Renewal of natural killer cells in mice having elevated natural killer cell activity

The present study was designed to examined the dynamics of splenic natural killer (NK) cells under two conditions of enhanced NK cell activity: (1) CBA/J mice given polyinosinic-polycytidylic acid (poly-I:C), an NK-cell-enhancing agent, and 62) untreated athymic nude (nu/nu) mice. The 'total NK cell activity' of the spleen (percentage specific lysis corrected for changes in organ cellularity) increased 5-fold and 2.7-fold after poly-I:C treatment for 1 day and 4 days, respectively. An injection of hydroxyurea (HU), a cell-cycle-toxic drug, given together with either poly-I:C or saline to CBA/J mice resulted in both cases in a 25% reduction in total NK cell activity 1 day later. This suggests that the renewal rate of nondividing NK cells is similar in poly-I:C-treated and saline-injected mice, and that the NK-enhancing effect of poly-I:C is not due to a stimulation of proliferation among NK cell precursors. HU administered simultaneously with poly-I:C or saline for 4 days eliminated NK cell activity in both cases, indicating that spleen NK cell activity is mediated almost entirely by newly formed (less than or equal to 4 days) cells. In nude mice, NK cell activity was assayed at various intervals after an HU depletion period of 2 days. NK depletion was initially more rapid in nu/nu mice than in control (nu/+) mice, although equally profound, and the subsequent recovery of NK cell activity after cessation of HU was also more rapid than in control (nu/+) mice.(ABSTRACT TRUNCATED AT 250 WORDS)     

Altered production and renewal of natural killer cells in B-lymphocyte-deficient CBA/N mice

The present study was designed to measure by quantitative and kinetic methods the production and renewal of natural killer (NK) cells in congenitally B-lymphocyte-deficient (CBA/N) mice. The total NK activity (percent specific lysis corrected for changes in whole organ cellularity) of the bone marrow and spleen of immunologically normal (CBA/CaJ) and CBA/N mice was assayed prior to and immediately after 48 h treatment (2 X/day, i.p.) with the cell cycle poison hydroxyurea (HU) and at various intervals throughout the subsequent post-HU recovery period. The total NK activity (TNKA) of untreated CBA/N bone marrow was 154% of that of CBA/CaJ bone marrow while the TNKA of CBA/N spleen was not significantly different (112%) from that of CBA/CaJ spleen. At the conclusion of 48 h HU, bone marrow TNKA of CBA/N and CBA/CaJ mice fell to 60 and 49%, respectively, of their saline-injected (2 X/day, i.p.) control levels, while spleen TNKA fell to 42 and 61%, respectively, of their saline-injected control levels. In the bone marrow, NK cell depletion in response to HU was more rapid in CBA/N mice (day 0.5 after HU) than in CBA/CaJ mice (day 2 after HU). TNKA of the spleen also decreased more rapidly in CBA/N mice (day 2 after HU) than in CBA/CaJ mice (day 3 after HU). The data indicate an enhanced production and turnover of NK cells in CBA/N mice relative to CBA/CaJ mice. Moreover, increased production and renewal of NK cells in CBA/N mice together with virtually unchanged levels of NK activity (112% of CBA/CaJ mice) in CBA/N mouse spleens indicate that mature lytic NK cells in CBA/N spleen but not bone marrow have a significantly shorter post-mitotic life span than do NK cells in the spleens of immunologically normal (CBA/CaJ) mice.     

Mycoplasma pulmonis infection augments natural killer cell activity in mice

The goal of this study was to determine if experimental Mycoplasma pulmonis infection augmented splenic natural killer (NK) cell activity in mice. A 4 hour 51Cr-release in vitro assay using YAC-1 tumor target cells was employed to measure splenic NK cell activity in C57BL/6J mice infected intraperitoneally with M. pulmonis and in uninfected controls. Transient augmentation of the NK cells was observed, peaking at day 3 postinoculation (PI) and gradually returning to normal levels by day 10 PI. Selective depletion studies showed that the cells responsible for killing target cells were NK cells. They were nonadherent to nylon wool, not susceptible to Thy-1.2 antibody and susceptible to asialo GM1 ganglioside antibody. Inadvertent augmentation of the NK cell system due to M. pulmonis infection may complicate the interpretation of research data, especially in immunology and cancer studies.     

Enhancement by interferon of natural killer cell activity in mice.

