Effect of injection of adult mouse peritoneal macrophages on suppressor cell activity in neonatal mice

Injection of adult mouse peritoneal exudate cells into newborn mice results in a premature decrease of splenic suppressor cell activity in the neonates. The effect becomes apparent 4–5 days after ip injection of 10–15 × 10⁶ thioglycollate-induced peritoneal exudate cells into mice on the day of birth. The macrophage in the peritoneal exudate is the responsible cell type. The effect is not H-2 restricted or strain limited. Heat-killed peritoneal exudate cells or peritoneal cells from unstimulated donors can also decrease neonatal suppressor cell activity prematurely. Adult spleen cells, injected into neonatal mice, do not affect suppressor cell activity. The data are discussed in light of the hypothesis that macrophages control suppressor activity in neonatal mice and that an increase in the number and/or function of macrophages shortly after birth results in a decrease in the number and/or function of suppressor cells, allowing for immunological competence to emerge.     

Protective effect of recombinant human interleukin-2 against lethal infection caused by Klebsiella pneumoniae

The effects of recombinant human interleukin-2 (rIL-2) administered prophylactically on the death of CBA/J mice challenged with Klebsiella pneumoniae 27 intraperitoneally were examined. rIL-2 administered subcutaneously at 20 micrograms per mouse for 7 days enhanced survival after a lethal challenge. The injection of anti-asialo GM1 antibody did not influence the effect of rIL-2. In mice given rIL-2, the number of peritoneal macrophages increased, and the infiltration of polymorphonuclear leukocytes (PMN) into the peritoneal cavity after the bacterial challenge was enhanced. In addition, adoptive transfer of sera and peritoneal exudate cells (PEC), consisting of an approximately equal number of macrophages and PMN, obtained from mice given rIL-2 enhanced resistance to a K. pneumoniae infection, compared with adoptive transfer of sera and PEC obtained from mice not given rIL-2. These results indicate that rIL-2 protects mice from a lethal challenge with K. pneumoniae, and suggest that the protective effect is due to an increase in the number of phagocytic cells and in the cooperative activity of the sera and the phagocytic cells.     

The chemiluminescence reactions of the blood neutrophils and of peritoneal exudate cells in Syrian hamsters to the intraperitoneal administration of cellular and microbial materials]

The luminol-dependent chemiluminescence (CL) activity of peritoneal exudate cells and blood neutrophils of Syrian hamsters inoculated intraperitoneally with heat-inactivated microbial particles of Candida albicans, (C. albicans), heated irradiated normal cells and native or heated irradiated malignant tumor cells was studied. The inoculation with particles of C. albicans and heated normal cells induced significant activation of CL of peritoneal exudate cells, but did not influence the CL reaction of blood neutrophils. The inoculation of animals with nonheated irradiated tumor cells led to increase of CL response of both peritoneal exudate cells and blood neutrophils. The inoculation with heated irradiated tumor cells did not activate CL of peritoneal exudate cells and led to slight, but long-lasting decrease of CL response of blood neutrophils.     

Peritoneal exudate cells. I. Growth requirement of cells capable of forming colonies in soft agar

Mouse peritoneal exudate cells induced by thioglycollate medium can form colonies in soft agar with a plating efficiency of about 5% (0.6%–10%). Cells from an unstimulated peritoneal cavity form no colonies or have a plating efficiency of less than 0.001 %. These colony-forming cells from the peritoneal exudate are similar to bone marrow colony-forming cells in vitro in that they both require a substance(s) present in conditioned medium from L-cells or mouse embryo fibroblasts or the serum from endotoxin-treated mice for the initiation and the continuation of their growth. However, peritoneal exudate colony-forming cells have a much longer initial lag period (10–14 days) and can survive longer in the absence of L-cell conditioned medium than bone marrow colony-forming cells. Only mononuclear cells, presumably macrophages, are observed in peritoneal exudate colonies, whereas bone marrow cell colonies contain both polymorphonuclear cells and macrophages.     

Local and systemic expression of inducible nitric oxide synthase in comparison with that of cyclooxygenase-2 in rat carrageenin-induced pleurisy.

Expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) is up-regulated in response to inflammatory stimuli. To evaluate the extent to which local pleural inflammation involves additional site in the pleural cavity and elsewhere, we investigated the time course of the levels of iNOS and its product in the inflammatory and other sites, and compared those with a level of COX-2 in rat carrageenin-induced pleurisy. The exudate and plasma NOx levels rose, reaching peaks at 9 and 14 h, respectively. Both COX-2 and iNOS became detectable in exudate leukocytes, their levels reaching peaks at 3 and 9 h after irritation, respectively. COX-2 was detectable mainly in neutrophils, but iNOS was detectable in both neutrophils and mononuclear leukocytes. Furthermore, iNOS became detectable in neutrophils and mononuclear leukocytes in enlarged parathymic lymph nodes from 3h in addition to those in peripheral blood and Kupffer cells from 3 to 14 h, respectively. The gene product is also detectable in thymic large dendritic cells of pleurisy-induced rats as well as normal control rats. COX-2 became detectable in stellar dendritic cells of the enlarged draining lymph nodes from 14 h. Thus, these gene products were induced in the immediate proximity of regional lymph nodes, and even at a considerable distance of liver by the local inflammatory stimulus. Although their expression pattern was quite different from each other, these gene products were detectable in phagocytic cells.     

Chemiluminescence response of beta-glucan stimulated leukocytes isolated from different tissues and peritoneal cavity of Dicentrarchus labrax

The respiratory burst of leukocytes isolated from sea bass (Dicentrarchus labrax) pronephros, peritoneal cavity (P.C.), spleen and blood, was measured by a chemiluminescence (CL) assay after stimulation with beta-glucan. The CL response by P.C. and pronephros leukocytes was significantly higher than that expressed by a similar number of cells separated from spleen and blood. This probably reflects the observation that the proportion of macrophages and neutrophils was highest in the populations of leukocytes from peritoneal cavity and pronephros. Comparative observations showed a higher degree of yeast phagocytosis by leukocytes taken from peritoneal cavity than the pronephros. Moreover phagocytic index evaluated by microscopical observations, indicated that peritoneal macrophages internalised more yeast cells than neutrophils (identified by the peroxidase reaction). Scanning electron microscopy observations were also carried out.Inhibition experiments by a myeloperoxidase inhibitor sodium azide, iodonium-diphenyl-chloride which inhibits NADPH-oxidase, and exogenous superoxide dismutase, which catalyses O-2 dismutation to H(2)O(2), supported the correlation between CL and respiratory burst. Treatment with ouabain and DNP suggested that in this response, Ca(++) pump channels and calmodulin are involved in a metabolic energy-dependent pathway.     

Role of prostaglandin H synthase-2 in prostaglandin E2 formation in rat carrageenin-induced pleurisy

Rat carrageenin-induced pleurisy was used to clarify the role of prostaglandin H synthase (PGHS)-2 in acute inflammation. Intrapleural injection of 0.2 ml of 2% λ-carrageenin induced accumulation of exudate and infiltration of leukocytes into the pleural cavity. When PGHS-1 and -2 proteins in the pleural exudate cells were analyzed by Western blot analysis, PGHS-2 was detectable from 1 hr after carrageenin injection. Its level rose sharply, remained high from 3 to 7 hr after injection, and then fell to near the detection limit. PGHS-1 was also detected, but kept almost the same level throughout the course of the pleurisy. Levels of prostaglandin (PG) E₂ and thromboxane (TX) B₂ in the exudate increased from hour 3 to hour 7, and then declined. Thus, the changes of the level of PGE₂ were closely paralleled those of PGHS-2.The selective PGHS-2 inhibitors NS-398, nimesulide and SC-58125 suppressed the inflammatory reaction and caused a marked decrease in the level of PGE₂ but not in those of TXB₂ and 6-keto-PGF_1α. These results suggest that the PGHS-2 expressed in the pleural exudate cells may be involved in PGE₂ formation at the site of inflammation.     

Polyester treatment in rats with a dorsal air pouch: chemotaxis in lavage fluid from the pouch]

Minced polyester threads introduced into peritoneal cavity of rats cause a granulomatous inflammation with evidence of macrophage stimulation. Chemotactic agents play an important role in the inflammatory reaction; they are released locally by cells involved in inflammation. In this paper the chemotactic effect of the peritoneal and subcutaneous air pouch fluids from rats bearing the polyester inflammatory process, have been studied on PMN cells "in vitro". The fluids were obtained by washing the cavity of untreated rats or rats injected with polyester, 7 days after the injection. The chemotactic response was assayed by employing modified chemotaxis Boyden chambers (Blind Well Neuro Probe) and polymorphonuclear cells from normal rats. Quantification of the migration was calculated by chemotactic index (A/B) (B = random migration, A = chemotaxis). The results demonstrate that a chemotactic activity is present in peritoneal and subcutaneous air pouch fluids following the inflammatory process. In conclusion the chronic inflammation determines the appearance of chemotactic factors for PMN cells, in the peritoneal cavity and in the air pouch, and the air pouch is a very convenient experimental system with the particular advantage that it permits easy repeated sampling of exudate during the course of an inflammatory response.     

