Two independent receptors allow selective target lysis by T cell clones

Production and characterization of a novel monoclonal antibody inhibitory for murine natural killer cell activity

Natural killer (NK) cells are considered to play an important role in tumor surveillance. The killing of tumor target cells by NK cells is the result of a complex series of sequential binding, signal processing and lytic events. However, the mechanism which NK cells use to recognize tumor targets is poorly understood. To further study the cell-surface molecules involved in tumor recognition, we immunized rats against cloned murine T cells with NK activity (DBA/2.1) and generated rat-mouse hybridomas which were screened for the ability to block lytic activity of DBA/2.1 effector cells. Culture supernatants from one IgM-producing hybridoma, designated S1C4, were found to consistently inhibit DBA/2.1-mediated lysis of YAC-1 target cells. Endogenous splenic NK activity was also diminished in the presence of S1C4 monoclonal antibody (mAb) while alloantigen-specific cytotoxic T lymphocyte (CTL) activity was not affected. S1C4 mAb appears to react with effector cell-surface structures involved in the recognition/adhesion phase of NK activity since pretreatment of effector cells with mAb S1C4 inhibits their ability to bind to YAC-1 target cells. ELISA studies revealed that the S1C4 antigen is expressed by a range of lymphoid cell lines, as well as by DBA/2.1 cells and fresh splenic NK cells. S1C4 mAb were shown to react with 22, 24, 30, and 46 kiloDalton (kDa) DBA/2.1 cell membrane components on immunoblots performed under reducing conditions. These structures do not correspond to any known recognition/adhesion molecules, suggesting that mAb S1C4 defines novel cell membrane components involved in NK cell function.

Rearrangement and expression of T-cell receptor genes in marmoset lymphoid cell lines with natural killer activity

To study the role of the T-3/T-cell receptor complex in mediating natural killer activity, we have examined T-cell receptor gene configuration and expression in five transformed marmoset cell lines with natural killer activity. All lines showed rearrangement of the beta chain of the T-cell receptor and expressed full size beta chain transcripts as well as full size delta chain transcripts. Only three lines contained full size T-cell receptor alpha chain transcripts. Of the three lines which had potential for encoding functional T-3/T-cell receptor complex, those which possessed natural killer activity also expressed T-gamma transcripts. In addition, one clone with high natural killer activity which did express high levels of T-gamma mRNA was unable to produce functional T-3/T-cell receptor complex due to truncated T-cell receptor alpha chain message. The results in this paper provide strong evidence that natural killer activity is dependent upon expression of the T-cell receptor gamma chain.

The role of T cell receptors in non-MHC-restricted cytotoxicity

The relationship between natural killer cells (NK) and cells of the T lineage has been obscured by the existence of poorly characterized clones of presumed NK origin. We have analyzed nine of these cloned cell lines displaying varying levels of cytotoxic activity against NKS YAC-1 target cells for rearrangement and expression of the genes encoding the alpha, beta, and gamma chains of the T cell receptor for antigen. Rearrangements at both the TcR beta and gamma loci were detected in all clones often at both alleles. Rearrangement patterns at the TcR beta locus were identical in several clones, despite different degrees of cytotoxicity. T cell receptor alpha, beta, and gamma genes were expressed as full length transcripts in all clones regardless of their levels of cytotoxic activity. To explore the involvement of cell surface molecules in the cytolytic events, studies were undertaken to determine whether cytotoxic activity could be inhibited by antibodies against CD3, LFA-I, and H-2KdDd. In two selected clones, both alpha and beta chains of the LFA-I molecule were expressed but only monoclonal antibodies against the alpha chain significantly blocked cytotoxicity. Cytotoxicity was also inhibited by monoclonal antibodies against epitopes of H-2KdDd and CD3, the extent of inhibition correlating with the level of surface expression on both clones. These data suggest that conventional alpha/beta heterodimers may be necessary but not sufficient for target cell recognition by these clones. Since T cell receptor rearrangement and expression occur normally in the T cell lineage but not the NK lineage, these results also indicate that a subpopulation of cells with non-MHC-restricted killer activity lies on the T cell differentiation pathway and is selected by in vitro growth with IL-2. The limited rearrangement pattern observed can be explained if only a small subpopulation of T cells is capable of non-MHC-restricted killing, and if certain rearrangements favor self-MHC recognition which is known to block cytolysis in the NK system.

Inhibition of natural killer cell cytotoxicity by a monoclonal antibody directed against adhesion-mediating protein gp 90 (CD18)

Contact is required between the effector natural killer (NK) or cytotoxic killer T cell and its corresponding target in order for efficient lysis to occur. Several surface molecules of different types are involved in this process. Here we could show that Fab fragments from a murine monoclonal antibody reacting with gp 90, the human leucocyte common antigen CD18, are extremely efficient in blocking human NK of killer T cells, regardless of whether the target has or does not have the antigen. In contrast, no impact of the antibody was observed when the effector cells were of murine origin, again regardless of whether the target cell did or did not display the leucocyte common antigen. The inhibition could be shown to occur at the level of blockage of target-conjugate formation. This means that the functional display of effector/target gp 90 on the effector but not the target cell is necessary for efficient lysis to occur both in NK and killer T cell systems.