Injection of mice with several interferon inducers, Newcastle Disease virus, polyinosinic-polycytidylic acid and tilorone resulted in an increase in spleen cell cytotoxicity for ⁵¹chromium-labeled mouse YAC tumor target cells in 4-hr in vitro assays. This increase in spleen cell cytotoxicity was abrogated by injection of mice with potent anti-mouse interferon globulin. Inoculation of mice with mouse interferon (but not human leucocyte or mock interferon preparations) also resulted in a marked enhancement of spleen cell cytotoxicity. The extent of enhancement of spleen cell cytotoxicity was directly proportional to the amount of interferon injected and a significant increase was observed after inoculation of as little as 10³ to 10⁴ units of interferon. An effect could be detected as soon as 1 hr after injection of interferon. The increase of spleen cell cytotoxicity after inoculation of an interferon inducer was not due to a localization and accumulation of cytotoxic cells in the spleen but reflected a general increase in cytotoxic cell activity in various lymphoid tissues (except the thymus). The splenic cytotoxic cells from interferon or interferon-inducer-injected mice had the characteristics of natural killer (NK) cells since (i) interferon enhanced spleen cell cytotoxicity in athymic (nu/nu) nude mice, (ii) classical spleen cell fractionation procedures by nylon wool columns, anti-Thy 1.2 serum plus complement, anti-Ig columns, and depletion of FcR+ rosette-forming cells, failed to remove the effector cells generated in vivo or in vitro. Therefore like NK cells, interferon-induced cytotoxic cells lack the surface markers of mature T and B lymphocytes, are not adherent, and are devoid of avid Fc receptors. Furthermore like NK cells, the spleen cells from interferon-treated mice lysed various target cells (known for their sensitivity to NK cells) without H-2 or species restriction. Incubation in vitro of normal spleen cells with interferon also resulted in an increase in cytotoxicity for YAC tumor cells. We conclude that interferon acts directly on NK cells and enhances the inherent cytotoxic activity of these cells.     

Heterogeneity of human natural killer cell populations.

Natural killing (NK) in human donors was determined by the ability of peripheral blood subpopulations to lyse the myeloid target, K562, in a 2 to 4 hr ⁵¹Cr release assay. The most active cell was a non-T cell which passed through nylon columns (representing 10 to 25% of column passed cells). A second column passed cell population, with characteristics of T lymphocytes (75 to 90% of column passed cells), was also capable of mediating natural killing. Non-T cells which were retained by the nylon columns (45 to 55% of adherent cells) lacked NK activity. However, nylon adherent T cells (45 to 55% of adherent cells) were consistently active in NK assays, illustrating an important subset of NK effector cell often overlooked. Both column passed and adherent T cells were further separated according to their ability to bind IgG or IgM immune complexes, showing that those mediating NK have receptors for IgG (Tγ+) but not for IgM (Tμ+).     

Chemotherapy-induced modulation of natural killer and lymphokine-activated killer cell activity in euthymic and athymic mice

Combinations of chemotherapy and interleukin-2 (IL-2) aimed at improving therapeutic efficacy in cancer patients have generally proved disappointing. Although chemotherapy blocks tumor growth and sometimes boosts immune functions, most drugs are immunosuppressive, at least transiently. Therefore, it is reasonable to assume that maximal exploitation of the immunostimulatory and antitumor activity of both modalities requires careful coordination of chemotherapy and IL-2 timing. We analyzed the temporal effect of 5-fluorouracil (5-FU, 100–120 mg/kg), cyclophosphamide (CY, 100 mg/kg), Adriamycin (8 mg/kg) and dacarbazine (100 mg/kg) on the activation of natural killer/lymphokine-activated killer (NK/LAK) cells by IL-2 in several strains of euthymic mice and in athymic nude mice. Following in vivo or in vitro exposure to IL-2 1–15 days after chemotherapy, the total lytic activity of the spleen and the number of LAK precursors (LAK-p) were measured. In euthymic mice injected with IL-2 (5×10⁴ Cetus units twice daily for 4–5 days), 5-FU augmented (up to 37-fold, days 1–9) and CY reduced (up to day 6) LAK activity, as compared with that in the IL-2 control. In bulk cultures containing IL-2 (1000 CU/ml, 3–4 days), both 5-FU and CY reduced LAK activity of euthymic mice splenocytes for up to 6 days after chemotherapy, which was followed on day 9 by full recovery. In splenocytes of nude mice, 5-FU increased and CY diminished LAK activation in bulk cultures, starting 3 days after chemotherapy. In athymic mice, 5-FU markedly augmented the total number of LAK-p/spleen (up to 30-fold, days 3–9), as determined by limiting-dilution cultures with IL-2 (for 7–8 days). In euthymic mice, in contrast, LAK-p levels decreased for up to 6–9 days after treatment with 5-FU, Adriamycin or dacarbazine, later recovering to pretreatment levels, whereas CY markedly increased LAK-p (up to 15-fold) when administered 6–12 days before limiting-dilution culture initiation. The effect of chemotherapy on LAK and NK activity was essentially similar. In other experiments, a subset of asialoGM1-LAK-p was found in the spleens of 5-FU-treated mice, but not in untreated mice. Our results suggest that the immunomodulatory effect of chemotherapy on NK/LAK activity in mice is variable and largely depends on the drug itself, the interval between chemotherapy and IL-2 administration, the strain of mice and the assay used.     