The in vitro killing of syngeneic cells by peritoneal cells from adjuvant-stimulated mice.

The cytotoxicity of peritoneal exudate cells from mice which had been injected with anaerobic coryneform organisms which have adjuvant activity was assessed by measuring the release of radioactive chromium from monolayers of whole mouse embryo cells. It was found that the peritoneal cells from adjuvant-stimulated mice were more cytotoxic than cells from normal mice. The increased cytotoxicity was present as early as 2 days after injection of the organisms, and was abolished by trypsin treatment of the peritoneal cells. The cytotoxic effect requires the presence of live peritoneal cells, and is more marked as the ratio of effector to target cells is increased. The plastic adherent cells of the peritoneal cell population are more effective in the cytotoxic reaction than are the non-adherent cells. The stimulated peritoneal cells can kill both syngeneic and allogeneic mouse embryo cells. Consideration is given to the possible mechanisms by which the increased cytotoxicity might be induced.     

Chemiluminescence response of β-glucan stimulated leukocytes isolated from different tissues and peritoneal cavity of Dicentrarchus labrax

The respiratory burst of leukocytes isolated from sea bass (Dicentrarchus labrax) pronephros, peritoneal cavity (P.C.), spleen and blood, was measured by a chemiluminescence (CL) assay after stimulation with β-glucan. The CL response by P.C. and pronephros leukocytes was significantly higher than that expressed by a similar number of cells separated from spleen and blood. This probably reflects the observation that the proportion of macrophages and neutrophils was highest in the populations of leukocytes from peritoneal cavity and pronephros. Comparative observations showed a higher degree of yeast phagocytosis by leukocytes taken from peritoneal cavity than the pronephros. Moreover phagocytic index evaluated by microscopical observations, indicated that peritoneal macrophages internalised more yeast cells than neutrophils (identified by the peroxidase reaction). Scanning electron microscopy observations were also carried out.Inhibition experiments by a myeloperoxidase inhibitor sodium azide, iodonium-diphenyl-chloride which inhibits NADPH-oxidase, and exogenous superoxide dismutase, which catalyses O−2 dismutation to H₂O₂, supported the correlation between CL and respiratory burst. Treatment with ouabain and DNP suggested that in this response, Ca⁺⁺ pump channels and calmodulin are involved in a metabolic energy-dependent pathway.     

Comparison of the responses of peritoneal macrophages from Japanese flounder (Paralichthys olivaceus) against high virulent and low virulent strains of Edwardsiella tarda

In vivo infection studies in Japanese flounder (Paralichthys olivaceus) demonstrated that the number of viable cells of the virulent strain (NUF251) of Edwardsiella tarda increased gradually in kidney and hepato-pancreas after intraperitoneal injection, but the low virulent strain (NUF194) did not. To gain insight into the virulence factors of E. tarda, in vitro responses of Japanese flounder (P. olivaceus) peritoneal macrophages to these strains were compared in terms of phagocytosis, bactericidal activity, and reactive oxygen species (ROS) generation as measured by chemiluminescence (CL) responses. Microscopic observation revealed that these two strains of E. tarda were phagocytosed by the peritoneal macrophages, and there was no significant difference in the mean numbers of ingested bacteria per macrophage between these strains. A gradual increase in the number of viable cells of the highly virulent strain within macrophages was observed during 9h post-phagocytosis, whereas no significant replication of the low virulent strain within macrophages was detected. These results suggest that the virulent strain of E. tarda has an ability to survive and replicate within macrophages, while the low virulent strain has no such ability. When the peritoneal macrophages were exposed to the opsonized low virulent E. tarda strain, a rapid increase in CL response was induced. However, the highly virulent strain caused only background level of CL response. By the subsequent stimulation with phorbol myristate acetate, the macrophages exposed to the virulent E. tarda strain showed extremely higher CL response than that of the one exposed to the low virulent E. tarda strain. These results suggest that the virulent E. tarda prevents the activation of ROS generation system during phagocytosis, though the system is still capable of responding to other stimulation. The virulent strain significantly reduced the CL response induced by xanthine/xanthine oxidase system, while the low virulent strain had almost no effect. Furthermore, the virulent strain showed greater resistance to H(2)O(2) than the low virulent strain. Our results suggest that the virulent strain of E. tarda is highly resistant to ROS, and such ability might allow the organism to survive and multiply within phagocytes, and may serve to disseminate E. tarda throughout the host during in vivo infection.     

