Hairy cell leukemia cells are relatively NK-insensitive targets

The hypothesis that interferon alpha (IFN alpha) has its beneficial effects in hairy cell leukemia by activating natural killer cells against hairy cells was examined. Leukemic cells from patients with hairy cell leukemia were tested for their susceptibility to lysis by fresh and IFN alpha activated peripheral blood mononuclear (PBMC) cells from normal donors. All hairy cells tested were relatively insensitive to cytolysis by PBMC and IFN alpha activated PBMC. The low levels of 51Cr release obtained with a few donors was due to lysis of leukemic cells, not residual normal cells, and was mediated by a natural killer cell (T cell receptor independent) mechanism. Chronic lymphatic leukemic cells before and after treatment with phorbol ester were also resistant to cytolysis. Hairy cells were not susceptible to lymphokine activated killer (LAK) cells but were sensitive to lysis by antibody and complement. The insensitivity to cell mediated cytolysis against hairy cells was shown by cold target inhibition to be a lack of target recognition by NK cells.     

Effect of simple sugars on natural killing: evidence against the involvement of a lectin like mechanism in target recognition

The spontaneous lysis of target cells sensitive to natural killer (NK) activity is accomplished in two distinct phases: (i) binding between target and effector cells and (ii) post-binding events leading to target cell destruction. To test the hypothesis that cell surface carbohydrate(s) might be involved in recognitive and/or lytic events, the binding and cytotoxicity of peripheral blood lymphocytes (PBL) towards NK sensitive K-562 targets was studied in the presence of simple sugars and after treatment of the targets with the antibiotic, tunicamycin. Lysis by peripheral blood lymphocytes was found to be inhibited by N-acetyl glucosamine, N-acetyl galactosamine and alpha-methyl mannoside in a dose-dependent manner under conditions where neither these sugars nor those (fucose, galactose) which had little effect on lysis inhibited the binding of effector cells to targets. Further, growth of K-562 in tunicamycin (which inhibits N-linked glycosylations occurring through the lipid intermediate pathway) with or without subsequent treatment with the enzyme neuraminidase, markedly reduced cell surface expression of sugars monitored by lectin binding. Treated cells showed no loss of NK susceptibility and were frequently more sensitive to lysis. Sugar inhibition profiles were the same as for untreated cells. These data suggest that carbohydrates are not the target sites of NK recognition but that simple sugars may have an inhibitory action at a later stage of the lytic process.     

Natural cytotoxic cell activity in the snake Psammophis sibilans.

Thymocytes, splenocytes and peripheral blood mononuclear cells (PBMC) of the snake Psammophis sibilans consistently killed the human erythroleukemic cells K562 in a 4 h assay as judged by lactate dehydrogenase enzyme release. PBMC and splenocyte natural cytotoxicity (NC) increased proportionally with increase in the effector/target cell ratio. Spontaneous killer cell activity was consistently 2-3 times higher in peripheral blood (PB) than in spleen. On the other hand, thymocytes displayed low, yet detectable, NC. In an attempt to define the cell subpopulation responsible for natural killer (NK) activity, PBMC were depleted of macrophages or B lymphocytes before use in NK cell assays against K562 cells. Depletion of macrophages did not impair NK activity thus suggesting that macrophages do not mediate spontaneous lysis in the present 4 h assay. Conversely, removal of B lymphocytes by panning onto dishes coated with monoclonal antibody against snake Ig significantly reduced, but did not eliminate, PBMC spontaneous cytotoxicity. These data suggest that T, B and perhaps distinct NK cells participate in spontaneous lysis. This suggestion was confirmed by studies of NC in thymus, spleen and PB the year round. Strong NC was detected during spring and autumn when high numbers of leukocytes including T and B cells can be recovered from spleen and PB. Negligible spontaneous cytotoxicity was observed during early and mid-summer and in winter, periods of the year when snakes are thymus-less and contain few T and B cells in peripheral lymphoid organs. These findings, the first to document natural cytotoxic activity in snakes, were discussed in relation to the issue of NK cell identity in vertebrates.     