Induction of natural killer cell activity in mice by injection of indomethacin

The natural killer (NK) cell system of mice in the peritoneal cavity is of very low to undetectable activity, and testing peritoneal NK cells is a useful model to study the influence of activating substances upon local injection. Injection of indomethacin at doses of 100-400 micrograms/mouse caused a marked activation of NK cell activity which was maximal at 3 days and lasted for a total of 6 days. A similar albeit less marked effect was observed with other cyclooxygenase inhibitors such as aspirin. Prostaglandin E2 reversed the activation of NK cells induced by injection of indomethacin. The cellular count of the peritoneal population was 2-fold elevated after indomethacin injection but the percentage of macrophages in the washed-out cell population was decreased from 60% (controls) to around 20%. The NK cell nature of the effector cells activated by indomethacin was substantiated by the finding that previous injection of anti-asialo GM1 antibody prevented activation. Interferon could not be detected in the peritoneal wash fluid after injection of indomethacin, suggesting interferon-independent activation. However, the possibility of small interferon quantities being locally produced could not be excluded. In further experiments we found after intraperitoneal injection of indomethacin not only cells that killed YAC-1 targets in a 4-hour assay but also killer cells that were insensitive to anti-asialo GM1 and killed P815 cells in an 18-hour assay. We assumed that these were macrophages and have done further experiments with in vitro grown bone-marrow-derived macrophages. These could be activated for killing of P815 targets by the addition of indomethacin, but (to a lesser degree) also for killing of YAC-1 lymphoma cells.     

Modulation of natural killer activity in mice following infection with Listeria monocytogenes

Mice that received a sublethal, intraperitoneal dose of viable Listeria monocytogenes, virulent strain 10403, exhibited a systemic increase in natural killer (NK) activity. The kinetics of the response differed with respect to the various effector cell populations analyzed. Resident peritoneal cells and peripheral blood leukocytes demonstrated high NK activity on Days 3, 7, and 10. Peak spleen and bone marrow NK activity was observed on Day 3, returning to normal levels by Day 7. In contrast, peritoneal exudate cells, elicited with proteose peptone, expressed enhanced NK activity for 60 days following infection with viable Listeria. Augmented NK activity was detected with all cell types as early as 12 hr after infection. The intraperitoneal injection of nonviable antigenic preparations derived from L. monocytogenes, strain 10403, resulted in the enhancement of peritoneal and splenic NK activity. In contrast, mice that received an intraperitoneal injection of avirulent Listeria, strain 19113, failed to express enhanced levels of NK activity. The genetic trait of anti-listerial resistance which is associated with non-H-2 linked genes was of no importance with respect to enhanced NK activity. Listeria-resistant C57BL/6J and Listeria-susceptible DBA/2J mice both produced systemic augmentation of NK activity following infection. NK activity was not abrogated by macrophage depletion or by treatment with anti-Thy 1.2 serum plus complement. These results confirm the potent immunostimulatory capacity of virulent Listeria for NK activity and provide further insight into the kinetics of this response in various lymphoid compartments. The protracted augmentation of NK activity of elicited peritoneal exudate cells as compared to nonelicited peritoneal cells in Listeria-primed mice suggests that the influx of inflammatory cells may provide NK-enriched and/or accessory populations for immunopotentiation of NK activity in inflammatory sites.     

