Natural killer, lymphokine-activated killer and interferon-gamma producing activities of peripheral blood- and regional lymph node-mononuclear cells in 23 cases of colorectal cancer

Natural killer (NK) activity, lymphokine-activated killer (LAK) activity and interferon-gamma (IFN-gamma) producing activity of peripheral blood mononuclear cells (PBMC) and regional lymph node mononuclear cells (LNMC) were studied in 23 previously untreated cases of colorectal cancer. NK and LAK activities were significantly lower in LNMC than in PBMC. Patients showed depressed NK and LAK activities in PBMC. In the Dukes C group, especially, both NK activity and LAK activity decreased compared to control patients. NK and LAK activities of PBMC decreased as the grade of invasion to lymphatic channels progressed. LAK activity positively correlated with NK activity in PBMC. Patients with high LAK activity showed high IFN-gamma production in both controls and Dukes A . B patients. However, in Dukes C patients, no relationship between LAK activity and IFN-gamma production was observed. We conclude that the depressed NK and LAK activities of PBMC reflect the local lymphatic invasion and that IFN-gamma involvement in LAK cell generation is impaired in advanced cancer patients. More fundamental studies should be carried out before clinical trials of adoptive immunotherapy using LAK cells, because LAK activity is not induced sufficiently in advanced cancer.     

Antilymphocyte globulin therapy enhances impaired function of natural killer cells and lymphokine activated killer cells in aplastic anaemia

MHC-unrestricted cytotoxic lymphocytes, namely natural killer (NK) and lymphokine activated killer (LAK) cells, have been implicated in the regulation of haemopoiesis. To investigate the possible role of these lymphocytes in the pathogenesis of aplastic anaemia (AA), we studied their functions in the peripheral blood mononuclear cells (PBMC) and bone marrow mononuclear cells (BMMC) of patients with AA treated with antilymphocyte globulin (ALG). Before treatment, both NK and LAK activities in the PBMC of 25 patients were low (NK = 1.9 +/- 2.1 x 10(3) LU/l) LAK = 4.7 +/- 3.6 x 10(3) LU/l) compared to normal (NK = 6.0 +/- 3.0 x 10(3) LU/l, LAK = 10.0 +/- 3.5 x 10(3) LU/l) or multiply transfused (NK = 7.8 +/- 6.6 x 10(3) LU/l, LAK = 25.2 +/- 13.6 x 10(3) LU/l) controls. The NK and LAK activities in the BMMC in AA patients were not significantly different from those in PBMC. In all patients with low LAK and NK activities pre ALG there was an increase in activity 2-24 weeks after therapy which eventually reached normal levels and which was maintained for up to 2 years. Analysis of lymphocyte phenotypes in AA patients before treatment showed both significantly low mean proportion and absolute numbers of CD16+ cells compared to normals, which increased after therapy. Changes in MHC-unrestricted cytotoxicity and lymphocyte phenotypes post therapy were not correlated with haemopoietic recovery. These data suggest that ALG treatment can enhance the functions of MHC-unrestricted lymphocytes independently from haemopoiesis. It is unlikely that these cells play a role in the pathogenesis of AA.     

Interleukin 2-activated killer cells in patients following transplants of soybean lectin-separated and E rosette-depleted bone marrow

During the early period following bone marrow transplantation before the immune system has reached full functional maturity, unprimed, nonspecific lytic systems may play a critical role as antiviral or antitumor effectors. The reconstitution of cells with this potential is of particular importance in recipients of bone marrow that has been depleted of mature T lymphocytes to prevent graft v host disease (GVHD). We examined the recovery of natural killer (NK) cells and interleukin 2 (IL 2)-augmented lymphokine-activated killer cells (LAK) in 48 patients at various intervals following transplantation of bone marrow depleted of mature cellular elements by treatment with soybean agglutinin and sheep RBCs (SBA-E- BMT). We found normal levels of both NK and LAK activity as early as 3 weeks following SBA-E- BMT. When compared with cells from controls, NK and LAK precursors from transplant recipients appeared to be activated in vivo in that freshly isolated peripheral blood mononuclear cells (PBMCs) from patients had an elevated cytolytic activity toward NK-insensitive targets and a more rapid response to activation by IL 2. In patients as well as controls, both LAK precursors and LAK effectors lacked antigens present on mature T lymphocytes (CD3, CD4, or CD8) but expressed antigens present on NK cells (CD2, CD16, and NKH1A). The LAK cells did not lyse either donor or host peripheral blood T cell targets. The activity of NK effectors but not LAK precursors survived the in vivo total body irradiation used for pretransplant conditioning in three patients studied. LAK precursors could be demonstrated as early as 18 days following transplant at a time when the bone marrow contained primarily donor- derived cells. Little or no LAK activity could be generated from cells of the SBA-E- BM graft itself, suggesting that LAK precursors differentiate rapidly from more primitive progenitors in the marrow graft. Thus, our data indicate that the NK and LAK lytic systems are among the earliest activities to recover during immune reconstitution following T cell-depleted BMTs.     

