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.     

Virus-induced enhancement of lymphocyte-mediated antibody-dependent cytotoxicity (ADCC) in vitro

The effect of Parotis virus on antibody-dependent cellular cytotoxicity in vitro (ADCC) of human lymphocytes was investigated in a 51Cr-release assay and, at the effector cell level, in an ADCC plaque assay. Target cells were bovine or chicken erythrocytes, which are not susceptible to natural cytotoxicity (NK) of human lymphocytes. They were not killed when incubated with virus-treated lymphocytes in the absence of antibodies. Treatment of the lymphocytes or the target cells with small amounts of virus, however, resulted in a very significant enhancement of ADCC. The same results were obtained with live or UV-inactivated virus, suggesting that enhancement was a passive phenomenon not requiring infection. Enhancement was already significant after 3 hr of incubation, indicating that it was independent of endogenously released interferon. Enhancement of ADCC by virus was due to effector cell recruitment rather than due to the increase of the cytotoxic potential of the individual K cell. The highest frequency of effector cells was present in Percoll fractions enriched in large granular lymphocytes (LGL). Virus treatment resulted in recruitment of effector cells carrying T cell markers such as the T3 antigen (OKT3+), receptors for sheep erythrocytes, or Fc receptors for IgM. In contrast, the absolute number of K cells carrying the HNK-1 marker (Leu-7) or receptors for C3 fragments was not changed by the virus. It is concluded that Parotis virus enhances ADCC by improving effector cell-target cell contacts, resulting in recruitment of effector cells with T cell characteristics. Recruitment is accompanied by a significant reduction of the antibody concentration needed for ADCC induction. This virus-mediated enhancement of ADCC may be of importance for protection of the host in the early phases of a virus infection in which the amounts of anti-viral IgG antibodies capable of inducing cellular cytotoxicity may yet be very small.     

Cell-mediated cytotoxicity against virus-infected target cells in humans. II. Interferon induction and activation of natural killer cells.

The mechanisms by which human lymphocytes lyse virus-infected allogeneic fibroblast cultures were analyzed with particular consideration of the role of anti-viral antibodies and interferon. Human cells infected with viruses were able to induce high levels of interferon upon contact with human lymphocytes. Interferon, whether produced by lymphocytes after direct infection with virus or induced upon exposure of lymphocytes to virus-infected fibroblasts, appeared to be responsible for enhancing the cytotoxic efficiency of the natural killer cell against the infected target. Activation of cytotoxic lymphocytes occurred as early as 6 hr after addition of interferon and increased up to 24 hr. Antibody-dependent cell-mediated cytotoxicity (Ab-CMC) could be easily induced by sensitization of infected target cells with antiviral antibodies and could be detected at 4 hr from the beginning of the cytotoxic test, before the effect of interferon on the natural killer cell was evident. However, the antibody-dependent effector cell was inactive after 4 hr of incubation. F(ab')2 fragments of rabbit anti-human IgG completely inhibited Ab-CMC but did not at all affect the spontaneous cytotoxic activity of the effector cells against virus-infected target.     

Mechanism of defective NK cell activity in patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex. II. Normal antibody-dependent cellular cytotoxicity (ADCC) mediated by effector cells defective in natural killer (NK) cytotoxicity