Letter From the Editor

ABSTRACT: The NK-susceptibility of trophoblast cells to allogeneic and autologous intraplacental natural killer (NK), antibody-dependent (K), and mitogen-induced cell-mediated cytotoxicity was studied, using untreated and neuraminidase-treated trophoblast cells from normal, full-term deliveries. The work was preceded by systematic studies of placental cell separation and labelling techniques, and the effects of these techniques on the NK target, K562. The results indicated that maternal NK cells are present among intraplacental lymphocytes, but that their activity is lower than that of peripheral blood lymphocytes and they are not stimulated by interferon to the same extent as peripheral blood lymphocytes (PBL). Trophoblast cells were rarely susceptible to allogeneic NK cells, with low cytotoxicity at high effector-target cell ratios in only two of five experiments. Interferon (IF)-boosted NK cells mediated some cytolysis of trophoblasts in three of four experiments, but high effector/target cell ratios were also required for the effect to be observed. The trophoblast cells could be lysed, however, by K cells and lectin-induced cytotoxicity. Removal of surface sialic acid by neuraminidase treatment of the trophoblast cells had little effect on the susceptibility of these cells to unstimulated NK cells (one of four experiments), but resulted in susceptibility to IF-boosted NK cells in four of four experiments. Normal trophoblast cells did not compete in IF-NK(K562) assays and neuraminidase-treated cells competed weakly in only one of three such experiments, indicating that the NK “target structure” is only weakly expressed on human trophoblast cells. Intraplacental lymphocytes lysed autologous trophoblast cells to a lower extent than allogeneic PBL. This lysis was markedly increased if antibody against the target cells was present in the assay. These data indicate that a) the trophoblast cell is susceptible to maternal cell-mediated lysis by several mechanisms that could potentially be activated in vivo, b) NK cells are present in the intraplacental lymphocyte pool, and c) the access of NK cells and interferon activated NK cells to the NK cell target structure is blocked by cell surface sialic acid residues. This target structure may be similar to that found on other susceptible cells, and in similarity to the tumor—NK interaction, the cell surface sialic acid is ineffective in blocking cytotoxicity if the appropriate antibody is present. Assuming NK cells mediate ADCC, this indicates that sialic acid does not mask the target site of the lytic molecule. These data are relevant to the understanding of the NK– target interaction in a situation where it is known that the target is nonself.     

Perforin dependence of natural killer cell-mediated tumor control in vivo

Adaptive immune surveillance by T cells against infections and tumors depends on the presence of antigenic peptides presented by major histocompatibility complex (MHC) molecules. If antigenic tumor-specific peptides or MHC class I molecules are absent, the adaptive T cell immune response fails. Natural killer (NK) cells seem to complement the specific T cells by recognizing target cells lacking MHC class I (e.g. RMA-S). The role of perforin, which is crucially involved in T cell and NK cell-mediated target cell lysis, was evaluated in mice lacking perforin with respect to their capacity to eliminate a syngeneic lymphoid tumor. Here, we show that growth of MHC class I^? RMA-S tumor cells in unprimed mice was controlled by NK cells through perforin-dependent cytotoxicity.     

Target-Effector Interaction in the Natural Killer Cell System

These results show that two subpopulations of target-binding cells (TBC) can be detected in the lymphoid organs of normal, nonimmunized mice. One cell type is not adherent to nylon wool columns and binds selectively to a large number of tumour cell targets which are susceptible to lysis by the natural killer (NK) cell. The rise and fall in the frequency of these nylon nonadherent TBC, with age, closely parallels the NK cell activity in these mice. Nylon nonadherent TBC were specific since they could be inhibited by subcellular sonicates of sensitive targets but not insensitive targets. The presence of these TBC in nude mice and their ability to pass through nylon wool columns is compatible with the suggestion that, like the NK cell, they may not be mature T cells, macrophages or B cells and hence represent a distinct but not yet defined subpopulation of lymphocytes. The genes controlling the frequency of TBC are inherited in a dominant fashion and are linked to the H-2 region. The strong correlation between the frequency of TBC in a population and the level of lysis provides strong indirect evidence that the TBC may represent, or be closely related to, the NK cell. In contrast, the second cell type(s), a nylon-adherent population, was not subject to any detectable genetic control and bound to targets nonspecifically. Furthermore, these nylon-adherent TBC differed from nylon-nonadherent TBC in their lack of correlation with lysis, age variations and organ distribution. We believe these observations provide the basis for the eventual understanding of the target structures and receptors involved in the NK cell system.     

