ESAT-6 and HspX Improve the Effectiveness of BCG to Induce Human Dendritic Cells-Dependent Th1 and NK Cells Activation

The limited efficacy of the BCG vaccine against tuberculosis is partly due to the missing expression of immunogenic proteins. We analyzed whether the addition to BCG of ESAT-6 and HspX, two Mycobacterium tuberculosis (Mtb) antigens, could enhance its capacity to activate human dendritic cells (DCs). BCG showed a weak ability to induce DC maturation, cytokine release, and CD4⁺ lymphocytes and NK cells activation. The addition of ESAT-6 or HspX alone to BCG-stimulated DC did not improve these processes, whereas their simultaneous addition enhanced BCG-dependent DC maturation and cytokine release, as well as the ability of BCG-treated DCs to stimulate IFN-γ release and CD69 expression by CD4⁺ lymphocytes and NK cells. Addition of TLR2-blocking antibody decreased IL-12 release by BCG-stimulated DCs incubated with ESAT-6 and HspX, as well as IFN-γ secretion by CD4⁺ lymphocytes co-cultured with these cells. Moreover, HspX and ESAT-6 improved the capacity of BCG-treated DCs to induce the expression of memory phenotype marker CD45RO in naïve CD4⁺ T cells. Our results indicate that ESAT-6 and HspX cooperation enables BCG-treated human DCs to induce T lymphocyte and NK cell-mediated immune responses through TLR2-dependent IL-12 secretion. Therefore ESAT-6 and HspX represent good candidates for improving the effectiveness of BCG vaccination.     

Human B cell activation by autologous NK cells is regulated by CD40-CD40 ligand interaction: role of memory B cells and CD5+ B cells.

NK cells are a subpopulation of lymphocytes characterized primarily by their cytolytic activity. They are recognized as an important component of the immune response against virus infection and tumors. In addition to their cytolytic activity, NK cells also participate either directly or indirectly in the regulation of the ongoing Ab response. More recently, it has been suggested that NK cells have an important role in the outcome of autoimmune diseases. Here, we demonstrate that human NK cells can induce autologous resting B cells to synthesize Ig, including switching to IgG and IgA, reminiscent of a secondary Ab response. B cell activation by the NK cell is contact-dependent and rapid, suggesting an autocrine B cell-regulated process. This NK cell function is T cell-independent, requires an active cytoplasmic membrane, and is blocked by anti-CD40 ligand (anti-CD154) or CD40-mIg fusion protein, indicating a critical role for CD40-CD40 ligand interaction. Depletion studies also demonstrate that CD5+ B cells (autoreactive B-1 cells) and a heterogeneous population of CD27+ memory B cells play a critical role in the Ig response induced by NK cells. The existence of this novel mechanism of B cell activation has important implications in innate immunity, B cell-mediated autoimmunity, and B cell neoplasia.     

A subset of CD16-natural killer cells without antibody-dependent cellular cytotoxicity function

The low affinity IgG receptor, CD16 (Fc gamma RIII), is expressed on almost all peripheral blood natural killer (NK) cells. A small subset of CD3- CD16- CD56+ NK cells, representing less than 1% of peripheral blood lymphocytes, expands during in vivo IL-2 treatment. To analyze this CD16- NK cell subset in more detail, NK clones have been generated. One of them (TNK2) has been used to study the function of these cells in more detail. It is demonstrated that TNK2 exerts normal NK activity and displays large granular lymphocyte morphology. Since this clone lacks CD16 expression, antibody-dependent cellular cytotoxicity cannot be exerted. CD16 monoclonal antibodies fail to induce cytotoxic activity against NK-resistant target cells. These studies reveal that the lack of CD16 detection is not due to the modulation or the stage of activation of these NK cells. TNK2 is representative of this small subset of peripheral blood NK cells, expanded during IL-2 treatment, which does not express Fc gamma RIII and therefore cannot perform antibody-dependent cellular cytotoxicity.     

