Enhancement Of Phagocytic Activity Research Articles

Mitochondrial Transfer via Tunneling Nanotubes is an Important Mechanism by Which Mesenchymal Stem Cells Enhance Macrophage Phagocytosis in the In Vitro and In Vivo Models of ARDS.

Mesenchymal stromal cells (MSC) have been reported to improve bacterial clearance in preclinical models of Acute Respiratory Distress Syndrome (ARDS) and sepsis. The mechanism of this effect is not fully elucidated yet. The primary objective of this study was to investigate the hypothesis that the antimicrobial effect of MSC in vivo depends on their modulation of macrophage phagocytic activity which occurs through mitochondrial transfer. We established that selective depletion of alveolar macrophages (AM) with intranasal (IN) administration of liposomal clodronate resulted in complete abrogation of MSC antimicrobial effect in the in vivo model of Escherichia coli pneumonia. Furthermore, we showed that MSC administration was associated with enhanced AM phagocytosis in vivo. We showed that direct coculture of MSC with monocyte‐derived macrophages enhanced their phagocytic capacity. By fluorescent imaging and flow cytometry we demonstrated extensive mitochondrial transfer from MSC to macrophages which occurred at least partially through tunneling nanotubes (TNT)‐like structures. We also detected that lung macrophages readily acquire MSC mitochondria in vivo, and macrophages which are positive for MSC mitochondria display more pronounced phagocytic activity. Finally, partial inhibition of mitochondrial transfer through blockage of TNT formation by MSC resulted in failure to improve macrophage bioenergetics and complete abrogation of the MSC effect on macrophage phagocytosis in vitro and the antimicrobial effect of MSC in vivo. Collectively, this work for the first time demonstrates that mitochondrial transfer from MSC to innate immune cells leads to enhancement in phagocytic activity and reveals an important novel mechanism for the antimicrobial effect of MSC in ARDS. Stem Cells 2016;34:2210–2223

The MAPK ERK5, but not ERK1/2, inhibits the progression of monocytic phenotype to the functioning macrophage

Intracellular signaling pathways present targets for pharmacological agents with potential for treatment of neoplastic diseases, with some disease remissions already recorded. However, cellular compensatory mechanisms usually negate the initial success. For instance, attempts to interrupt aberrant signaling downstream of the frequently mutated ras by inhibiting ERK1/2 has shown only limited usefulness for cancer therapy. Here, we examined how ERK5, that overlaps the functions of ERK1/2 in cell proliferation and survival, functions in a manner distinct from ERK1/2 in human AML cells induced to differentiate by 1,25D-dihydroxyvitamin D3 (1,25D). Using inhibitors of ERK1/2 and of MEK5/ERK5 at concentrations specific for each kinase in HL60 and U937 cells, we observed that selective inhibition of the kinase activity of ERK5, but not of ERK1/2, in the presence of 1,25D resulted in macrophage-like cell morphology and enhancement of phagocytic activity. Importantly, this was associated with increased expression of the macrophage colony stimulating factor receptor (M-CSFR), but was not seen when M-CSFR expression was knocked down. Interestingly, inhibition of ERK1/2 led to activation of ERK5 in these cells. Our results support the hypothesis that ERK5 negatively regulates the expression of M-CSFR, and thus has a restraining function on macrophage differentiation. The addition of pharmacological inhibitors of ERK5 may influence trials of differentiation therapy of AML.

Dynamics of milk leukocytes in response to a biological response modifier during bovine subclinical mastitis

The objective of this study was to evaluate the dynamics of milk leukocyte in response to intramammary infusion of a biological response modifier (BRM) prepared from Nocardia globerula during bovine subclinical mastitis (SCM). The somatic cell count (SCC), total bacterial count (TBC) in milk, cyclooxygenase (COX) activity, phagocytic activity, production of hydrogen peroxide (H2O2) and myeloperoxidase (MPO) activity in milk leukocytes were evaluated after intramammary infusion of BRM in quarters inflicted with SCM. Intramammary infusion of BRM significantly enhanced the SCC in earlier phase with subsequent reduction on day 7 after initiation of treatment. Similarly, the COX activity in milk cell lysate increased on day 3 and reduced on day 5. However reduction in TBC could be observed from day 3 onwards. The phagocytic activity of milk leukocytes was lower in mastitic cows whereas, significant enhancement in phagocytic activity was recorded in post treated cows with BRM. Similarly, the H2O2 production and MPO activities in milk leukocytes were enhanced significantly in diseased cows in response to BRM infusion. Significant enhancement of phagocytic activity, H2O2 and MPO activities indicate the priming of resident milk leukocytes in response to BRM infusion. Initial influx of SCC and raised COX activity also indicate the immunomodulatory activity of BRM. Reduction of TBC could be due to increased leukocytosis or direct microbicidal activity of activated milk cells. In the present study, the biological activity of BRM at standardized dose against bovine SCM is reported for the first time. Development of such therapy is warranted to reduce the drug resistance of microorganisms and contamination of milk with antibiotic residue.