Lymphocyte subpopulations in man: induction of cytotoxic T lymphocytes in vitro by allogeneic B and T cells

Testing B and T cells as allogeneic stimulators of cytotoxic T lymphocytes in primary as well as secondary in vitro cultures, reveals that fresh, nonactivated B cells isolated from peripheral blood have an enhanced cytotoxic T cell stimulating capacity compared to T cells, although target determinants are present both on B and T cell blasts. Similarly, the capacity of T and B cells to stimulate proliferation in MLC is also quantitatively different. These results are in accordance with the hypothesis concerning the in vitro generation of alloreactive cytotoxic T lymphocytes, which postulates concurrent stimulation by strong lymphocyte activating determinants and target determinants for the generation of cytotoxic effector T lymphocytes, as both determinants are simultaneously found on B lymphocytes. Three cell experiments performed by coculturing allogeneic stimulating B and T cells with responding T cells show that strong lymphocyte activating determinants found on B cells enhance the cytotoxicity against target determinants on cocultured B cells but not on cocultured T cells, indicating qualitative differences between target determinants on B and T cells with respect to specific CTL stimulating capacity. Furthermore, primed resting CTLs in secondary cultures could unspecifically be restimulated by third party B cells or pokeweed mitogen. These results are the basis for a hypothesis concerning activation of CTLs, postulating nonspecific triggering of cytotoxic precursor cells by lymphocyte activating properties intrinsic to target determinants (TD) on B cells, preferentially activating clones of cytotoxic cells. The clonal proliferation is further unspecifically amplified by products of the T cell recognition of strong lymphocyte activating determinants (LAD).     

Evolutionary Conservation of a Human Function-Associated Molecule on Murine Natural Killer Cells: Expression and Function

Using a novel anti-natural killer (NK) cell monoclonal antibody (MoAb), we have recently identified an evolutionary conserved function-associated molecule (FAM) present on fish, rat and human NK cells. This molecule is involved in NK cell function as anti-FAM MoAbs inhibit cytotoxicity, stimulate lymphokine secretion and inhibit conjugate formation between effector cells and target cells. We now have examined murine NK cells for the presence of this structure. It was observed by two-colour flow cytometric analysis that the anti-FAM MoAb 5C6 specifically bound to a subpopulation of nylon wool non-adherent splenic lymphocytes (19–20%). The expression of the FAM molecule was restricted to NK cells that expressed the NK1.1 antigen. Neither T cells, B cells, nor macrophages reacted with the anti-FAM MoAb. Analysis of FAM expression in various lymphoid tissues revealed that splenocytes expressed the greatest numbers of MoAb(+) cells. Generation of lymphokine-activated killer (LAK) cells and adherent tymphokine-activated killer (ALAK) cells resulted in higher levels of FAM expression. The anti-FAM MoAb 5C6 also detected the presence of FAM on fresh SCID NK cells. It was demonstrated that the anti-FAM MoAb 5C6 inhibited the lysis of target cells by endogenous NK cells, activated NK cells, 5d LAK cells, ALAK cells and SCID NK cells. Moreover, conjugate assays demonstrated involvement of this molecule in recognition between NK cells and target cells.     

