H-2 antigen expression and sensitivity of BL6 melanoma cells to natural killer cell cytotoxicity

The sensitivity of H-2b-high and H-2b-low variants of BL6 melanoma to the cytotoxic action of NK and lymphokine-activated killer cells was investigated. BL6 mouse melanoma cells lack detectable H-2Kb and had low levels of expression of H-2Db Ag. The BL6T2 variant cells, obtained after treatment of BL6 cells with mutagen N-methyl-N-nitro-N'-nitro-soguanidine, had relatively high levels of expression of class I H-2b Ag. Poly(I:C)-stimulated spleen cells of nude mice were highly cytotoxic for BL6T2, whereas H-2b-low BL6 cells were less sensitive to NK activity in an 18-h 51Cr-release assay. Similar results were obtained after 4-h incubation of radio-labeled tumor cells with IL-2-activated effector cells. In contrast, both lines were equally sensitive to lysis by purified granules derived from rat large granular lymphocytes (LGL) or by macrophages. By using various clones selected from BL6 or BL6T2 cells, it was found that BL6 or BL6T2 clones with low H-2b Ag expression were less sensitive to lysis by NK cells than H-2b-high clones. After IFN treatment of either BL6 or BL6T2, the target cells became more resistant to lysis by either NK cells or by purified LGL granules. IFN-treated BL6 cells had substantially increased expression of H-2b Ag and in this respect became similar to untreated BL6T2. However, IFN-treated BL6 cells were more resistant than BL6T2 cells to lysis by NK cells and LGL granules, suggesting that augmentation of H-2b Ag expression and NK resistance could be two independent IFN-induced effects. With a cold target inhibition assay, it was found that BL6T2 or its H-2 positive clones were highly competitive and inhibited the cytotoxic activity of NK and lymphokine-activated killer cells against radiolabeled YAC-1 and BL6T2, whereas BL6 cells or H-2-negative clones of BL6T2 and BL6 lines showed poor competitive ability. Thus, our data indicate that the NK resistance of H-2-low BL6 cells may be due to a paucity of NK recognizable determinants. N-Methyl-N-nitro-N'-nitroguanidine treatment of BL6 melanoma cells was associated with an increase in class I H-2b Ag expression and NK sensitivity, suggesting the involvement of class I MHC Ag in the sensitivity of tumor cells to NK cell-mediated cytotoxicity.     

Paclitaxel probably enhances cytotoxicity of natural killer cells against breast carcinoma cells by increasing perforin production

Paclitaxel, a semisynthetic taxane, is one of the most active chemotherapeutic agents for the treatment of patients with breast cancer. We focused on the effect of paclitaxel on the cytotoxicity of natural killer (NK) cells. NK cells were purified by negative selection with magnetic beads from peripheral blood mononuclear cells of healthy volunteers. A human breast carcinoma cell line BT-474 and an NK cell–sensitive erythroleukemia cell line K562 were used as targets. Cytotoxicity of NK cells was determined by ⁵¹Cr-release assay with labeled target cells. Paclitaxel (1–100 nM) did not affect cellular viability, and significantly enhanced cytotoxicity of NK cells in a dose-dependent manner. Although paclitaxel did not affect Fas-ligand expression of NK cells, paclitaxel induced mRNA and protein production of perforin, an effector molecule in NK cell–mediated cytotoxicity. Concanamycin A, a potent inhibitor of the perforin-mediated cytotoxic pathway, inhibited paclitaxel-dependent NK cell–mediated cytotoxicity. Furthermore, paclitaxel induced activation of nuclear factor B (NF-B) in NK cells. NF-B inhibitor pyrrolidine dithiocarbamate significantly suppressed both paclitaxel-induced perforin expression and NK cell cytotoxicity. Our results show for the first time that paclitaxel enhances in vitro cytotoxicity of human NK cells. Moreover, our results suggest a significant association between enhanced NK cell cytotoxicity, increased perforin production, and NF-B activation.     