Dynamics of milk leukocytes in response to intramammary infusion of amoxicillin plus sulbactam during bovine subclinical mastitis

The therapeutic potential of amoxicillin plus sulbactam and its effect on dynamics of milk leukocytes in bovine sub clinical mastitis were investigated in this study. Therapeutic efficacy was measured by somatic cell count and total bacterial count of the milk, whereas, the dynamics of milk leukocytes were assessed by measuring phagocytosis, hydrogen peroxide production, myeloperoxidase and lactoperoxidase enzyme levels in the milk leukocytes. Forty-five crossbred cows were randomly divided into three equal groups. Group I consisting of 15 cows served as healthy control, whereas 30 cows (sub clinical mastitis) were randomly divided into Groups II and III on the basis of positive reactions in the California Mastitis Test. Group II cows received 300mg of amoxicillin plus sulbactam twice daily for three days and Group III received sterile 5 ml phosphate buffer saline (pH 7.4) for three days. Both treatments were administered via the intramammary route. Observations were made up to 15 days after initiation of treatment. The results revealed a pronounced drop in somatic cell count and total bacterial count, whereas significant (P < 0.05) enhancement of phagocytic activity (42.20%), hydrogen peroxide production (29.46%), myeloperoxidase (49.27%) and lactoperoxidase (147.10%) enzyme levels in the milk leukocytes in Group II cows during post treatment periods were observed. Such changes were statistically non-significant in Group III cows. The results of the present study indicate that intramammary use of amoxicillin plus sulbactam augments the bactericidal function of milk leukocytes during bovine sub clinical mastitis and demonstrate the strong therapeutic potential against bovine subclinical mastitis.

In situ forming microparticle implants for delivery of sex steroids in fish: Modulation of the immune response of gilthead seabream by testosterone

Current knowledge on the sensitivity of marine fish to androgenic environmental chemicals is limited, despite the growing interest in the effects of endocrine disrupting chemicals. To study in vivo the effects of testosterone (T) on the fish immune response, we used a microencapsulation implant technique, the in situ forming microparticle system, containing 1mg T/kg body weight (T-ISM), in adult specimens of gilthead seabream (Sparus aurata L.), a species of great economic interest. We demonstrated that implants themselves (without T) have no significant effect on most of the parameters measured. In T-ISM implanted fish, T serum levels reached supraphysiological concentrations accompanied by a slight increase in 11-ketotestosterone and 17β-estradiol levels 21days post-implantation (dpi). Liver and head-kidney samples were processed 7 and 21 dpi to assess T-ISM effect on (i) the mRNA expression of genes involved in the metabolism of steroid hormones and in the immune response, and (ii) phagocyte activities. The expression profile of cytokines, chemokines and immune receptors was altered in T-ISM implanted animals that showed an early pro-inflammatory tendency, and then, a mixed pro-/anti-inflammatory activation during longer exposure. Furthermore, the enhancement of phagocytic activity and the production of reactive oxygen species by leukocytes 21 dpi in T-ISM implanted specimens suggest fine modulation of the innate immune response by T. Taken together, these data demonstrate for the first time the feasibility of using ISM implants in an aquatic species, and provide new data on the role played by T on the immune response in fish.