Our studies and other investigations have shown that NK effector cells can also mediate antibody-dependent cellular cytotoxicity (ADCC) through the use of the Fc gamma receptor on the NK cell membrane. Peripheral blood lymphocytes (PBL) derived from patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex exhibit a poor NK activity due to a defective "trigger" required for activation in the lethal hit stage of the NK lytic pathway. Consequently, it was important to delineate whether the defect in AIDS NK cells affected the ADCC function. By using the 51Cr-release assay, the ADCC cytotoxic activity of AIDS PBL was found to be within the normal range, despite the absence of significant NK activity. Several experiments corroborated that the same effector cells mediate both NK CMC and ADCC. Depletion of Fc gamma R-bearing cells resulted in elimination of both the ADCC and NK cytotoxic functions. Single cell analyses, using one- and two-target cell conjugates, revealed that the frequency of ADCC effector:target conjugates and the frequency of killer cells from AIDS PBL were comparable to the frequencies seen in the normal controls. However, when mixtures of NK and ADCC targets were used to form mixed two-target conjugates, the AIDS effector cells lysed only the bound ADCC target, whereas the normal effector cells lysed both the bound NK and ADCC targets. These results demonstrate clearly that the same NK/K effector cells from AIDS PBL, defective in NK activity, are not impaired in mediating ADCC activity. These findings were supported by the demonstration that AIDS PBL stimulated with ADCC targets, but not with NK targets, released NK cytotoxic factors, postulated mediators of the NK CMC reaction. These findings indicate that the NK/K cells in AIDS are triggered normally for ADCC activity but are not triggered for NK activity. Furthermore, the results indicate that the lytic machinery is not impaired in the AIDS NK/K cells.     

Analysis of effector cells in human antibody-dependent cellular cytotoxicity with murine monoclonal antibodies

We examined purified human large granular lymphocytes, peripheral monocytes, and T cells for their ability to mediate antibody-dependent cellular cytotoxicity (ADCC) with murine monoclonal antibodies. We also evaluated the effects of pretreatment of cells with interleukin 2 and interferon to augment ADCC activity. MB3.6, a murine monoclonal antibody directed against the GD3 ganglioside, induced high levels of ADCC. This ADCC was mediated predominantly, if not completely, by human killer cells (large granular lymphocytes) whereas other effector cell populations demonstrated no significant cytotoxic activity in 6- or 18-hr assays. The IgG2a an anti-melanoma antibody 9.2.27 generated low or no ADCC with most normal donors or melanoma patients. IL 2 was a very potent booster of ADCC activity. Interferon alpha also was effective, whereas interferon gamma did not augment but rather inhibited reactivity. We tested a large panel of antibodies of various isotype against colon carcinoma cells and found that gamma-3 isotype antibodies more frequently generated ADCC and produced higher levels of cytotoxic activity than did IgG1 or IgG2 antibodies. It appears that a variety of parameters can affect ADCC reactions, including the type of effector cell and its level of activation, the isotype of the antibody, and properties of the target cell line such as its susceptibility to lysis.     

Evidence that natural cytotoxicity and antibody-dependent cellular cytotoxicity are mediated in humans by the same effector cell populations.

The present study strongly suggests that, in humans, natural killer (NK) activity and antibody-dependent cell-mediated cytotoxicity (ADCC) are mediated by the same effector cell population. This is supported by two different experimental approaches. First, competition for NK effector cells was accompanied by simultaneous inhibition of ADCC activity. Target cells sensitive to NK activity were capable of inhibiting specifically an ADCC assay in cold target competition experiments. Second, specific removal of NK cells on monolayers formed by target cells sensitive to NK activity caused simultaneous depletion of ADCC effector cells. In association with the removal on the monolayers of effector cells for ADCC as well as NK activity, we also found a significant depletion of cells bearing Fc gamma receptors.     

Inhibitory Fc receptors modulate in vivo cytotoxicity against tumor targets

Inhibitory receptors have been proposed to modulate the in vivo cytotoxic response against tumor targets for both spontaneous and antibody-dependent pathways. Using a variety of syngenic and xenograft models, we demonstrate here that the inhibitory FcgammaRIIB molecule is a potent regulator of antibody-dependent cell-mediated cytotoxicity in vivo, modulating the activity of FcgammaRIII on effector cells. Although many mechanisms have been proposed to account for the anti-tumor activities of therapeutic antibodies, including extended half-life, blockade of signaling pathways, activation of apoptosis and effector-cell-mediated cytotoxicity, we show here that engagement of Fcgamma receptors on effector cells is a dominant component of the in vivo activity of antibodies against tumors. Mouse monoclonal antibodies, as well as the humanized, clinically effective therapeutic agents trastuzumab (Herceptin(R)) and rituximab (Rituxan(R)), engaged both activation (FcgammaRIII) and inhibitory (FcgammaRIIB) antibody receptors on myeloid cells, thus modulating their cytotoxic potential. Mice deficient in FcgammaRIIB showed much more antibody-dependent cell-mediated cytotoxicity; in contrast, mice deficient in activating Fc receptors as well as antibodies engineered to disrupt Fc binding to those receptors were unable to arrest tumor growth in vivo. These results demonstrate that Fc-receptor-dependent mechanisms contribute substantially to the action of cytotoxic antibodies against tumors and indicate that an optimal antibody against tumors would bind preferentially to activation Fc receptors and minimally to the inhibitory partner FcgammaRIIB.     

