow-derived stromal cell line results in the generation of mature NK cells. Thus, the restriction point to T and NK lymphocyte destinies from a multipotent progenitor stage is marked by a thymus-induced differentiation step.

Clonal Proliferation and Cytokine Requirement of Murine Progenitors for Natural Killer Cells

We have established a clonal cell culture system that supports the proliferation of committed natural killer (NK) cell progenitors of mice to investigate the pathway and cytokine regulation of NK cell development. Day 14 fetal thymocytes cultured in methylcellulose with interleukin-7 (IL-7), IL-15, and steel factor (SF ) formed diffuse colonies that could not be classified to known colony types. Single-cell origin of the colonies was established by micromanipulation of the colony-forming cells. Cells in the colonies are very blastic, showing no cytoplasmic differentiation, and express Ly5, Thy-1, and CD25 but not myeloid, B, mature T, or NK cell markers. The cells lack T, B, and myeloid potentials but can differentiate to mature NK cells in fetal thymus organ culture, suggesting that the colonies consist of NK committed progenitors. Examination of the minimal cytokine requirement for the NK colony formation showed that IL-7 and SF are indispensable for the formation of immature NK cell colonies. Both IL-2 and IL-15 increased the frequency of colonies. In contrast to IL-2, IL-7, and IL-15, IL-4 strongly inhibited the formation of the colonies. This quantitative clonal culture will provide a useful means to examine the mechanism of NK cell development.

Interleukin-15-induced maturation of human natural killer cells from early thymic precursors: selective expression of CD94/NKG2-A as the only HLA class I-specific inhibitory receptor

Immature postnatal thymocytes were shown to contain precursors which, under suitable culture conditions, give rise to phenotypically and functionally mature natural killer (NK) cells. Here, we analyzed the effect of different cytokines for their ability to induce the expression of HLA class I-specific inhibitory receptor(s) during the process of NK cell development from immature thymocytes. From thymocyte cell suspensions depleted of CD2+, CD3+, CD4+, CD8+, CD56+, and CD16+ cells, we further removed cells expressing HLA class I-specific inhibitory receptors including CD94/NKG2-A, p58.1, and p58.2 by immunomagnetic bead separation. The resulting cells did not contain any of the above NK receptors as determined by immunofluorescence and flow cytometric analysis, as well as by reverse transcriptase polymerase chain reaction (RT-PCR) amplification using appropriate sets of primers. Although different cytokines have been used, including interleukin (IL)-7, stem cell factor (SCF), IL-2, and IL-15, only IL-2 or IL-15 induced cell proliferation when used alone. Moreover, maturation towards CD3- CD56+ cells displaying cytolytic activity against the HLA class I- targets K562 or 221 was detectable in cultures containing IL-15 used alone or in combination with IL-7 or SCF. On the other hand, these CD3- CD56+ cell populations did not lyse HLA class I+ target cells, including autologous PHA blasts. Analysis of the expression of the various HLA class I-specific inhibitory NK receptors revealed the presence of high proportions of CD94/ NKG2-A+ cells, while the NK receptors belonging to the Ig superfamily were undetectable both by immunofluorescence and by RT-PCR analysis. The expression of CD94/NKG2-A appeared to be responsible for the inability of cells to lyse HLA class I+ target cells. Thus, addition of anti-CD94 monoclonal antibodies of IgM isotype resulted in lysis of autologous target cells. The use of 221 cells transfected with different HLA class I alleles as target cells confirmed the broad class I specificity of CD94/NKG2-A receptor. Our experiments indicate that IL-15 provides an appropriate stimulus to the expression of CD94/NKG2-A, but not of other class I-specific NK receptors in the process of maturation of NK cells from thymocyte precursors.

Development and function of T lymphocytes and natural killer cells after bone marrow transplantation of severely immunodeficient mice

Bone marrow (BM) transplantation experiments were performed in a strain of CD3 epsilon-transgenic mice, termed tg epsilon 26, which are completely deficient in T-cell and natural killer (NK) cell development. We found that an interaction of stromal cells and prothymocytes is required for the induction of a cortical thymic microenvironment. This induction takes place in a time window from fetal development to early neonates. Although the thymic environment is not required for NK-cell development, we found that aberrantly educated alpha beta or gamma delta T lymphocytes can influence NK-cell ontogeny. Surprisingly, BM transplantation of tg epsilon 26 fetuses and neonates results in normal T-cell development, but very low levels of NK cells. The poor NK-cell reconstitution in fetal and neonatal stages could be explained by an inefficient migration of hematopoietic progenitor cells to the BM. By contrast, migration of the progenitor cells to the thymus was efficient to initiate T-cell development. BM transplantation of adult tg epsilon 26 mice resulted in abnormal T-cell development which, in turn, caused an inflammatory bowel disease (IBD) in the recipient mice. Studies in these BM chimeras have revealed that both alpha beta and gamma delta T cells can be pathogenic and, further, that Th1-like cytokines produced by these cells are causal factors in the pathogenesis of IBD.