Studies on the relationship of human natural killer and lymphokine-activated killer cells with lysosomal staining and analysis of surface marker phenotypes

We have previously demonstrated that natural killer (NK) cells are lysosome-rich and stain more intensely with lysosomotropic agents such as neutral red and quinacrine (Qu) than do non-NK cells. In this study we combined the quantitation of Qu staining with surface marker staining to define subpopulations of NK cells. While all NK activity was contained within the Qu+ population, most but not all NK cells expressed the surface marker CD16. A subpopulation of NK cells was found to be Qu+CD16- composed of medium- to large-sized cells with a granular appearance on Giemsa staining. Culture with interleukin-2 (IL-2) induced enhanced cytotoxicity in peripheral blood lymphocytes (PBL) against NK-sensitive and NK-resistant tumor cells. Like NK cells, these lymphokine-activated killer (LAK) cells were predominantly Qu+CD16+. However, some LAK cells were Qu+CD16-. The Qu+CD16+ cells were typical large granular lymphocytes (LGL). The Qu+CD16- cells were also large lymphocytes, more than 50% of which were proliferating. However, the granulation in some Qu+CD16- cells, as detected by Giemsa staining, was more prominent and numerous than others in the same population. No LAK activity was ever detected in Qu- cells, which were uniformly small lymphocytes. Quantitation of Qu staining in effector cells was therefore demonstrated to have a good correlation with NK and LAK functions, and with surface markers can help to characterize both types of cells. Moreover, these results indicate that both NK and LAK populations include a small subset of CD16- cells in each.     

Infection of human natural killer (NK) cells with replication-defective human T cell leukemia virus type I provirus. Increased proliferative capacity and prolonged survival of functionally competent NK cells.

Human T-cell leukemia virus type I (HTLV-I) can infect a variety of human cell types, but only T lymphocytes are efficiently immortalized after HTLV-I infection. This study reports an attempt to infect and to immortalize NK cells with HTLV-I. Co-cultivation of freshly isolated NK cells with a HTLV-I-producing T cell line did not result in NK cell infection. However, NK cells activated with an anti-CD16 mAb and co-cultivated with a HTLV-I-producing T cell line were reproducibly infected by HTLV-I. HTLV-I infection was documented in NK cell lines and clones by the detection of defective integrated provirus by both Southern blot and polymerase chain reaction analysis. Although HTLV-I-infected NK cells produced viral proteins, they did not produce infectious viral particles. HTLV-I-infected NK cells were phenotypically indistinguishable from their uninfected counterparts (CD16+, CD2+, CD56+, CD3-). They also retained the ability to mediate both natural and antibody-dependent cell cytotoxicity. The IL-2-dependent proliferation of HTLV-I-infected NK cells was significantly greater than that of uninfected NK cells. The doubling time of this infected population was reduced from 9 days to 3 days, and the overall survival of the culture in the absence of restimulation was extended from 5 wk to 18 wk. Unlike T lymphocytes, HTLV-I-infected NK cells were not immortal, implying a fundamental difference between these two lymphocyte populations.     

Herpes simplex virus antigens directly activate NK cells via TLR2, thus facilitating their presentation to CD4 T lymphocytes

NK cells infiltrate human herpetic lesions, but their role has been underexplored. HSV can stimulate innate immune responses via surface TLR2, which is expressed on monocyte-derived dendritic cells (DCs) and NK cells. In this study, UV-inactivated HSV1/2 and immunodominant HSV2 glycoprotein D peptides conjugated to the TLR2 agonist dipalmitoyl-S-glyceryl cysteine stimulated CD4 T lymphocyte IFN-γ responses within PBMCs or in coculture with monocyte-derived DCs. NK cells contributed markedly to the PBMC responses. Furthermore, NK cells alone were activated directly by both Ags, also upregulating HLA-DR and HLA-DQ and then they activated autologous CD4 T lymphocytes. Using Transwells, Ag-stimulated NK cells and CD4 T lymphocytes were shown to interact through both cell-to-cell contact and cytokines, differing in relative importance in different donors. A distinct immunological synapse between Ag-stimulated NK cells and CD4 T lymphocytes was observed, indicating the significance of their cell-to-cell contact. A large proportion (57%) of NK cells was also in contact with CD4 T lymphocytes in the dermal infiltrate of human recurrent herpetic lesions. Thus, NK cells stimulated by TLR2-activating HSV Ags can present Ag alone or augment the role of DCs in vitro and perhaps in herpetic lesions or draining lymph nodes. In addition to DCs, NK cells should be considered as targets for adjuvants during HSV vaccine development.     