Effect of surgical stress on murine natural killer cell cytotoxicity

Natural killer cell cytotoxicity (NKCC) against tumors may be important in preventing in vivo solid tumor dissemination. Multiple animal models demonstrate increased rates of tumor dissemination after surgical stress; previously, we have observed that surgical stress impairs murine NKCC. Because of the importance of surgery in the control of solid tumors, it appeared valuable to examine the mechanism underlying surgical stress impairment of NKCC. The results of this study demonstrate that postsurgical suppression of NKCC begins as early as 2 hr after murine hind limb amputation, reaches nadir at 4 days, and does not recover to control level until postoperative day 12. Anesthetic treatment alone does not cause comparable NKCC suppression. The suppression of NKCC accompanied changes in both splenic size and morphology. The immune suppression was observed in multiple compartments including peripheral blood, bone marrow, and spleen. Mixing experiments demonstrated that surgical stress per se generated a suppressor cell population affecting NKCC. The observed suppression apparently required cell-to-cell contact, because supernatants from 4 and 18 hr cultures of suppressor cells did not cause suppression. The observed suppression was prevented by perioperative treatment with the pyrimidinone analog 2-amino-5-bromo-6-phenyl-4-pyrimidinol. These preclinical observations point to the future prospect of NK-specific perioperative immunotherapy that may help prevent possible tumor dissemination from occurring at the time of surgery.     

Depression of murine natural killer cell cytotoxicity by isobutyl nitrite

Summary We have investigated the effect of isobutyl nitrite on murine NK-cell antitumor-directed cytotoxicity. This agent has been suggested as one of the factors underlying immunodeficiency syndrome (AIDS) in man. We demonstrated that two injections, each of 0.25 ml isobutyl nitrite, resulted in significant depression of endogenous splenic and peripheral blood natural killer (NK) cell cytotoxicity against T-cell lymphoma, YAC-1. In addition to endogenous NK cells, activity of pyrimidinol-activated NK cells was also substantially depressed by this agent. The latter observation is of the utmost importance, since it suggests that the attempt to augment NK-cell activity (to promote resistance to infections and malignancies) could fail in patients with AIDS who are isobutyl nitrite users. Isobutyl nitrite was NK-cell-suppressive not only after in vivo administration but, most importantly, also after inhalation. This indicates that isobutyl nitrite, via its NK-cell suppressive effect, could contribute to immunodeficiency in AIDS. Studies on the mechanism of NK-cell depression by isobutyl nitrite demonstrated that the NK-cell tumor-binding properties as well as NK-cell cytotoxic potential were substantially depressed. Mixing experiments failed to reveal any regulation by suppressor cell activities. The results of these studies clearly indicate that isobutyl nitrite is an immunosuppressive agent and that its use should be avoided. Abbreviations used: NK, natural killer; S-RPMI 1640, supplemented tissue culture medium 1640; HEPES, N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid; TBC, tumor-binding cells; C-TBC, cytotoxic tumor-binding cells; AIPP, 2-amino-5-iodo-6-phenyl-4 pyrimidinol; LU, lytic units; T:E, target-to-effector; AIDS, acquired immunodeficiency syndrome     

Tumor cell differentiation modulates susceptibility to natural killer cells

Induced differentiation in three human cell lines altered their sensitivity specifically to human natural killer (NK) cells by affecting their expression of NK target antigens. Differentiation of HL-60, a promyelocytic leukemia cell line, and the erythroleukemic cell line K562 was accompanied by a concomitant decrease in susceptibility to NK-mediated lysis whereas induction of MeWo melanoma cells resulted in an enhanced sensitivity to lysis. Our findings suggest that target cell susceptibility to NK-mediated lysis may in part be dependent on the stage of differentiation of the tumor cell target.     

Similarities and distinctions between murine natural killer cells and lymphokine-activated killer cells

Pretreatment of mice with rabbit anti-asialo GM1 removes both natural killer (NK) effector cells and NK cells responsive to interleukin 2 (IL-2). Spleen cells from these mice do possess normal lymphokine-activated killer (LAK) activity. Young mice (less than 3 weeks of age) do not have NK activity and do not possess IL-2-inducible NK effector cells. Similarly to anti-asialo GM1-treated mice, LAK cells can be generated from these mice. While these experiments indicate clear distinctions between a certain level of NK and LAK precursors, the distinctions are not as clear when analyzing mice congenitally deficient in NK cells. Beige mice which lack NK effector cells and IL-2-inducible NK cells also lack the ability to generate LAK cells. The relationships and differences between NK- and LAK-cell precursors and effectors are discussed.     

