Acute Chlamydia pneumoniae infection in the pathogenesis of autoimmune diseases.

Autoimmune diseases have several etiologies. Acute Chlamydia pneumoniae (C. pneumoniae) infection may be involved in the pathogenesis of several autoimmune diseases. In this study, 82 patients with several autoimmune diseases and 70 controls were enrolled, and acute C. pneumoniae infection has been evaluated by monitoring the levels of IgM antibody. Chlamydia pneumoniae IgM positive results were observed in 29% (P < 0.05) of the patients with several autoimmune diseases and in 10% of the controls. Chlamydia pneumoniae IgM positive cases were more frequent among the patients with rheumatoid arthritis (RA; 30%, P < 0.05), systemic lupus erythematosus (SLE; 28.0%, P < 0.05), dermatomyositis/polymyositis (23%, NS), myeloperoxidase-antineutrophil cytoplasmic autoantibody (MPO-ANCA)-associated vasculitis (33%, NS), adult onset of Still's disease (29%, NS) and giant cell arteritis/Takayasu arteritis (50%, NS) than among the controls. This positive frequency was statistically significant in RA and SLE. These results suggest that acute C. pneumoniae infection is probably involved in the pathogenesis of autoimmune diseases.

Unrelated bone marrow transplantation for Epstein-Barr virus-associated T/NK-cell lymphoproliferative disease.

Epstein-Barr virus (EBV)-associated T/NK-cell lymphoproliferative disease (LPD) has been linked to several different disorders, including chronic active EBV infection, EBV-associated hemophagocytic syndrome, hypersensitivity to mosquito bites, hydroa vacciniforme, aggressive NK-cell leukemia, and nasal/nasal-type NK-cell lymphoma. In most instances, these disorders are refractory to conventional treatments and have a poor prognosis. Here, we report a new treatment strategy for EBV-associated T/NK-cell LPD, consisting of immunochemotherapy, intensive combination chemotherapy, and stem cell transplantation. The five patients studied, two with T-cell and three with NK-cell LPD, lacked a human leukocyte antigen-matched, related donor, and therefore received bone marrow grafts from HLA-matched, unrelated donors. The preconditioning regimen consisted of total-body irradiation (12 Gy), etoposide (900 mg/m(2)), and cyclophosphamide (120 mg/kg) or melphalan (210 mg/m(2)). All patients had residual LPD by a quantitative PCR technique prior to transplantation. After unrelated bone marrow transplantation (UBMT), four of the five patients remain in continuous complete remission at a median of 19 months, without detectable EBV-DNA in peripheral blood. Thus, UBMT appears to be a reasonable option for the treatment of patients with EBV-associated T/NK-cell LPD. Detection of EBV-DNA by PCR offers an important tool for assessing minimal residual disease in patients with EBV-associated T/NK-cell LPD.

Successful allogeneic stem cell transplantation from an unrelated donor for aggressive Epstein-Barr virus-associated clonal T-cell proliferation with hemophagocytosis.

We present here a case of aggressive Epstein-Barr virus (EBV)-associated clonal T-cell proliferation with hemophagocytosis that was successfully treated by allogeneic stem cell transplantation using an unrelated donor. A 17-year-old woman was admitted into the hospital with a high fever and liver dysfunction. Laboratory data including bone marrow aspiration revealed hemophagocytic syndrome with proliferation of immature T-lymphoid cells. The clonal proliferation of EBV-infected T cells was confirmed by Southern blot analysis using a terminal-repeat probe from the EBV genome and also by demonstrating T cell-receptor beta gene rearrangement. Intensive immunochemotherapy consisting of cyclosporin A, vincristine, etoposide, and high-dose methylprednisolone did not control the disease and relapse occurred repeatedly. Therefore, during remission after chemotherapy according to the CHOP-E regimen, the patient underwent allogeneic bone marrow transplantation (BMT) from an HLA-matched, unrelated donor. Donor selection was performed with help from the Japanese Association for Marrow Donor Program (JMDP). The patient has remained in good condition without recurrence of disease for 18 months after BMT. Allogeneic BMT is the treatment of choice for aggressive EBV-associated hemophagocytic lymphohistiocytosis even in the case where an HLA-matched sibling donor is not available, especially when the patient is refractory to intensive chemotherapy and/or there is a ready recurrence of disease after conventional therapy.

A novel natural killer cell line (KHYG-1) from a patient with aggressive natural killer cell leukemia carrying a p53 point mutation.