Suppression of natural killer cytolytic activity in mice undergoing pulmonary granulomatous inflammation

CBA/J mice undergoing pulmonary granulomatous inflammation exhibited depressed NK cytolytic activity. Granulomas induced by i.v. embolization of Schistosoma mansoni eggs (hypersensitivity type) or Sephadex beads (foreign body type) both caused reduced NK activity, although hypersensitivity granulomas induced a significantly higher level of NK suppression. Kinetic analysis of hypersensitivity lesions at 4, 8, 16, and 32 days post-embolization indicated that NK activity was significantly suppressed by day 8, maximally suppressed by day 16 (at the peak of the inflammatory response) then returned to near control values by day 32 (as the granulomas resolved). Suppression of NK activity ranged from three- to 15-fold in different experiments. NK cells obtained from both spleen and peripheral blood demonstrated reduced NK activity with kinetic patterns similar to the granuloma NK cells. Suppression was not due to reduced splenic NK cells as the frequency of YAC-1 binding cells, as well as asialo GM1+ or laminin+ cells remained constant over the entire study period. Suppression of NK activity did not appear to be due to serum components or suppressor cells present in the spleen preparations. However, the suppression of NK activity could be reversed by overnight incubation of spleen cells at 25 or 37 degrees C or daily treatment of the mice with indomethacin. Suppression also appeared relatively specific for NK cells as the generation and expression of cytotoxic T lymphocyte activity was not affected.     

Mutations in mice that influence natural killer (NK) cell activity

A series of mutations in mice was tested for splenic NK-cell activity against YAC-1 target cells. Mutations at six loci that reduce NK-cell activity in the homozygous state were identified, including beige (bg), hairless (hr), motheaten (me), obese (ob), steel (Sl) and, to a lesser extent, dominant spotting (W). Motheaten mice displayed the most profound NK-cell deficiency, with NK-cell activity virtually absent. Two mutations, nude (nu) and lymphoproliferation (Ipr), produced elevated NK-cell-mediated lysis. The double homozygous recessivenu/nu bg/bg nude-beige mouse was viable and NK-cell-deficient, with activity slightly higher than that of +/?bg/bg beige littermate controls. Pigmentation mutants related to beige, including pale ears (ep), pearl (pe), and ruby eyes (ru ^2J ) did not dramatically influence NK-cell levels. Unlike the obese gene, other mutations leading to obesity, diabetes (db) and yellow (Asuy), did not impair NK-cell function. The possible site of gene action of these mutants in the NK-cell pathway is discussed.     

Influence of dietary restriction and aging on natural killer cell activity in mice

Natural killer cells (NK) are believed to defend against tumor growth. Because rodents subjected to dietary restriction without malnutrition live longer and develop spontaneous tumors less often or later in life than unrestricted controls, we measured NK activity in restricted and in unrestricted mice. An age-related decline in NK responses to YAC-1 tumor target cells was detected in both groups. NK responses for control mice were highest in 2- to 3-mo-old mice, sharply reduced in middle-age mice (14 to 15 mo), and slightly reduced further in old mice (30 to 33 mo). At all ages the response of restricted mice was less than that of controls. However, after injection with Poly I:C (which increases NK activity), old restricted mice showed NK cytolysis not different from young mice on either diet, and substantially higher responses than old unrestricted mice. In addition, restricted mice showed increased in vitro generation of cytotoxic T lymphocytes (CTL) to YAC-1 and P815 compared with age-matched controls. Restricted mice may better resist cancer via an NK system very responsive to induction signals coupled with a CTL system more effective than that of unrestricted controls.     

Genetic control of the natural killer cell activity in SJL and other strains of mice