An absence of T cells in murine bone marrow allografts leads to an increased susceptibility to rejection by natural killer cells and T cells

The mechanisms behind the increased incidence of marrow graft failure in recipients that receive allogeneic marrow depleted of T cells were studied. Recipient mice were lethally irradiated and challenged with bone marrow cells (BMC) from C.B-17 +/+ (+/+) donors. Radioisotope 125IUdR incorporation was assessed 5 to 7 days after transfer to determine the extent of engraftment. Some groups received BMC in which the T cells were removed by treatment with antibody and C. In addition, some groups received BMC from T cell-deficient C.B-17 scid/scid (SCID) mice to determine the postulated need for donor T cells in hematopoiesis and engraftment. In a model system that distinguishes between possible host NK cell and radioresistant T cell-mediated rejection of marrow allografts, it was determined that the absence of donor T cells in a marrow graft does not affect engraftment in syngeneic recipients. However, both host NK cell and radioresistant T cell rejection was markedly enhanced when SCID BMC or BMC from C.B-17 +/+ donors that had T cells removed by antibody and complement were infused into irradiated allogeneic recipients. Furthermore, the addition of alloreactive thymocytes as a source of T cells could abrogate this increased susceptibility of the BMC to host rejection mechanisms. As determined by histology and 59Fe uptake, the addition of thymocytes resulted in enhanced erythropoiesis. These results suggest that the increased incidence of marrow graft failure when BMC depleted of T cells are used is a result of active rejection by host effector cells and that the adverse effect of marrow T cell depletion can be reversed by the addition of thymocytes.     

Overexpression of human CUTA isoform2 enhances the cytotoxicity of copper to HeLa cells

Copper, an essential transient element, can be toxic to cells when present in excess. Altered copper homeostasis is involved in pathological events of many diseases. Human CUTA isoform2 is a member of cation tolerance protein (CutA1) family. In this study, we examined the effect of CUTA isoform2 overexpression on copper toxicity. It was shown that overexpressed CUTA isoform2 sensitized HeLa cells to copper toxicity by promoting copper-induced apoptosis. The inhibition effect of excessive copper on cell proliferation was also enhanced by overexpressed CUTA isoform2. So CUTA isoform2 was implicated to be involved in the cytotoxicity of copper.     

Antibody-dependent cellular cytotoxicity and natural killer cytotoxicity of peritoneal cells from nude mice to herpes simplex virus-infected cells

Nude BALB/c mice (athymic) were more susceptible to fatal herpes simplex virus (HSV) than normal BALB/c mice (P = 0.002). The peritoneal cells of nude mice mediated levels of antibody-dependent cellular cytotoxicity (ADCC) of equal or greater magnitude than cells from normal BALB/c, heterozygote nu/+, or C57BL/6 mice. Unstimulated natural killer cytotoxicity of peritoneal cells from nude mice was higher (P less than 0.05) than that mediated by cells from C57BL/6 mice. Nude mice failed to make anti-HSV ADCC antibody 6 to 14 days post HSV inoculation, at times when nu/+, BALB/c, and C57BL/6 mice produced antibody. Passive reconstitution of nude mice with high titer intraperitoneal anti-HSV immune globulin provided circulating anti-HSV ADCC antibody and significant protection against lethal HSV infection.     

Overexpression of Hsp27 affects the metastatic phenotype of human melanoma cells in vitro

Overexpression of the small heat shock protein Hsp27 has been shown by us to inhibit the in vitro proliferation rate and to delay tumor development of a human melanoma cell line (A375) in nude mice. We hypothesized that Hsp27 may influence the neoplastic phenotype. In the present study Hsp27 transfectants from this cell line were analyzed for various cellular aspects associated with the metastatic process. We found that Hsp27-overexpressing clones exhibited an altered cellular morphology as compared with control transfected cells. The Hsp27-positive cells tended to develop an epithelial-like phenotype growing in clusters and were characterized by a loss of transcytoplasmic stressfibers. In parallel, Hsp27-expressing cells lost the ability to form colonies in soft agar. The invasive potential was studied in vitro by the use of a reconstituted extracellular matrix-coated filter (Matrigel). Compared with controls, Hsp27-overexpressing cells showed decreased cell invasiveness through Matrigel. A correlation between invasion and activation of matrix metalloproteinases (MMPs) has been shown in several cell models. Secretion of MMPs (MMP-2 and MMP-9) was studied by gelatin-substrate zymogram analysis, as well as by a sensitive gelatinase activity assay. The Hsp27-transfected A375 melanoma cell line showed decreased secretion of MMP-2 and MMP-9 as compared with the control transfected cells. Integrins are adhesion receptors and function in cell invasion by mediating cell movement on matrix molecules and by regulating the expression of MMPs. Both fluorescence-activated cell sorter analysis and immunofluorescence analysis revealed a loss of alpha(v)beta3 integrin in Hsp27-transfected cell colonies. Our results demonstrate that Hsp27 overexpression has a profound impact on several parameters regulating the invasive and metastatic potential of melanoma cells in vitro.     