Immunomodulatory active compounds from Tinospora cordifolia

Ethnopharmacological relevanceTinospora cordifolia mentioned as “Rasayana” is extensively used in various herbal preparations for the treatment of different ailments for its general tonic, antiperiodic, antispasmodic, antiinflammatory, antiarthritic, antiallergic and antidiabetic properties. It is extensively used in Ayurveda due to its potential in improving the immune system and the body resistance against infections. Aim of the studyThe aim of the study was to isolate and characterise the immunomodulatory active compounds of Tinospora cordifolia. Materials and methodsThe immunomodulatory activity of different extracts, fractions and isolated compounds in relation to phagocytosis and reactive oxygen species production in human neutrophil cells have been investigated using the PMN phagocytic function studies, NBT, NO and chemiluminescence assay. ResultsThe results obtained indicate that ethyl acetate, water fractions and hot water extract exhibited significant immunomodulatory activity with an increase in percentage phagocyctosis. Chromatographic purification of these fraction led to the isolation of a mixture of two compounds 2, 3 isolated for the first time from natural source and five known compounds 1, 4–7 which were characterized as 11-hydroxymustakone (2), N-methyl-2-pyrrolidone (3), N-formylannonain (1), cordifolioside A (4), magnoflorine (5), tinocordiside (6), syringin (7) by nuclear magnetic resonance (NMR) and mass spectrometry (MS) and comparing the spectral data with reported one. Cordifolioside A and syringin have been reported to possess immunomodulatory activity. Other five compounds showed significant enhancement in phagocytic activity and increase in nitric oxide and reactive oxygen species generation at concentration 0.1–2.5μg/ml. ConclusionsSeven immunomodulatory active compounds belonging to different classes have been isolated and characterised indicating that the immunomodulatory activity of Tinospora cordifolia may be attributed to the synergistic effect of group of compounds.

Use of garlic, Allium sativum, to control Aeromonas hydrophila infection in rainbow trout, Oncorhynchus mykiss (Walbaum)

Garlic, Allium sativum, which was fed at 0, 0.05, 0.1, 0.5 and 1.0 g per 100 g of feed for 14 days to rainbow trout, Oncorhynchus mykiss (Walbaum), led to control of experimental infection with Aeromonas hydrophila. At doses of 0.5 and 1.0 g garlic per 100 g of feed, there was a reduction in mortalities to 4% compared with the controls (88%). Moreover, there was a significant increase in growth, feed conversion and protein efficiency. There was stimulation of the number of erythrocytes and leucocytes, a significantly higher haematocrit, enhancement of phagocytic activity, respiratory burst, lysozyme, anti-protease and bactericidal activities following feeding with garlic.

Curcumin enhances non-inflammatory phagocytic activity of RAW264.7 cells

Present study was performed to assess the effect of curcumin treatment on macrophage functions using RAW264.7 cells, a murine macrophage cell line. Phagocytic activity of RAW264.7 cells was enhanced by the treatment with curcumin for 48h while the nitric oxide synthesis from RAW264.7 cells following lipopolysaccharide exposure was blocked. The incubation of RAW264.7 cells with curcumin dose-dependently inhibited the stimulatory responses of macrophage triggered by lipopolysaccharide; the enhanced secretion of inflammatory cytokines such as TNF-α and IL-1β and the up-regulated expression of surface antigens like CD14 and CD40. Curcumin alone, however, was able to increase the basal level of TNF-α secretion and elevated markedly the expression of CD14 and slightly CD40. The marked enhancement of both phagocytic activity and CD14 was detectable as early as 75min after curcumin treatment which is the minimum time period required for the phagocytosis and CD14 measurement, suggesting a signaling pathway distinct from that triggered by apoptotic cells. In conclusion, this study elucidates that curcumin treatment enhances the phagocytic activity with blocking nitric oxide synthesis, a scavenger function of macrophages in non-inflammatory condition. In addition, this enhancement of phagocytic activity is triggered directly by the signals from curcumin itself not by apoptotic cells.

P38 MAPK contributes to CD54 expression and the enhancement of phagocytic activity during macrophage development

p38 is a subfamily of the mitogen-activated protein kinase (MAPK) superfamily with four isoforms. It has been well established that p38 plays a central role in the production of inflammatory molecules and is therefore required for the activation of macrophages in response to inflammatory stimuli. However, little is known about the roles of p38 in macrophage development. The difficulty to get mice deficient in multiple p38 isoforms complicates the study of p38 in macrophage development. With the model of bone marrow-derived murine macrophages and highly selective p38α/β inhibitors SB203580 and SB239063, here we report that macrophage colony-stimulating factor (M-CSF) induces p38 activation during macrophage development. Inhibition of p38 activity showed minor effects on macrophage proliferation or survival, and did not block CD14, F4/80 expression. However, p38 inhibitors resulted in a significant reduction in CD54 expression and impaired phagocytic activity. Taken together, our data suggest that p38 contributes to macrophage development.

