Immunological characteristics of leukemia and lymphoma in allogeneic cell immunity: growth of syngeneic tumors in rats immunized with allogeneic cells

The growth of transplanted tumors was strongly inhibited in syngeneic Wistar King Aptekman (WKA) rats immunized with allogeneic tumor cells from Donryu rats. This phenomenon of non-specific immunity against tumors is referred to as "allogeneic cell immunity". However, an exception to the "allogeneic cell immunity" was observed in leukemias and lymphomas. Four transplanted leukemia or lymphoma lines were not inhibited in syngeneic rats immunized with allogenic tumor cells. Furthermore, immunization with allogeneic leukemic cells had only a relatively weak inhibitory effect upon a syngeneic fibrosarcoma and no inhibitory effect upon leukemias. In WKA rats immunized with allogeneic lymphoid cells from Donryu rats, the growth of fibrosarcoma, but not of lymphoma, was inhibited. Using transplantation experiments, both fibrosarcoma and lymphoma were defined as antigenic tumors in WKA rats. Transplantation of mixtures of syngeneic tumor cells and allogeneic tumor cells in WKA rats confirmed the above findings. These results revealed that leukemia and lymphoma differ from non-leukemic tumors in regard to "allogeneic cell immunity".     

The effect of the transfer of lak cells,combined with alloimmunization,on the pulmonary metastases of rat mammary carcinoma

Alloimmunization was combined with lymphokine activated killer (LAK) cells to assess its effect on mammary carcinoma in rats. The animals were injected with both irradiated allo-splenocytes and syngeneic LAK cells. Metastatic lung nodules were markedly reduced using combined therapy when compared with the transfer of LAK cells or alloimmunization alone. IL-2 activity in the serum of alloimmunized rats could be detected. This activity, maintained in vivo for one week, may be responsible for enhancing the antitumor effect of transferred LAK cells.     

Morphological and immunological characteristics of a rat choriocarcinoma

A post-gestational rat choriocarcinoma has been studied for its immunological and morphological characteristics. The tumor can be transplanted in syngeneic (WKA/H rats) as well as in allogeneic rats (BN, R/A). Even after immunization of the allogeneic hosts with tissues of the WKA/H rats, the development of the tumor was not inhibited. This neoplasm is composed of giant cells, surrounding spaces filled with blood, and of proliferating cytotrophoblasts. Ultrastructurally, these cells are very similar to those found in normal rat placenta. In some rats lung metastases were also observed. The tumor is hormonally active as demonstrated by proliferation of and secretions from the mammary glands in tumor-bearing animals and by the presence of delta 5-3 beta-hydroxysteroid dehydrogenase in the giant cells of the neoplasms. These 2 findings indicate that the rat choriocarcinoma cells secrete lactogen and might produce progesterone, like cells of rat placenta. All the described features of this tumor point to its similarity to human choriocarcinoma.     

In vivo and in vitro effect of adoptive immunotherapy of experimental murine brain tumors using lymphokine-activated killer cells

Adoptive immunotherapy for the experimental murine brain tumor was investigated by using lymphokine-activated killer (LAK) cells both in vitro and in vivo. Supernatants of 48-h culture medium of spleen cells from Wistar rats in the presence of concanavalin A were used as interleukin 2 (IL-2). LAK cells were generated by cocultivation of spleen cells from Fischer rats with IL-2 with the peak reactivity on Day 2 or 3 of culture. Lytic activity was observed against not only syngenic tumor cells but also allogenic and xenogenic tumor cells, while no lytic activity was observed against normal brain cells. The cell depletion test, dye exclusion test, and immunofluorescence method using monoclonal antibodies revealed that LAK cells partially belonged to the population of the activated T-cell group, but the precursor cells did not react with any monoclonal antibodies used. On the basis of these results in vivo study was performed. LAK cells and immune spleen cells were adoptively transferred to the rats i.v. or intratumorally (i.t.) on the seventh day after the inoculation of T9, a gliosarcoma induced by methylcholanthrene from Fischer rats, into the right basal ganglia. Then the survival rate and necrotic foci were compared between the groups treated with those cells and the control. The survival rate of the groups treated with LAK cells was significantly higher than that of the control (administered i.v.; P less than 0.01, administered i.t.; P less than 0.05). But the treatment with immune spleen cells was not effective. The incidence and area of necrotic foci in the tumors treated with LAK cells were greater than those of the others. Microautoradiography was also performed using [3H]thymidine-labeled LAK cells, which were administered i.v. to the models on the 14th day after the inoculation of T9. It was revealed that LAK cells accumulated in the lung shortly after the administration and then in the liver and spleen, especially in the white pulp. IL-2 inhibitor activity of the sera from the tumor-bearing rats was greater than that of normal rats (P less than 0.001), but it was depressed markedly by cyclophosphamide (P less than 0.01). The adoptive transfer of LAK cells may be one of the effective treatments of malignant brain tumor. The nature of IL-2 inhibitors is necessary to be clarified for more effective immunotherapy.     