High resolution mapping of the binding site on human IgG1 for Fc gamma RI, Fc gamma RII, Fc gamma RIII, and FcRn and design of IgG1 variants with improved binding to the Fc gamma R

Immunoglobulin G (IgG) Fc receptors play a critical role in linking IgG antibody-mediated immune responses with cellular effector functions. A high resolution map of the binding site on human IgG1 for human Fc gamma RI, Fc gamma RIIA, Fc gamma RIIB, Fc gamma RIIIA, and FcRn receptors has been determined. A common set of IgG1 residues is involved in binding to all Fc gamma R; Fc gamma RII and Fc gamma RIII also utilize residues outside this common set. In addition to residues which, when altered, abrogated binding to one or more of the receptors, several residues were found that improved binding only to specific receptors or simultaneously improved binding to one type of receptor and reduced binding to another type. Select IgG1 variants with improved binding to Fc gamma RIIIA exhibited up to 100% enhancement in antibody-dependent cell cytotoxicity using human effector cells; these variants included changes at residues not found at the binding interface in the IgG/Fc gamma RIIIA co-crystal structure (Sondermann, P., Huber, R., Oosthuizen, V., and Jacob, U. (2000) Nature 406, 267-273). These engineered antibodies may have important implications for improving antibody therapeutic efficacy.     

Studies on cytotoxicity generated in human mixed lymphocyte culture

Summary High levels of cytotoxic activity against the natural killer (NK) cell-sensitive target K562 and the NK-resistant target UCLA-SO-M14 (M14) can be generated in vitro either by mixed lymphocyte culture (MLC) or by culture of lymphocytes in interleukin 2 (IL2) (lymphokine activated killer (LAK) cells). The purpose of this study was to identify similarities and differences between MLC-LAK and IL2-LAK cells and allospecific cytotoxic T cells. Induction of cytotoxicity against K562 and M14 in both culture systems was inhibited by antibodies specific either for IL2 or the Tac IL2 receptor. Like NK effector cells, the precursors for the MLC-LAK cells were low density large lymphocytes. However these precursors differed from the large granular lymphocytes that mediated NK cytolysis in sensitivity to the toxic lysosomotropic agent L-leucine methyl ester (LME). The resistance of the MLC-LAK precursors to LME indicated that the precursors included large agranular lymphocytes. Although interferon-gamma (IFN-gamma) is produced in MLC and in IL2 containing cultures, it is not required for induction of either type of cytotoxic activity. Neutralization of IFN-gamma in MLC-and IL2-containing cultures with specific antibodies had no effect on the induction of cytotoxic activities. Both allospecific cytotoxic T lymphocyte (CTL) and LAK activities were enhanced by IL2 and IFN-gamma at the effector cell stage. However, the mechanism of cytolysis was different in the two systems. NK- and MLC-induced LAK activities were independent of CD3-T cell receptor complex while CTL activity was blocked by monoclonal antibodies specific for the CD3 antigen. These results suggest that NK and the in vitro induced LAK cytotoxicities are a family of related functions that differ from CTL. Furthermore, MLC-induced and IL2-induced cytotoxicities against K562 and M14 appear to be identical.This work was supported by NIH grant CA34442     