Endogenous lymphokine activated killer cell activity and cytogenetic response in chronic myelogenous leukaemia treated with α-interferon

The capacity of α-interferon (α-IFN) to induce lymphokine activated killer (LAK) cytotoxicity in the absence of interleukin-2 (IL2) has prompted us to test whether or not its ability to reduce dramatically the number of Ph1 ⁺ clones in chronic myelogenous leukaemia (CML) patients is in part mediated through the generation of natural killer (NK) or LAK activity. The latter were tested using NK-sensitive (K562) and NK-resistant (Raji) cell lines in a target-cell colony-growth inhibition assay. Effector cells (E) were patient blood mononuclear cells (MC) without in vitro activation prior to their coculture with targets (T). Out of 16 patients tested so far, three failed to undergo cytogenetic remission under α-IFN therapy. No NK nor LAK cells could be detected in the MC from two of them while the other displayed NK activity within upper normal limits. 13 patients underwent complete (eight) or partial (five) cytogenetic remission together with significantly high NK and/or LAK activity as compared to normal controls. These observations could favour the hypothesis of an indirect effect of α-IFN on leukaemic cells, mediated by cells involved in immune surveillance.     

Distinct characteristics of lymphokine-activated killer (LAK) cells derived from patients with B-cell chronic lymphocytic leukemia (B-CLL). A factor in B-CLL serum promotes natural killer cell-like LAK cell growth

We show that lymphokine-activated killer (LAK) cell precursors derived from patients with B-cell chronic lymphocytic leukemia (B-CLL) and cultured in the presence of recombinant interleukin-2 and normal human serum (NHS), develop into primarily NK cell-like (CD 57+) LAK cells, whereas identically prepared LAK cell precursors from normal subjects develop into mainly T cell-like (CD 3+, CD 8+) LAK cells. B-CLL LAK cells exhibited greater proliferative capacity than did normal LAK cells. When normal LAK cells were grown in B-CLL serum instead of NHS, their proliferation increased; NK cell levels also increased to those found in B-CLL LAK cells, suggesting that B-CLL serum contains a factor that promotes NK cell-like growth, LAK cells derived from normal or B-CLL patients demonstrated similar lytic activity toward K562 and Raji cells. Growth in B-CLL serum did not reduce their lytic potential. Thus, the altered phenotype and growth exhibited by B-CLL LAK cells and normal LAK cells grown in B-CLL serum does not lead to abnormalities in their cytolytic functions. We propose instead that the predominance of NK-like cells in B-CLL LAK cell populations and the presence of an NK cell-like growth factor in B-CLL serum reflect abnormalities related to NK cell-mediated B-cell regulation; ie, either inhibition of normal B-cell growth and/or growth stimulation of the leukemic clone in B-CLL.     

Lymphokine-activated killer cell and natural killer cell activities in patients with systemic sclerosis.

OBJECTIVE. To determine the ability of T lymphocytes and natural killer (NK) cells from patients with systemic sclerosis (SSc) to respond to cytokines and to generate immune effector cells. METHODS. The numbers and percentages of peripheral blood T and NK cells were examined by 2-color flow cytometry, and NK and lymphokine-activated killer (LAK) cell function were measured in 4-hour 51Cr-release assays, in 34 patients with SSc. The patients were categorized into 3 subgroups: 10 had diffuse cutaneous disease of less than or equal to 3 years disease duration, 11 had diffuse cutaneous SSc of greater than 3 years duration, and 13 had limited cutaneous disease. RESULTS. Baseline and activated NK and T cell numbers and NK activity were normal in SSc patients. However, mean LAK activity was significantly depressed in all SSc subgroups. CONCLUSION. Decreased LAK cell function, despite normal numbers of circulating T and NK cells, indicates that SSc patients have poor ability to produce effector cells in response to interleukin-2.     