Increased sensitivity of murine leukemia virus-infected tumor cells to lymphocyte-mediated cytotoxicity

The cytotoxic sensitivity of murine leukemia virus (MuLV)-infected and noninfected fibrosarcoma cells in syngeneic inbred WKA/Hok rats was compared by in vitro cell-mediated 51Cr release cytotoxicity assay. A highly significant increase in cytotoxic sensitivity of target cells was observe in MuLV-infected tumor cells as compared with noninfected cells when spleen cells from syngeneic tumor-bearing hosts (TBH) were used as a source of effector lymphocytes. The cytotoxicity of spleen cells against MuLV-infected tumor cells was specifically directed to the tumor-associated antigen (TAA), but not to the virus-associated antigen. However, there was no quantitative difference in the amount of TAA on the cell membranes between virus-infected and noninfected tumor cells as measured by a quantitative absorption test of anti-TAA serum. The cytotoxic activity of spleen cells from TBH against MuLV-infected tumor cells was abrogated by the treatment of anti-T-serum plus complement and significantly decreased after trypsin treatment. Spleen cells from normal rats given injections of immune sera from TBH acquired the cytotoxic activity against MuLV-infected tumor cells.     

HTLV-1-associated Myelopathy/Tropical Spastic Paraparesis-like Rats by Intravenous Injection of HTLV-1-producing Rabbit or Human T-Cell Line into Adult WKA Rats

We intravenously injected Ra-1 cells or MT-2 cells into female adult WKA rats. Spastic paraparesis mainly in the hind-limbs was observed in 1 out of 2 Ra-1 cell-injected WKA rats and in 3 out of 8 MT-2 cell-injected WKA rats 20 27 months after injection. The main neuropathological finding was symmetrical white matter degeneration with mononuclear cell infiltration of the spinal cord, similar to that of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients, and degeneration of nerve roots and peripheral nerves. Antibodies against HTLV-1 antigens were detected in plasma and cerebrospinal fluid from these HAM/TSP-like rats. HTLV-1 provirus was detected from the peripheral blood mononuclear cells of one of these rats 20 months after injection. Interestingly, spastic paraparesis was not observed in F344 rats.     

树突状细胞诱导的LAK细胞对肺癌的生长抑制作用

目的　研究肺癌细胞裂解物负载的树突状细胞 (DC)诱导的淋巴因子激活杀伤细胞 (LAK )对肺腺癌裸鼠移植瘤的生长抑制作用。方法　肺腺癌患者手术切除标本接种裸鼠皮下建立肺腺癌移植瘤模型 ,同时将手术切除标本用反复冻融的方法制备肿瘤细胞裂解物 ;同一患者外周血分离得到的单个核细胞中 ,贴壁的细胞加入DC生长因子 (DCGF)培养得DC ,未贴壁的细胞加入rhIL 2培养得LAK细胞。用肿瘤细胞裂解物负载自体DC ,诱导LAK细胞生成DC LAK细胞 ,注射荷瘤裸鼠腋下以观察DC LAK细胞对肺癌移植瘤的生长抑制作用。结果　用肺癌手术标本能成功建立裸鼠移植瘤模型。DC LAK组、LAK组、DC组和生理盐水对照组肿瘤瘤重平均值分别为 0 .47、1.0 5、1.3 0和 1.5 8g ,DC LAK组、LAK组和DC组肿瘤生长抑制率分别为 70 .3 %、3 3 .5 %和 17.9% ,DC LAK细胞具有抑制裸鼠肺癌移植瘤生长的作用而且强于LAK细胞 (P <0 .0 5 )。结论　通过荷瘤裸鼠体内抗瘤实验证实了DC LAK细胞的抗肺癌作用明显高于LAK细胞 ,提示DC LAK细胞治疗是更为有效的抗肺癌生物治疗方法 ,为DC疫苗用于肺癌的临床治疗提供依据     