We present the establishment of a natural killer (NK) leukemia cell line, designated KHYG-1, from the blood of a patient with aggressive NK leukemia, which both possessed the same p53 point mutation. The immunophenotype of the primary leukemia cells was CD2+, surface CD3-, cytoplasmic CD3epsilon+, CD7+, CD8alphaalpha+, CD16+, CD56+, CD57+ and HLA-DR+. A new cell line (KHYG-1) was established by culturing peripheral leukemia cells with 100 units of recombinant interleukin (IL)-2. The KHYG-1 cells showed LGL morphology with a large nucleus, coarse chromatin, conspicuous nucleoli, and abundant basophilic cytoplasm with many azurophilic granules. The immunophenotype of KHYG-1 cells was CD1-, CD2+, surface CD3-, cytoplasmic CD3epsilon+, CD7+, CD8alphaalpha+, CD16-, CD25-, CD33+, CD34-, CD56+, CD57-, CD122+, CD132+, and TdT-. Southern blot analysis of these cells revealed a normal germline configuration for the beta, delta, and gamma chains of the T cell receptor and the immunoglobulin heavy-chain genes. Moreover, the KHYG-1 cells displayed NK cell activity and IL-2-dependent proliferation in vitro, suggesting that they are of NK cell origin. Epstein-Barr virus (EBV) DNA was not detected in KHYG-1 cells by Southern blot analysis with a terminal repeat probe from an EBV genome. A point mutation in exon 7 of the p53 gene was detected in the KHYG-1 cells by PCR/SSCP analysis, and direct sequencing revealed the conversion of C to T at nucleotide 877 in codon 248. The primary leukemia cells also carried the same point mutation. Although the precise role of the p53 point mutation in leukemogenesis remains to be clarified, the establishment of an NK leukemia cell line with a p53 point mutation could be valuable in the study of leukemogenesis.

[Intravascular large B-cell lymphoma associated with hypoalbuminemia and hypoxemia].

A 67-year-old man was referred to our hospital for treatment of hemophagocytic syndrome. Hypotension, hypoxemia, pleural effusion, severe anasarca, and splenomegaly were noticed at the time of admission. Laboratory findings showed anemia (7.7 g/dl), thrombocytopenia (4.5 x 10(4)/microliter), an increase of serum LDH (1,466 IU/L) and severe hypoalbuminemia (1.9 g/dl). Bone marrow aspiration revealed an increase of reticulum cells with active hemophagocytosis and the presence of immature lymphocytes (6.0%). Lymphoma was suspected, but effective chemotherapy could not be performed because of progressive hypoxemia and severe hypoalbuminemia, and the patient died of the disease 2 weeks after admission. Autopsy revealed large lymphoid cells packed within systemic vessels as well as invasion into organs such as the liver, lungs, and spleen. The postmortem diagnosis was intravascular large B-cell lymphoma. Hypoalbuminemia and hypoxemia appear to be important clinical features of intravascular large B-cell lymphoma.

Distinct genetic involvement of the TP53 gene in therapy-related leukemia and myelodysplasia with chromosomal losses of Nos 5 and/or 7 and its possible relationship to replication error phenotype.

We examined chromosomes and molecular aberrations in 21 patients with therapy-related leukemias (t-AML) or myelodysplastic syndromes (t-MDS). All patients showed abnormal karyotypes, and chromosomal losses of No. 5 and/or No. 7 (-5/5q- and/or -7/7q-) were identified in 12 patients. Among these 12, six patients (50%) harbored a TP53 mutation, and two of five examined showed microsatellite instability, suggesting replication error (RER+) phenotype. Meanwhile, among the other nine patients without -5/5q- and/or -7/7q-, none harbored a TP53 mutation, and none of five examined showed RER+ phenotype. Thus, TP53 mutations and RER+ phenotype were preferentially associated with specific chromosomal losses in t-AML/MDS. We then screened for mutational events in representative DNA mismatch repair genes; exons 5-7 and 12-15 of the hMSH2 gene and exon 9 of hMLH1. Notably, two unrelated patients showing RER+ phenotype had an identical missense alteration at codon 419 of hMSH2 in their marrow cells and fibroblasts, which were not found in 120 DNA samples from healthy volunteers or patients with other hematological disorders. Consequently, this study revealed a possible relationship of RER+ phenotype accompanying an hMSH2 alteration to the development of therapy-related AML/MDS in association with TP53 mutations and specific chromosomal losses, and suggests that some patients may be predisposed to myelodysplasia after chemotherapy for their primary tumor.