Effect of arginase on splenic killer cell activity in patients with gastric cancer

Arginase has been detected in high levels in gastric cancer tissues. The effect of arginase on the activities of splenic natural killer (NK) cell, phytohemagglutinin activated killer (PAK) cell, and interleukin-2 activated killer (LAK) cell in patients with gastric cancer (N=12) was evaluatedin vitro. These activities in patients (N=10) with trauma and benign lesions were used as control. The splenic NK and PAK cell activities in patients with gastric cancer were significantly lower than in the controls (P<0.05), whereas LAK cell activity did not have significant difference. Arginase inhibited all splenic killer cell activities to a similar degree between patients with gastric cancer and the controls. The inhibition was dose-related. These data suggest that arginase may play a positive role in the spread of gastric cancer cells. However, LAK may be a potential approach of immunoadoptive therapy in the future.This study was supported by a grant from National Science Council of the Republic of China (NSC 77-0412-B075-61).     

Lymphokine activated killer (LAK) cells in patients with gastrointestinal cancer.

Lymphokine activated killer (LAK) cells are a recently described cellular immune phenomenon with exciting potential for the treatment of tumours arising from solid organs. A comparison of some aspects of LAK cell precursors and LAK cell function was undertaken in 44 control subjects and 44 preoperative patients suffering from gastrointestinal cancer (20 localised and 24 advanced). Lymphokine activated killer cell precursor (natural killer (NK) cell) activity was significantly diminished in patients with advanced tumours (p less than 0.02) as was fully mature LAK cell activity against an NK resistant target cell (p less than 0.012). T-lymphocyte responses were not significantly different between the three groups. The reduced LAK cell generation was associated with a significantly diminished proliferative response of LAK precursors to stimulation with high dose IL-2 in vitro (p less than 0.012). Impaired LAK cell generation may explain the failure of adoptive cellular immunotherapy with LAK cells in some patients with advanced gastrointestinal cancer and prompts the search for means of augmenting this activity in such patients.     

Interleukin 2 induction of lymphokine-activated killer (LAK) activity in the peripheral blood and bone marrow of acute leukemia patients. I. Feasibility of LAK generation in adult patients with active disease and in remission

The feasibility of in vitro interleukin 2 (IL-2) activation and expansion of mononuclear cells (MNCs) derived from adult patients with acute myelogenous leukemia (ANLL) was studied. Patients' natural killer (NK) and lymphokine-activated killer (LAK) cell activity was compared with that of normal donors in terms of: (a) cytolytic activity (four- hour 51Cr release assay) against an NK-sensitive target (K562), NK- resistant targets (Raji/Daudi), and fresh/cryopreserved autologous and allogeneic leukemic blasts; (b) proliferation and expansion in culture with 1,000 U/mL recombinant IL 2 (rIL 2); and (c) the cell surface phenotype of the cultured cells. In 21 of 24 patients with active disease (AP) MNCs derived from the peripheral blood (PBL) or bone marrow (BM) could be cultured and expanded in the presence of rIL 2. These cultures initially contained between 30% and 50% blasts, and during 2 to 4 weeks of culture destruction of blasts and enrichment of up to 60% in cells with the morphology of large granular lymphocytes (LGLs) was observed. Expansion in culture varied between two- and 100- fold. MNCs from all patients in remission (RP) could be activated by rIL 2 and expanded up to 30-fold after 1 to 3 weeks in culture. NK activity of fresh PBLs from AP was significantly lower than in normal controls, whereas NK activity of RP was within the normal range. High levels of postactivation NK and LAK activity on K562/Raji/Daudi and on fresh/cryopreserved leukemic blasts was generated in approximately 50% of cases of AP and in most RP. Cell surface phenotype studies showed that cultured cells derived from ANLL patients were significantly enriched (up to 40%) in NKH-1 (Leu 19) positive cells, with RP LAK cells also expressing a high proportion of CD16 positive cells (up to 40%). This study has shown that it is feasible to activate and significantly expand killer cells derived from active disease and remission ANLL patients during 1 to 3 weeks culture with IL 2 with good maintenance of cytolytic activity. Both initial NK activity and LAK generation was optimal in remission patients. Based on data from this study, a clinical protocol has been developed for treatment of early relapse ANLL patients with LAK cells cultured for 1 to 3 weeks and systemic IL 2.     