The Induction of Enhanced Antitumor Effect against a Nonimmunogenic Tumor by Highly Immunogenic Variants Obtained by Mutagen Treatment

Nonimmunogenic 1767-3 fibrosarcoma was treated with the mutagen N-methyl-N'-nitro-N-nitrosoguanidine, and stable variant cell clones (M-clones) were obtained that were able to elicit an immunological rejection response in syngenic C3H mice. Mice immunized with some M-clones were protected against a challenge from the original nonimmunogenic fibrosarcoma. Furthermore, when spleen cells of immunized syngenic mice were restimulated in vitro with M-clones, cytotoxic T lymphocytes (CTL) were obtained that were able to lyse not only M-clones but also the original nonimmunogenic tumor. These in vivo and in vitro results demonstrate the immunogenicity of M-clones and the existence of a singular antigenic specificity between the original nonimmunogenic tumor and M-clones. For the purpose of application of this mutagen treatment to cancer therapy, we combined it with lymphokine-activated killer (LAK) adoptive immunotherapy (AIT). With interleukin 2 and in vitro stimulation with highly immunogenic variant clones, we tried to induce transfer cells that had not only nonspecific LAK cells but also CTL with specific immunity against the original nonimmunogenic tumor. Successful results were obtained in the LAK AIT models. These findings indicate that an immunotherapy of human cancers that are thought to be weakly or nonimmunogenic may be possible by the application of this approach to LAK AIT.     

The induction of enhanced antitumor effect against a nonimmunogenic tumor by highly immunogenic variants obtained by mutagen treatment

Nonimmunogenic 1767-3 fibrosarcoma was treated with the mutagen N-methyl-N'-nitro-N-nitrosoguanidine, and stable variant cell clones (M-clones) were obtained that were able to elicit an immunological rejection response in syngenic C3H mice. Mice immunized with some M-clones were protected against a challenge from the original nonimmunogenic fibrosarcoma. Furthermore, when spleen cells of immunized syngenic mice were restimulated in vitro with M-clones, cytotoxic T lymphocytes (CTL) were obtained that were able to lyse not only M-clones but also the original nonimmunogenic tumor. These in vivo and in vitro results demonstrate the immunogenicity of M-clones and the existence of a singular antigenic specificity between the original nonimmunogenic tumor and M-clones. For the purpose of application of this mutagen treatment to cancer therapy, we combined it with lymphokine-activated killer (LAK) adoptive immunotherapy (AIT). With interleukin 2 and in vitro stimulation with highly immunogenic variant clones, we tried to induce transfer cells that had not only nonspecific LAK cells but also CTL with specific immunity against the original nonimmunogenic tumor. Successful results were obtained in the LAK AIT models. These findings indicate that an immunotherapy of human cancers that are thought to be weakly or nonimmunogenic may be possible by the application of this approach to LAK AIT.     

Inhibition by fibrin coagulation of lung cancer cell destruction by human interleukin-2-activated killer cells.

We examined the effect of fibrin coagulation on tumor cytotoxicity mediated by human lymphokine (IL-2)-activated killer (LAK) cells. LAK cells were induced from peripheral blood mononuclear cells (MNC) by culture with recombinant IL-2 for 4 or 5 days, and LAK cell-mediated cytotoxicity against tumor cells was assessed by 51Cr release assay in the presence or absence of plasma from normal subjects and lung cancer patients. Plasma did not affect the phase of induction of LAK activity by IL-2, but dose-dependently inhibited the effector phase of LAK cell-mediated cytotoxicity against Daudi cells. Similar inhibition of LAK cell-mediated cytotoxicity was observed on pretreatment of Daudi cells and human lung cancer cell lines with human fibrinogen plus thrombin. A parallel relationship was found between the amount of fibrinogen in plasma of lung cancer patients and inhibition of LAK cytotoxicity. This inhibition was reduced by addition of anticoagulants (heparin or argatroban). These findings suggest that fibrin coagulation on tumor cells protects them from LAK cell-mediated tumor cytotoxicity in malignant lesions and that a combination of an anticoagulant drug and IL-2/LAK therapy may be effective for treatment of lung cancer patients.     

Correlation between concanavalin A agglutinability and cytotoxic sensitivity to antiserum against tumor-associated antigen in rat fibrosarcoma cells.