Reactivity of monoclonal antibodies against human leucocyte antigens with lymphocytes of non-human primate origin

The phylogenetic distribution of antigens present on human lymphocytes was investigated by incubating human or simian cells with murine anti-human monoclonal antibodies and then determining the level of reactivity with a radiolabelled anti-murine IgG reagent. The monoclonal antibodies used were specific for a T-cell antigen, lymphoid and lymphoid:myeloid antigens, Ia antigens, and beta 2 microglobulin. The cells examined included B- and T-lymphoblastoid cell lines and fresh peripheral blood lymphocytes separated by sheep erythrocyte rosetting into T-cell and non T-cell fractions. Results of these studies showed that the antibodies gave complete cross-reactivity with gorilla and chimpanzee cells while B-cell lines of orangutan origin had lost lymphoid and beta 2 microglobulin markers. Gibbon cells and cells of Old World and New World monkeys reacted strongly only with monoclonal antibodies against Ia antigenic determinants. These Ia antigens were found on the non T-cell fraction of fresh peripheral lymphocytes, on B-cell lines and on some virus induced T-cell tumour lines. Immunoprecipitation analysis using the anti-Ia antibodies showed a degree of molecular diversity on owl monkey and marmoset cells compared to the Ia antigens associated with human cells.     

In vivo modulation of thymus-derived lymphocytes with monoclonal antibodies in mice. III. Spontaneous and natural cytotoxic effector cells

Cytotoxic effector cell populations in murine spleen can be characterized by the phenotype of the cytotoxic cells or the nature of target cells. Lytic events can be antigen-specific, MHC-restricted and clonal, or target cell-specific but apparently non-MHC-restricted. Two cytotoxic effectors of this latter category are spontaneous and natural killers. Normal spleen cells from (BALB/c X DBA/2J)F1 mice (CDF1) cultured without added antigen develop a population of Thy-1+ spontaneous cytotoxic lymphocytes (SCTL) that lyse the DBA/2J mastocytoma P815, as well as the BALB/c-derived plasmacytomas MOPC-11 and SP2/0. Cold target competition experiments reveal the BALB/c-derived plasmacytomas MOPC-11, SP2/0, J558 and the A strain-derived T cell lymphoma YAC-1, but not normal lymphoblasts, block the lysis of P815 target cells. Thus, while these tumour cells appear to express common antigens which are recognized by SCTL cells, plasmacytomas such as J558 are not susceptible to lysis by SCTL. The relationship of SCTL to natural killer (NK) cells was examined. In-vivo treatment of mice with monoclonal anti-Thy-1 antibody leads to a rapid loss of SCTL and precursors from the spleen, but there is a concomitant increase in NK cell activity.     

Interaction of natural killer cells with MHC class II: reversal of HLA-DR1-mediated protection of K562 transfectant from natural killer cell-mediated cytolysis by brefeldin-A

Major histocompatibility complex (MHC) class I antigens on tumour cell surfaces have been shown to modulate target susceptibility to natural killer (NK) cell-mediated lysis in some, although not all, systems investigated. MHC class II expression may also affect NK cell function, but the mechanism by which MHC class II antigen regulates NK cell activity has not been fully examined. In this study we induced HLA-DR1 expression by gene transfection into the classic NK-sensitive K562 cell line to study the interaction of NK cells with MHC class II molecules and the effect of brefeldin-A (BFA), an endogenous antigen-processing pathway blocker, on NK–target cell interaction. We demonstrated that the expression of HLA-DR1 on the cell surface reduced K562 cell susceptibility to NK lysis by peripheral blood monuclear cells and a NK cell line. The effect was demonstrable in prolonged (8 hr) cytotoxicity assays and was blocked by pretreatment of target cells with anti-HLA-DR antibody. Treatment of K562 DR transfectant with BFA abrogated the resistance of K562 transfectant to NK-mediated cytolysis. These findings indicate that HLA class II molecules regulate NK cell function and target recognition, and suggest that endogenous peptides presented through MHC molecules are responsible for regulating NK cytolysis.     

Natural killer cells in peripheral blood and the mixed lymphocyte response: interaction with the transferrin receptor.