Murine natural killer (NK) cell activity against lymphoma targets can be classified into three major functional phenotypes, i.e., low, inducible, and high, according to the levels of endogeneous activity and the extent of augmentation by interferon (IFN) or IFN inducers, as previously described. The prototype strains identifying these three phenotypes are SJL, A.SW, and B10.S, respectively, all bearing the H-2s haplotype. In the present study, the genetic basis of the low phenotype of SJL mice was examined further. F1 hybrid offspring of crosses between SJL and a strain with the high NK phenotype (B10.S, B10.D2, B10, C3H/HeN, or D1.LP) invariably expressed the high NK phenotype, indicating recessiveness of the low phenotype. Crosses between SJL and another low NK strain, such as A/J, A/HeN, or I/St, resulted in offspring of either the inducible or the high NK phenotype. Such genetic complementation between the low NK pairs indicates that the low phenotype of SJL and that of the other strains have different genetic bases. F1 hybrid mice between SJL and an inducible strain, A/WySn, were inducible, but those between SJL and the second inducible strain, A.SW, had the high NK phenotype. Thus, the congenic A/WySn and A.SW have distinct genotypes resulting in the same inducible phenotype. According to analyses of the segregating offspring from backcrosses of (SJL X B10.S)F1 mice to SJL, a single gene difference is responsible for the low endogenous level of NK activity in SJL as opposed to the high endogenous level in B10.S, and that the difference in three genes accounts for the poor responsiveness of NK cells to IFN in SJL mice. Studies of the two congenic lines of SJL, i.e., SJL-Igha and SJL-nu, indicated that the Igh locus is irrelevant for the low NK phenotype of SJL, but the nu locus clearly is relevant; SJL mice homozygous for the nu allele were phenotypically inducible in contrast to the nu/+ or +/+ mice which are low. The nu gene homozygosity rendered SJL mice responsive to IFN, not only in NK activity against lymphomas but also in ADCC activity against antibody-coated lymphoma cells.     

Studies on the mechanism of low natural killer cell activity in infant and aged mice

NK activity in mice is high between about 6 and 10 weeks of age. In contrast, infant mice and mice older than 12-14 weeks of age usually have quite low or undetectable NK activity. Studies were performed to analyze the mechanisms underlying this characteristic age-related regulation of NK activity. Spleen cells from infant mice did not develop appreciable NK activity upon incubation for 12-18 h with either interferon (IFN) or interleukin-2 (IL-2). Analysis of the frequency of IL-2-dependent progenitors of NK cells, in a limiting dilution assay, also indicated that the spleens of infant mice are deficient in precursors of NK cells. In contrast, spleen cells from old mice (30 weeks old) developed substantial levels of NK activity upon incubation with either IFN or IL-2, and they showed a frequency of IL-2-dependent progenitors of effector cells that was similar to that of young mice. Both infant and old mice had plastic-adherent suppressor cells in their spleens, which could strongly inhibit NK activity. In addition, both infant and old mouse spleen cells contained nonadherent suppressor cells, which had a higher density on Percoll gradients than NK cells. Thus, several factors appear to contribute to the age-related regulation of NK activity in mice.     

Induction of natural killer effectors from human thymus with recombinant IL-2

The NKH1 Ag is expressed on all cells in human peripheral blood capable of mediating spontaneous non-MHC restricted cytolytic function (i.e., natural killing). The majority of NK cells do not express CD3 Ag and do not express TCR gene products. However, approximately 20 to 25% of NKH1+ cells coexpress CD3 and TCR proteins. Both NKH1+CD3+ and NKH1+CD3- effectors can proliferate in response to IL-2 which also results in enhancement of cytolytic function. In the present studies, we examined thymocytes after incubation with rIL-2 for the presence of NKH1+ cells and for the development of non-MHC restricted cytolytic function. NKH1+ cells and NK activity could not be detected in fresh thymus. After culture with rIL-2 only, NK activity appeared in 3 days, reached a maximum after 7 days, and was effective against a panel of NK-sensitive targets. NK activity was correlated with the expression of NKH1 on the surface of in vitro proliferating thymocytes and immunofluorescent cell sorting demonstrated that almost all cytolytic activity was mediated by NKH1+ cells. As expected given the thymic origin of these cells, the majority of NKH1+ cells in culture expressed CD3. However, all cultures contained NKH1+CD3- effector cells which represent 15 to 40% of the NKH1+ population. As in peripheral blood, both NKH1+CD3- and NKH1+CD3+ exhibited non-MHC-restricted cytotoxicity, but only CD3+ effectors could be inhibited by anti-T3 mAb. These findings demonstrate that rIL-2 alone can induce subpopulations of thymocytes to proliferate, to express the NKH1 marker and become NK active in vitro. Furthermore, they suggest that the thymus which plays a role in the differentiation of NKH1+CD3+ NK effectors may also play a role in the differentiation or maturation of NKH1+CD3- NK effectors.     