Differential sensitivity to natural cell-mediated cytotoxicity of two rat colon adenocarcinoma variants differing in their tumorigenicity: identification of the effector cells as natural killer cells

Summary DHD/K12 TRb (PROb) and DHD/K12 TSb (REGb) are two cancer cell variants originating from the same rat colon adenocarcinoma. They differ in their tumorigenicity: when inoculated into syngeneic BDIX rats, PROb cells induce progressive tumors whereas REGb cells induce tumors which always regress. As previously described, there is an inverse relation between their tumorigenicity and their susceptibility to NCMC mediated by syngeneic spleen or peripheral blood lymphocytes: PROb cells are significantly less sensitive to NCMC than REGb cells. This suggests a role for NCMC in the regression of REGb tumors. In this work the BDIX NCMC effector cells active in vitro against REGb cells were identified as NK cells according to four criteria: (1) efficacy in a 4-h ⁵¹Cr release assay, (2) sensitivity to anti-asGM1 antibody plus complement, (3) LGL morphology, and (4) ability to bind with the same affinity REGb and YAC-1 cells. In spleen, these NK cells were heterogeneous with respect to their asGM1 surface density and their morphology. PROb cells were not lysed by these NK cells in a short-term cytotoxicity assay, but only in a 16-h assay. It was shown that PROb and REGb cells were bound with the same affinity by NK cells, thus they certainly differ in their ability to resist to NK lytic mechanisms. This difference could play a role in the different tumorigenicity of the two variants. Abbreviations used: NK, natural killer; NC, natural cytotoxic; NCMC, natural cell-mediated cytotoxicity; asGM1, asialo GM1; LL, large lymphocytes; LGL, large grnular lymphocytes; LAL, large agranular lymphocytes; PBMNC, peripheral blood mononuclear cells; E:T, effector to target cell ratio; C:H, cold to hot cell ratio; FBS, fetal bovine serum     

NK Cell Terminal Differentiation: Correlated Stepwise Decrease of NKG2A and Acquisition of KIRs

Background

Terminal differentiation of NK cells is crucial in maintaining broad responsiveness to pathogens and discriminating normal cells from cells in distress. Although it is well established that KIRs, in conjunction with NKG2A, play a major role in the NK cell education that determines whether cells will end up competent or hyporesponsive, the events underlying the differentiation are still debated.

Methodology/Principal Findings

A combination of complementary approaches to assess the kinetics of the appearance of each subset during development allowed us to obtain new insights into these terminal stages of differentiation, characterising their gene expression profiles at a pan-genomic level, their distinct surface receptor patterns and their prototypic effector functions. The present study supports the hypothesis that CD56^dim cells derive from the CD56^bright subset and suggests that NK cell responsiveness is determined by persistent inhibitory signals received during their education. We report here the inverse correlation of NKG2A expression with KIR expression and explore whether this correlation bestows functional competence on NK cells. We show that CD56^dimNKG2A⁻KIR⁺ cells display the most differentiated phenotype associated to their unique ability to respond against HLA-E+ target cells. Importantly, after IL-12 + IL-18 stimulation, reacquisition of NKG2A strongly correlates with IFN-γ production in CD56^dimNKG2A⁻ NK cells.

Conclusions/Significance

Together, these findings call for the reclassification of mature human NK cells into distinct subsets and support a new model, in which the NK cell differentiation and functional fate are based on a stepwise decrease of NKG2A and acquisition of KIRs.     

Phenotypic studies of natural killer cell subsets in human transporter associated with antigen processing deficiency

Peripheral blood natural killer (NK) cells from patients with transporter associated with antigen processing (TAP) deficiency are hyporesponsive. The mechanism of this defect is unknown, but the phenotype of TAP-deficient NK cells is almost normal. However, we noticed a high percentage of CD56(bright) cells among total NK cells from two patients. We further investigated TAP-deficient NK cells in these patients and compared them to NK cells from two other TAP-deficient patients with no clinical symptoms and to individuals with chronic inflammatory diseases other than TAP deficiency (chronic lung diseases or vasculitis). Peripheral blood mononuclear cells isolated from venous blood were stained with fluorochrome-conjugated antibodies and the phenotype of NK cells was analyzed by flow cytometry. In addition, (51)Chromium release assays were performed to assess the cytotoxic activity of NK cells. In the symptomatic patients, CD56(bright) NK cells represented 28% and 45%, respectively, of all NK cells (higher than in healthy donors). The patients also displayed a higher percentage of CD56(dim)CD16(-) NK cells than controls. Interestingly, this unusual NK cell subtype distribution was not found in the two asymptomatic TAP-deficient cases, but was instead present in several of the other patients. Over-expression of the inhibitory receptor CD94/NKG2A by TAP-deficient NK cells was confirmed and extended to the inhibitory receptor ILT2 (CD85j). These inhibitory receptors were not involved in regulating the cytotoxicity of TAP-deficient NK cells. We conclude that expansion of the CD56(bright) NK cell subtype in peripheral blood is not a hallmark of TAP deficiency, but can be found in other diseases as well. This might reflect a reaction of the immune system to pathologic conditions. It could be interesting to investigate the relative distribution of NK cell subsets in various respiratory and autoimmune diseases.     