乙酰胆碱对自然杀伤细胞活性的影响

目的:观察乙酰胆碱(ACh)对自然杀伤(NK)细胞活性的影响,并初步探讨其作用的受体机制.方法:根据不同的实验目的,选择ACh、胆碱能受体激动剂和拮抗剂分别作用于NK细胞,以乳酸脱氢酶(lactate dehydrogenase,LDH)自然释放法检测不同实验条件下NK细胞杀伤肿瘤靶细胞(Yac)的活性.结果:ACh、M受体激动剂毛果芸香碱和N受体激动剂烟碱在10-10～10-6mol/L浓度范围内都能显著抑制NK细胞杀伤肿瘤细胞的活性.M受体拮抗剂阿托品(10-8和10-7mol/L)能完全阻断同浓度ACh抑制NK细胞活性的作用;但N受体拮抗剂筒箭毒碱(10-8和10-7mol/L)不能阻断同浓度ACh抑制NK细胞活性的作用.结论:ACh可抑制NK细胞对肿瘤细胞的杀伤作用,此作用主要由淋巴细胞上的M受体和N1受体介导.     

Prostaglandin-mediated inactivation of natural killer cells in the murine decidua

Previous studies from this laboratory have demonstrated a large influx of null lymphocytes into the murine decidua during pregnancy. We had also shown that trophoblast cells of the murine placenta bear target structures recognized by NK cells. Since NK lineage cells belong to the null category of lymphocytes, we examined whether cells of this lineage appear in the murine decidua, and if so, whether their activity is locally regulated by NK suppressor cells. We further investigated the identity of the suppressor cells as well as their suppressor products. NK lineage cells, irrespective of their activation status, were identified morphologically in radioautographic preparations as the non-T, non-B (null) lymphocytes capable of binding YAC-1 lymphoma targets. NK activity of nucleated cells was measured with a 4-hr 51Cr-release assay against labeled YAC-1 targets. Studies with outbred CD1 mice, and to a smaller extent, inbred CBA mice revealed that the incidence of NK lineage cells remained fairly constant within the decidua throughout pregnancy, but their activity decreased steadily to negligible levels by Day 12-14 of gestation. This was found to result from an inactivation caused by NK-suppressor cells in the decidua. A mixing of Ficoll-Paque-separated nucleated cells of the decidua with normal splenic effector cells (at 1:1 ratio) led to a suppression of their NK activity tested immediately or after a 20-hr coculture. This suppression was MHC unrestricted. Suppressor cells were identified both in plastic nonadherent fraction highly enriched for typical decidual cells as well as in the plastic adherent fraction containing decidual cells and macrophages. Addition of indomethacin (10(-5) M), an inhibitor of prostaglandin synthesis, or anti PGE2 antibody, revived the NK activity in the mixed population, as well as in the decidua, suggesting a PGE2-mediated suppression. High levels of PGE2 were detectable in decidual cell supernatants with a sensitive radioimmunoassay. Addition of pure PGE2 (10(-7)-10(-6) M) but not PGF2 alpha (10(-6) M) during the NK assay or to the effector cells for a 20-hr period prior to the assay led to an inhibition of NK activity. These results reveal that NK cells appearing in the murine decidua are progressively inactivated by PGE2 produced by decidual cells and decidual macrophages.     