Damselfish with neurofibromatosis exhibit cytotoxicity towards retrovirus infected cells

Lymphocytes from tumor-bearing damselfish are cytotoxic towards target cell lines derived from damselfish neurofibromatosis. These cell lines contain at least one retrovirus which appears to be related to the etiology of the disease. The current studies were designed to characterize the effectors of this cytotoxic reaction. Data presented here show that cells separated using an antibody (5C6.10.4) directed towards non-specific cytotoxic cells of catfish sequesters all antitumor activity in the 5C6.10.4 negative population. Thus, damselfish 5C6.10.4 positive cells bind to tumor targets, but do not contribute to target cell death. In contrast, 5C6.10.4 positive cells are cytotoxic towards xenogeneic erythrocytes. Cytotoxicity of splenocytes from animals inoculated with virus purified from the 88-503 cell line suggested that prior exposure to the retrovirus enhanced reactivity, especially towards 88-503. In addition, cytotoxicity was significantly greater in tumor homogenate injected animals that resisted tumor development for more than 5 months as compared to those that developed tumors quickly. Lastly, cytotoxic responsiveness towards primary cultures of mock and virus infected autologous and allogeneic cells implies that the cytotoxic effector is directed towards retrovirus infected cells.

Natural killer cell lymphoma/leukemia: pathology and treatment

Malignancies arising from cells of putative natural killer (NK) cell origin have increasingly been recognized as distinct clinicopathological entities. These malignancies are marked by tumour cells with NK cell characteristics, including the immunophenotype of CD2+, surface CD3-, cytoplasmic CD3 epsilon+, CD7 +/-, and CD56+, and the genotype of germline T cell receptor gene. A consistent association with monoclonal Epstein-Barr virus infection in the tumour cell has been observed. These tumours are now regarded as putative NK cell lymphoma/leukemia. Pathologically, tumour cells show variable cytological appearances, with frequent angiocentricity and angioinvasion, associated with zonal necrosis. Clinically, most cases occur in the nasal area and upper aerodigestive tract. However, occurrence in non-nasal sites such as the skin, gastrointestinal tract and testis is also observed. A particularly aggressive form of NK lymphoma/leukemia presents fulminantly as disseminated disease sometimes with a leukemic phase. All types of NK lymphoma/leukemia have an extremely poor prognosis with a median survival of less than a year. New modalities of treatment, including the use of high dose chemotherapy and stem cell rescue may be needed to improve treatment outcome.

The binding site of NK receptors on HLA-C molecules

The protection of cells expressing class I HLA molecules from NK lysis is mediated by natural killer cell inhibitory receptors (NKIR). Using site-directed mutagenesis, residues on HLA-C that determine the locus specificity (alphaVal-76), allotype group specificity (a dimorphism alphaAsn-80/Lys-80), and affinity of NKIR binding (a second pair of dimorphisms, alphaAla-73, Asp-90 or alphaThr-73, Ala-90) have been identified. Thus the "footprint" of the NKIR on the alpha1 helix of the class I MHC molecule HLA-C and its associated beta strands are similar in position to the site occupied by superantigens on and behind the alpha1 helix of the class II MHC molecule HLA-DR1, but further toward its C-terminus. The intermediate affinity binding of NKIR to HLA-C, determined by alpha73 and alpha90, has an essential role in preventing cross-reactivity and ensuring the availability of NK cells for immunosurveillance; low affinity and high affinity mutants are both physiologically impaired.

Phenotypic and Functional Evidence for the Expression of CD4 by Hematopoietic Stem Cells Isolated From Human Fetal Liver