Several reports suggest that natural killer (NK) cells recognize the transferrin receptor (TFR) as a target for killing, and that natural cytotoxicity may be involved in the control of stem cell proliferation in bone-marrow. This study tested whether NK-cell recognition of the TFR on activated lymphocytes plays a role in the control of peripheral immune responses. Six lymphoid lines were created from a single individual, and used as targets for cytotoxicity assays, using either peripheral blood mononuclear cells, or mixed lymphocyte reaction (MLR)-derived effectors. The cells responsible for killing were predominantly Leu-11+Leu-7+ NK cells, though CD3+ cells accounted for about 25% of cytotoxicity from MLR. No correlation was observed between TFR density and NK susceptibility when using all six cell lines. Specifically increasing the density of TFR on a single cell line failed to increase susceptibility to NK, suggesting that the TFR does not act as a major target for natural cytotoxicity directed at lymphoid cells. Furthermore, the relatively low levels of killing observed indicate that activated NK populations that accumulate at sites of immune response are unlikely to play a direct immunoregulatory role.     

Selective elimination through a cytolytic mechanism of bovine serum albumin-specific T helper lymphocytes by T suppressor cells with the same antigen specificity

An antigen-specific T suppressor cell clone isolated from a CBA/J mouse tolerized to low doses of bovine serum albumin (BSA) has previously been analyzed with regard to its effector functions. The T suppressor cell clone HF1 specifically inhibits T helper cell responses to the antigen. It also has characteristic cytolytic activity which can neither be classified as cytotoxic T cell nor as natural killer cell activity. Since this lytic capacity might be of relevance in immunoregulation, it has now been studied in more detail. For that purpose BSA-specific T cell lines have been isolated from immune CBA/J mice in order to test them in 51Cr-release assays as possible targets for HF1 T suppressor cells. Two T cell lines, both BSA specific and restricted to recognition of I-Ek major histocompatibility complex determinants, have been selected for the studies because one is a helper cell (83/1), the other a suppressor cell type (83/2). HF1 T cells are able to lyse cells of line 83/1 but not those of line 83/2. Control experiments show that 83/1 cells are not a natural killer cell target and that on the other hand 83/2 cells are susceptible to lysis in an alloreactive BALB/c anti-CBA/J cytotoxic T cell response. The extent of lysis of 83/1 T cells by HF1 T cells changes with time after antigenic stimulation. The lysis is based on direct effector: target cell interaction and not caused by soluble mediators. The data are discussed with regard to the effector function of a type of T suppressor cells which expresses I-A and I-E molecules and whose proliferation is restricted to the recognition of I-A or I-E determinants.     

Tumor cell lysis by T cells distinct from NK cells and alloantigen-specific cytotoxic T cells

The generation and mechanism of tumor cell lysis by cytotoxic T cells derived from natural killer cell (NK) and allospecific cytotoxic T cell (CTL)-depleted precursors were examined. NK cells and the precursors of alloantigen-specific CTL were deleted from human peripheral blood lymphocytes by preincubation with L-leucyl-L-leucine methyl ester (Leu-Leu-OMe). Following phytohemagglutinin activation, CD3(+), CD4(+) or CD8(+), CD11b(-), CD16(-), and NKH1(-) killer cells capable of lysing a broad spectrum of tumor targets were generated. Cytolysis was not strictly lectin dependent as similar killer cells were generated by activating Leu-Leu-OMe-treated T cells with immobilized monoclonal antibodies to the CD3 molecular complex. The rate of tumor cell lysis by these mitogen-activated T cells was slower than that mediated by CD3(-) NK cells. Tumor cell lysis by mitogen-activated killers was inhibited by anti-CD3 but was not restricted by major histocompatibility complex antigen expression on target cells or by CD4/CD8 expression on effectors. Although similar to NK cells in susceptibility to anti-LFA-1 inhibition of killing, these mitogen-activated killer cells were more sensitive to the inhibitory effects of anti-CD2 than were CD3(-)-activated NK-like cells. Thus, tumor cell lysis by CD3(+) cytotoxic cells generated from Leu-Leu-OMe-treated lymphocytes appears to be mediated in part by mechanisms distinct from those employed by CD3(-) NK cells or antigen-specific CTL.     