Activation of autoreactive cytolytic T lymphocyte clone against human melanoma by anti-T3 monoclonal antibody and autologous accessory cells

A cytotoxic T-lymphocyte (CTL) clone Tc1.8 was derived in a limiting dilution culture from a single cell that was derived from melanoma-involved lymph node lymphocytes activated in in vitro coculture against the autologous melanoma cells (VIP). The clone Tc1.8 (T3+, T8+, T4-, and Leu7-) expressed restricted cytolytic activity against only the autologous target VIP. As it aged in continuous culture containing interleukin 2, Tc1.8 lost cytolytic activity. The cytolytic function could be restored, however, with monoclonal antibody (MoAb) against T3 (OKT3) or with F(ab')2 fractions of OKT3, and upon restimulation with irradiated accessory cells. OKT3-mediated reinduction of cytotoxicity by the aged Tc1.8 could not be achieved if the T3 molecules were modulated from the effector cell surface following overnight incubation of Tc1.8 with saturating concentrations of OKT3 MoAb. Following reactivation with OKT3 Tc1.8 gained cytolytic function against NK targets in addition to VIP. Reactivation with F(ab')2 fractions of OKT3 and with autologous accessory cells, however, maintained its restricted antigen fidelity. The NK-like activity of Tc1.8 upon reactivation with OKT3 resulted from conjugate formation between the activated Tc1.8 and NK targets via the activating ligand itself. Thus, upon stimulation with anti-T3 MoAb and with autologous accessory cells, independently, the autoreactivity could be restored in an aged and inactive CTL clone.

Class I H-2Db determinants are not involved in hybrid resistance to parental H-2b/Hh-1b bone marrow allograft

Hybrid resistance to parental H-2b bone marrow grafts is directed to a cell surface structure controlled by the Hh-1 locus in or near the H-2D region. The nature of this surface structure is not known. Since homozygosity at the class I H-2D locus or loci in this haplotype would seem a necessary but not sufficient condition for the grafts' susceptibility to resistance, we tested whether the expression of this phenotype is dependent on the expression of class I H-2Db determinants. Cloned variants of H-2b tumor RBL-5 were obtained by immunoselection for the absence of H-2Db expression, as determined by the inability to bind specific antibody and to induce or react with alloreactive cytotoxic T lymphocytes. The three clones used in this study were H-2Db negative but H-2Kb positive and were natural killer cell resistant. When tested in vivo as competitive inhibitors the variant cells were capable of blocking hybrid resistance to parental H-2b bone marrow grafts as were unselected H-2Db-positive parental line cells. Therefore, H-2Db expression is irrelevant for Hh-1b expression. An incidental observation was that YAC-1 cells, a non-H-2b tumor with pronounced susceptibility to natural killing, were able to block hybrid resistance. This reactivity, not observed in our previous studies, raises the possibility that at least some of the effector cells are cross-reactive or capable of dual recognition.

Cloned "anomalous" killer cells derived from allogeneic mixed leukocyte culture

In addition to allospecific cytotoxic lymphocytes, cytolytic effector cells capable of killing a broad range of targets are generated during mixed leukocyte culture (MLC). These cells, which have been previously called anomalous killer cells, are a distinct functional subset separate from natural killer cells or allospecific cytotoxic lymphocytes but display many characteristics of lymphokine-activated killers. In order to isolate anomalous killer cells for detailed analysis, we generated the cytolytic effectors from an allogeneic MLC using heat-inactivated stimulators. This treatment of the stimulator population abrogated the generation of classical allospecific cytotoxic lymphocytes but allowed the generation of anomalous killer cells which were subsequently cloned via limiting dilution. The clones derived by this method displayed the functional properties of anomalous killers seen in bulk MLCs. The clones demonstrated potent cytolytic activity against both NK-sensitive and NK-resistant tumor targets in vitro and also suppressed tumor growth in vivo. Ultrastructural studies revealed features similar to those of cloned antigen-specific cytolytic cells and clones with NK-like function. The cells expressed surface glycoproteins associated with both NK and T lymphocytes including Thy-1, Ly-2, T200, Qa-5, asialo GM1, and the antigens defined by the NK alloantisera NK-2.1 and NK-3.1. These cells may play an important role during early phases of the immune response, since cytolytic cells of broad specificity may protect the host until classical cytotoxic lymphocytes with restricted specificity are generated.

