Regulation Of Granulopoiesis Research Articles

Adenosine potentiates stimulatory effects on granulocyte-macrophage hematopoietic progenitor cells in vitro of IL-3 and SCF, but not those of G-CSF, GM-CSF and IL-11

The aim of the studies was to ascertain if adenosine is able to co-operate with selected hematopoietic growth factors and cytokines, namely with granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), stem cell factor (SCF), interleukin-3 (IL-3), and interleukin-11 (IL-11), in inducing the growth of colonies from hematopoietic progenitor cells for granulocytes and macrophages (GM-CFC) from normal bone marrow cells in vitro. Adenosine was found not to produce any colonies when present in the cultures as the only potential stimulator. All the tested cytokines and growth factors were observed to induce the growth of distinct numbers of GM-CFC colonies, with the exception of IL-11. When suboptimal concentrations of the evaluated cytokines and growth factors were tested in the cultures in which various concentrations of adenosine were concomitantly present, mutually potentiating effects were found in the case of IL-3 and SCF. These results confirm the role of adenosine in regulation of granulopoiesis and predict IL-3 and SCF as candidates for further in vivo studies of their combined administration with adenosine.

Critical role for granulocyte colony-stimulating factor in inflammatory arthritis.

Granulocyte colony-stimulating factor (G-CSF) is a well known regulator of granulopoiesis, but the role of endogenous G-CSF in inflammatory joint disease has not been explored. We studied the response of G-CSF-deficient mice in experimental models of joint inflammation. We show that G-CSF deficiency protects mice from acute and chronic arthritis. Reduced severity was associated with blunted mobilization of granulocytic cells from the bone marrow and less cellular infiltrate and cellular activation in inflamed joints. We also demonstrate that G-CSF blockade in established collagen-induced arthritis in WT mice markedly reduces disease manifestations and is as effective as tumor necrosis factor blockade. Our results reveal a critical role for G-CSF in driving joint inflammation and highlight G-CSF as a potential therapeutic target in inflammatory joint diseases, such as rheumatoid arthritis.

Functional dissociation between proforms and mature forms of proteinase 3, azurocidin, and granzyme B in regulation of granulopoiesis.

We previously demonstrated that secreted proform(s) of the neutrophil serine protease PR3 (proteinase 3) can down-modulate the fraction of normal human colony-forming unit granulocyte-macrophage (CFU-GM) in S-phase, whereas PR3 extracted from mature neutrophils lacks this ability. The objective of this study was to characterize the structural and functional dissociation between secreted proforms and granule-stored mature forms and to extend the investigation to other related hematopoietic serine proteases. Conditioned media containing secreted proteases from transfectant cell lines with stable expression of human PR3, neutrophil elastase, cathepsin G, azurocidin, and granzymes A, B, H, K, and M were tested for their ability to reduce the fraction of normal human CFU-GM in S phase. Furthermore, recombinant PR3, azurocidin, and granzyme B with defined N-terminal propeptides, and the respective mature forms without propeptide, were functionally characterized. In addition to PR3, secreted proforms of azurocidin and granzymes A, B, H, K, and M, but not cathepsin G or neutrophil elastase, have S-phase reducing activity. This activity is restricted to the dipeptide proforms, whereas mature forms without propeptide have no S-phase reducing activity. On the other hand, only the mature forms of PR3 and granzyme B could bind the serine protease inhibitor diisopropylfluorophosphate (DFP), or aprotinin in the case of azurocidin. We also demonstrate that granulocyte colony-stimulating factor-stimulated CD34+ cells and interleukin-2-stimulated lymphocytes secrete active proforms of PR3 and granzyme B, respectively. These results demonstrate distinctive functional and conformational differences between proforms and mature forms of these hematopoietic serine proteases and suggest novel growth regulatory mechanisms in granulopoiesis.

Granulocyte Colony-stimulating Factor Induces Erk5 Activation, Which Is Differentially Regulated by Protein-tyrosine Kinases and Protein Kinase C: REGULATION OF CELL PROLIFERATION AND SURVIVAL

Granulocyte colony-stimulating factor (G-CSF) plays a major role in the regulation of granulopoiesis. Treatment of cells with G-CSF has been shown to activate multiple signal transduction pathways. We show here that Erk5, a novel member of the MAPK family, and its specific upstream activator MEK5 were activated in response to incubation of cells with G-CSF. Different from other members of the MAPK family including Erk1/2, JNK, and p38, maximal activation of Erk5 by G-CSF required the C-terminal region of the G-CSF receptor. Genistein, a specific inhibitor of protein-tyrosine kinases, blocked G-CSF-induced Erk5 activation. In contrast, inhibition of protein kinase C activity increased G-CSF-mediated activation of Erk5 and MEK5, whereas stimulation of protein kinase C activity inhibited activation of the two kinases by G-CSF. The proliferation of BAF3 cells in response to G-CSF was inhibited by expression of a dominant-negative MEK5 but potentiated by expression of a constitutively active MEK5. Expression of the constitutively active MEK5 also increased the survival of BAF3 cells cultured in the absence of or in low concentrations of G-CSF. Together, these data implicate Erk5 as an important signaling component in the biological actions of G-CSF.