[Extramedullary relapse in the external auditory canal in a patient with acute promyelocytic leukemia treated with all-trans retinoic acid and autologous peripheral blood stem cell transplantation].

A 41-year-old man was given a diagnosis with of acute promyelocytic leukemia (APL) in August 1994. A chromosome analysis showed 46, XY, t(15; 17) and 47, XY, idem, +8 at that time. Because initial induction chemotherapy (BHAC-DMP) has not been successful, the patient was given 45 mg/m2 of all-trans retinoic acid (ATRA) and achieved complete remission (CR) after 26 days on this regimen. Following intensified chemotherapy, he received an autologous peripheral blood stem cell transplant (PBSCT) with high-dose busulfan and cyclophosphamide in April 1995. Competitive RT-PCR for PML-RAR alpha mRNA did not find any of APL cells in the collected stem-cell fraction. Although the patient remained in CR without therapy, a myeloblastoma was found in his left external auditory canal in August 1996. Recurrence in bone marrow, moreover, was discovered the following month. A chromosome analysis of bone marrow cells showed 47, XY, t(15; 17), +8 at this time. Thus, the extramedullary relapse developed after autologous PBSCT. This case provides information linking ATRA to the development of extramedullary relapse in patients with APL.

Enhancement of cytolytic activity of human peripheral blood lymphocytes by sodium periodate (IO4) possible involvement of protein kinase C.

Sodium periodate (IO4) exerts a number of biological effects including the enhancement of lymphocyte activation. In this study, we investigated its effects on cytotoxicity of human peripheral blood lymphocytes (PBL) and explored the mechanism whereby it exerted these effects. In vitro treatment of human PBL with IO4 augmented their cytotoxicity against K562 myelogenous leukemia cells. IO4 oxidative treatment increased the frequency of effector-to-target cell binding. It also increased cellular ATP levels in effector cells, suggesting that the post-binding cytolytic functions of these cells were also enhanced after treatment with IO4. Moreover, IO4 treatment significantly increased the protein kinase C (PKC) activity of effector cells and induced the translocation of activity in the membrane fraction from the cytosol. H-7, a potent PKC inhibitor, significantly reduced this enhancement of membrane-associated PKC activity at 10 microM and significantly reduced the enhanced cytotoxicity of PBL at the same concentration. These results indicated that IO4 enhanced the binding capacity and post-binding cytolytic functions of PBL and that PKC activation was one mechanism to explain the IO4-induced cellular activation.

Therapy-related leukemia and myelodysplasia following oral administration of etoposide for recurrent breast cancer.

We report a high risk of therapy-related acute myeloid leukemia and myelodysplastic syndrome (t-AML/MDS) in patients receiving oral administration of etoposide for recurrent breast cancer. We examined 119 patients with recurrent disease. Patients were initially treated with anthracyclines, cyclophosphamide, or cisplatin with or without radiation before etoposide treatment. Etoposide was used as the final drug in most cases. Twenty-four patients were treated with the oral administration of etoposide (50 or 100 mg/day for 5-7 days at 4-week intervals). Three cases of t-AML/MDS developed among those 24 patients exposed to etoposide. In contrast, the development of t-AML/MDS was not observed in the other 95 patients not treated with etoposide. Our data suggest that there is a substantial risk of secondary leukemia with oral administration of etoposide for a prolonged period as well as i.v. schedules.

MRP-1/CD9 and KAI1/CD82 expression in normal and various cancer tissues.