An ip transplantation of 3-methylcholanthrene-induced, transplanted fibrosarcoma KMT-17 cells (1 X 10(8)) grew rapidly and killed syngeneic WKA rats in 3-4 days. Agglutinability induced by concanavalin A (Con A) and antigenic expression of KMT-17 cells were investigated in relation to days after ip transplantation. Agglutinability was highest in 1-day-old cells and lowest in 3-day-old cells. The agglutinability of 3-day-old cells increased again when these cells were transplanted into normal rats. The cytotoxic sensitivity of tumor cells to antiserum against tumor-associated surface antigen (TASA) changed simultaneously with the degree of Con A agglutinability. This phenomenon disappeared after artificial infection of tumor cells with Friend murine leukemia virus. The result of the quantitative absorption test at 4 degrees C overnight was that 1- and 3-day-old cells did not differ in their absorbing capacities to anti-TASA sera. However, when the absorption test was done at 37 degrees C for 60 minutes, 1-day-old cells had approximately 16 times more absorbing capacity than 3-day-old cells. However, the cytotoxic sensitivity to antiserum against histocompatibility antigen did not change, regardless of the number of days after ip transplantation. Analysis based on the quantitative absorption test revealed no difference in antibody-absorbing capacities between 1- and 3-day-old cells at both 4 degrees C and 37 degrees C. The relationship between Con A agglutinability and cytotoxic sensitivity to anti-TASA serum is discussed from the viewpoint of "lateral receptor mobility" on the cell surface.     

Inhibition by Fibrin Coagulation of Lung Cancer Cell Destruction by Human Interleukin-2-activated Killer Cells

We examined the effect of fibrin coagulation on tumor cytotoxicity mediated by human lymphokine (IL-2)-activated killer (LAK) cells. LAK cells were induced from peripheral blood mononuclear cells (MNC) by culture with recombinant IL-2 for 4 or 5 days, and LAK cell-mediated cytotoxicity against tumor cells was assessed by ⁵¹Cr release assay in the presence or absence of plasma from normal subjects and lung cancer patients. Plasma did not affect the phase of induction of LAK activity by IL-2, but dose-dependently inhibited the effector phase of LAK cell-mediated cytotoxicity against Daudi cells. Similar inhibition of LAK cell-mediated cytotoxicity was observed on pretreatment of Daudi cells and human lung cancer cell lines with human fibrinogen plus thrombin. A parallel relationship was found between the amount of fibrinogen in plasma of lung cancer patients and inhibition of LAK cytotoxicity. This inhibition was reduced by addition of anticoagulants (heparin or argatroban). These findings suggest that fibrin coagulation on tumor cells protects them from LAK cell-mediated tumor cytotoxicity in malignant lesions and that a combination of an anticoagulant drug and IL-2/LAK therapy may be effective for treatment of lung cancer patients.     

Interleukin-2-inducible Killer Activity and Its Regulation by Blood Monocytes from Autologous Lymphocytes of Lung Cancer Patients

The ability of blood lymphocytes of newly diagnosed lung cancer patients to respond to interleukin 2 (IL-2) to become IL-2-activated killer (LAK) cells and its regulation by autologous monocytes were examined. LAK activity was measured by ³¹Cr release assay. The abilities of lymphocytes among blood mononuclear cells (MNC) of subjects of different ages without malignancies to generate LAK activity against NK-cell resistant Daudi cells and lung adenocarcinoma (PC-9) cells were very similar. The LAK activity of blood MNC of lung cancer patients was also nearly the same as that of blood MNC of control subjects. There was no significant difference in IL-2-inducible LAK activity between MNC of patients with small cell lung cancer (SCLC) and those of patients with non-SCLC. Monocytes and lymphocytes were separated from blood MNC on a one-step Percoll gradient. Monocytes of lung cancer patients were found to augment in vitro induction of LAK activity by IL-2 of autologous blood lymphocytes. In contrast, endotoxin-stimulated monocytes suppressed LAK induction of autologous lymphocytes of cancer patients. These findings suggest that administration of IL-2 and LAK cells induced in vitro may be of benefit in the treatment of lung cancer.     

Effect of Krestin (PSK) on the induction of IL-2 activated killer cells.