Expression of the CD4 antigen was observed on human fetal liver, fetal bone marrow (BM), and umbilical cord blood progenitors expressing high levels of CD34. Using clonal and liquid-culture assays, CD4+ CD34++ Lin− (lineage = CD3, CD8, CD10, CD14, CD15, CD16, CD19, CD20, and glycophorin A) fetal liver progenitors were found to have a greater proliferative potential than CD4− CD34++ Lin− progenitors, whereas the CD4− fraction was more enriched for erythroid progenitors. Both the CD4+ and the CD4− progenitor subpopulations also gave rise to multilineage engraftment upon transplantation into human fetal bone fragments, supportive of B-lymphoid and myeloid growth, or into human fetal thymic fragments, supportive of T-cell growth, implanted in scid/scid (SCID) mice. However, in SCID-hu mice transplanted with graded doses of donor cells ranging from 2.0 × 102 to 2.0 × 104 cells, BM reconstitution by the CD4+ fraction of CD34++ Lin− cells was more frequent than by the CD4− fraction when low numbers of cells were injected. These functional data strongly suggest that stem cells reside among CD4+ CD34++ Lin− fetal liver cells. This hypothesis was further supported by the observations that CD4+ CD34++ Lin− fetal liver cells were enriched for CDw90+ (Thy-1), CD117+ (kit), CD123+, HLA-DR+, CD7−, CD38−, CD45RA−, CD71−, CD115− (fms), and rhodamine 123dull cells, a phenotypic profile believed to represent fetal stem cells. Furthermore, all CD4+ CD34++ Lin− fetal liver cells also expressed CD13 and CD33.

Abnormal development of intestinal intraepithelial lymphocytes and peripheral natural killer cells in mice lacking the IL-2 receptor beta chain

The interleukin-2 receptor beta chain (IL-2R beta) is expressed on a variety of hematopoietic cell types, including natural killer (NK) cells and nonconventional T lymphocyte subsets such as intestinal intraepithelial lymphocytes (IEL). However, the importance of IL-2R beta-mediated signaling in the growth and development of these cells has yet to be clearly established. We have investigated IEL and NK cells in mice deficient for IL-2R beta and describe here striking defects in the development of these cells. IL-2R beta-/- mice exhibited an abnormal IEL cell population, characterized by a dramatic reduction in T cell receptor alpha beta CD8 alpha alpha and T cell receptor gamma delta lymphocytes. This selective decrease indicates that IEL can be classified into those whose development and/or differentiation is dependent on IL-2R beta function and those for which IL-2R beta-mediated signaling is not essential. NK cell development was also found to be disrupted in IL-2R beta-deficient mice, characterized by a reduction in NK1.1+CD3- cells in the peripheral circulation and an absence of NK cytotoxic activity in vitro. The dependence of NK cells and certain subclasses of IEL cells on IL-2R beta expression points to an essential role for signaling through this receptor, presumably by IL-2 and/or IL-15, in the development of lymphocyte-subsets of extrathymic origin.

Lymphoblastic lymphoma expressing natural killer cell phenotype with involvement of the mediastinum and nasal cavity

Most CD56+ lymphomas display polymorphic and angiocentric/angiodestructive histologic features and are closely related to Epstein-Barr virus (EBV) infection. We report a 47-year-old Japanese man with CD56+ lymphoma that showed histologic features of lymphoblastic lymphoma with mediastinal and nasal involvement and an aggressive course. A sample specimen showed the histology of lymphoblastic lymphoma with a positive reaction for terminal deoxynucleotidyl transferase (TdT) but no angiocentric/angiodestructive features. Transmission electron microscopy revealed a few membrane-bound electron-dense granules in their cytoplasm. Immunohistochemically, lymphoma cells exhibited CD56+ cytoplasmic CD3 (cCD3)+ TdT+. A Southern blot analysis showed no integration of EBV and human T-lymphotrophic virus 1 (HTLV-1) and no rearrangement of the T-cell receptors or immunoglobulin heavy chain genes. This unusual lymphoblastic lymphoma exhibiting cCD3 + CD56 + TdT + TCR- is assumed as an immature or progenitor natural killer cell lineage.

Structure and function of the CD94 C-type lectin receptor complex involved in recognition of HLA class I molecules

A multigene family of immunoglobulin superfamily (Ig-SF) killer cell inhibitory receptors (KIRs) specifically recognize HLA class I molecules, while the interaction with H-2 products is mediated by members of the murine Ly49 C-type lectin family. A common structural feature of these receptors with inhibitory function is the presence of cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that couple them to SHP phosphatases. Strong support for the involvement of the CD94 C-type lectin receptor complex in NK cell-mediated recognition of Bw6+ HLA-B, HLA-A and HLA-C alleles has been obtained. The cloned CD94 molecule covalently assembles with at least two different glycoproteins (43 kDa and 39 kDa) to form functional receptors. NK cells inhibited upon HLA recognition express the CD94/p43 dimer, whose specificity for HLA molecules partially overlaps the Ig-SF receptor system. By contrast, NK clones bearing the homologous CD94/p39 receptor are triggered upon its ligation by CD94-specific mAbs. Remarkably, a set of Ig-SF receptors (p50) homologous to p58 KIRs also display an activating function. CD94-associated molecules belong to the NKG2 family of C-type lectins; the NKG2-A gene encodes for the p43 subunit, which contains cytoplasmic ITIMS. Expression of the different CD94 heterodimeric receptors will enable precise analysis of their putative interaction with HLA class I molecules.