Overexpression of the cochaperone CHIP enhances Hsp70-dependent folding activity in mammalian cells

CHIP is a cochaperone of Hsp70 that inhibits Hsp70-dependent refolding in vitro. However, the effect of altered expression of CHIP on the fate of unfolded proteins in mammalian cells has not been determined. Surprisingly, we found that overexpression of CHIP in fibroblasts increased the refolding of proteins after thermal denaturation. This effect was insensitive to geldanamycin, an Hsp90 inhibitor, and required the tetratricopeptide repeat motifs but not the U-box domain of CHIP. Inhibition of Hsp70 chaperone activity abolished the effects of CHIP on protein folding, indicating that the CHIP-mediated events were Hsp70 dependent. Hsp40 competitively inhibited the CHIP-dependent refolding, which is consistent with in vitro data indicating that these cofactors act on Hsp70 in the ATP-bound state and have opposing effects on Hsp70 ATPase activity. Consistent with these observations, CHIP overexpression did not alter protein folding in the setting of ATP depletion, when Hsp70 is in the ADP-bound state. Concomitant with its effects on refolding heat-denatured substrates, CHIP increased the fraction of nascent chains coimmunoprecipitating with Hsc70, but only when sufficient ATP was present to allow Hsp70 to cycle rapidly. Our data suggest that, consistent with in vitro studies, CHIP attenuates the Hsp70 cycle in living cells. The impact of this effect on the fate of unfolded proteins in cells, however, is different from what might be expected from the in vitro data. Rather than resulting in inhibited refolding, CHIP increases the folding capacity of Hsp70 in eukaryotic cells.     

Cellular tumorigenicity in nude mice. Role of susceptibility to natural killer cells

The athymic nude mouse is a useful animal model for assaying the neoplastic growth potential in vivo of animal cells transformed in vitro. Despite the demonstrated absence of thymus-dependent immunological functions, however, the nude mouse has now been shown to reject transplants of certain highly malignant heterologous tumors. In addition, a few transformed mammalian cell lines that exhibit all or most of the cellular phenotypes usually associated with malignancy fail to grow as tumors when injected into nude mice. In a continuing study to identify the in vitro phenotypes associated with tumor-forming ability in vivo, we investigated the role of cellular susceptibility to the naturally occurring, thymus-independent lymphocytes (natural killer or NK cells) in determining tumor induction by animal cells in nude mice. A representative collection of animal cells (ranging from normal human diploid cell strains to highly tumorigenic clonal cell lines, either transformed in vitro or derived from experimental tumors) was tested to see if the ability of cells to form tumors is consistently correlated with their susceptibility to NK cell-mediated lysis measured in vitro with splenic leukocytes from nude mice. If the physiological role of the NK cells in vivo were to recognize, and possibly to destroy, incipient tumor cells in situ, a direct association between cellular tumorigenicity and susceptibility to NK activity, might be expected. If, on the other hand, the formation of growing tumors by animal cells in nude mice depended on their ability to escape the cytolytic activity of NK cells, cellular tumorigenicity would be associated with cellular resistance to NK cells. Results obtained in this study failed to confirm either of these associations. Thus, cellular suscepbibility to NK cells, at least as determined by direct cytotoxicity assay in vitro, is not a useful predictive indicator of cellular tumorigenicity in nude mice.     

The role of natural killer cells in experimental murine salmonellosis

This study was designed to determine if murine natural killer (NK) cells play a role in host protection against a Salmonella typhimurium challenge infection. Outbred ICR mice injected intravenously with either attenuated (RIA strain) or virulent (SR-11 strain) salmonellae elicited enhanced killing of YAC-1 targets, which was maximal at 24 h after challenging. When NK cells were depleted with antiasialo GM1 prior to challenging, the splenic bacterial numbers were significantly less in this group of mice compared to sham-injected and challenged animals. The rabbit antiasialo GM1 sera had no detectable direct or indirect effect on the salmonellae. Our results indicate that the NK or natural suppressor cells may be functioning as down-regulators.     

Recognition of vaccinia virus-infected cells by human natural killer cells depends on natural cytotoxicity receptors

Natural Killer (NK) cells are important in the immune response to a number of viruses; however, the mechanisms used by NK cells to discriminate between healthy and virus-infected cells are only beginning to be understood. Infection with vaccinia virus provokes a marked increase in the susceptibility of target cells to lysis by NK cells, and we show that recognition of the changes in the target cell induced by vaccinia virus infection depends on the natural cytotoxicity receptors NKp30, NKp44, and NKp46. Vaccinia virus infection does not induce expression of ligands for the activating NKG2D receptor, nor does downregulation of major histocompatibility complex class I molecules appear to be of critical importance for altered target cell susceptibility to NK cell lysis. The increased susceptibility to lysis by NK cells triggered upon poxvirus infection depends on a viral gene, or genes, transcribed early in the viral life cycle and present in multiple distinct orthopoxviruses. The more general implications of these data for the processes of innate immune recognition are discussed.     