The effects of Krestin (PSK) on the generation of lymphokine-activated killer (LAK) cells were examined in tumor-bearing mice. BALB/c mice were inoculated subcutaneously with methylcholanthrene-induced fibrosarcoma (Meth A) cells, and PSK was administered intraperitoneally every other day. The reduced LAK activity in tumor-bearing mice was restored by the administration of PSK. Since involvement of the humoral immunosuppressive factor in the impairment of LAK activity has been suggested, the effect of PSK on the impaired LAK activity in the presence of an immunosuppressive factor isolated from the ascites of X5563 (plasmacytoma)-inoculated mice was examined. The activity reduced by the immunosuppressive factor in an in vitro induction of LAK was restored by incubation with PSK. The antimetastatic effect of IL-2 was also augmented by its combined use with PSK. The data provide a rational basis for using PSK in combination with recombinant IL-2 in cancer immunotherapy.     

Granulocyte-Macrophage Colony-stimulating Factor Augments Lymphokine-activated Killer Activity from Pleural Cavity Mononuclear Cells of Lung Cancer Patients without Malignant Effusion

The role of recombinant granulocyte-macrophage colony-stimulating factor (GM-CSF) in augmentation of lymphokine-activated killer (LAK) cell induction by interleukin-2 (IL-2) from pleural cavity mononuclear cells (PCMNCs) was examined in sixteen patients with resectable primary lung cancer not associated with malignant effusion. None of the patients had received any anticancer therapy prior to this study. Incubation of PCMNCs of patients without malignant effusion with GM-CSF for 4 days in the presence of IL-2 resulted in a significant increase in LAK activity against natural killer-resistant Daudi cells. This result was obtained by using the 4 h ⁵¹Cr-release assay. PCMNCs and blood mononuclear cells (BMNCs) were harvested simultaneously from pleural cavity lavage fluid and peripheral blood in lung cancer patients. The LAK activity developed from PCMNCs and BMNCs following incubation with IL-2 for 4 days, but the LAK activity from PCMNCs was significantly lower than that from BMNCs ( P < 0.05). Incubation of PCMNCs with GM-CSF augmented the LAK activity from PCMNCs to a level as high as that from BMNCs. These results suggest that the combined use of GM-CSF with IL-2 may result in augmentation of LAK activity developed from PCMNCs of lung cancer patients without malignant effusion.     

人LAK细胞对肺腺癌靶细胞的杀伤活性及形态学观察

LAK activity could be induced by IL-2 from human PBL. After co-culture of LAK cells with Anip973 human lung adenocarcinoma cells, LAK-tumor conjugates were microscopically observed. Formation of the conjugates paralleled well with the expression of LAK activity (P less than 0.001), suggesting that LAK cells' killing of Anip973 cells depended on their close contact. Under electron microscope, LAK cells had various shapes and certain degrees of motility. Ten minutes after co-culture, close binding between microvilli of LAK cells and tumor cells could be seen. Four hours after co-culture, Anip973 cells bound with LAK cells developed numerous membrane blebs with cytoplasm and nucleus very condensed, which was followed by destruction of cell structure and appearance of cell debris. These features are consistent with apoptosis rather than a cytolytic mechanism of cell death. These findings suggest that LAK cells share certain common features with CTL and NK cells on killing tumor cells.     

Interleukin-2-inducible killer activity and its regulation by blood monocytes from autologous lymphocytes of lung cancer patients.

The ability of blood lymphocytes of newly diagnosed lung cancer patients to respond to interleukin 2 (IL-2) to become IL-2-activated killer (LAK) cells and its regulation by autologous monocytes were examined. LAK activity was measured by 51Cr release assay. The abilities of lymphocytes among blood mononuclear cells (MNC) of subjects of different ages without malignancies to generate LAK activity against NK-cell resistant Daudi cells and lung adenocarcinoma (PC-9) cells were very similar. The LAK activity of blood MNC of lung cancer patients was also nearly the same as that of blood MNC of control subjects. There was no significant difference in IL-2-inducible LAK activity between MNC of patients with small cell lung cancer (SCLC) and those of patients with non-SCLC. Monocytes and lymphocytes were separated from blood MNC on a one-step Percoll gradient. Monocytes of lung cancer patients were found to augment in vitro induction of LAK activity by IL-2 of autologous blood lymphocytes. In contrast, endotoxin-stimulated monocytes suppressed LAK induction of autologous lymphocytes of cancer patients. These findings suggest that administration of IL-2 and LAK cells induced in vitro may be of benefit in the treatment of lung cancer.     