Development of natural killer cells in human thymocyte culture: regulation by accessory cells

In vitro culture of human thymocytes resulted in the development of cells with natural killer (NK) activity and the acquisition of a pan-NK antigen (NKH1) by a large number of thymocytes. The ability to kill the NK-sensitive target, K562, was restricted to thymocytes expressing the NKH1 antigen. All NKH1+ thymocytes displayed a mature T cell phenotype, T3+T11+T8+T4-. Both the acquisition of NK activity and the development of cells with the NKH1+ phenotype could be suppressed by culturing thymocytes in the presence of adherent mononuclear cells. These results suggest that adherent accessory cells have the ability to regulate the development of T cell lineage NK cells.

Constitutive functional heterogeneity without detectable somatic mutation of antigen receptor genes in helper T cell clones: possible regulation by novel HLA class II "DY" determinants

The majority of interleukin 2-dependent HLA class II alloreactive human CD4-positive helper T-cell clones (TCC) presented the phenomenon of constitutive intraclonal heterogeneity, as reflected by limiting dilution analysis of changing frequencies of autonomously proliferative cells within the monoclonal population. Moreover, at 30-35 population doublings, these TCC lost their allospecific proliferative and helper capacity (phase I) and instead acquired strong antigen-non-specific suppressive activity (phase II). The TCR was still expressed at the same level on the cell surface. As shown by stable and identical rearrangement profiles of their T-cell receptor beta- and gamma-chain genes in both phases I and II, this constitutive change of function was probably not due to somatic mutation of the genes coding for the antigen-specific receptor. Moreover, antigen specific reactivity was retained in phase II TCC by the criterion of specifically stimulated secretion of granulocyte/macrophage colony stimulating factor. The preprogrammed intra-clonal functional flexibility is explained here by invoking a novel regulatory mechanism mediated by a fourth group of HLA class II determinants. When TCC were used as stimulators with the appropriate primed responders, it could be demonstrated that phase I autonomously proliferative non-suppressive T-helper lines failed to express certain novel lymphocyte activating determinants (LADs), whereas after their acquisition of suppressive function (phase II) these LADs were present. These stimulatory moieties appeared to represent a novel class II-like structure as established by serology, immunochemistry and functional characterization employing monoclonal antibodies to block stimulation. These operationally designated "DY" determinants are primarily and thus far exclusively involved in the induction of antigen non-specific suppressor cells. It is proposed that the intra-clonal changes of T-cell function described here are mediated by a switch in the utilization of qualitatively distinct class II-like restriction elements. The functional status of the phase I TCC can be altered by a switching over to the engagement of "DY" determinants in phase II. Thus, an autostimulatory and self-maintaining suppressive network may operate in extended inductive phases of human HLA restricted T-cell responses.

Human lymphokine-activated killer cells (LAK cells) as a potential immunotherapeutic modality

Lymphokine-activated killer cells (LAK) are cytolytic lymphocytes with the unique capacity of killing NK-resistant fresh human tumor cells in short-term assays. LAK appear to kill autologous tumors as well as TNP-modified self and allogeneic tumors with complete crossreactivity, both at the population and clonal level. Initial studies on the classification of LAK conclude that LAK are distinct from the classical NK and T-lymphocyte systems based on a number of criteria including surface phenotype, activation conditions, and spectrum of susceptible target cells. LAK kill rasoncogene-transfected fibroblasts in a manner similar to fresh tumors. As yet, the target cell determinant responsible for susceptibility to LAK lysis is unknown, but cell-surface proteins are definitely involved. Activation of LAK requires only IL-2, and is most efficient using serum-free conditions. Because interleukin-2 alone is sufficient for LAK activation, we have tested in vitro whether fresh PBL could be activated in the presence of tumor, as might be desired in vivo. LAK activation was greatly suppressed by tumor presence. LAK activation is also suppressed by hydrocortisone, but not cyclosporine A. Because of the above and other findings, we have initiated a clinical protocol to test whether LAK made from brain-tumor patients' PBL could eliminate residual glioma tumor cells. Autochthonous LAK, plus rIL-2 to maintain lytic ability, are injected during surgery. Preclinical studies in a rat glioma model have shown this approach to be safe. Eleven glioma patients have been injected intracerebrally with IL-2 and/or LAK with no immediate or long-term (14 months follow-up) adverse effects. Much work is needed to understand the LAK phenomenon and to resolve its potential usefulness in cancer therapy as well as its inherent biologic role.