Activation of cloned human natural killer cells via Fc gamma RIII

The Fc gamma RIII (CD16) Ag on human NK cells involved in antibody-dependent cellular cytotoxicity has been demonstrated to be an important activation structure. The present studies were carried out to further characterize the functional role of the CD16 Ag and the mechanisms whereby cytotoxicity is activated by using human NK clones. In phenotypic studies Fc gamma RIII was found to be expressed heterogeneously on various human cloned NK cells. Expression on CD3- and CD3+ clones varied with the donor and mAb used for detection. Functional data demonstrated that cytotoxicity against NK-resistant target cells can be induced in CD3-CD16+ NK clones and CD3+CD16+ clones with NK activity when various CD16 mAb were used. CD16 antibodies but not reactive isotype control antibodies induced cytotoxicity. In contrast to complete CD16 antibodies F(ab')2 fragments were not able to activate the cytotoxic mechanism. Both an antibody against FcR on the target cell (Fc gamma RII) and a CD11a antibody blocked induction of cytotoxicity. These results suggest that three steps are critical for activation of CD16+ cells via Fc gamma RIII: 1) specific binding of CD16 antibodies to Fc gamma RIII on effector cells irrespective of the epitope recognized; 2) cross-linking of effector cell CD16 Ag through binding of the Fc site of CD16 antibodies via corresponding FcR on the target cell membrane; and 3) interaction of CD11a/18 molecules with the target cell membrane.     

Association of decreased natural and antibody-dependent cellular cytotoxicity and production of natural killer cytotoxic factor and interferon in neonates

Cord blood lymphocytes (CBL) were compared with adult peripheral blood lymphocytes (a-PBL) for their: (i) natural killer (NK) and antibody-dependent cellular cytotoxic (ADCC) activities, (ii) target-binding capacity, (iii) ability to induce soluble natural killer cytotoxic factor (NKCF), (iv) interferon (IFN)-, interleukin 2 (IL-2)-, and lectin-induced augmentation of NK activity, and (v) ability to produce IFN against tumor targets in vitro. CBL depleted of adherent cells and Percoll-separated, NK-enriched subpopulations demonstrated significantly lower NK, ADCC, and target-binding activities compared to a-PBL. CBL produced significantly lower levels of NKCF directed against K562 tumor targets in comparison with a-PBL. Although the NK activity of CBL was not stimulated by either IFN or IL-2 to the same levels shown by a-PBL, the percentage enhancement of cytotoxicity of CBL by IFN and IL-2 was greater than that of a-PBL. Lectin-induced enhancement of cytotoxicity was significantly greater for CBL in comparison with a-PBL. Further, the ability of CBL lymphocytes to produce IFN-gamma in vitro against K562 target cells was significantly lower than that of adult PBL. These studies suggest an association between decreased NK, ADCC, and target-binding activities, induction of NKCF and IFN production by CBL, and increased susceptibility of neonates to infection.     

Human monocyte cytotoxicity to tumor cells. I. Antibody-dependent cytotoxicity.

Recent investigations examining mononuclear cell antibody-dependent cell-mediated cytotoxicity against tumor cell lines suggest that K lymphocytes and not monocytes are active in this cytotoxic reaction. We have found, however, that in an allogeneic assay system, human monocyte monolayers as well as lymphocytes mediate substantial lysis of 51Cr-labeled antibody-coated CEM lymphoblast tumor cells. This cytotoxicity is temperature-dependent and rapid, with most 51Cr release occurring in the first 4 hr of co-incubation. Interaction between target cell-bound antibody and the monocyte Fc receptor is necessary as demonstrated by the marked fall in antibody-dependent cell-mediated cytotoxicity (ADCC) produced by staphylococcal protein A, high concentrations of nonspecific immunoglobulin, and dilution of the target cell antiserum. Morphologic and functional characteristics of the monocyte-monolayer preparations establish their relative purity (greater than 95%) and indicate that monocytes and not contaminating lymphocytes are responsible for tumor cell lysis. Furthermore, preincubation of monocyte and lymphocyte preparations with latex particles or low concentrations of immunoglobulin distinguished monocyte from lymphocyte ADCC. Thus, normal human monocytes have the capacity to carry out antibody-dependent cytotoxicity against nucleated malignant target cells.     