Effects of allogeneic LAK cells on the function of immune system of the recipients

SHR rats, 1 week of age, were i.p. inoculated with allogeneic JB LAK cells generated by 3-day incubation of spleen cells of JB rats in CM-IL 2. The spleens were removed 4 and 12 weeks later and their cells were tested for proliferative potential to mitogens and their response and production of IL2. The results demonstrated that the allo-LAK recipients showed no signs of acute or chronic GvH disease but showed immune response equivalent to or higher than the normal controls. Severe functional defects were observed in the newborn SHR rats after i.p. inoculation of allogeneic fresh spleen cells from JB rats. The results strongly suggest that allogeneic LAK cells be ineffective in terms of induction of GvH disease or GvH associated immunodeficiencies (GvHID). Therefore, this might be promising in clinical trials on cancer adoptive immunotherapy with allogeneic, instead of autologous, LAK cells.     

Enhancement of therapeutic effect of interleukin-2 on spontaneous pulmonary metastases of Lewis lung carcinoma by killer helper factor associated with increased induction of killer activity

Killer helper factor (KHF) was previously found to be produced by a human T cell hybridoma, 24A . CA2. We studied the therapeutic effects of interleukin-2 (IL-2) and KHF on the inhibition of pulmonary metastases of syngeneic Lewis lung carcinoma (3LL) in C57BL/6N mice. Multiple subcutaneous (sc) injections of IL-2 plus KHF had significantly more effect than injections of IL-2 alone in inhibiting spontaneous pulmonary metastases and prolonging survival of the mice. The effect of KHF with IL-2 on induction of lymphokine (IL-2)-activated killer (LAK) activity against P-29 cells was examined in the murine system. Spleen cells generated LAK activity after incubation for 4 days with more than 500 U/ml of IL-2. In contrast, KHF alone did not render spleen cells cytotoxic. The combination of these lymphokines at subthreshold concentrations, however, resulted in significant in vitro induction of LAK activity. The LAK activity of splenocytes incubated with IL-2 plus KHF was maximal after 4 days, and persisted for longer than that of cells treated with IL-2 alone. The LAK cells induced by KHF plus IL-2 were also cytotoxic to FBL and YAC-1 cells. Moreover, spleen cells of mice bearing lung metastases could be induced to the cytotoxic state by sc injections of IL-2 plus KHF. These results indicate that combination treatment with IL-2 and the new lymphokine KHF should be useful clinically in inducing LAK activity for inhibition of pulmonary metastases.     

Augmentation of lymphokine-activated killer cell cytotoxicity by monoclonal antibodies against human small cell lung carcinoma

This study examined the lymphokine-activated killer (LAK) cell cytotoxicity on monoclonal antibody (MoAb)-bound tumor cells from the human small cell lung carcinoma cell lines H69 and H128. LAK cells were generated from normal peripheral blood mononuclear cells by incubation with interleukin 2 for 3 or more days. Cells from the LAK culture were cytotoxic to natural killer-sensitive (K562, 84% cytotoxicity) and natural killer-resistant (Daudi, 85%; H69 and H128, 69% and 97%, respectively) cell lines, and to freshly excised human lung (49%) and breast (57%) tumors. LAK cytotoxicity to H69 or H128 cells was significantly augmented by target cell preincubation with the small cell lung carcinoma-reactive MoAbs 1096 (increases of up to 271%) or 5023 (up to 223%). SCLC 5023 or 1096 did not enhance LAK cytotoxicity to Daudi cells of lymphoblastoid origin. Pretreatment of LAK cells with an anti-Fc receptor antibody blocked MoAb augmentation by 1096 or 5023 (but not LAK cytotoxicity), suggesting that LAK-MoAb interaction may be mediated by Fc binding. LAK activity coincided with emergence of a large cell [interleukin 2-stimulated large mononuclear leukocyte (LML)] subset expressing the CD16 and NKH-1 surface determinants. Serial immunophenotyping of the LAK cell culture harvested at Days 3, 5, and 7 indicated that the level of LAK cytotoxicity, with or without MoAb augmentation, correlated with frequency of NKH-1-reactive LMLs. These observations support the hypothesis that LAK cytotoxicity is mediated by a NKH-1-reactive LML subpopulation. Antitumor cytotoxicity may be augmented by tumor-reactive MoAbs through Fc binding to this LML subset.