Lymphokine activated killer (LAK) cell activity in the peripheral blood lymphocytes of systemic sclerosis (SSc) patients

Lymphokine activated killer (LAK) cells, which arise from interleukin-2 (IL-2) activation of natural killer (NK) cells, are capable of lysing NK-resistant cell targets, including endothelial cells (EC). Since EC cytotoxicity is postulated to play a role in the pathogenesis of systemic sclerosis (SSc), experiments were performed to measure LAK activity in the peripheral blood lymphocytes (PBL) of 10 SSc patients and 10 normal controls. SSc patients had no significant spontaneous cytotoxicity against NK-resistant cell targets, including EC. After IL-2 stimulation in vitro, SSc patients and normal controls demonstrated cytotoxicity toward NK-resistant cell targets, including EC. This LAK-mediated EC cytotoxicity was actually lower for SSc patients than for normal controls. These studies do not preclude a role for LAK-mediated EC cytotoxicity in the pathogenesis of SSc, but demonstrate that LAK cells are not spontaneously present in circulating PBL.     

Influence of Autologous Blood Transfusion on Natural Killer and Lymphokine-Activated Killer Cell Activities in Cancer Surgery

Background and objectives: Immunosuppression associated with blood transfusion may influence postoperative infection rates. It may also affect the prognosis of patients treated surgically for colorectal cancer. To control this effect, study protocols have applied autologous blood donation programs, which are thought to be immunologically neutral. However, evidence has emerged that blood donation itself might have suppressive effects on natural killer (NK) cell activities. At present, there are no data available on the effects of autologous blood transfusion on NK or lymphokine-activated killer (LAK) cells. This might be of interest as LAK cells may be active in tumor control. Materials and methods: 26 patients who underwent surgical resection for colorectal cancer, were assigned at random into two groups: (1) autologous blood donation and transfusion, or (2) allogeneic blood transfusion. NK and LAK activities were determined before blood donation, at surgery, and on the 3rd and 8th postoperative day. Results: Blood donation induced a small decrease in NK and LAK activities. The postoperative courses of the two groups differed. In the allogeneic group, NK activity (?50%, p = 0.018) and LAK activity decreased (?60.7%, p = 0.043), whereas in the autologous group the decline in LAK was less pronounced (?33.7%, p = 0.091), and their NK activity even increased (+17.4%, p = 0.315). NK activity was modulated differently in the two study groups (0.0036). Differences in LAK activities were found between the 3rd and 8th day postoperatively (p = 0.354). Conclusions: In patients receiving autologous blood transfusion, postoperative suppressed NK and LAK activities were modulated. This implies that autologous blood transfusion is not immunologically neutral, but has an intrinsic immunomodulatory potential.     

Lymphokine-activated killer cell and natural killer cell activities in patients with systemic sclerosis

Objective. To determine the ability of T lymphocytes and natural killer (NK) cells from patients with systemic sclerosis (SSc) to respond to cytokines and to generate immune effector cells. Methods. The numbers and percentages of peripheral blood T and NK cells were examined by 2-color flow cytometry, and NK and lymphokine-activated killer (LAK) cell function were measured in 4-hour ⁵¹Cr-release assays, in 34 patients with SSc. The patients were categorized into 3 subgroups: 10 had diffuse cutaneous disease of ≤3 years disease duration, 11 had diffuse cutaneous SSc of >3 years duration, and 13 had limited cutaneous disease. Results. Baseline and activated NK and T cell numbers and NK activity were normal in SSc patients. However, mean LAK activity was significantly depressed in all SSc subgroups. Conclusion. Decreased LAK cell function, despite normal numbers of circulating T and NK cells, indicates that SSc patients have poor ability to produce effector cells in response to interleukin-2.     