Differences in initial rate of intracellular killing of Salmonella typhimurium by granulocytes of Salmonella-susceptible C57BL/10 mice and Salmonella-resistant CBA mice

The contribution of granulocytes to differences in the innate susceptibility of mouse strains to infection by Salmonella typhimurium was assessed on the basis of the size and composition of the inflammatory exudate after i.p. injection of bacteria and the intracellular killing of the bacteria by exudate peritoneal cells and blood granulocytes of resistant CBA and susceptible C57BL/10 mice. The increase in the numbers of both peritoneal granulocytes and macrophages 24 hr after i.p. injection of various numbers of live S. typhimurium was two to four times higher in C57BL/10 mice (p less than 0.05) than in CBA mice. However, despite the larger number of phagocytes in the inflammatory exudate, the numbers of viable S. typhimurium in the peritoneal cavity 24 hr after injection was higher (p less than 0.01) in C57BL/10 mice than in CBA mice. Because the proportion of noningested bacteria was similar in the two mouse strains (less than 30%), these findings indicate a difference in the rate of intracellular killing of the bacteria by exudate peritoneal cells (greater than 75% granulocytes) of the two mouse strains. Subsequent determination of the initial rate of intracellular killing (Kk) of S. typhimurium revealed that after phagocytosis of the bacteria in vivo, exudate peritoneal granulocytes (harvested 24 hr after i.p. injection of 10(3) live S. typhimurium) of CBA mice killed S. typhimurium twice as efficiently (Kk = 0.014 min-1; p less than 0.01) as exudate granulocytes of C57BL/10 mice (Kk = 0.008 min-1) did. Similarly, the initial rate of intracellular killing of the ingested S. typhimurium by blood granulocytes of CBA mice (Kk = 0.017 min-1) was two times higher (p less than 0.01) than that of C57BL/10 mice (Kk = 0.007 min-1). These findings may be specific for S. typhimurium, because L. monocytogenes were killed with equal efficiency by exudate granulocytes and blood granulocytes of these mouse strains (p greater than 0.20). The results of the present study are relevant with respect to the innate resistance of mice to S. typhimurium, particularly during the initial phase of infection when the inflammatory exudate contains predominantly granulocytes.     

Comparative study on peroxidatic activity in inflammatory cells on cutaneous and peritoneal implants

Summary Inflammatory reactions were evoked by simultaneous implantation of pieces of Melinex plastic in the subcutaneous tissues of the dorsum and in the peritoneal cavity of rats. The cellular composition of the Melinex-adherent cells and their peroxidatic (PO) activity were investigated in relation to the duration of implantation. Several striking differences were found between the subcutaneous and peritoneal implants. On the 7th and 14th days, multinucleated giant cells were abundantly present on the subcutaneous implants, whereas they were relatively rare on the peritoneal implants. The subcutaneous implants bore no mast cells and only a few eosinophilic granulocytes, but both types of cell were observed frequently on the peritoneal implants.Macrophages and multinucleated giant cells on the subcutaneous implants show PO activity only in the granules or are PO negative. On the peritoneal implants three types of macrophages can be distinguished: exudate macrophages which have PO activity restricted to granules or are PO-negative; macrophages with PO activity in granules and both the rough endoplasmic reticulum (RER) and nuclear envelope; and resident macrophages with PO activity only in the RER and nuclear envelope. In addition, two types of multinucleated giant cells are found, one with and the other without PO activity in the RER and nuclear envelope. Multinucleated giant cells with PO activity in the RER and nuclear envelope as well as exudate macrophages with PO activity in the RER and nuclear envelope were mainly found 32 h and 3 days after implantation of the Melinex in the peritoneal cavity. These findings are discussed in the light of current knowledge of the PO activity in macrophages and multinucleated giant cells. It is concluded that the appearance of PO activity in the RER and nuclear envelope of exudate macrophages and multinucleated giant cells is in all probability a transient phenomenon, and that there is no objective evidence to support the opinion that exudate macrophages with PO activity in the RER and nuclear envelope are transitional cells between exudate and resident macrophages.     