Leukemic cells arise from cloned cytotoxic lymphocytes during cell culture

In spite of many promising attempts to apply T cell clones to questions of in vitro and in vivo function of T cells it is still unclear to what extent continuous propagation of T lymphocytes in vitro effects their original properties. This study describes the appearance of malignant cells from long-term cultured C57BL/6 (B6) cytotoxic T lymphocytes (CTL). Four out of five T cell lines (CTLL.1,3,4,5) representing distinct stages of development of T effector cells in vitro were repeatedly cloned and all five CTLL were tested for various cellular parameters. It is shown that transformation of H-Y-specific CTLL into malignant cells in vitro was accompanied by alterations in growth characteristics, successive loss of specificity and cytolytic function and by quantitative changes in the expression of cell surface markers. Whereas growth of the H-Y-specific CTLL (CTLL.1) was dependent on antigen and concanavalin A (Con A) supernatant (Con ASN) the CTLL variants could be either maintained in Con ASN alone (CTLL.3) or in the absence of both antigen and lymphokine sources (CTLL.4,5). CTLL.1 was cytolytic for male B6 target cells and lysed P815 tumor targets in the presence but not the absence of lectin. In contrast, CTLL.3 lost its original specificity but lysed P815 cells in the absence or presence of lectin. CTLL.2 representing an intermediary stage showed cytolytic activity on both male B6 and P815 target cells. In contrast, CTLL.4 and CTLL.5 lost the ability to lyse any of the indicated target cells. Although all CTLL expressed the surface markers Thy-1, Lyt-2, Kb, Db and interleukin 2 receptor (IL 2 R), Thy-1 and Lyt-2 markers were drastically reduced and Kb/Db and IL 2 R structures significantly increased on CTLL.4 and CTLL.5 compared to CTLL.1,2,3. In addition, multiple karyotypic alterations including the appearance of metacentric chromosomes were observed in long-term cultured CTLL. Investigations on the expression of the alpha-, beta-, and gamma-chains of the T cell antigen receptor in CTLL.1-5 indicate that all three chains were expressed as mRNA irrespective of whether the lymphocytes expressed their original specificity and/or function. However, distinct beta variable chain genes were used by H-Y-specific CTLL and its long-term culture variants CTLL.2 and CTLL.3 suggesting that the expression of the new specificity was accompanied by the rearrangement of a new beta-chain gene in T effector cells.(ABSTRACT TRUNCATED AT 400 WORDS)

T cell receptor gene rearrangements in cells with natural killer activity in the mouse

Cell-mediated recognition can operate at different levels of complexity and specificity based largely on the time of appearance of effector mechanisms during the course of evolution. Antigen-specific cytotoxic T lymphocytes require both T cell receptor genes and lectin-like cell adhesion molecules (LFA-1, LFA-2, lymphocyte function-associated) to initiate and maintain stable effector target cell conjugates. Natural killer (NK) cells, on the other hand, do not require expression of T cell receptor genes in the recognition and killing of tumor cells and virally infected cells. Adhesion is mediated by a family of glycoprotein molecules, of which the LFA-1 and LFA-2 molecules appear as the most likely candidates. NK-mediated cytolysis proceeds in the absence of MHC restriction, but nevertheless appears to be triggered by depressed levels of self MHC products on the cell surface of target cells. Finally, interleukin 2-dependent, cloned cell lines with NK-like cytotoxic activity should no longer be considered as bona-fide NK cells but rather reclassified as a subset of T cells which displays NK function.

Involvement of the T cell antigen receptor and of Lyt-2 in the cytotoxic function of aged killer (AK) T cells

Aged killer (AK) T cells are antigen-independent, IL-2-requiring variants of antigen-dependent CTL clones that have lost their original antigen specificity and have acquired, instead, specific cytotoxicity for P815 target cells. In this report we study whether AK cells use a similar or a different target cell recognition system than that of bona fide CTL. To this end, we selected from a cloned AK line variants that are partially or completely deficient in specific target recognition and/or in cytotoxic function, and analyzed these variants for expression of the T cell antigen receptor and of Lyt-2. Variants were selected from the prototype AK line (Cl 96) with specific, as well as lectin-facilitated, cytotoxicity for P815 tumor cells. Variants could be grouped into four types with increasing degrees of functional deficiency, which correlated with loss of T cell receptor and/or loss of Lyt-2. In short, loss of Lyt-2 was reflected in loss of specific target recognition, and loss of the T cell antigen receptor was reflected in loss of all cytotoxic activity. We conclude from these results that both Lyt-2 and the T cell antigen receptor are required for specific target cell recognition and the T cell antigen receptor is, in addition, required for cytotoxic function. Moreover, since AK cells express a somatically acquired specificity that differs from that of their clonal precursors, it appears that cytotoxic T cells may change their antigen receptor from one specificity to another during tissue culture.