Human Cytotrophoblastic Cells Absorb the NK Blocking Activity of Monoclonal BA11

PROBLEM: R80K is a polymorphic alloantigeneic protein present on human placental trophoblast and on paternal B lymphocytes and monocytes. This protein, unlike the former candidate TLX antigen, stimulates a protective maternal immune response in vivo. A murine monoclonal BA11 antibody, directed against R80K, prevents abortion in three murine pregnancy-failure models and inhibits human and murine NK activity. We attempted to define the target of BA11 in the human NK assay system. METHODS: A CELISA method was used to detect R80K antigen on the surface of different cells using the BA11 antibody. The effect, on human peripheral blood NK activity against K562, by BA11 before and after absorption by different cells, including the K562 target, was determined. RESULTS: R80K was detected on term placental syncytio and cytotrophoblast and on BeWo cells, by CELISA. BA11 suppressed NK lysis of K562 cells in a dose-dependent manner. Absorption of the BA11 by BeWo and by cytotrophoblastic cells significantly decreased the NK-inhibitory activity. There was minimal absorption by K562 and BA11-pretreated K562 cells remained susceptible to NK lysis. By contrast, BA11-pretreated peripheral blood cells lost all NK activity. CONCLUSIONS: The inhibition of NK killing of K562 cells by BA11 is more complex than a simple masking of a trophoblast cell-associated molecule in K562 necessary for recognition in NK cells.     

Natural killing and growth inhibition of K562 cells by subpopulations of mononuclear cells as a function of target cell proliferation.

K562 cells of different proliferative activity were tested for their capability to form lytic and non-lytic conjugates in agarose with peripheral blood mononuclear cells (MNC). Simultaneously, the conjugating MNC were analysed in suspension by monoclonal antibodies (mAb) defining subsets of T cells, natural killer (NK) cells, and monocytes (OKT 8, OKT 4, Leu 7, OKM 1, Mo 2). It is demonstrated that the pattern of conjugating (binding, lysis) is dependent upon the proliferative status of the target cell population. Target cells of intermediate division rate are optimally lysed by MNC of NK phenotype, whereas targets of high and low division rate bind cells predominantly of T cell phenotype, but are not killed. Non-cytolytic conjugating MNC are shown to inhibit significantly the cell cycle progression of their bound tumour target cells. There is evidence of an additional cytostatic effect on non-bound K562 cells in agarose dishes containing effector MNC, possibly mediated by soluble factors released from NK cells. Adherent monocytes contribute only little to cytolysis and cytostasis in our testing system. There is no correlation between the expression of transferrin receptors on target cells as determined by the OKT9 mAb and the extent of killing. Transferrin receptors may, however, be involved in the step of target cell recognition by MNC exhibiting the OKT 8+ phenotype.     

Heterogeneity in the activation requirements of T cells stimulated by phytohaemagglutinin.

Natural killer (NK) activity is inhibited by the contact of peripheral blood lymphocytes with primary monolayer cell cultures of both benign and malignant origin. In this study the effect of interferons (IFNs) on the inhibition has been analysed. Both alpha IFN- and gamma IFN-pretreated peripheral blood lymphocytes are effectively inhibited by monolayer target cells. IFN treatment of lymphocytes does not change cytotoxicity against the inhibitory target cells, although it enhances reactivity against NK-sensitive target cells. Treatment of monolayer cells with interferons significantly reduces their inhibitory capacity. However, diminished inhibition of NK activity by the IFN-treated target cells is not associated with increased lysis, probably due to their decreased sensitivity to natural killer cytotoxic factors (NKCF). In 18.5% of the cases studied, monolayer target cells induced endogenous IFN production in lymphocytes. In these cases no inhibition of the NK activity of the effector cells was seen. According to the results of this paper, IFNs have a dual effect on the NK regulatory system: they enhance the NK activity of the effector cells against NK-sensitive target cells, and they change the NK resistant target cells in a way that makes them less inhibitory to NK activity but simultaneously more resistant to the toxic factors secreted by NK cells.     