Bispecific monoclonal antibody regulation of Fc gamma RIII-directed tumor cytotoxicity by large granular lymphocytes.

Bispecific monoclonal antibodies (BsMAbs) prepared by somatic cell fusion bind monovalently to their targets and yet are extremely potent enhancers of target cell lysis by relevant effector cells. The mechanisms underlying this efficiency are not known. To investigate this property, we studied the ability of selected antibodies to modulate potentiation of tumor lysis by a bispecific antibody (CL158) which targets Fc gamma RIII-expressing cells, via the 3G8 epitope, to malignant cells expressing CA19-9 antigen. Antibodies directed against the 3G8 and B73.1 epitopes of Fc gamma RIII efficiently inhibited BsMAb-mediated SW948 tumor cell lysis by interleukin-2 (IL-2)-activated lymphocytes (PBLs). Unexpectedly, Leu 19 antibody reversed antibody-dependent but not antibody-independent lysis of 51Cr-labeled SW948 cells by IL-2-activated PBLs in a concentration-dependent fashion. Leu 19 binds to CD56, a neural cell adhesion molecule (N-CAM) isoform expressed by large granular lymphocytes (LGLs). The effects of Leu 19 on bispecific antibody promotion of lysis were due to competition for binding to the 3G8 epitope of Fc gamma RIII and led to inhibition of binding between LGLs and SW948 cells. Leu 19 did not inhibit antibody-dependent lysis by the monospecific, bivalent IgG2a variant of CA19-9 antibody. These studies show that competition assays can be useful in dissecting the relevant mechanisms underlying BsMAb-promoted lysis. Steric constraints between effector cell trigger molecules (i.e., Fc gamma RIII) and CAM such as N-CAM may regulate the function of these molecules. Understanding the roles of diverse CAM in this phenomenon will facilitate efforts to expand and use defined effector cell populations with maximal lytic potential and to identify potentially responsive tumor phenotypes.     

Lymphocyte-mediated cytotoxicity to autologous cells infected with measles virus. II. Specificity of the cytotoxic reaction and characterization of the effector cells involved.

The mechanism of lymphocyte-mediated cytotoxicity to cells infected with measles virus was investigated. Cytotoxicity was measured in a direct assay, immediately after the isolation of lymphocytes from human peripheral blood; mononuclear leukocytes, infected with measles virus in vitro, served as autologous target cells. Virus-specific cytotoxicity required the presence of both IgG antibodies against measles virus and of effector lymphocytes. The effector lymphocytes had Fc receptors and were mainly present in a fraction of non-T lymphocytes. Monocytes were not cytotoxic but rather inhibitory. These results indicate that lysis of virus-infected cells in this direct assay is due to antibody-dependent cellular cytotoxicity (ADCC), caused by K cells. Control experiments showed that the virus-infected target cells were sensitive to incubation with human serum or IgG, resulting in a nonspecific increase of ⁵¹Cr release; however, this did not affect the results of K-cell cytotoxicity. Maximal virus-specific lysis by ADCC did not reach the level obtained by complement-dependent cytotoxicity. Possible explanations for this difference are discussed.     

A role for protein tyrosine kinase activity in natural cytotoxicity as well as antibody-dependent cellular cytotoxicity. Effects of herbimycin A.