Distinct T-cell proliferative responses to 13762A rat mammary adenocarcinoma and derived clones

We examined the in vitro responses of immune lymphocytes to the tumor antigens of the syngeneic rat mammary adenocarcinoma 13762A. This tumor readily metastasizes to lymph node and lungs and is poorly immunogenic. Rats were immunized with a highly immunogenic clone (18A) which was isolated as a spontaneous variant from the parental 13762A tumor. Clone 18A grew progressively in irradiated rats but regressed completely in normal rats. Animals immune to 18A tumor were also immune to parental 13762A. Lymphocytes obtained from the spleen and peritoneum of immune rats were tested for specific proliferation to parental 13762A tumor and clone 18A to determine whether similar cross-reactivity to these tumors occurred in vitro. We found an anatomical difference in localization of immune lymphocytes which reacted to the two tumor cell lines. Immune peritoneal exudate cells (PEC) responded strongly to clone 18A but poorly to 13762A, while immune spleen cells from the same animals responded predominantly to 13762A tumor. After 7 days culture, PEC proliferating in response to clone 18A contained 84-95% W3/25+ T-helper cells, and only 5-8% OX8+ cytotoxic/suppressor cells, while analogous cultures of spleen cells responding to parental 13762A tumor consisted of 60-80% W3/25+ cells and 20-23% OX8+ cells. Immune spleen cell cultures stimulated with 13762A tumor generated cytotoxic lymphocytes which specifically lysed both parental 13762A and clone 18A cells. We conclude that despite cross-reactivity in vivo and in vitro, antigens present on 13762A and 18A tumor cells stimulated different subsets of immune T cells.

Cytotoxic capabilities of lymphocytes from patients with the acquired immunodeficiency syndrome

Lymphocytes from 16 AIDS patients were tested in the cell-mediated lympholysis assay (CML). The ability to produce alloreactive cytotoxic lymphocytes in vitro was found to be substantially reduced when compared with concomitantly investigated normal controls. Addition of interleukin 2 (IL-2) to the inducer cultures increased the cytotoxic activity, but not to normal levels. The CML response did not correlate with the relative or absolute number of Leu 3+ cells or the proliferation in effector suspensions. The ability to produce cytotoxic cells in CML, and the degree of potentiation by IL-2, was positively correlated with the absolute number of Leu 2+ cells in peripheral blood of the patients, which was below normal in 56% of the patients. It is suggested that the low CML in AIDS patients is primarily caused by defective T-cell help. In addition patients with decreased absolute numbers of Leu 2+ cells may have a reduced number of CTL precursors. The natural killer (NK) activity of AIDS lymphocytes was reduced, but could be improved by incubation with IL-2 in vitro. The mononuclear cells from the patients showed a decreased ability to respond and to stimulate in the mixed lymphocyte culture. In one of the AIDS patients, the CML was found to induce autoreactivity in vitro.

Degradation of specificity in cytolytic T lymphocyte clones: two broad specificity, H-2-independent recognition systems, one natural killer-like, develop during culture, in addition to the clonally distributed antigen-specific receptor

Ly-2+ CBA mouse T lymphocytes stimulated with concanavalin A in limiting dilution culture produce clones of cytotoxic T lymphocytes (CTL) which, although initially specific, eventually lyse a wide range of target cells. The nature of the recognition system for this apparently "nonspecific" cytolysis was examined using a range of tumor cells as labeled targets and as cold target inhibitors. Most syngeneic and allogeneic murine tumor cells were lysed but the degree of lysis varied, even for different sublines of the same tumor. All tumor cells cold target inhibited their own lysis, and cross-inhibited lysis of other targets to varying degrees. The recognition stage of "nonspecific" cytolysis appeared to be independent of target cell H-2 expression; some H-2-negative murine target cells were lysed and some were not, but all gave cold target inhibition of "nonspecific" cytolysis. Xenogeneic tumor cells were resistant to lysis, but some nevertheless gave cold target inhibition of the "nonspecific" cytolysis of murine targets. A study of the specificity of cold target cross-inhibition revealed two distinct patterns of recognition which existed simultaneously in "nonspecific" CTL; one was like that of natural killer cells and was directed to targets such as YAC-1, the other was distinct from that of natural killer cells and was directed to targets such as P815. Thus, murine CTL may express three distinct receptors, the clonally distributed, H-2-restricted, antigen-specific T cell receptor and two different "broad-range" receptors common to most clones.

The analysis of an anti-fluorescein response at limit dilution: the effect of lymphokine preparations on the apparent specificity

The specificity of cytotoxic T cells in an anti-fluorescein response has been analysed at limit dilution. According to the ability to discriminate between fluoresceinated and non-fluoresceinated targets, the apparent specificity of CTL depends on the quality and quantity of the lymphokine preparations used to support the response. An acid-labile component of the factor preparations contributes to the generation of non-specific CTL. No cytotoxicity against NK-sensitive target cells was detected in the 'non-specific' component of the responses.