B cell induction of IL-13 expression in NK cells: role of CD244 and SLAM-associated protein

NK cells are an important component of the innate immune system that can also interact with B cells in a mutually productive manner. We have previously shown that activated B cells can induce NK cells to up-regulate their secretion of IFN-gamma. In this study, we show that B cells, and, particularly, marginal zone B cells, can, in addition, induce NK cells via direct cell-cell interactions to express mRNA encoding the Th2 cytokine IL-13. The induction of NK cell IL-13 mRNA expression requires the ligation of the CD244 receptor by the CD48 ligand on B cells via signaling pathways that depend upon expression of the X-linked lymphoproliferative disease gene product, SH2D1A/DSHP/SAP (SLAM-associated protein, or SAP) in NK cells. Thus, the positive signals attributed to the B cell activation of CD244 on murine NK cells appears to be more similar to the activity of CD244 on human cells. The induction of IL-13 mRNA by B cells may account for the effect of NK cells on the generation of Th2-type responses in the presence of some adjuvants.     

Lectin-dependent and anti-CD3 induced cytotoxicity are preferentially mediated by peripheral blood cytotoxic T lymphocytes expressing Leu-7 antigen

Monoclonal antibodies against the CD3 antigen and certain lectins can induce interleukin 2 dependent antigen-specific T cell clones to mediate non-antigen specific cytotoxicity. On the basis of this observation, we predicted that it may be possible to identify cytotoxic T lymphocytes (CTL) in peripheral blood without knowing the antigen specificity of these in vivo primed CTL. By using this strategy, peripheral blood lymphocytes were separated into low and high-density fractions on Percoll gradients and were tested for cytotoxic activity in the presence or absence of concanavalin A (Con A) or anti-Leu-4 antibody. Lectin-dependent cellular cytotoxicity (LDCC) and anti-CD3 induced cytotoxicity against both natural killer (NK)-insensitive and NK-sensitive targets were exclusively mediated by low-density CD3+ T lymphocytes. Additional studies indicated that low-density CD3+ T lymphocytes co-expressing Leu-7 antigen preferentially mediated this activity, although in some individuals, significant activity was also observed in the low-density T cells lacking Leu-7. In contrast, high-density CD3+ T lymphocytes and CD16+ (Leu-11+) NK cells (both Leu-7 and Leu-7+) did not mediate nonantigen-specific cytotoxicity under these conditions. The finding that NK cell-mediated cytotoxicity was unaffected by these lectins refutes the hypothesis that lectin-dependent cellular cytotoxicity is simply a result of effector and target agglutination. T cell-mediated cytotoxicity was both lectin and antibody specific. Phytohemagglutinin, Con A, and pokeweed mitogen induced cytolytic activity in the Leu-7+ T cells, whereas wheat germ agglutinin did not. Of the antibodies against T cell-associated differentiation antigens (anti-Leu-2,3,4, and 5), only anti-Leu-4 induced cytotoxicity. This anti-CD3-induced cytotoxicity was essentially completely inhibited by the presence of anti-LFA-1 or anti-CD2 monoclonal antibodies, implicating these molecules in the triggering process. A proportion of the CD3+, Leu-7+ CTL expressed HLA-DR antigens, indicating possible in vivo activation. Because previous clinical studies have indicated that lymphocytes with this phenotype may be elevated in clinical situations associated with immunosuppression and chronic viral infection, this unique subset of CD3+ T lymphocytes may represent a population of in vivo primed CTL possibly against viral antigens.     

IL-7 up-regulates IL-4 production by splenic NK1.1+ and NK1.1- MHC class I-like/CD1-dependent CD4+ T cells.