Evaluation of natural killer cell activity

Natural killer (NK) cells are being appreciated not only for their ability to recognize and lyse tumor cells and virus-infected cells but also for their immunoregulatory properties. NK cells provide a first line of defense against invading pathogens with a two pronged attack, lysis of infected cells and secretion of cytokines and chemokines with potent antipathogen effects. This article describes the standard chromium release assay, which measures the ability of NK cells derived from the peripheral blood to lyse appropriate target cells.     

Enhancement of natural killer cell activity in mice by treatment with a thymic factor

Summary The administration of a thymic factor, thymostimulin (TP-1), to mice resulted in considerable augmentation of natural killer (NK) cell activity as measured in a short-term assay against ⁵¹Cr-labeled YAC-1 target cells. Conditions suitable for detection of the thymostimulin-induced boosting of NK included multiple daily exposures to TP-1 (50 g/kg), and peak levels of reactivity were observed at 2–4 days after discontinuation of treatment. A strict age-dependency of the effect was also observed, with optimal augmentation of NK-cell activity when TP-1 was administered to mice at 4–6 weeks of age. The effect was not limited to TP-1 treatment but was also observed on administration of another thymic factor (thymosin ₁). The activated cells responsible for the increased natural cell-mediated cytotoxicity appeared to be typical murine NK cells, judging by both functional and antigenic criteria.     

Resorption of CBA/J x DBA/2 mouse conceptuses in CBA/J uteri correlates with failure of the feto-placental unit to suppress natural killer cell activity

Lipid extraction was used to study the natural killer (NK) suppressive activity of individual feto-placental units. Normal pregnancies showed a lipophilic NK cell suppressive factor that was gestational day specific. Feto-placental units from CBA/J x DBA/2 pregnancies were deficient in the NK cell suppressive factor when compared to normal CBA/J x BALB/c pregnancies. The frequency of non-suppressive feto-placental units from CBA/J x DBA/2 pregnancies correlated with the frequency of feto-placental units infiltrated with NK cells and the frequency of spontaneous resorption. Our results implicate a deficiency of NK suppressive activity in the feto-placental unit as a contributing factor in spontaneous fetal resorption.     

Analysis of natural killer activity and natural killer cell subsets in patients with bladder cancer

Summary In order to analyze the state of the natural resistance system of bladder cancer patients in vivo, we measured natural killer (NK) activity and NK cell subsets of peripheral blood lymphocytes (PBL) from 46 patients with bladder cancer and 25 age- and sex-matched healthy volunteers. The mean NK activity in patients with lowstage bladder cancer was similar to that in the controls, while NK activity in patients with high-stage bladder cancer was significantly depressed. The mean proportions of Leu7⁺ cells in patients with both low-stage and highstage bladder cancer were significantly higher than that in the controls. The mean proportion of Leu11a⁺ cells in patients with low-stage bladder cancer was similar to that in the controls, while in patients with high-stage bladder cancer it was significantly higher. This study demonstrates the abnormal immunological state of bladder cancer patients; namely, abnormalities exist not only in NK activity but also in the proportions of circulating NK cell subsets.     

Lysosomal enzymes and natural killer activity

The natural killer activity (NKA) of human mononuclear cells and the activity of the lysosomal enzymes of these cells (arylsulfatase, acid phosphatase and beta-glucuronidase) has been studied in norm and under human lung cancer. The mononuclear cells were isolated from peripheral blood of 10 healthy donors and 20 patients with lung cancer of II-III stages. Under the action of mononuclear cells on the target cells (human erythroleukosis cells K-562 labeled with 3H-uridine) the NKA of mononuclear cells of patients was seen to decrease (cytotoxic index = 54.8 +/- 6.4%), in comparison with that of healthy donors (cytotoxic index = 65.1 +/- 4.5%). Simultaneously a decrease in arylsulfatase activity (0.05 +/- 0.01 nmoles/10(6) cells/min) was found in comparison with the control value (0.11 +/- 0.01 nmoles/10(6) cells/min). In 2-3 weeks after the operation the NKA value (cytotoxic index = 50.2 +/- 5.8%) was restored and arylsulfatase activity (0.09 +/- 0.02 nmoles/10(6) cells/min) was increased. There was no correlation between the NKA value and the activities of acid phosphatase and beta-glucuronidase. The parallelism observed between changes in NKA value and arylsulfatase activity may suggest a possible participation of this enzyme in the killing mechanism at the stage of cerebroside sulfate ester degradation of the target cell membrane to initiate the lytic events.