Human NK cells: their ligands, receptors and functions

The expression, or lack thereof, of class I MHC glycoproteins has a marked influence on natural killer cell function. Cells which do not express class I MHC molecules are susceptible to lysis by NK cells, and transfection of these targets with class I MHC genes can render these cells resistant to NK attack. This inhibition of NK-killing is mediated by a novel family of receptors expressed mainly on NK cells, but also found on some T-cells. The function of these class I MHC binding receptors when expressed on T-cells is discussed also and a novel co-stimulatory activity of NKAR described. Lastly, a novel mechanism by which human cytomegalovirus evades immune surveillance by NK cells is documented.

Peripheral expression of Jak3 is required to maintain T lymphocyte function

The Jak family tyrosine kinase, Jak3, is involved in signaling through cytokine receptors that utilize the common gamma chain (gammac), such as those for IL-2, IL-4, IL-7, IL-9, and IL-15. Recent studies of Jak3-deficient mice and humans have demonstrated that Jak3 plays a critical role in B and T lymphocyte maturation and function. The T lymphocyte defects in Jak3-deficient mice include a small thymus, a decrease in peripheral CD8+ cells, an increase in the surface expression of activation markers, and a severe reduction in proliferative and cytokine secretion responses to mitogenic stimuli. To determine whether the peripheral T lymphocyte defects result from aberrant maturation in the thymus or from the absence of Jak3 protein in peripheral T cells, we generated reconstituted mice that express normal levels of Jak3 protein in the thymus but lose Jak3 expression in peripheral T cells. Jak3 expression in the thymus restores normal T cell development, including CD8+, gammadelta, and natural killer cells. However, the loss of Jak3 protein in peripheral T cells leads to the Jak3-/- phenotype, demonstrating that Jak3 is constitutively required to maintain T cell function.

Delayed maturation of CD4- CD8- Fc gamma RII/III+ T and natural killer cell precursors in Fc epsilon RI gamma transgenic mice

Fc epsilon RI gamma (gamma) is a member of a group of related proteins (the zeta-family dimers) that function as signal-transducing components of both Fc receptors and the T cell antigen receptor (TCR). Analysis of gamma expression during fetal thymus ontogeny revealed that it is expressed in early thymocytes, before the initiation of clonotypic TCR-alpha and TCR-beta gene rearrangement but is down-regulated in most adult thymocytes. To explore a possible role for gamma in thymocyte development, we generated transgenic mice in which this protein was overexpressed at all stages of ontogeny. Overexpression of gamma inhibited the maturation of T cells as well as natural killer (NK) cells. The developmental effects were transgene dose related and correlated with markedly delayed maturation of fetal CD4-CD8- FcRII/III+ thymocytes, cells thought to include the progenitors of both T and NK cells. These results suggest that the zeta and gamma chains serve distinctive functions in thymocyte development and indicate that Fc receptor(s) may play an important role in regulating the differentiation of early progenitor cells within the thymus.

Essential requirement of an invariant V alpha 14 T cell antigen receptor expression in the development of natural killer T cells

NK1.1+ T [natural killer (NK) T] cells express an invariant T cell antigen receptor alpha chain (TCR alpha) encoded by V alpha 14 and J alpha 281 segments in association with a limited number of V betas, predominantly V beta 8.2. Expression of the invariant V alpha 14/J alpha 281, but not V alpha 1, TCR in transgenic mice lacking endogenous TCR alpha expression blocks the development of conventional T alpha beta cells and leads to the preferential development of V alpha 14 NK T cells, suggesting a prerequisite role of invariant V alpha 14 TCR in NK T cell development. In V beta 8.2 but not B beta 3 transgenic mice, two NK T cells with different CD3 epsilon expressions, CD3 epsilon(dim) and CD3 epsilon(high), can be identified. CD3 epsilon(high) NK T cells express surface V alpha 14/V beta 8 TCR, indicating a mature cell type, whereas CD3 epsilon(dim) NK T cells express V beta 8 without V alpha 14 TCR and no significant CD3 epsilon expression (CD3 epsilon(dim)) on the cell surface. However, the latter are positive for recombination activating gene (RAG-1 and RAG-2) mRNA, which are only expressed in the precursor or immature T cell lineage, and also possess CD3 epsilon mRNA in their cytoplasm, suggesting that CD3 epsilon(dim) NK T cells are the precursor of V alpha 14 NK T cells.