NK T cells are an unusual subset of T lymphocytes. They express NK1. 1 Ag, are CD1 restricted, and highly skewed toward Vbeta8 for their TCR usage. They express the unique potential to produce large amounts of IL-4 and IFN-gamma immediately upon TCR cross-linking. We previously showed in the thymus that the NK T subset requires IL-7 for its functional maturation. In this study, we analyzed whether IL-7 was capable of regulating the production of IL-4 and IFN-gamma by the discrete NK T subset of CD4+ cells in the periphery. Two hours after injection of IL-7 into mice, or after a 4-h exposure to IL-7 in vitro, IL-4 production by CD4+ cells in response to anti-TCR-alphabeta is markedly increased. In contrast, IFN-gamma production remains essentially unchanged. In beta2-microglobulin- and CD1-deficient mice, which lack NK T cells, IL-7 treatment does not reestablish normal levels of IL-4 by CD4+ T cells. Moreover, we observe that in wild-type mice, the memory phenotype (CD62L-CD44+) CD4+ T cells responsible for IL-4 production are not only NK1.1+ cells, but also NK1.1- cells. This NK1.1-IL-4-producing subset shares three important characteristics with NK T cells: 1) Vbeta8 skewing; 2) CD1 restriction as demonstrated by their absence in CD1-deficient mice and relative overexpression in MHC II null mice; 3) sensitivity to IL-7 in terms of IL-4 production. In conclusion, the present study provides evidence that CD4+MHC class I-like-dependent T cell populations include not only NK1.1+ cells, but also NK1.1- cells, and that these two subsets are biased toward IL-4 production by IL-7.     

Natural killer cytolytic activity is associated with the expression of killer cell immunoglobulin-like receptors on peripheral lymphocytes in human

Although it has been shown that killer cell immunoglobulin-like receptors (KIRs) on peripheral lymphocytes are upregulated by interleukin-2 (IL-2), which activates natural killer (NK) activity, it has not been demonstrated whether the expression of KIRs is related to NK activity. Therefore, we investigated the association between the KIR expression on lymphocytes and NK activity. CD158a/b expression on lymphocytes obtained from 37 subjects was analyzed using flow cytometry. Simultaneously, NK activity was measured each sample using a 51Cr-release assay. Additionally, lymphocytes were cultured in RPMI 1640 medium with or without IL-2 for 48 h, and then their CD158a/b expression and NK activity was analyzed. CD158a/b expression was significantly correlated with NK activity. Especially, the percentage of CD16+CD158a+ and CD8+CD158a/b+ cells in lymphocytes showed a highly significant correlation with NK activity. However, analysis of CD8+ and CD16+ cells revealed that there was only a significant correlation between the percentage of CD8+CD158a+ cells among only CD8+ cells and NK activity. The upregulation of CD16+CD158a+/b+ cells in response to IL-2 tended to be related to the increase of NK activity, but the relationship was not significant. In conclusion, the level of KIR expression was correlated with NK activity, and IL-2 treatment resulted in an increase of NK activity as well as KIR expression, suggesting that upregulation of KIRs enhances the ability to sort target cells, such as virus-infected cells from uninfected cells, according to major histocompatibility complex class I expression.     

Dendritic cells mediate NK cell help for Th1 and CTL responses: two-signal requirement for the induction of NK cell helper function

Early stages of viral infections are associated with local recruitment and activation of dendritic cells (DC) and NK cells. Although activated DC and NK cells are known to support each other's functions, it is less clear whether their local interaction in infected tissues can modulate the subsequent ability of migrating DC to induce T cell responses in draining lymph nodes. In this study, we report that NK cells are capable of inducing stable type 1-polarized "effector/memory" DC (DC1) that act as carriers of NK cell-derived helper signals for the development of type 1 immune responses. NK cell-induced DC1 show a strongly elevated ability to produce IL-12p70 after subsequent CD40 ligand stimulation. NK-induced DC1 prime naive CD4+ Th cells for high levels of IFN-gamma, but low IL-4 production, and demonstrate a strongly enhanced ability to induce Ag-specific CD8+ T cell responses. Resting NK cells display stringent activation requirements to perform this novel, DC-mediated, "helper" function. Although their interaction with K562 cells results in effective target cell killing, the induction of DC1 requires a second NK cell-activating signal. Such costimulatory signal can be provided by type I IFNs, common mediators of antiviral responses. Therefore, in addition to their cytolytic function, NK cells also have immunoregulatory activity, induced under more stringent conditions. The currently demonstrated helper activity of NK cells may support the development of Th1- and CTL-dominated type 1 immunity against intracellular pathogens and may have implications for cancer immunotherapy.     