Cytokines and Peroxisome Proliferator-Activated Receptor γ Ligand Regulate Phagocytosis by Pancreatic Stellate Cells

Pancreatic stellate cells have been characterized as the major source of extracellular matrix and cytokine production in the pancreas. This study showed that pancreatic stellate cells have a phagocytic function. The morphological features of periacinar phagocytic cells were investigated by immunohistochemically staining serial sections of the pancreas from male WBN/Kob rats and an animal model of acute pancreatitis for glial fibrillary acidic protein and alpha-smooth muscle actin. Pancreatic stellate cells were assayed for phagocytic activity by incubating them with senescent polymorphonuclear neutrophils or fluorescence-labeled latex beads in the presence or absence of cytokines, growth factors, and peroxisome proliferator-activated receptor gamma ligand. The role of CD36 and peroxisome proliferator-activated receptor gamma in phagocytosis was investigated by blocking endogenous CD36 and peroxisome proliferator-activated receptor gamma activity with anti-CD36 antibody and peroxisome proliferator-activated receptor gamma small interfering RNAs, respectively. Phagocytic cells were observed in areas of inflammation, and they were identical to the glial fibrillary acidic protein-positive and alpha-smooth muscle actin-positive cells, thus suggesting that they were pancreatic stellate cells. Aged polymorphonuclear neutrophils were ingested into the cytoplasm of the pancreatic stellate cells. Transforming growth factor beta, tumor necrosis factor alpha, and interleukin 1beta decreased the phagocytic activity of pancreatic stellate cells, whereas troglitazone induced a dose-dependent increase in both phagocytic activity and expression of CD36. Blockade of CD36 reduced troglitazone-induced phagocytosis. Silencing of the peroxisome proliferator-activated receptor gamma gene decreased phagocytosis and expression of CD36. Pancreatic stellate cells act as resident phagocytic cells, and CD36 promotes troglitazone-induced phagocytic activity via peroxisome proliferator-activated receptor gamma transactivation. Because phagocytosis is essential to limit the extent of inflammation, enhancement of phagocytic activity may provide an important approach to the treatment of pancreatic diseases.

Enhancing phagocytic activity of hemocytes and disease resistance in the prawn Penaeus merguiensis by feeding Spirulina platensis

Exposing the prawn Penaeus merguiensis to the bacteria Vibrio harveyi and Escherichia coli for an hour or feeding the prawns with Spirulina (Arthrospira) platensis (0.3% w/w feed) enhanced the phagocytic activity of their hemocytes. Improvement of the phagocytic activity was primarily through the activation of the hemocytes. The activated phagocytic hemocytes had a higher capacity to engulf foreign agents, such as bacteria, and a higher rate of phagocytosis. The phagocytic enhancement effect peaked on the fourth day of feeding with Spirulina. In the in vitro study, the granular cells from prawns took 45–60 min to complete the process of degranulation. Pre-exposure to Salmonella typhimurium and Bacillus subtilis did not result in enhancement of phagocytic activity of hemocytes. Only 10% prawns fed with Spirulina died in the first 14 days when challenged by V. harveyi at a concentration of 1 × 104CFUs mL−1, while all control prawns (basal feed without Spirulina) died within 14 days.

Immunomodulatory activity of Withania somnifera

Administration of an extract from the powdered root of the plant Withania somnifera was found to stimulate immunological activity in Babl/c mice. Treatment with five doses of Withania root extract (20 mg/dose/animal; i.p.) was found to enhance the total WBC count (17 125 cells/mm 3) on 10th day. Bone marrow cellularity (27×10 6 cells/femur) as well as α-esterase positive cell number (1800/4000 cells) also increased significantly ( P<0.001) after the administration of Withania extract. Treatment with Withania extract along with the antigen (SRBC) produced an enhancement in the circulating antibody titre and the number of plaque forming cells (PFC) in the spleen. Maximum number of PFC (985 PFC/10 6 spleen cells) was obtained on the fourth day. Withania extract inhibited delayed type hypersentivity reaction in mice (Mantoux test). Administration of Withania extract also showed an enhancement in phagocytic activity of peritoneal macrophages (76.5 pigmented cells/200) when compared to control (31.5/200 cells) in mice. These results confirm the immunomodulatory activity of W. somnifera extract, which is a known immunomodulator in indigenous medicine.

Granulocyte-macrophage colony-stimulating factor and fluconazole enhance anti-cryptococcal activity of monocytes from AIDS patients.