A dynamic model of ex vivo granulopoiesis

Abstract Evaluating kinetics in hematopoietic cultures is complicated by the distribution of cells over various stages of differentiation and by the presence of cells from different lineages. Growth factors and other parameters can greatly affect the lineage and maturation stage of a culture. In order to resolve the kinetics and more clearly define the differential effects of O 2 tension (pO 2 ), pH, and IL-3 on granulopoiesis, a mathematical model-based approach was undertaken. Granulocytic differentiation is described within a continuous, deterministic framework in which cells develop from primitive granulocytic progenitors to mature neutrophils. The model predicts distributed cell populations by incorporating independent rates of growth, death, differentiation, and transition between quiescence and active cycling. The response of these four model processes to changes in the culture environment was examined. Model simulations of experimental data revealed that (i) pO 2 effects are exerted only on the growth rate, (ii) pH effects between pH 7.25 and 7.4 on growth and differentiation are coupled; however, with increasing pH values, especially at pH 7.6, the death rate for cells in the early stages of differentiation becomes increasingly significant, and (iii) the absence of IL-3 markedly enhances the rate of differentiation through the myeloblast window in the differentiation pathway. The combined effects of these environmental factors can be predicted based on changes in the model parameters derived from the individual effects. Integrating experimental data with mathematical modeling can be used to elucidate the mechanisms underlying the regulation of granulopoiesis by pO 2 , pH, IL-3, and other conditions, and to improve culture strategies.

Regulation of granulopoiesis by transcription factors and cytokine signals.

The development of mature granulocytes from hematopoietic precursor cells is controlled by a myriad of transcription factors which regulate the expression of essential genes, including those encoding growth factors and their receptors, enzymes, adhesion molecules, and transcription factors themselves. In particular, C/EBPalpha, PU.1, CBF, and c-Myb have emerged as critical players during early granulopoiesis. These transcription factors interact with one another as well as other factors to regulate the expression of a variety of genes important in granulocytic lineage commitment. An important goal remains to understand in greater detail how these various factors act in concert with signals emanating from cytokine receptors to influence the various steps of maturation, from the pluripotent hematopoietic stem cell, to a committed myeloid progenitor, to myeloid precursors, and ultimately to mature granulocytes.

A Secreted Proform of Neutrophil Proteinase 3 Regulates the Proliferation of Granulopoietic Progenitor Cells

AbstractMyeloid leukemia cells, the human promyelocytic cell line HL-60, and a subpopulation of normal marrow cells produce a leukemia-associated inhibitor (LAI) that reversibly downmodulates DNA synthesis of normal granulopoietic progenitor cells colony-forming unit granulocyte-macrophage (CFU-GM). We isolated an active 125-kD component of LAI from HL-60 conditioned medium (CM), subjected it to cyanogen bromide cleavage and show by amino acid sequencing of the resulting peptides that it consists of a complex of the serine proteinase inhibitor α1-antitrypsin and a 31-kD fragment that retained the S-phase inhibitory activity, but resisted sequencing. This finding suggested that the 31-kD fragment originated from one of the neutrophil serine proteases (ie, elastase, proteinase 3, or cathepsin G) produced by normal promyelocytes, as well as HL-60 cells, for storage in primary granules and partly secreted during synthesis as enzymatically inactive proforms. Immunoblot analysis showed that the 125-kD complex contained proteinase 3 (PR3), and immunoprecipitation of PR3 from HL-60 CM abrogated the S-phase inhibitory activity, whereas immunoprecipitation of cathepsin G or elastase did not. Immunoprecipitation of PR3 from CM of a subpopulation of normal marrow cells also abrogated the S-phase inhibitory effect. Furthermore, CM from rat RBL and murine 32D cell lines transfected with human PR3 both reduced the fraction of CFU-GM in S-phase with 30% to 80% at 1 to 35 ng/mL PR3, whereas CM of the same cells transfected with cathepsin G or elastase did not. Also, an enzymatically silent mutant of PR3 exerted full activity, showing that the S-phase modulatory effect is not dependent on proteolytic activity. Amino acid sequencing of biosynthetically radiolabeled PR3 showed that PR3 from transfected cells is secreted after synthesis as proforms retaining amino terminal propeptides. In contrast, mature PR3 extracted from mature neutrophils has only minor activity. The inhibitory effect of secreted PR3 is reversible and abrogated by granulocyte (G)- or granulocyte-macrophage colony-stimulating factor (GM-CSF). Experiments with highly purified CD34+ bone marrow cells suggested that PR3 acts directly on the granulopoietic progenitor cells. These observations suggest a role for PR3 in regulation of granulopoiesis, and possibly in suppression of normal granulopoiesis in leukemia.