Cellular and biochemical responses of human T lymphocytes stimulated with streptococcal M proteins

Purified group A streptococcal M proteins, pep M5 and pep M6, bearing heart cross-reactive epitopes were compared with pep M24, which lacks such epitopes, in their ability to induce functional differentiation of human T lymphocytes. Lymphocytes activated by pep M5 and pep M6 demonstrated cytotoxic activity against cultured heart cells, whereas pep M24-activated cells differentiated into suppressor T cells, which specifically blocked cytotoxic T lymphocytes against cultured human myocardial cells and not NK cell activity against K562 cells. Pep M5 and not pep M24 induced an increase in the number of CD4, 4B4, helper/inducer T cells. In addition, these M proteins appear to induce different biochemical changes in T lymphocytes. Both pep M5 and pep M24 induced the phosphorylation of a 35-kDa cytoplasmic protein; however, only pep M5 induced the phosphorylation of a 28-kDa membrane protein, primarily in CD4 T cells. These data indicate that the virulent M protein Ag of group A streptococci may exert their effect on the human immune system via different mechanisms. Determining these mechanisms and the biochemical pathways involved in T cell differentiation triggered by these Ag may be important in understanding the pathogenesis of post-streptococcal diseases.     

Robust NK cell-mediated human immunodeficiency virus (HIV)-specific antibody-dependent responses in HIV-infected subjects

Antibody-dependent cellular cytotoxicity (ADCC) is a potentially effective adaptive immune response to human immunodeficiency virus (HIV) infection. The study of ADCC responses has been hampered by the lack of simple methods to quantify these responses and map effective epitopes. We serendipitously observed that standard intracellular cytokine assays on fresh whole blood from a cohort of 26 HIV-infected subjects identified non-T lymphocytes expressing gamma interferon (IFN-gamma) in response to overlapping linear peptides spanning HIV-1 proteins. The effector cells were CD3(-) CD4(-) CD8(-) CD14(-) CD2(+) CD56(+/-) NK lymphocytes and degranulated granzyme B and perforin in response to antigen stimulation. Serum transfer assays demonstrated that the specific response was mediated by immunoglobulin G. Fresh blood samples from half of the HIV-infected cohort demonstrated robust HIV peptide-specific IFN-gamma expression by NK cells, predominately to Env, Pol, and Vpu HIV-1 proteins. Responses were readily mapped to define minimal epitopes utilizing this assay. Antibody-dependent, HIV-specific NK cell recognition, involving components of both innate and adaptive immune systems, represents a potentially effective immune response to induce by vaccination.     

Regulation of CD3- lymphocyte function with an antibody against the IL-2 beta chain receptor: modulation of NK and LAK activity and production of IFN gamma

To elucidate the role of interleukin 2 (IL-2) activation in CD3- lymphocytes, we examined the ability of monoclonal antibody (MAb) TU27, developed against the IL-2 receptor (IL-2R) p75 protein (IL-2R beta), to block lymphocyte activation with exogenous IL-2, as well as its innate ability to activate lymphocytes as a result of its surface ligand interaction. The binding of the TU27 MAb and the results of 125I-IL-2 cross-linking experiments suggest that the IL-2R beta chain is expressed primarily on CD3-, CD56+ lymphocytes; although the protein was also detected in a small portion of CD3+ cells, its expression appeared to be donor dependent. In the present study, we found that TU27 totally blocked natural killer (NK) cell activation in a 4-h assay but had no effect on basal levels of NK activity. When treatment was extended to 24 to 72 h, the MAb was able to block the induction of both NK and lymphokine-activated killer (LAK) activity. Of interest was the observation that MAb treatment alone augmented NK activity and subsequent interferon gamma (IFN gamma) production in CD3- lymphocytes but did not activate LAK activity or induce cell growth. Collectively, these results indicate that TU27 not only reacts with p70-75 IL-2R beta but can abrogate IL-2 binding and subsequent activation events. In addition, some CD3- lymphocyte functions (e.g., NK activity and IFN gamma secretion) are directly induced by the binding of MAb to p70-75 through signals that only partially mimic IL-2.