As part of our evaluation of MRP-1/CD9 and KAI1/CD82 as prognostic predictors among patients with cancer, we have extended our studies to solid tumors of a variety of anatomical sites. Normal tissues were included for comparison. Immunohistochemical techniques were used throughout. Our results indicate that MRP-1/CD9 was strongly expressed by many normal tissues, including the epithelium of the gastrointestinal tract, alveolar epithelium of the lung, urothelium and smooth muscle. Expression was weak in the pituitary gland, spleen and hepatocytes, and absent in testes and spinal cord. KAI1/CD82 was also expressed by many normal tissues, but was absent in some MRP-1/CD9-positive tissues (e.g., smooth muscle, adrenal cortex, urothelium, myelin of peripheral nerves, epithelium of amnion). On the other hand, KAI1/CD82 was strongly expressed in spinal cord gray matter, which was MRP-1/CD9-negative. Expression of these glycoproteins was detected in almost all types of tumors examined. In certain cancers, MRP-1/CD9 and KAI1/CD82 positivity was inversely related to lymph node involvement. Whereas lymph node metastases were present in 22.2% of lung cancer patients whose tumors were MRP-1/CD9 and KAI1/CD82-positive, 65.5% of patients with MRP-1/CD9 and KAI1/CD82-reduced/negative tumors had lymph node metastases. A similar inverse relationship was seen in colon cancer and breast cancer patients with respect to MRP-1/CD9 expression. The present data, together with our previous results suggest that evaluating the MRP1/CD9 and KAI1/CD82 status of cancers of the lung, breast and colon may provide useful information on the metastatic potential of the tumors.

Heat shock enhances the susceptibility of tumor cells to lysis by lymphokine-activated killer cells.

OBJECTIVE: To determine whether heat-treated thyroid cancer cells augment the susceptibility of target cells to lysis by autologous lymphokine-activated killer cells. DESIGN: Peripheral blood lymphocytes from patients with thyroid cancer were incubated with recombinant interleukin 2 (100 U/mL) for 7 days, and thyroid cancer cells obtained from surgical specimens were heated at 44 degrees C for 20 minutes and incubated at 37 degrees C for 18 hours before performing the radioactive chromium Cr 51 release assay. RESULTS: The susceptibility of heat-treated thyroid cancer cells to lysis by autologous and allogeneic lymphokine-activated killer cells was significantly greater than that of untreated tumor cells. The mechanism of enhanced susceptibility was unclear. However, the effect depended on de novo protein synthesis, because inhibition of RNA synthesis by dactinomycin completely abolished the heat-enhanced susceptibility of tumor cells. CONCLUSION: Immunotherapy combined with hyperthermia may be useful in management of thyroid cancer.

Relationship between cellular glutathione level and susceptibility to LAK killing in human pharyngeal carcinoma cell line.

We examined the relationship between cellular glutathione (GSH) level and susceptibility to lymphokine-activated killer (LAK) cell-mediated cytolysis in KB human pharyngeal carcinoma cells. Treatment of KB cells with D,L-buthionine-S,R-sulfoximine (BSO), a gamma-glutamyl cysteine synthetase blocker, resulted in decreased total intracellular GSH levels associated with increased susceptibility to LAK killing. In contrast, treatment with oxothiazolidine-4-carboxylate (OTZ, a precursor of cysteine), which is known to increase cellular GSH level, decreased the susceptibility of KB cells to LAK killing. Both agents had no effects on binding frequency of KB cells to LAK cells. These results suggest that intracellular GSH in tumor cells play a protective role against LAK mediated cytolysis, specially in the post-binding killing phase.

Swainsonine augments the cytotoxicity of human lymphokine-activated killer cells against autologous thyroid cancer cells.

OBJECTIVE: Swainsonine (SW), an inhibitor of mammalian Golgi alpha-mannosidase II, blocks the processing of high mannose to complex type oligosaccharides. In this study, the effect of SW on the cytotoxicity of lymphokine-activated killer (LAK) cells against autologous thyroid cancer was investigated. DESIGN: Peripheral blood lymphocytes from patients with thyroid cancer were incubated with recombinant interleukin 2 (100 U/mL) and 0.5 mg/L of SW for 7 days, and thyroid cancer cells obtained from surgical specimens were pretreated with SW (0.5 mg/L) for 18 hours. The cytotoxicity of SW-treated LAK cells against tumor cells tested in a standard 4-hour radioactive chromium Cr 51 release assay. RESULTS: The cytotoxicity of SW-treated LAK cells against autologous thyroid cancer cells was found to be significantly greater than that of standard LAK cells incubated with interleukin 2 alone. The N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester esterase activity of LAK cells, this activity being a cytotoxic factor that is necessary for the lethal hit stage, was also increased by SW treatment. Further, thyroid cancer cells incubated with SW, as compared with nontreated tumor cells, showed much higher susceptibility to LAK killing. CONCLUSIONS: Our results suggest that SW might have potential immunomodulatory properties in the treatment of thyroid cancer.

Deacetylase activity of human tumor cells producing immunosuppressive aminosugars: its possible role in resistance to cell-mediated cytotoxicity.