To investigate the effect of human recombinant granulocyte-macrophage colony-stimulating factor (hrGM-CSF) and fluconazole on anti-cryptococcal activity of monocytes from AIDS patients and normal subjects. The effect of GM-CSF and fluconazole on fungistatic and fungicidal activity of monocytes was studied in an in vitro system. Monocytes were treated in vitro with hrGM-CSF and fluconazole or either agent alone for 24 or 48 h, and fungistatic and fungicidal activity was evaluated in a colony-forming unit inhibition assay. CD11b/CD18 expression in monocytes was measured by flow cytometry analysis. Superoxide anion generation by peripheral blood monocytes was measured in the presence of pre-opsonized zymosan. Defective antifungal capacity of monocytes from AIDS patients was observed. GM-CSF treatment of monocytes from AIDS patients increased fungistatic activity, and the combination of hrGM-CSF and fluconazole resulted in fungicidal activity. The mechanisms involved in the GM-CSF-mediated effect appeared to be mediated by (i) enhancement of phagocytic activity, (ii) increase of superoxide anion generation, and (iii) upregulation of CD11b/CD18 expression on the monocyte surface. Our data highlight the effect of GM-CSF on anti-cryptococcal activity of human monocytes and show a synergistic effect of GM-CSF with fluconazole, suggesting a new therapeutic strategy in the treatment of cryptococcosis.

Iron Regulates Microglial Cell-Mediated Secretory and Effector Functions

Iron homeostasis and macrophage physiology are tightly intertwined. In the present study, we evaluated the influence of iron loading on the constitutive and interferon-γ (IFN-γ) plus lipopolysaccharide (LPS)-induced functional and secretory properties of microglial cells, using thein vitroestablished murine cell line BV-2. We demonstrate that iron augments the basal and IFN-γ plus LPS-enhanced anti-Candida albicansactivity exerted by BV-2 cells and that the phenomenon occurs with no enhancement of phagocytic activity. Furthermore, when the secretory properties of IFN-γ plus LPS-treated BV-2 cells were assessed, we found that tumor necrosis factor remains unchanged while nitric oxide production is significantly reduced in iron-loaded cells. The addition of the iron chelator deferiprone (L1) reverts the effects of iron on BV-2 functional and secretory properties. These data suggest that iron differently affects secretory and effector functions of BV-2 microglial cells, thus implying that iron interferes with murine microglial cell physiology.

Role of nitric oxide and melanogenesis in the accomplishment of anticryptococcal activity by the BV-2 microglial cell line

In the present paper, we investigated the involvement of cryptococcal melanogenesis and macrophage nitric oxide (NO) production in the accomplishment of anticryptococcal activity by microglial effector cells, using the murine cell line BV-2. We demonstrate that the constitutive levels of anticryptococcal activity exerted by BV-2 cells is significantly enhanced upon interferon γ plus lipopolysaccharide treatment. The phenomenon, which occurs with no enhancement of phagocytic activity, is associated with the production of high levels of NO and is abolished by addition of G-monomethyl- l-arginine. Comparable patterns of results are observed employing either unopsonized or opsonized microbial targets, the latter microorganisms being markedly more susceptible to BV-2 cell antimicrobial activity. Furthermore, melanization of Cryptococcus neoformans significantly reduces its susceptibility to BV-2 antimicrobial activity, regardless of the fact that activated macrophages or opsonized microorganisms have been employed. In conclusion, our results provide evidence that NO-dependent events are involved in the fulfillment of anticryptococcal activity by activated microglial cells and that fungal melanization is a precious escamotage through which C. neoformans overcomes host defenses.

Enhancement of in vitro nonspecific immune functions by African plant extracts

AbstractThe in vitro immunomodulatory activities of extracts from African medicinal plants have been studied. Certain extracts exerted enhancing effects in a concentration dependent manner on immune cells from three different strains of mice. Among these were an enhancement of phagocytic activity of peritoneal macrophages and tumoricidal potential of macrophages, and stimulation of the oxidative burst of macrophages and granulocytes. In contrast, the proliferative capacity of lymphocytes was only marginally affected. Enhanced macrophage and granulocyte activation was also detected using leucocytes of the LPS nonresponder strain C3H/HeJ. From the latter it can be concluded that the observed stimulatory effects of the plant extracts in vitro were not due to contamination with lipopolysaccharides.