Differential effects of interleukin-15 and interleukin-2 on differentiation of bipotential T/natural killer progenitor cells

Bipotential T/natural killer (NK) progenitor cells are destined to differentiate mainly into T cell receptor (TCR) alpha beta and TCR gamma delta cells in a thymic microenvironment, whereas extrathymically they selectively develop into NK cells. The exact environmental conditions that are required for differentiation into these three leukocyte populations are largely unknown. In this report, we have investigated and compared the effect of interleukin (IL)-15 and IL-2 in this process. The IL-15 receptor is composed of the gamma and beta chains of the IL-2 receptor (IL-2R gamma and IL-2R beta) and of a specific alpha chain (IL-15R alpha). Here, it is shown that IL-15 mRNA is mainly expressed in thymic epithelial stromal cells, whereas IL-2 mRNA is exclusively expressed in thymocytes. IL-2R beta-expressing cells were present in the fetal thymus with a CD25-CD44+Fc gamma R+HSA-/low TCR- phenotype, which is characteristic of progenitor cells. These cells also expressed IL-15R alpha messenger RNA. Sorted IL-2R beta + TCR- cells differentiated into TCR alpha beta and TCR gamma delta cells after transfer to alymphoid thymic lobes, whereas culture of the same sorted cells in cell suspension in the presence of IL-15 resulted in the generation of functional NK cells. This shows that IL-2R beta +TCR- cells of the fetal thymus contain bipotential T/NK progenitors. Addition of low concentrations of IL-15 to fetal thymic organ culture (FTOC) resulted in an increase of all T cell subpopulations. The largest expansion occurred in the TCR gamma delta compartment. In contrast, low concentrations of IL-2 did not result in a higher total cell number and did not induce outgrowth of TCR gamma delta cells. High concentrations of IL-15 blocked TCR alpha beta development and shifted differentiation towards NK cells. Differentiation towards TCR gamma delta cells still proceeded. High concentrations of IL-2 similarly induced development into NK cells, but the cell number was fourfold lower than in IL-15 cultures. Importantly, blocking of IL-2R alpha in IL-2-treated FTOC resulted in a drastic increase in cell number, indicating that IL-2R alpha negatively regulates cell expansion. Collectively, these experiments provide direct evidence that IL-15 and IL-2 differentially affect the differentiation of bipotential T/NK progenitors.

Human thymocyte responsiveness to stem cell factor: synergy with interleukin-2 for the generation of NK/LAK cytotoxicity

Human recombinant stem cell factor (SCF) increases the viability and cell size of a subset of thymocytes in vitro, but does not independently induce phenotypic changes on thymocytes indicative of T cell differentiation. The SCF-responsive thymocytes have characteristics of large granular cells, that do not express T, B or NK cell-related antigens, and are primarily found in immature thymocyte subsets. These large granular thymocytes do not display cytotoxic activity. However, SCF acts synergistically with IL-2 in the generation of cytotoxic effector cells from thymocyte precursors. Synergy in cytotoxicity is observed to both NK-sensitive and NK-resistant targets. Studies of the SCF receptors on thymocytes show that receptors are expressed on mature 'bright' CD3+ cells, immature 'dim' CD3+ cells as well as CD3- cells. IL-2 increases the frequency of SCF receptor-positive cells in cultured thymocytes, which may explain its synergy with SCF in the generation of NK/LAK cytotoxicity. These data show that SCF enhances the functional development of thymic NK/LAK cells in vitro.

Differences in T-cell-receptor gene rearrangement and transcription in nasal lymphomas of natural killer and T-cell types: implications on cellular origin

Although nasal lymphomas showing midfacial destructive lesions had been classified as T-cell lymphomas, their exact cellular origin is still unclear. Although they usually express a restricted number of T-cell-related antigens, namely, CD2, CD43, and CD45RO, other pan-T or subset-T-lineage antigens, such as CD3 (membrane), CD5, CD4, CD8, and CD7, are frequently absent. Conversely, they often express a natural killer (NK) cell-associated antigen, CD56, but lack other mature NK markets. To study their cellular origin further, the authors analyzed T-cell receptor (TCR) gene transcription in three cases of nasal lymphomas and correlated the findings with the phenotype and gene rearrangement data. Two cases of nasal lymphomas with CD2+CD3(Leu4)-CD19-CD56+ phenotype were shown to express truncated 1.0-kb Tbeta and multiple unrearranged Tdelta transcripts with germline TCR beta, gamma, delta, and immunoglobulin heavy-chain joining region (JH) genes, consistent with NK cell lineage. In contrast, one case of nasal lymphoma with CD2+CD3(Leu4)+CD8+CD19-CD56+ phenotype expressed full-length Talpha, Tbeta, and Tgamma transcripts rearranged TCR beta, gamma, and deleted TCR delta genes, indicating T-lineage, These results support the view that nasal lymphomas can separated into NK-cell and T-cell neoplasms, based on differences genotypic characteristics. The possibility of these tumors being derived from a putative common precursor cell merits further investigation.