In the present study, we examined the presence of deacetylases capable of producing free hexosamines, which we have shown earlier to be immunosuppressive against human natural killer (NK) cell-mediated cytotoxicity, from N-acetylhexosamines in human tumor cells. When human NK-resistant colon cancer cells (Colo-320DM) were incubated with acetyl-D-[1,6-3H(N)]glucosamine, a significant conversion to [3H]glucosamine occurred. Deacetylation was demonstrated as a change of the substrate radioactivity into free glucosamine trapped by a cation exchange resin, and this was subsequently confirmed by paper chromatography. This deacetylase activity was detected in other NK-resistant tumor cell lines, especially in freshly isolated human renal and breast cancer cells and testicular seminoma cells. However, no deacetylase activity was detected in NK-sensitive target cells such as K562, MOLT-4, or HL-60 cells. The ability to produce free hexosamines from N-acetylated aminosugars may provide a new mechanism for the escape of tumor cells from the attack of immune effector cells such as NK cells.

Dimethyl sulfoxide (DMSO) increases expression of sialyl Lewis x antigen and enhances adhesion of human gastric carcinoma (NUGC4) cells to activated endothelial cells.

Dimethyl sulfoxide (DMSO) exerts a number of biological effects including the promotion of cell differentiation in cultured cells. In this study, we examined the effect of DMSO on the adhesion of tumor cells to endothelial cells. In vitro treatment of human gastric adenocarcinoma (NUGC4) cells with DMSO resulted in increased adhesion to interleukin-I (IL-I)-activated human endothelial cells compared with DMSO-untreated NUGC4 cells. In flow cytometry, treating NUGC4 cells with DMSO enhanced the expression of sialyl Lewis x (sialyl Le(x)) and sialyl dimeric Le(x) antigens on their surface. Also, the binding of Limulus polyphemus agglutinin (LPA), which specifically binds to cell-surface sialic acids, was increased by DMSO. The adhesion of DMSO-treated NUGC4 cells to activated endothelial cells was blocked by neuraminidase pre-treatment of tumor cells or by antibody against either endothelial leukocyte adhesion molecule-I (ELAM-I) or sialyl Le(x). Thus, it is suggested that enhanced adhesion following DMSO treatment is mediated by the interaction of sialyl Le(x) expressed on NUGC4 cells with ELAM-I of endothelial cells. The modulation of sialyl Le(x) antigen by DMSO provides a useful system for studying the regulatory mechanism of Lewis-related carbohydrate antigens and also for understanding the metastatic properties of cancer cells.

The presence of concanavalin-A(Con-A)-like molecules on natural-killer (NK)-sensitive target cells: their possible role in swainsonine-augmented human NK cytotoxicity.

In the present study we examined the expression of concanavalin-A(Con-A)-like molecules on natural-killer (NK)-sensitive target cells and investigated their possible role in the human NK-cell phenomenon. The incubation of either peripheral-blood lymphocytes (PBL) or large granular lymphocytes (LGL) with swainsonine (SW), an inhibitor of mannosidase II, resulted in the augmentation of cytotoxicity against K562 leukemia cells. The enhanced cytotoxicity was associated with increased binding of fluorescein isothiocyanate-conjugated Con-A to SW-treated effector cells, and immunofluorescence staining of the target K562 cells using goat anti-Con-A antibody (Ab) showed a significant positive shift in the flow cytometric pattern. Electrophoretic separation and immunoblotting analysis revealed that 4 components with a molecular weight of approximately 95, 80, 60 and 50 kDa were recognized by anti-Con-A Ab from the detergent-extract of K562 cells. The addition of Con-A during the antibody incubation step of the Western blotting abolished their expression, thus excluding non-specific binding of the antibody. The addition of Con-A also strongly inhibited the cytotoxicity of SW-treated effector cells (PBL or LGL) against K562 cells, and this inhibition was abolished by 40 mM alpha-methyl-mannopyranoside (alpha-MM), which binds to Con-A. Furthermore, Con-A increased the binding frequency of SW-treated LGL to K562, in spite of the inhibited cytotoxicity, and this effect could be neutralized by the further addition of alpha-MM. Our results suggest that Con A-like molecules might play an important role in cell-cell interactions between SW-treated effector cells and NK target cells.

Lysis of fresh human tumor cells by autologous peripheral blood lymphocytes and tumor-infiltrating lymphocytes activated by PSK.