Phytohemagglutinin-induced proliferation and cytolytic activity in T3+ but not in T3- cloned T lymphocytes requires the involvement of the T3 antigen for signal transmission

Proliferation and the cloning efficiency of T3+ but not T3- T cells are increased by the addition of lectins (phytohemagglutinin; PHA) to the culture medium. In contrast to that of T3+ cloned cell lines, the cytolytic activity of T3- clones is not enhanced by PHA, as we report here. We have investigated the effects of anti-T3 monoclonal antibody (MAb) and PHA on the proliferative capacity and cytolytic activity of various T3+ and T3- clones and cells to determine the possible involvement of the T3 receptor in these processes. We found that, in addition to inhibition of allospecific cytotoxicity, anti-T3 MAb can induce and/or enhance nonspecific cytolytic activity against particular target cells in cloned allospecific cytotoxic T cells (CTL) following preincubation of the effector cells with PHA or anti-T3. This enhancement of cytolytic activity is seen in T3+ but not T3- activated killer (AK) clones or fresh T3- natural killer (NK) cells and depends on the concentrations of anti-T3 MAb or PHA used. We conclude that the T3-Ti antigen-receptor complex is involved in the transmission of the activation signals by anti-T3 and PHA.

Participation of natural killer cells in the recovery of mice from visceral leishmaniasis

After infection with the protozoan parasite Leishmania donovani, C57BL/6J bg/bg (beige) mice, which are deficient in natural killer (NK) activity, were unable to control splenic parasite loads relative to phenotypically normal C57BL/6J bg/+ and +/+ mice, particularly beyond 21 days of infection. When beige mice were injected intravenously with 2 or 3 X 10(6) syngeneic, cloned NK cells (NKB61B10 cell line), they displayed splenic parasite burdens which did not differ significantly from those of normal controls. In C57BL/6 +/+ mice rendered NK deficient by split-dose irradiation (four weekly, 200-rad doses of gamma irradiation beginning at 4 weeks of age) splenic and hepatic parasite levels were significantly higher than those in nonirradiated controls at 15 days of infection and beyond. In both sets of experiments, relative degrees of hepato- and splenomegaly were not sufficient to account for differences in parasite burdens among NK-deficient and normal mice. Taken together, the results of these experiments suggest that NK cells may contribute to parasite elimination during the acquired-resistance phase of L. donovani infection in mice.

Reversibility of lymphokine-induced NK-like activity in virus-specific cytotoxic T-lymphocyte clones

A limiting dilution microculture system, supplemented with a source of interleukin-2 (IL-2), was employed to evaluate the frequency of Moloney-murine leukaemia/sarcoma virus (M-MuLV/M-MSV)-specific cytotoxic T-lymphocyte precursors (CTL-p) which also exhibited NK-like activity. Spleen cells, obtained from M-MuLV/M-MSV regressor mice, were restimulated in bulk secondary mixed leucocyte-tumour cell cultures (MLTC), and subsequently plated in a culture medium supplemented with two different supernatants (SN) produced following PMA-stimulation of the same EL-4 thymoma cell line. SN 20, obtained from the cell line maintained in vitro, contained IL-2 and only negligible amounts (less than 3 U/ml) of interferon (IFN), while SN 19, obtained after passage of the ascitic form of EL-4 thymoma in syngeneic mice, contained both IL-2 and IFN in high titres. The frequency of CTL-p specific for MBL-2 lymphoma cells was high and comparable in cultures supplemented with both SN (1/2 X 84 cells and 1/2 X 40 cells, respectively), while the frequency of CTL-p directed against NK-susceptible YAC-1 target cells was low in SN 20 (1/90 cells) and high in SN 19 (1/5 X 40 cells). An analysis of individual microcultures established at low cell dose (1 cell/well) indicated that specific and NK-like activity could be ascribed to the same precursor cells. Furthermore, using different long-term CTL clones, we observed that, after passage in SN 20, double-reactive clones gradually lose the capacity to lyse NK-susceptible targets, while most of MBL-2 specific clones acquired NK-like activity following a few passages in SN 19. Therefore, the induction of NK-like activity is reversible and may be modulated by soluble factors present in supernatant in which CTL clones are maintained. Double-reactive clones were unable to lyse NK-resistant allogeneic tumour cells or normal syngeneic blast cells. A few clones cross-reacting with H-2d alloantigens also exhibited NK-like activity when maintained in SN 19. The different pattern of CTL clone activity was associated with a morphological change in the clones themselves: the acquisition of double activity was accompanied by an increase in cell size and the appearance of numerous cytoplasmic granules. All CTL clones were phenotypically Thy-1+ and Lyt-2+ on indirect immunofluorescence and complement-dependent cytotoxicity investigation.(ABSTRACT TRUNCATED AT 400 WORDS)

Change in functional phenotype of cloned human alloreactive cytolytic T cells

Alloreactive T cell clones primed in vivo were tested for the expression of T cell differentiation antigens CD2, CD3, CD4, and CD8. Each of 29 different clones were found to express CD2 and CD3, but were variable in their expression of CD4 (7 positive clones) and CD8 (15 positive clones). Six clones were positive for both CD4 and CD8. One of the 29 clones expressed neither CD4 or CD8. Over a period of 12-18 weeks of culture, these clones began to lose their alloreactivity but acquired NK-like activity. By changing the concentration of TCGF, the "allo" and "NK-like" lytic activities could be modulated. After 18 weeks of culture, these clones lost their alloreactive specificity, but not their NK activity. The expression of surface markers was unchanged. CD2 and CD3 molecules were determined to play a role in both the alloreactive and NK activity of these clones.