Thymic lymphoblastic lymphoma of committed natural killer cell precursor origin: a case report

BACKGROUND

Recently, it was demonstrated that the human fetal thymocyte contains a bipotential progenitor capable of both T lymphocyte and natural killer (NK) cell differentiation. However, prior to this report a malignant neoplasm arising from these cells had not been documented.

METHODS

A Japanese female age 38 years was examined by morphology of light and electron microscopy, immunohistochemistry, 3-color flow cytometry, cytotoxic assay, and Southern blotting.

RESULTS

The patient presented with a mediastinal mass and pleural effusion. Leukemic progression was identified following chemotherapy and complete clinical remission. Immunophenotyping of lymphoma revealed CD45++, c-kit dim+, terminal deoxynucleotidyl transferase (TdT)-<+, CD38++, CD34+<++, CD33+<-, CD13dim+approximately+, HLA-DR+, CD7+, cytoplasmic CD3 (cCD3)+, surface CD3 (sCD3)-, CD2dim+, CD56+, CD16-, CD11b+, CD57-, CD1a-, CD5-, TCR alpha beta-, TCR gamma delta-, CD4-, CD8-, CD28-, CD10-, CD19-, CD20-, CD22-, surface immunoglobulins-, and CD14-. Functional NK activity of the lymphoma cells was extremely low. DNA analysis revealed no gene rearrangement in TCR beta, gamma, and delta or immunoglobulin heavy and light chain genes.

CONCLUSIONS

Lymphoma cells of this case were derived from a distinct subtype of lymphocyte that originate from a thymic precursor committed to NK cell differentiation. This category is different from those of thymic T or precursor B cell pheno-/genotype.

Murine natural killer cell differentiation: past, present, and future

Natural killer cells are bone marrow-derived lymphocytes capable of lysing a variety of target cells without prior exposure. While the biological activities and function of mature NK cells have been extensively investigated, the differentiation of NK cells from primitive hematopoietic stem cells is poorly understood. Recently, we have reported on the identification of a highly enriched bone marrow population capable of repopulating recipient mice with mature NK cells. In this review, we will summarize our findings and those of others in an attempt to clarify the current status of murine natural killer cell differentiation.

Continuous ambulatory peritoneal dialysis impairs T lymphocyte selection in the peritoneum

Peritoneal lymphocytes (PCL) of 45 healthy individuals, four uremic patients with end-stage renal disease (ESRD) and 25 long-term continuous ambulatory peritoneal dialysis (CAPD) patients were characterized by flow cytometry to investigate whether CAPD alters the phenotype of PCL. B lineage cells constitute a minority of PCL (2.5% of cells). Although the majority of peritoneal T cells expressed alpha beta T cell receptor (TcR), 7% expressed gamma delta TcR, a proportion which was significantly higher than that in peripheral blood (PBMC) (approximately 4%). The majority of PCL T cells exhibited markers of the thymus-dependent lineage (CD2, CD3, TcR alpha beta, CD8 alpha beta or CD4) and surface antigens associated with memory and activation (CD45RO, CD11a, CD18, CD49d, HLA-DR). An average of 75% of both CD4+ and CD8+ PCL T cells of healthy subjects and CAPD patients were CDw60+, thus characterizing the T cell subset containing the helper activity for the mitogen-driven B cell differentiation. CD44s was abundantly expressed on PCL T cells. In contrast to PCL T cells of healthy subjects peritoneal T lymphocytes of CAPD patients exhibited CD44 splice variants containing products of exon-v9 and the proportion of CD44v9+ cells correlated with the frequency of peritonitis episodes the patients had gone through. The majority of PCL T cells of both healthy subjects and CAPD patients were CD8+. A large proportion of CD8+ PCL T cells from healthy subjects expressed the homodimeric CD8 alpha alpha isoform; however, such cells were not found in CAPD patients. In healthy subjects mRNA for the recombination activating gene 1 (RAG-1) was detectable in a PCL population containing CD7-CD34+ and CD7+CD34+ cells. In contrast, neither mRNA transcripts of the RAG-1 gene nor CD34+ cells were detectable in PCL of CAPD patients.