The protein-bound polysaccharide PSK was tested for the ability to induce in vitro autologous tumor killing (ATK) activity in human cancer patients. Peripheral blood lymphocytes (PBL) and tumor-infiltrating lymphocytes (TIL) demonstrated various levels of cytotoxicity against autologous, freshly isolated tumor cells. When PBL and TIL were cultured overnight with PSK, ATK activity was induced in previously non-reactive cases and augmented in previously reactive samples. The PSK effect was observed with PSK concentrations of 10-100 micrograms/ml that could be obtained in the blood of cancer patients who received standard oral administration of PSK. The manifestation of PSK-induced ATK required active cell metabolism and RNA and protein syntheses, but not DNA synthesis of lymphocytes. PSK-induced enhancement of ATK was not abrogated by monoclonal antibodies (mAb) directed against interferon (IFN) alpha or IFN gamma. In addition, mAb that neutralized interleukin-2 (IL-2) or mAb reactive with alpha-chain or beta-chain of IL-2 receptors (IL-2R) had no effect on PSK-induced ATK activity. Supernatants from PSK-stimulated lymphocyte cultures did not induce ATK. Cell fractionation experiments revealed that CD3-CD16+ large granular lymphocytes (LGL) and/or CD3+CD16- T lymphocytes were responsible for both spontaneous and PSK-induced ATK. PSK-activated LGL, but not T lymphocytes expressed lysis of fresh allogeneic tumor cells. These results indicate that PSK activates PBL and TIL to exhibit ATK independently of IL-2/IL-2R systems.

Prediction of postoperative clinical course by autologous tumor-killing activity in lung cancer patients.

Fifty patients with primary localized lung cancer were tested at the time of surgery for the ability of their lymphocytes to kill autologous, freshly isolated tumor cells, and the assay was evaluated for prognostic significance. Peripheral blood lymphocytes of 27 patients (54%) demonstrated significant autologous tumor-killing activity in 6-hour 51Cr-release assays. Twenty-three of the 27 patients with autologous tumor-killing activity remained tumor free and survived more than 5 years after curative surgery, while all 23 who were negative for autologous tumor-killing activity relapsed by 18 months after surgery and died within 42 months after surgery. The differences in survival curves for the two groups were highly significant (P less than .00003). Autologous tumor-killing activity was not correlated with natural killer (NK) cell activity against K562 human myeloid leukemia cells or proliferation of lymphocytes stimulated with autologous, freshly isolated tumor cells in mixed culture. There were no differences in total survival between patients with positive results and those with negative results in tests of NK cell activity and autologous mixed lymphocyte-tumor culture reaction. These results indicate that autologous tumor-killing activity is a meaningful prognostic indicator and provide evidence for immunological control of tumor growth and metastasis. According to our preliminary data, it is unlikely that lung cancer patients who remain tumor free after 60 months of follow-up will develop recurrence or die from the disease. We are conducting a study to determine whether induction of autologous tumor-killing activity before surgery, by treatment with biological response modifiers,can improve the clinical outcome in patients who do not naturally have this potential.

Reduction by OK-432 of the monolayer contact-mediated inhibition of human natural killer cell activity.

In the present study we investigated the effect of OK-432, a streptococcus preparation, on the contact-mediated inhibition of human NK activity by primary cultures of monolayer cells. Either peripheral blood lymphocytes (PBL) or large granular lymphocytes (LGL) were incubated (2 x 10(6) cells/ml, total volume 2 ml) on confluent monolayer cells (uvea-derived fibroblasts, uvea-derived melanoma cells, or renal carcinoma cells) for 18 h in 24-well plates, washed twice, and tested for cytotoxicity against K562, a human myelogenous leukemia cell line, in a 4 h 51Cr-release assay. After contact with monolayer cells, NK activity of both PBL and LGL was significantly reduced. When these effector cells were preincubated with 0.1 U/ml of OK-432 for 18 h and then tested for the sensitivity to contact-mediated inhibition, the inhibition was significantly reduced. The pretreatment of monolayer cells with OK-432 or the addition of OK-432 into the coculture wells (of effector cells and monolayer cells) also significantly reduced the contact-mediated inhibition. Moreover, OK-432 (0.1 U/ml) reestablished the inhibited NK activity of PBL. These results suggest that OK-432 might enable NK cells to escape from the contact-mediated inhibition by monolayer cells and thus provide an additional potential mechanism for the observed clinical effectiveness of OK-432 reported by many groups.