Decreased interleukin-2 receptor β chain expression by peripheral blood lymphocytes in chronic liver disease

To investigate the decrease in natural killer (NK) activity in chronic liver disease, interleukin-2 receptor beta chain (IL-2R beta) expression was assessed by peripheral blood lymphocytes (PBL) using flow cytometry and an IL-2R beta chain-specific mouse monoclonal antibody. The percentage of IL-2R beta chain-positive PBL was significantly decreased in patients with chronic viral hepatitis, liver cirrhosis and hepatocellular carcinoma in comparison with normal controls (P less than 0.01). Among chronic viral hepatitis patients, it was significantly less in those with chronic active hepatitis than in those with chronic persistent hepatitis (P less than 0.05). Two-colour flow cytometry revealed that the IL-2R beta chain was mainly expressed by CD8+ or CD16+ cells in both the controls and the liver disease patients. CD8dull+ cells (NK cells) constituted more than 60% of the CD8+ cells expressing the IL-2R beta chain. Expression of the IL-2R beta chain with CD8 or CD16 was also significantly decreased in chronic liver disease patients compared with controls. In chronic viral hepatitis, there was a significant correlation between NK activity and the percentage of IL-2R beta+ PBL (P less than 0.001, r = 0.916), as well as between NK activity and the percentage of PBL co-expressing both the IL-2R beta chain and CD16 (P less than 0.001, r = 0.850). These findings suggest that decreased expression of the IL-2R beta chain by PBL may result in diminished NK activity in chronic liver disease.     

Intestinal lymphokine-activated killer cells in inflammatory bowel disease

The role of non-specific cytotoxicity in the pathogenesis of inflammatory bowel disease (IBD) was investigated by assaying the natural killer (NK) and lymphokine-activated killer (LAK) cell activity of lamina propria mononuclear cells (LPMC) from 22 specimens of intestinal mucosa affected by IBD. Only minimal levels of NK activity were detected against K562 cells, as well as colon carcinoma cells, adenoma cells and fibroblasts freshly isolated from the intestinal mucosa. Culture of LPMC from IBD in the presence of interleukin-2 (IL-2) generated LAK cells that mediated high levels of activity against K562 cells and against neoplastic epithelial cells and fibroblasts derived from the intestinal mucosa. A group of 20 histologically normal specimens of intestinal mucosa showed similar levels of LAK activity against the K562 and intestinal cell targets. The minimal mucosal NK activity in IBD suggests that the cytotoxic properties of NK cells are not important in the pathogenesis of IBD. The presence of LAK precursor cells in the inflamed mucosa of IBD and their ability to lyse biologically relevant targets in vitro suggests that LAK cells have the potential to contribute to intestinal mucosal injury in IBD.     

Timing of Effective Natural Killer Cell Collection in Cancer Patients After High Dose Chemotherapy,Compared to Peripheral Blood Stem Cell Collection

Abstract: Natural killer (NK) and lymphokine-activated killer (LAK) activities in patients with seminoma were determined, and the timing of effective NK cell collection was evaluated, compared to autologus peripheral blood stem cell (PBSC) collection. After chemotherapy, patients were injected with granulocyte-colony stimulating factor (G-CSF). CD56 cells decreased in parallel with white blood cells and then were restored. After discontinuing the G-CSF, the CD56 cells increased and reached peak 3–5 days after PBSC collection. The peak showed higher levels than those at premedication. Recovery-peripheral blood lymphocytes (PBL) which were obtained 4 days after PBSC collection showed higher NK and LAK activities than the PBL obtained before chemotherapy (Before-PBL). They exhibited a high expression of CD2, CD 18, CD29, and CD49d. Furthermore, the LAK cells generated from the Recovery-PBL could aquire 9.6 folds of killer unit compared with the Before-PBL-LAK. These results suggest that, in order to obtain large numbers of NK cells, Recovery-PBL collection may be more advantageous than PBL collection in no relation with the chemotherapy.     

Spontaneous and interferon-induced natural cytotoxicity in Crohn's disease

Natural killer cell (NK) activity and antibody-dependent cellular cytotoxicity (ADCC) exerted by peripheral blood mononuclear cells (PMNC) were investigated in 53 patients with Crohn's disease (CD). NK activity and ADCC were found to be significantly reduced in patients with CD (p less than 0.0005) as compared to healthy controls. Both effector cell functions increased after in vitro treatment of PMNC with gamma-interferon, but did not reach the levels found in controls (p less than 0.0005). Neither NK activity nor ADCC was significantly influenced by therapy with corticosteroids. Moreover, the reduced serum zinc levels in patients with CD, which have been shown to be associated with impaired immune function, did not influence the lytic effector cell mechanism assayed either. Finally, no association could be found between NK cell activity or ADCC and CD activity index, the extent of the disease and several laboratory parameters of inflammation. We conclude that patients with CD have a reduced lytic effector cell function which remains uninfluenced by corticosteroid treatment and seems to be present independently of disease activity.     