Expression of functional molecules by human CD3- decidual granular leucocyte clones

Cell surface and cytoplasmic antigen expression by 35 CD3- decidual granular leucocyte (DGL) clones, derived from human endometrial tissue in the first trimester of pregnancy, has been compared with both that of fresh CD3- decidual leucocytes and that of CD3- peripheral blood natural killer (PBNK) cell clones (n = 12). The majority of DGL clones retained the antigenic phenotype of fresh cells, although CD103 (HML-1) was expressed on 50% of DGL clones but only 17% of fresh DGL. Both cytoplasmic CD3 zeta and CD3 epsilon chains were detected in > 90% of DGL clones in the absence of cell surface CD3. Cytoplasmic CD3 zeta was present in almost all fresh CD3- DGL, whereas CD3 epsilon was not. Most DGL clones did not express surface Fc gamma receptors I-III (CD64, -32 and -16, respectively) and complement receptors (CR) types 1 and 2 (CD35 and 21, respectively), but 43% expressed CR3 (CD11b/18); in contrast, all PBNK clones were CR3+. The NK cell-associated molecules Kp43 (CD94) and the p58 molecule recognized by the HP3E4 monoclonal antibody were both present on a higher proportion of CD3- PBNK (91% and 50%, respectively) than DGL clones (31% and 14%, respectively), despite expression of CD94 by > 90% of fresh CD56+ decidual leucocytes. Five of 35 CD3- DGL clones expressed cytoplasmic CD3 zeta in the absence of expression of CD2, CD16 or the p58 molecule recognized by HP3E4. These variations between CD3- DGL and PBNK cell clones in expression of functional molecules may be related to previously reported differences in major histocompatibility complex-non-restricted cytotoxic activities between these two cell types.

Receptors for HLA class-I molecules in human natural killer cells

Natural killer cells are likely to play an important role in the host defenses because they kill virally infected or tumor cells but spare normal self-cells. The molecular mechanism that explains why NK cells do not kill indiscriminately has recently been elucidated. It is due to several specialized receptors that recognize major histocompatibility complex (MHC) class I molecules expressed on normal cells. The lack of expression of one or more class I alleles leads to NK-mediated target cell lysis. During NK cell development, the class I-specific receptors have adapted to self-class I molecules on which they recognize epitopes shared by groups of class I alleles. As such, they may fail to recognize either self-molecules that bound unusual peptides or allogeneic class I molecules unrelated to self-alleles. Different types of receptors specific for groups of HLA-C or HLA-B alleles have been identified. While in most instances, they function as inhibiting receptors, an activating form of the HLA-C-specific receptors has been identified in some donors. Molecular cloning of HLA-C- and HLA-B-specific receptors has revealed new members of the immunoglobulin superfamily with two or three Ig-like domains, respectively, in their extracellular portion. While the inhibiting form is characterized by a long cytoplasmic tail associated with a nonpolar transmembrane portion, the activating one has a short tail associated with a Lys-containing transmembrane portion. Thus, these human NK receptors are different from the murine Ly49 that is a type II transmembrane protein characterized by a C type lectin domain. A subset of cytolytic T lymphocytes expresses NK-type class I-specific receptors. These receptors exert an inhibiting activity on T cell receptor-mediated functions and offer a valuable model to analyze the regulatory mechanisms involved in receptor-mediated cell activation and inactivation.

Epstein-Barr virus in non-Hodgkin's lymphomas of the upper respiratory tract: association with sinonasal localization and expression of NK and/or T-cell antigens by tumour cells

Fifty-five cases of non-Hodgkin's lymphoma (NHL) of the upper respiratory tract, comprising 27 sinonasal (SN) and 28 Waldeyer's ring (WR) NHL, were investigated for expression of Epstein-Barr virus (EBV)-encoded EBER transcripts and latent membrane protein-1 (LMP-1) by RNA in-situ hybridization (RISH) and immunohistochemistry, respectively. Thirty-two cases were B-cell tumours (10 SNHLs and 22 WRNHLs) and 23 cases expressed natural killer (NK) and/or T-cell antigens (17 SNHLs and 6 WRNHLs). EBER transcripts were detected in tumour cells in 19 lymphomas expressing NK and/or T-cell antigens (16/17 SHNHLs and 3/6 WRNHLs) but in only 2/32 B-NHLs (1/10 SNHLs and 1/22 WRNHLs). LMP-1 expression was found in tumour cells in the 19 EBER-positive tumours expressing NK and/or T-cell antigens but in none of the B-cell lymphomas. All the LMP-1-positive lymphomas expressed the CD30 molecule in tumour cells. These results indicate that in lymphomas of the upper respiratory tract, EBV is strongly associated with sinonasal localization and expression of NK and/or T-cell antigens by tumour cells. EBV can also be detected in some cases of WRNHLs expressing NK and/or T-cell antigens, whereas it is rarely found in B-cell SNHLs and WRNHLs. Furthermore, the detection of the LMP-1 protein in tumour cells in most SNHLs and some WRNHLs expressing NK and/or T-cell antigens, in view of the LMP-1 transforming potential, suggests that EBV may play a role in the pathogenesis of these lymphomas.