NK cells, CD3- large granular lymphocytes, have diverse means by which they lyse targets, including antibody-dependent cellular cytotoxicity. The low affinity receptor for the Fc portion of Ig (Fc gamma RIIIA), like the TCR, is a multimeric receptor complex coupled to a protein tyrosine kinase. In the present study, we observed that inhibition of tyrosine kinase activity by herbimycin A interferes with receptor-mediated phosphorylation of a variety of substrates and mobilization of intracellular calcium. Fc gamma RIIIA induced IL-2R alpha-chain expression was also extremely sensitive to herbimycin A as was antibody-dependent cellular cytotoxicity, in fact more so than receptor-mediated phosphorylation and calcium mobilization. In contrast to Fc gamma RIIIA, the surface molecules and biochemical mechanisms involved in NK cytotoxicity and lymphokine-activated killing are not well characterized. Interestingly, however, herbimycin A also blocks these modes of cytolysis, implicating a role for tyrosine kinase function in these processes. Whether FcR-mediated signaling and receptor-mediated signaling involved in NK activity share specific biochemical intermediates is not known, but the involvement of tyrosine kinase function in the latter means of cytotoxicity may provide novel avenues for understanding the biochemical basis of this perplexing cellular function.     

Lysis of dengue virus-infected cells by natural cell-mediated cytotoxicity and antibody-dependent cell-mediated cytotoxicity

Peripheral blood mononuclear cells (PBMC) from humans without antibodies to dengue 2 virus lysed dengue 2 virus-infected Raji cells to a significantly greater degree than uninfected Raji cells. The addition of mouse anti-dengue antibody increased the lysis of dengue-infected Raji cells by PBMC. Dengue 2 immune human sera also increased lysis of dengue-infected Raji cells by PBMC. These results indicate that both PBMC-mediated cytotoxicity and antibody-dependent cell-mediated cytotoxicity (ADCC) can cause significant lysis of dengue-infected Raji cells. The lysis of infected Raji cells in the ADCC assay correlated with the dilution of dengue-specific antibody which was added, indicating the dengue virus specificity of the lysis of dengue virus-infected Raji cells. Alpha interferon (IFN alpha) was detected in the culture supernatant of PBMC and dengue-infected Raji cells. However, enhanced lysis of dengue-infected Raji cells by PBMC may not be due to the IFN produced, because neutralization of all IFN activity with anti-IFN alpha antibody did not decrease the lysis of dengue-infected cells, and effector cells pretreated with exogenous IFN alpha also lysed dengue-infected cells to a greater degree than uninfected cells. The effector cells responsible for lysis of dengue virus-infected Raji cells in the natural killer and ADCC assays were analyzed. Nonadherent PBMC caused more lysis than did adherent cells. Characterization of nonadherent cells with monoclonal antibodies showed that the predominant responsible effector cells were contained in OKM1+ and OKT3- fraction in the natural killer and ADCC assays.     

Antibody-dependent cellular cytotoxicity effector cells lack Ia antigens.

Surface immunoglobulin (sIg)-positive and sIg-negative subpopulations of macrophage-depleted murine splenic lymphocytes were obtained by Sephadex anti-Fab immunoabsorbent fractionation. These lymphocyte subpopulations were analyzed for the presence of Thy 1 and Ia alloantigens and also for Fc receptors by fluorescence microscopy. Concurrently, these lymphocyte subpopulations were studied for effector cell activity in antibody-dependent cellular cytotoxicity (ADCC). Effector cells mediating ADCC were contained in the sIg-negative lymphocyte subpopulation and sIg-positive lymphocytes did not mediate cytotoxicity. The majority of sIg-positive lymphocytes were found to bear Ia antigens and Fc receptors, and these cell surface structures were associated in that treatment of these cells with anti-Ia sera inhibited binding of complexed immunoglobulin to Fc receptors. In contrast, most sIg-negative, Thy 1-negative lymphocytes lacked Ia Antigens, and the Fc receptors detected on such cells were not blocked by anti-Ia sera. In addition, a small subpopulation of sIg-negative, Ia antigen-positive, Fc receptor-positive lymphocytes was found. Elimination of this subpopulation of Ia antigen-positive cells from sIg-negative lymphocytes, by treatment with anti-Ia serum and complement, did not diminish ADCC effector cell activity in the resultant cell population when compared with untreated sIg-negative lymphocytes. Thus, in murine spleen, nonphagocytic mononuclear cells that lack both sIg and Ia antigens were shown to mediate ADCC.     