Recent progresses in the phenotypic and functional characterization of human cytolytic T lymphocytes

Over the past several years, a number of major advances have been made in our understanding of the phenotypic and functional heterogeneity of human T lymphocytes. Most of these advances have resulted from: 1. the availability of monoclonal antibodies directed to T cell surface antigens; 2. the development of sensitive microassays to measure lymphocyte function in vitro; 3. the refining of T cell cloning technology. The most relevant studies in delineating how human T cell functions have been based on the analysis of T cell clones with specific cytolytic activity. These studies allowed the recent molecular definition of structures serving as specific antigen receptor on T cells as well as the understanding of the role of accessory molecules involved in specific receptor activity such as T3, T8 and T4 antigens. The development of microculture conditions allowing clonal proliferation of all human T lymphocytes, whether resting or activated, has provided a major tool for the precise definition of the frequency and subset distribution of T cells with different functional capabilities. Studies along this line have shown that precursors of cytolytic T lymphocytes (CTL-P) represent about one third of peripheral blood human T lymphocytes; in addition, all T8+ were CTL-P, whereas CTL-P were relatively rare among normal T4+ lymphocytes. As much as 20% cytolytic T lymphocytes were found to be capable to release interleukin-2 and B cell growth factor, thus indicating that cytolytic and helper function may be associated at the single cell level.

Natural resistance of lethally irradiated F1 hybrid mice to parental marrow grafts is a function of H-2/Hh-restricted effectors

The natural resistance of F1 hybrid mice against parental bone marrow grafts is thought to be mediated by natural killer (NK)-like effector cells. However, unlike the NK cell activity against a wide range of tumors and normal cells, hybrid resistance is characterized by the immunogenetic specificity controlled by a set of unique noncodominant genes denoted as Hh. Two alternative hypotheses can account for the specificity. Thus, the specificity may reflect either the Hh restriction of effectors or the Hh gene control of mechanisms regulating non-Hh-restricted effector activity. In this study, therefore, we tested the recognition specificity of putative effectors mediating hybrid resistance in lethally irradiated H-2b/d and H-2b/k F1 hybrid mice to the engraftment of parental H-2b bone marrow. As a direct means of defining the effector specificity, rejection of parental bone marrow grafts was subjected to competitive inhibition in situ by irradiated tumor cells. Of the 16 independent lines of lymphoma and other hemopoietic tumor cells tested, the ability to inhibit hybrid resistance was the exclusive property of all tumors derived from mice homozygous for the H-2Db region, regardless of whether the tumor cells were susceptible or resistant to NK cell-mediated cytotoxicity in vitro. Four cell lines heterozygous for the H-2Db were noninhibitory, including one that is susceptible to natural killing. Pretreatment of the F1 hosts with an interferon inducer augmented the resistance with no alteration in the recognition specificity of effector cells. Therefore, natural resistance to parental H-2b bone marrow grafts was mediated by effectors restricted by the H-2Db/Hh-1b gene(s), and not by the nonrestricted NK cells detectable in conventional in vitro assays.

Aged murine killer T-cell clones acquire specific cytotoxicity for P815 mastocytoma cells

T-cell clones that grow continuously in tissue culture have become a major tool for studying the properties of T lymphocytes. It is therefore important to know to what extent such clones resemble their normal counterparts. Several reports have appeared recently which demonstrate that long-term T-cell lines may lose the specificity for which they were initially selected and acquire cytotoxic activity to a variety of targets, typical of the activity displayed by natural killer cells. We now report a number of instances in which murine cytotoxic T-cell clones have lost their original specific cytotoxic activity but have acquired strong specific cytotoxic activity for P815 mastocytoma target cells. Loss of the original specificity was usually observed after continuous in vitro cultivation for more than 6 months. We propose that this novel type of cytotoxicity should be called aged killer activity.