Effects of Chlamydia trachomatis infection on the expression of natural killer (NK) cell ligands and susceptibility to NK cell lysis

Natural killer (NK) cells are an important component of the immediate immune response to infections, including infection by intracellular bacteria. We have investigated recognition of Chlamydia trachomatis (CT) by NK cells and show that these cells are activated to produce interferon (IFN)-gamma when peripheral blood mononuclear cells (PBMC) are stimulated with CT organisms. Furthermore, infection of epithelial cell lines with CT renders them susceptible to lysis by human NK cells. Susceptibility was observed 18-24 h following infection and required protein synthesis by the infecting chlamydiae, but not by the host cell; heat or UV inactivated chlamydiae did not induce susceptibility to NK cell lysis. CT infection was also shown to decrease the expression of classical and non-classical major histocompatibility complex (MHC) molecules on infected cells, thus allowing recognition by NK cells when combined with an activating signal. A candidate activating signal is MICA/B, which was shown to be expressed constitutively on epithelial cells.     

Cellular distribution of a natural killer cell tumour recognition-related surface antigen in purified human lymphocytes.

Natural killer (NK) cells are large granular lymphocytes capable of human leucocyte antigen (HLA) unrestricted killing of tumour cells. A putative NK cell tumour-recognition molecule (NK-TR) was previously isolated and cloned. The predicted primary structure of the NK-TR revealed that the amino terminus of the protein shared high homology with cyclophilin proteins. In this study, we used rabbit antibodies directed against synthetic peptides corresponding to amino acids 476-497 of the NK-TR protein, to examine the expression of the NK-TR antigen in freshly purified human lymphocytes. Cell-surface staining experiments using these peptide antibodies indicated the presence of the NK-TR protein on the surface of human CD3+ T-cell populations purified from peripheral blood. There were individual donor differences in the levels of cell-surface expression of this antigen ranging from 35 to 90% in T lymphocytes and, NK cells purified from different healthy volunteers. The immunoreactivity of our peptide antibodies in immunoprecipitation showed that the NK-TR-related protein expressed in purified T cells is similar to that expressed in NK cells in terms of its electrophoretic mobility. Cell-surface staining experiments using the peptide antibodies revealed that the NK-TR-related protein is more abundantly expressed on the surface of purified T cells compared with NK cells. Northern blot analysis of the mRNA species transcribed in human lymphocytes revealed abundant expression of NK-TR-specific mRNA species in purified T cells. Furthermore, another mRNA species smaller than 7 kb was detected in both NK and T-cell populations of lymphocytes freshly isolated from peripheral blood. Expression at the cell surface of a cyclophilin-homologous protein in purified human T lymphocytes may indicate another function for the reported NK-TR protein, that is, distinct from tumour-cell recognition and cytosis.     

Rat natural killer cell and cytotoxic T cell lysis of H-2-negative murine embryonal carcinoma cells

H-2-lacking murine embryonal carcinoma (EC) cells have been proposed as universal targets for natural killer (NK) effectors from different species because their killing appeared to be uncomplicated by potential T cell effector mechanisms (Stern, P. L. et al., Int. J. Cancer 1981. 27:679). While some previous studies had shown that murine cytotoxic T cells were unable to lyse EC cells, rat T killers are shown here to be active against these targets and to be distinguishable from NK cells. Percoll density fractionation of rat peripheral blood lymphocytes enriches in parallel for NK-mediated lysis of both EC or YAC target cells. These NK cells unlike T cells, do not mediate lectin-dependent and cell-mediated cytotoxicity (LDCC) of NK-insensitive target cells. This procedure is thought to reveal the total cytolytic potential of stimulated T cell populations, regardless of specificity. In contrast to previous results with mice, we found that allogeneically primed rat cytotoxic T cells can kill murine EC cells in LDCC and, further, that rat cytotoxic T cells, generated by stimulation with mouse spleen cells in vitro, can lyse murine EC cells directly. This demonstration of T cell lysis of EC cells suggests that either there is a novel mechanism of lysis operating without requirement for major histocompatibility complex (MHC) structures, or EC cells express some hitherto unidentified MHC-like structures on their cell surface.