Cytotoxic T cells in the peritoneal cavity of mice infected with ectromelia virus.

Peritoneal exudate cells from mice infected with ectromelia virus were cytotoxic for virus-infected target cells as measured in a 51Cr release assay. Cytotoxic activity seemed to be T cell-dependent as it was largely abolished by treatment with anti-theta serum and complement but was not impaired by macrophage depletion. The kinetics of development of cytotoxicity in the peritoneal cavity lagged behind spleen cytotoxicity by 1-2 days. Peak activity in peritoneal cells was present about 6 days after intravenous infection with virus. These studies suggest that macrophages present in the free peritoneal cell populations of ectromelia-infected mice are not cytotoxic for virus-infected target cells. The effect of macrophages in virus clearance is therefore likely to be due to phagocytic rather than cytotoxic effects.     

Effects of increased major histocompatibility complex dosage on chicken monocyte-macrophage function

The influence of major histocompatibility complex (B complex) dosage on monocyte-macrophage function was examined using 4- to 6-week-old trisomic strain chickens. Di- (B15B15), tri- (B15B15B15), and tetrasomic (B15B15B15B15) progeny were produced from trisomic x trisomic crosses. Although mononuclear leukocytes from tetrasomics exhibited enhanced chemotactic activity in response to both f-met-leu-phe and Enterobacter cloacae culture supernatant as compared with that of cells from other groups, the ability to generate peritoneal exudate cells in response to intraperitoneal Sephadex stimulation was similar in all groups. Among peritoneal exudate cells, tetrasomic birds produced a significantly lower percentage of adherent macrophages with a higher proportion of Fc receptor-positive and CMTD-2-reactive macrophages than either disomic or trisomic chickens. Both tetrasomic and trisomic peritoneal macrophages exhibited a reduced phagocytic activity for unopsonized but not opsonized SRBC than was found with disomic macrophages. Thus, the number of major histocompatibility complex copies present in cells appears to influence monocyte-macrophage function.     

Litomosoides sigmodontis: dynamics of the survival of microfilariae in resistant and susceptible strains of mice

Litomosoides sigmodontis in the BALB/c mouse is the only model of filariasis which allows the observation of the complete development in an immunocompetent mouse. In this study, we injected microfilariae (mf) intravenously, as well as into the pleural cavity, the site of natural release of mf from adult female worms, and followed the kinetics of elimination within the host. In susceptible BALB/c mice, mf circulated at high levels in the blood. In contrast, in C57BL/6 mice, which are refractory to full development, mf were eliminated rapidly from the peripheral blood. However, 6 days after intrapleural injection, viable larvae could be found in the pleural cavity and lung capillaries of both susceptible and resistant strains. The numbers of mf in the pleural cavity and lung capillaries in individual mice were significantly correlated, but not dependent on strain or peripheral microfilaraemia. Thus, although C57BL/6 mice showed enhanced production of nitric oxide by pleural exudate cells and a faster change in the numbers of circulating leukocytes after injection, rapid killing of mf by cell or nitric oxide-mediated mechanisms were not the reason for the different outcome. Furthermore, 3 h after iv injection, only a small percentage of mf could be recovered from the peripheral circulation, indicating the presence of a reservoir for mf containment. In conclusion, injected mf showed disparate dynamics of persistence within susceptible and resistant hosts, which is similar to the disparate outcome of natural infections with L. sigmodontis. This difference became obvious within 1 day after injection. The lung capillary system plays obviously a crucial part in regulation of microfilaremia. Our model also provides a possible means to explain frequent cases of occult infections in human filariasis.     

Inflammatory process caused by intraperitoneal injection of polyester in the rat: chemotactic activity in lavage fluid from the cavity

Minced polyester threads introduced into peritoneal cavity of guinea pigs or rats cause a granulomatous inflammation with evidence of macrophage stimulation. Chemotactic agents play an important role in the inflammatory reaction; they may be exogenous and/or endogenous. These are released locally by the cells involved in inflammation. In this paper the chemotactic effects of the peritoneal fluids from rats bearing the polyester inflammatory process, have been studied on PMN cells "in vitro". The peritoneal cavity fluids were obtained by washing the cavity of untreated rats or rats intraperitoneally injected with polyester, 1, 3, 7, 14 days after the intraperitoneal injection. The chemotactic response was assayed by employing modified chemotaxis Boyden chambers (Blind Well Neuro Probe) and polymorphonuclear leukocytes from normal or treated rats. Quantification of the migration was calculated by chemotactic index (A/B) (B = random migration, A = chemotaxis). The results demonstrated that the peritoneal fluids taken 3 and 7 days after the intraperitoneal polyester injection, elicit an evident chemotaxis response greater than that showed by peritoneal fluids from control rats. It is suggested that chemotactic factors can be produced and released by mononuclear cells involved in the inflammatory process.     