Natural killer (NK) cell activity and its in vitro response to interferon-alpha(Le) in chronic liver diseases and hepatocellular carcinoma

The natural killer (NK) cell activity of peripheral blood lymphocytes (PBL) in patients with various chronic liver diseases, and its in vitro response to human interferon-alpha(Le) were investigated using a 16-h 51-Cr releasing cytotoxicity assay against YAC-1 or RSa target cells. The NK cell activity was found to be higher in chronic active hepatitis (CAH) and liver cirrhosis (LC) patients without HCC, whereas it was slightly lower in LC patients with hepatocellular carcinoma (HCC), than in normal controls. By the addition of IFN-alpha(Le) in vitro, the NK cell activity was clearly and dose-dependently augmented, even in chronic liver diseases, as well as in normal controls. The magnitude of this augmentation by 10,000 IU/ml of IFN-alpha(Le) in the various chronic liver diseases was not significantly different from that in normal controls. The results suggested that the response of NK cells to IFN-alpha(Le) is not impaired even in chronic liver disease conditions, while the level of NK cell activity may vary according to the type of chronic liver disease and may decrease in patients with HCC.     

Association of natural killer cell immune recovery with a graft-versus-leukemia effect independent of graft-versus-host disease following allogeneic bone marrow transplantation

 There is good evidence that T lymphocytes play an important role in the graft-versus-leukemia (GVL) effect following allogeneic bone marrow transplantation (BMT) for hematologic malignancies. However, the role of natural killer (NK) cells in GVL is less clear. To further investigate a possible association of NK cells with GVL we studied 15 patients undergoing BMT for chronic myeloid leukemia (CML), correlating T-cell (CD4+ and CD8+) and NK-cell (CD16+56+) recovery with relapse and graft-versus-host disease (GVHD). Patients were studied on three occasions up to 9 months after BMT, for lymphocyte surface phenotype and for spontaneous and IL-2-stimulated (LAK cell) cytotoxic function. Circulating CD8+ and NK but not CD4+ cell numbers were significantly lower in five patients who relapsed compared with those remaining in remission after BMT (mean 0.03 vs 0.32×10⁹/l, p=0.002 for CD8+ cells; mean 0.03 vs 0.11×10⁹/l, p=0.002 for NK cells). There was no correlation of CD4+, CD8+, or NK cell numbers and development of grade-II or more acute GVHD. Spontaneous NK cytotoxic function rose to within the normal range in the first month after BMT. LAK function remained low during the study period. These results link NK cell recovery more closely with a GVL than with a GVH effect. Received: 1 May 1996/Accepted: 19 September 1996     

Fibrin formation inhibits the in vitro cytotoxic activity of human natural and lymphokine-activated killer cells

The biological significance of the interaction of the haemostatic factors with tumour cells remains unclear. It has been hypothesized that fibrin deposition around tumour cells could help those cells to escape destruction by cytotoxic effector cells. To obtain direct evidence in support of this possibility, the effect of fibrin formation on in vitro cytotoxicity of human natural killer (NK) or lymphokine-activated killer (LAK) cells was investigated by comparing their cytotoxic activity with various human tumour cell lines in the presence of human serum or plasma. The data demonstrate that pre-incubation of human tumour cells with plasma, but not serum, substantially diminished or completely abrogated the cytotoxic effects of these killer cells. This was shown to be due to fibrin formation. The degree of coagulation and the number of radioactive tumour cells trapped in the clot correlated with the extent of inhibition of NK or LAK cytotoxicity. Abrogation of LAK activity was also observed when the effector cells were pre-exposed to plasma or when effector and target cells were simultaneously mixed with plasma and trapped in a fibrin clot. Similar results were obtained when, instead of whole plasma, the cytotoxic effect of LAK and NK cells was studied in the presence of fibrinogen and thrombin. When heparin was added, fibrin formation was prevented and no inhibition of LAK/NK cell cytotoxicity was observed. In studies of the mechanisms of inhibition of LAK cell activity by fibrin, target - effector cell conjugate formation was found to be blocked. When plasma was added post-binding (15-30 min after mixing effector and target cells) although coagulation occurred, no effect on cytotoxicity was observed, supporting the conclusion that fibrin interfered with binding rather than the lytic phase of cytotoxic cell activity. Thus, the present data demonstrate that fibrin deposition around tumour and/or effector cells can protect tumour cells from immune destruction and diminish the efficiency of the cytotoxic LAK/NK cells.