Bromelain activates murine macrophages and natural killer cells in vitro

The innate immune response is critical for effective immunity against most pathogens. In this study, we show that bromelain, a mixture of cysteine proteases, can enhance IFN-gamma-mediated nitric oxide and TNFalpha production by macrophages. Bromelain's effect was independent of endotoxin receptor activation and was not caused by direct modulation of IFN-gamma receptors. Instead, bromelain either enhanced or acted synergistically with IFN-gamma receptor-mediated signals. These effects were seen in both RAW 264.7, a macrophage cell line, and primary macrophage populations. Bromelain also increased IL-2- and IL-12-mediated IFN-gamma production by NK cells. These results indicate a potential role for bromelain in the activation of inflammatory responses in situations where they may be deficient, such as may occur in immunocompromised individuals.     

Lysis of fresh murine mammary tumor cells by syngeneic natural killer cells and lymphokine-activated killer cells

Summary We have compared the ability of natural killer (NK) cells from two substrains of C3H mice that differ with respect to their susceptibility to the development of mammary adenocarcinomas to lyse fresh syngeneic mammary tumor cells. Single cell suspensions of mammary tumors from retired breeder females were used as targets in 22-h ⁵¹Cr-release cytotoxicity assays with syngeneic NK cells. Tumor cell suspensions were prepared by enzymatic digestion of finely minced tissue followed by centrifugation through a discontinuous Percoll gradient. Effector cells were prepared by passing spleen cells over nylon wool followed by centrifugation through Percoll fraction 7. Syngeneic NK cells had significant levels of lysis against 5/8 tumors studied. NK cells from low risk animals (C3Heb/FeJ) consistently demonstrated greater cytotoxicity against tumor cell preparations than did effectors from the high tumor substrain (C3H/OuJ). Study of cytocentrifuge preparations stained with Wright-Giemsa revealed that the two substrains were identical with respect to the number of azurophilic granules present in the cytoplasm of their NK cells. We have also shown that lymphokine-activated killer (LAK) cells can be generated from splenocytes in C3H mice. While LAK cells from both substrains were capable of lysing fresh syngeneic mammary tumor cells in vitro, LAK cells from the animals at high risk for the formation of mammary adenocarcinomas had greater cytotoxicity against tumor cell suspensions than LAK cells from the low tumor substrain.     

Age-related variation in the proportion and activity of murine liver natural killer cells and their cytotoxicity against regenerating hepatocytes

We investigated the distribution of liver NK cells in mice of various ages and their cytotoxicity against regenerating hepatocytes. Liver NK cells were identified by asialo GM1 antibody in mononuclear cell suspension from the liver, whereas NK activity was assayed against YAC-1 target cells. Mononuclear cells in the liver consisted of more than 25% NK cells with potent NK activity in C3H/He mice, 8 wk of age. The strain-specific distribution (C3H/He greater than C57BL/6 greater than DBA/2) of liver NK cells was the same as those in the spleen and blood. The proportion of liver NK cells and the level of NK activity in C3H/He mice were further demonstrated to vary depending on age, in that both the proportion and the function were generated at 4 wk of age, reached a maximum between the 6th and 8th wk, and then rapidly decreased around the 9th wk. The appearance of an increased number of NK cells in the liver seemed to coincide with the slowing of the rapid increase of murine liver weight. We then investigated whether liver NK cells mediated their cytotoxicity against regenerating hepatocytes. Both specific 51Cr-release assay and single cell cytotoxicity assay demonstrated that liver NK cells were significantly cytotoxic against regenerating hepatocytes in partially hepatectomized liver, but to a lesser extent against normal hepatocytes in resting liver. Morphologic study revealed that normal liver predominantly consisted of hepatocytes with binuclei (greater than 60%) but that regenerating liver mainly consisted of hepatocytes with a single nucleus (greater than 70%). One-nucleus hepatocytes were more susceptible to the cytotoxicity of liver NK cells. A comparative study of restoration kinetics of the liver weight and the number of liver NK cells after partial hepatectomy also showed a unique relationship. These results raise the possibility that liver NK cells might be responsible for regulating hepatocyte growth.