The effect of the supernatants obtained fromSporothrix schenkii andCandida albicans culture on the generation of reactive oxygen species by polymorphonuclear leukocytes

The effect of the supernatants obtained from the liquid culture medium ofSporothrix schenkii andCandida albicans on the generation of superoxide anion (O ₂ ^– and hydroxyl radicals OH_., the elements of reactive oxygen species (ROS), and chemilimunescence (CL), a measure of several ROS, by polymorphonuclear leukocytes (PMNs) was examined. In our study, it was shown that the supernatant ofS. schenkii increased all types of ROS generation examined and CL, while that ofC. albicans increased OH_. generation and CL. The effect of the supernatants ofS. schenkii on OH_. generation and CL and that ofC. albicans on CL were most remarkable when the supernatant obtained 8 weeks after the inoculation was used. The supernatant ofS. schenkii was shown to be a much more potent stimulant than the supernatant ofC. albicans. This ROS-stimulating effect of the supernatant ofS. schenkii was heat stable but not dialyzable. These findings suggest the possible role of ROS produced by infiltrated PMNs in the inflammatory skin lesions induced byS. schenkii.     

Proinflammatory role of glucocorticoid-induced TNF receptor-related gene in acute lung inflammation

Glucocorticoid-induced TNFR-related gene (GITR) participates in the immune/inflammatory response. Because GITR expression has been described in cells other than T lymphocytes, we investigated whether it also modulates acute inflammatory response. Using GITR-deficient (GITR(-/-)) mice, we analyzed the role of GITR in the development of carrageenan-induced lung inflammation (pleurisy) by studying several proinflammatory markers 2-8 h after carrageenan injection. When compared with GITR(+/+), GITR(-/-) mice exhibited decreased production of turbid exudate containing a lower number of leukocytes. This was correlated with the reduction of inflammatory markers (including TNF-alpha, IL-1beta, myeloperoxidase, inducible NO synthase, and cyclooxygenase 2) in the pleural exudate and/or in the lung. Moreover, endothelial cells expressed lower levels of adhesion molecules. In lungs of GITR(+/+) mice, GITR ligand expression was not modulated during pleurisy, while that of GITR increased, as a consequence of increased infiltration by GITR-expressing cells and of GITR up-regulation in macrophages and endothelial cells. Finally, cotreatment of GITR(+/+) mice with carrageenan and Fc-GITR fusion protein decreased the number of inflammatory cells (pleural macrophages and lung neutrophils) as compared with carrageenan treatment alone, confirming that GITR plays a role in the modulation of pleurisy.     

Augmentation of mouse natural killer cell activity by LS 2616, a new immunomodulator

The quinoline-3-carboxamide LS 2616 administered to mice in drinking water increased spontaneous cytotoxicity against YAC-1 cells in a dose-dependent manner. The enhancement of spontaneous cytotoxicity was found to be mediated by NK cells, as judged by their lack of adherence to nylon wool columns, relative resistance to treatment with antibodies to Thy-1.2 and complement, and almost total abrogation after depletion of asialo-GM1+ cells. Enhancement of NK activity was evident after 2 days of treatment, was maximal after 4 days, and remained elevated during the 14-day exposure period studied. NK activity returned to control levels 4 days after cessation of treatment. NK activity was significantly increased in spleen, peripheral blood, lymph nodes, and bone marrow of LS 2616-treated mice, while activity in peritoneal exudate cells and thymus remained low. LS 2616 was able to elevate NK activity in several mouse strains studied, including mice homozygous for the beige gene. Serum interferon levels were not increased during treatment with LS 2616. Combined injection of the interferon inducer Poly I:C and LS 2616 did not increase NK activity above that of animals injected with Poly I:C alone. However, Poly I:C, in contrast to LS 2616, increased NK activity in peritoneal exudate cells. Studies at the single cell level revealed that LS 2616 increased NK activity by increasing the number of lytically active cells via recruitment of new target-binding cells and not by increasing the lytic activity of pre-existing binders.