Assignment of human natural killer (NK)-like cells to the T cell lineage. Single allospecific T cell clones lyse specific or NK-sensitive target cells via distinct recognition structures

The aim of the present study was to define the cell lineage of mixed lymphocyte culture (MLC)-induced natural killer (NK) effector cells. Human MLC cells were plated under limiting microculture conditions in the presence of irradiated spleen cells and interleukin 2-containing supernatant. After 18 days, microcultures were scored for proliferation and for cytolytic activity against specific lymphoblasts and NK-sensitive K562 target cells. About 1 in 7 and 1 in 5 proliferating microcultures had specific or NK-like cytolytic activity, respectively. Moreover, several microcultures exhibited dual (specific and NK-like) cytolytic activity, even when they had been established at relatively low numbers of responding cells/well (0.5-0.25) to ensure a high probability of monoclonality. Direct evidence for the existence of cytolytic effector cells with dual activity was achieved by using clones derived from single MLC T cells by micromanipulation. Out of 26 cytolytic clones so derived, 16 exhibited specific cytolytic activity, whereas 22 lysed K562 target cells. More interestingly, 12 of these 26 clones were active against both types of target cells. Only one of these clones was able to lyse autologous or unrelated target cells. In contrast, all such clones lysed the NK-sensitive cell lines G11, MOLT-4, Raji, Daudi, Chang and T-24. Addition of saturating amounts of B9-4 monoclonal antibody in the lytic assays resulted in the inhibition of the specific cytolysis, but not the NK-like activity of clones with dual cytolytic activity. It thus appears that (a) alloreactive cytotoxic T lymphocytes can mediate both specific and NK-like cytolysis and (b) two independent recognition structures are involved in this dual activity.

Neoplastic cells as targets of spontaneously cytotoxic lymphocytes: studies with natural killer-like cell lines

Native natural killer (NK) cells comprise a heterogeneous family of lymphocytes distributed among several organs, which display spontaneous cytotoxic reactions directed against a broad range of tumor targets. In these studies, murine cell lines have been established in vitro following the selective expansion of bone marrow- and spleen-derived killer progenitors in culture medium supplemented with interleukin-2. Several clones of independent origin have been characterized in order to determine the extent of their phenotypic and functional diversity. With few exceptions most of them were found to be highly effective in lysing a variety of tumor cell lines, to share common cell surface alloantigens, lectin-binding receptors, and cytochemical markers. The presence of prominent azurophilic cytoplasmic granules is the most characteristic ultrastructural feature of these cells. In attempting to elucidate the nature of membrane components specifically recognized by NK cells we compared several isogenic tumor cell variants selected on the basis of their differential NK susceptibility, immunogenicity, metastatic potential or resistance to cytotoxic plant lectins. Sialylated glycoconjugates exposed on the external face of the tumor cell membrane appear to be essential determinants in the interaction between NK cells and their targets. Permanent cell lines retaining most of the functional attributes of endogenous NK cells may prove instrumental in understanding their role during tumor progression.

Immunological surveillance of tumors in the context of major histocompatibility complex restriction of T cell function

The immunological surveillance hypothesis was formulated prior to the realization of the fact that an individual's effector T cells generally only see neoantigen if it is appropriately presented in the context of self MHC glycoproteins. The biological consequence of this mechanism is that T lymphocytes are focused onto modified cell-surface rather than onto free antigen. The discovery of MHC-restricted T cell recognition, and the realization that T cell-mediated immunity is of prime importance in promoting recovery from infectious processes, has thus changed the whole emphasis of the surveillance argument. Though the immunological surveillance hypothesis generated considerable discussion and many good experiments, there is no point in continuing the debate in the intellectual context that seemed reasonable in 1970. It is now much more sensible to think of "natural surveillance" and "T cell surveillance," without excluding the probability that these two systems have elements in common. We can now see that T cell surveillance probably operates well in some situations, but is quite ineffective in many others. Part of the reason for this may be that the host response selects tumor clones that are modified so as to be no longer recognized by cytotoxic T cells. The possibility that this reflects changes in MHC phenotype has been investigated, and found to be the case, for some experimental tumors. In this regard, it is worth remembering that many "mutations" in MHC genes that completely change the spectrum of T cell recognition are serologically silent. The availability of molecular probes for investigating the status of MHC genes in tumor cells, together with the capacity to develop cloned T cell lines, monoclonal antibodies to putative tumor antigens, and cell lines transfected with genes coding for these molecules, indicates how T cell surveillance may profitably be explored further in both experimental and human situations.

Studies on chemically induced rat tumors. II. Partial protection against syngeneic lethal tumors by cloned syngeneic cytotoxic T lymphocytes

In the present article we describe studies on a chemically induced sarcoma in DA rats. This tumor expresses a unique antigen which can be demonstrated by both syngeneic antibodies and cytotoxic T cells. We have established cytotoxic T-cell lines (CTLs) specific for the tumor and with high efficient killing capacity in vitro. When testing for the ability of such CTLs to inhibit tumor out-growth in vivo, we found that they had to be inoculated together with the tumor and in the presence of T-cell growth factor to provide any significant degree of protection. We thus believe not only that there is a requirement for addition of CTL-stimulating lymphokines in vivo, but also that the CTLs fail to move from one site in vivo to attack relevant tumor cells at another site. No evidence was obtained that the CTLs gradually could acquire such migratory ability in vivo.