Contribution of Lysophosphatidylcholine-Treated Nonadherent Cells to Mechanism of Macrophage Activation

Lysophosphatidylcholine (lyso-PC), a product of inflammation, stimulates (in vivo) mouse peritoneal macrophages to ingest target cells via Fc receptors. In vitro treatment of macrophages with lyso-Pc was unable to enhance ingestion activity. When a mixture of macrophages and nonadherent (B and T) cells was treated with 20 micrograms of lyso-Pc/ml for 30 min, a greatly enhanced Fc-mediated ingestion was observed at about 3 hr after treatment, suggesting that nonadherent cells contributed to activation mechanism of macrophages. The accumulated evidence suggests that treated B cells collaborated with untreated T cells in a stepwise fashion for the exchange of a signaling factor(s) for macrophage activation. When conditioned medium prepared by stepwise cultivation from treated B cells to untreated T cells was used for cultivation of untreated macrophages, a markedly enhanced Fc-mediated ingestion was observed. However, cultivation of macrophages with stepwise conditioned medium of treated T cells and untreated B cells produced no significant enhancement of phagocytic activity. Therefore, we concluded that lyso-Pc-treated B cells initiated the macrophage activation process by releasing and transmitting a signaling factor to T cells, and, in turn, the T cells modified the factor or supplied a new factor capable of the ultimate activation of macrophages for ingestion capacity. This lyso-Pc-induced factor(s) appears to be distinct from the established interleukins 1 and 2.

Detection of different macrophage‐activating factor and interferon activities in supernatants of chicken lymphocyte cultures1

Chicken spleen lymphocytes were cultivated under various conditions in order to produce and characterise functionally avian lymphokines. Biological properties of lymphokine-containing culture supernatants were evaluated with regard to their antiviral activity and their effects on cultured bone marrow-derived chicken macrophages, including induction of cytostatic activity, enhancement of phagocytic activity towards vital Candida albicans, and giant cell formation. Optimal doses of concanavalin A (ConA) for stimulation of lymphocytes varied between 2.5 and 40 microg/ml, depending on the cell concentration and presence or absence of serum. Lymphokine production occurred even without exogenous mitogen stimulation when cells were cultured at sufficiently high concentrations (1 to 2 x 10(7) cells/ml). Cytostasis-inducing capacity of lymphokine preparations against lymphoblastoid MDCC-RP1 cells was always combined with the presence of antiviral activity. Experimental results suggested that these two activities had to be attributed to at least two distinct lymphokines, i.e. macrophage-activating factor and interferon (IFN). ConA stimulation of lymphocytes from a single donor appeared to be the appropriate signal for production of IFN-gamma. Endogenous stimulation in mixed lymphocyte cultures more appropriately triggered production of IFN-alpha or IFN-ss, although production of IFN-gamma was not completely suppressed. Phagocytic activity of macrophages could also be increased by a cytokine present in conditioned media from confluent cultures of chicken embryo fibroblasts, chicken kidney cells, or bone marrow-derived chicken macrophages. In lymphokine preparations, this mediator may-even be a cofactor necessary for induction of multinucleated giant cell formation of cultured macrophages.

The Enhancement of Phagocytic Activity of Cultured Smooth Muscle Cells by Dimethyl Sulfoxide

The Enhancement of Phagocytic Activity of Cultured Smooth Muscle Cells by Dimethyl Sulfoxide

Stimulation of leukocyte phagocytic activity by the platelet release reaction

The influence of material released by activated human platelets on the phagocytic activity of human neutrophils and monocytes has been investigated under conditions where there was a low background level of exposure of the leukocytes to platelet release products (PRPr). Incubation with PRPr prepared from 400 X 10(9)/l platelets resulted in an approximate 3-fold increase in the proportion of leukocytes which ingested particles. Stimulation was produced by dilutions of PRPr down to 1:4 to indicate that phagocytic activity was affected by material released by concentrations of platelets within the physiological range. This response appeared to reflect stimulation of the overall phagocytic capability of the leukocytes as equivalent ingestion was observed with particles which possessed or lacked the capacity to bind complement to their surface. Enhancement of phagocytic activity was evidently secondary to induction of a change in leukocyte behaviour which was sustained in the absence of continued exposure to preparations of PRPr. These observations extend the range of human biological processes known to be responsive to PRPr, and have implications for the assessment of leukocyte phagocytic activity in vitro as this particular property could be modified by contact with PRPr during the isolation of leukocytes from peripheral blood.