Induction of nonspecific cytotoxicity by monoclonal anti-T3 antibodies

The effects of monoclonal anti-T3 antibodies on the effector phase of cytotoxic T lymphocytes (CTL) were studied with respect to antigen-specific and antigen-nonspecific lysis of different target cells. Anti-T3 antibodies inhibited the antigen-specific lysis by CTL generated in mixed lymphocyte cultures (MLC), but they concomitantly augmented the nonspecific killing of third-party cells such as the cell lines Daudi, Raji, and K562. This nonspecific cytotoxicity was induced by various anti-T3 antibodies, whereas antibodies reactive with other antigens expressed on the cytotoxic effector cells lacked any such activity. Anti-T3 antibodies induced nonspecific cytotoxicity only when activated T cells, obtained by primary MLC, by repeated restimulation, or after cloning, were used. The antibodies had no effect on unstimulated peripheral T lymphocytes or thymocytes. The inhibition of the antigen-specific lysis and the induction of nonspecific lysis by anti-T3 was dose dependent, and both effects occurred at the same concentration range of anti-T3. F(ab')2 fragments of anti-T3 inhibited the specific lysis but were not able to induce cytotoxic activity, indicating that this induction is an Fc-dependent process. When different target cells were tested, only Fc receptor-positive cells were susceptible for this nonspecific cytotoxicity. Thus, anti-T3 antibodies have a dual effect on effector CTL: they inhibit antigen-specific lysis and concomitantly induce nonspecific lysis in an Fc-dependent way.     

The role of apoptosis in antibody-dependent cellular cytotoxicity

Apoptosis in three lymphoma cell lines has been studied following cytotoxicity induced in vitro by normal human blood lymphocytes utilizing either natural killer (NK) or antibody-dependent cellular cytotoxic (ADCC) mechanisms. Guinea-pig L₂C leukaemic lymphocytes, but not the human cell lines Daudi and Jurkat, revealed a degree of time- and temperature-dependent apoptotic death upon simple culture in vitro. NK cytotoxicity at low effector: target ratios (E: T) induced both release of⁵¹Cr and apoptosis. However NK cytotoxicity at higher E : T, and ADCC at all E : T, increased the level of⁵¹Cr release while reducing the level of apoptosis. The findings were consistent with the apoptotic process being cut short by intervention of necrotic death. The same characteristics accompanied ADCC whether the effectors were recruited by Fc regions of antibody coating the targets, or by bispecific antibodies attaching one arm to the targets and the other to Fc receptors type III on effectors. This finding, and the high level of cytotoxicity elicited by the bispecific method, confirm the belief that NK cells, in addition to exerting NK cytotoxicity, represent the principal effectors for ADCC among blood mononuclear cells. Our results suggest that NK cells have both apoptotic and necrotic mechanisms available for killing their targets, but use only the latter for ADCC.     

Different membrane anchors of Fc gamma RIII (CD16) on K/NK-lymphocytes and neutrophils. Protein- vs lipid-anchor

Fc gamma RIII (CD16), the type three receptor for the Fc portion of IgG, is expressed on neutrophils, killer (K)/NK lymphocytes and macrophages. K/NK lymphocyte Fc gamma RIII, which plays a role in antibody-dependent cellular cytotoxicity, is an efficient signal transducing molecule, whereas neutrophil Fc gamma RIII, which plays a role in immune-complex clearance, seems less efficient in signal transduction. Neutrophil Fc gamma RIII has been reported to be a glycan-phosphatidylinositol-anchored membrane protein. Our studies suggest that K/NK lymphocyte Fc gamma RIII is protein-anchored rather than glycan-phosphatidylinositol-anchored. That is, K/NK lymphocyte Fc gamma RIII was resistant to phosphatidylinositol-specific phospholipase C and surface expression of Fc gamma RIII was not affected on K/NK lymphocytes from patients with paroxysmal nocturnal hemoglobinuria, a disorder of hemopoietic stem cells resulting in deficient expression of glycan-phosphatidylinositol-anchored proteins. Different membrane anchoring mechanisms of the Fc gamma RIII may account for different consequences of the ligand binding to two cell types.