CD16-mediated p21ras activation is associated with Shc and p36 tyrosine phosphorylation and their binding with Grb2 in human natural killer cells

The Src homology (SH) 2/SH3 domain-containing protein Grb2 and the oncoprotein Shc have been implicated in a highly conserved mechanism that regulates p21ras activation. We investigated the involvement of these adaptor proteins in the signaling pathway induced by CD16 or interleukin (IL) 2R triggering in human natural killer (NK) cells. Both p46 and p52 forms of Shc were rapidly and transiently tyrosine phosphorylated upon CD16 or IL-2 stimulation with different kinetics. Shc immunoprecipitates from lysates of CD16- or IL-2-stimulated NK cells contained Grb2 and an unidentified 145-kD tyrosine phosphoprotein. Grb2 immunoprecipitates from anti-CD16-stimulated NK cells contained not only Shc, but also a 36-kD tyrosine phosphoprotein (p36). The interaction between Grb2 and Shc or p36 occurred via the Grb2SH2 domain as indicated by in vitro binding assays using a bacteriologically synthesized glutathione S-transferase-Grb2SH2 fusion protein. We also present evidence that p21ras is activated by CD16 and IL-2R cross-linking. Accumulation of guanosine triphosphate-bound Ras was detected within 1 minute and occurred with kinetics similar to inductive protein tyrosine phosphorylation and Grb2 association of Shc and p36 adaptor proteins.

Haemopoietic progenitor cell differentiation: flow cytometric assessment in bone marrow and thymus

We have recently shown that expression of any of the lineage-associated molecules CD2, CD7, CD10, CD19 or CD33 does not ensure lineage-commitment of CD34+ progenitor cells. Further, normal progenitor cells and leukaemic blast cells have been shown to coexpress molecules associated with more than one haemopoietic lineage. Five-dimensional flow cytometric analysis of normal bone marrow cells was exploited to investigate the hypothesis of a developmental stage in haemopoiesis comprising CD34+ cells coexpressing CD2, CD5, CD7, CD10, CD19 and CD33 or any combination of these molecules. We report on a subpopulation of CD34+ bone marrow cells constituting < 5% of the CD34+ cells and characterized by extensive coexpression of several molecules associated with the B lymphoid, T lymphoid and myeloid lineages. There is every probability that some cells display the CD34+ CD2+ CD5+ CD7+ CD10+ CD19+ CD33+ phenotype. Studies on postnatal thymocytes suggest that this may be the phenotype or one of a few phenotypes of a candidate thymus-seeding progenitor cell population. Finally, our findings that CD34+ as well as CD34+ CD5+ thymocytes can be driven into non-T-lymphoid differentiation by cytokines, support the notion that the thymus is seeded by uncommitted progenitors.

The enigma of the natural killer cell

Natural killer (NK) cells are controlled by receptors specific for polymorphic determinants of class I molecules of the major histocompatibility complex (MHC). The contrasting properties of NK and cytotoxic T cell (CTL) class I receptors provide complementarity in the cytolytic lymphocyte response to viruses, tumours and transplants. Whereas human NK cell class I receptors consist of immunoglobulin domains, their mouse counterparts resemble C-type lectins. This difference may reflect the receptors' diverse and rapidly evolving class I ligands.

Human T, B, natural killer, and dendritic cells arise from a common bone marrow progenitor cell subset

The early stages of lymphoid cell formation were studied by testing the differentiative potential of phenotypically defined subsets of CD34+ bone marrow cells. A subpopulation of CD34+ Lin- CD45RA+ cells expressing CD10 was isolated by flow cytometry. Such cells are CD38+, HLA-DR+, do not express significant levels of Thy-1 and c-kit, lack erythroid, myeloid, megakaryocytic potential, and give rise only to lymphoid T, B, natural killer (NK), and dendritic cells (DC) in kinetics and titration experiments. Limiting dilution analysis demonstrates the existence of multipotential B/NK/DC progenitor clones in the CD34hi Lin-CD10+ adult bone marrow cell population. Thus, nonprimitive progenitors for lymphoid cells and for DCs can be distinct from those of myeloid, megakaryocytic, and erythroid cells, implying that the DC lineage is developmentally more closely related to the lymphoid lineage than to the myeloid lineage. This study provides new insights into the organization and development of the human lympho-hematopoietic system.