A Secreted Proform of Neutrophil Proteinase 3 Regulates the Proliferation of Granulopoietic Progenitor Cells

Myeloid leukemia cells, the human promyelocytic cell line HL-60, and a subpopulation of normal marrow cells produce a leukemia-associated inhibitor (LAI) that reversibly downmodulates DNA synthesis of normal granulopoietic progenitor cells colony-forming unit granulocyte-macrophage (CFU-GM). We isolated an active 125-kD component of LAI from HL-60 conditioned medium (CM), subjected it to cyanogen bromide cleavage and show by amino acid sequencing of the resulting peptides that it consists of a complex of the serine proteinase inhibitor 1-antitrypsin and a 31-kD fragment that retained the S-phase inhibitory activity, but resisted sequencing. This finding suggested that the 31-kD fragment originated from one of the neutrophil serine proteases (ie, elastase, proteinase 3, or cathepsin G) produced by normal promyelocytes, as well as HL-60 cells, for storage in primary granules and partly secreted during synthesis as enzymatically inactive proforms. Immunoblot analysis showed that the 125-kD complex contained proteinase 3 (PR3), and immunoprecipitation of PR3 from HL-60 CM abrogated the S-phase inhibitory activity, whereas immunoprecipitation of cathepsin G or elastase did not. Immunoprecipitation of PR3 from CM of a subpopulation of normal marrow cells also abrogated the S-phase inhibitory effect. Furthermore, CM from rat RBL and murine 32D cell lines transfected with human PR3 both reduced the fraction of CFU-GM in S-phase with 30% to 80% at 1 to 35 ng/mL PR3, whereas CM of the same cells transfected with cathepsin G or elastase did not. Also, an enzymatically silent mutant of PR3 exerted full activity, showing that the S-phase modulatory effect is not dependent on proteolytic activity. Amino acid sequencing of biosynthetically radiolabeled PR3 showed that PR3 from transfected cells is secreted after synthesis as proforms retaining amino terminal propeptides. In contrast, mature PR3 extracted from mature neutrophils has only minor activity. The inhibitory effect of secreted PR3 is reversible and abrogated by granulocyte (G)- or granulocyte-macrophage colony-stimulating factor (GM-CSF). Experiments with highly purified CD34+ bone marrow cells suggested that PR3 acts directly on the granulopoietic progenitor cells. These observations suggest a role for PR3 in regulation of granulopoiesis, and possibly in suppression of normal granulopoiesis in leukemia.

Endogenous granulocyte colony-stimulating factor production by peripheral blood mononuclear cells in persons with acquired immunodeficiency syndrome

Granulocyte colony-stimulating factor (G-CSF) is essential for the regulation of granulopoiesis. A deficiency of endogenously produced G-CSF may account for the relative neutropenia, impaired neutrophil function, and other neutrophil-related responses to infection observed in patients with acquired immunodeficiency syndrome (AIDS). In the present study, heparin-anticoagulated whole blood was obtained from 10 patients with advanced human immunodeficiency virus (HIV) and a diagnosis of AIDS and 9 healthy volunteers. Peripheral blood mononuclear cells (PBMCs) were isolated by density-gradient centrifugation, stimulated with lipopolysaccharide at various concentrations, and kept in culture. G-CSF concentrations in culture supernatants were assayed by enzyme-linked immunosorbent assay after 24 hours. The PBMCs from patients with HIV showed decreased G-CSF production at baseline and at all concentrations of endotoxin, reaching significance at 10,000 ng/mL. The mean peak G-CSF level attained by PBMCs from patients with AIDS at 1000 ng/mL endotoxin was less than the level attained by PBMCs obtained from volunteers at an endotoxin concentration of 50 ng/mL. This decreased G-CSF production in response to endotoxin may explain attenuated neutrophil responses to acute bacterial infections.

Serum Concentrations of Granulocyte Colony-Stimulating Factor in Healthy Term and Preterm Neonates and in Those With Various Diseases Including Bacterial Infections

The neonate is uniquely susceptible to severe and overwhelming bacterial infections. One of the most important deficits in the neonatal host defense system seems to be a quantitative and qualitative deficiency of the myeloid and the phagocytic system. Future optimal therapy of neonatal sepsis may include the use of adjuvant immunologic therapy. Granulocyte colony-stimulating factor (G-CSF) has been shown to induce neutrophilia and to enhance mature effector neutrophil function. To evaluate the role of G-CSF with respect to infection, we examined serum levels of G-CSF in term and preterm neonates, using an enzyme-linked immunosorbent assay method. G-CSF levels in healthy neonates showed peak levels up to 7 hours after birth, followed by an increase in total neutrophil cell (TNC) counts. Both G-CSF levels determined between 4 and 7 hours after birth and peak TNC counts correlated with the gestational age of the neonates. The state of nutrition, maternal treatment with glucocorticoids, maternal infection and hypertension, and the mode of delivery influenced peak G-CSF levels. Neonates with signs of infection between 4 and 7 hours after birth had higher levels of G-CSF than did healthy neonates (1,312 +/- 396 pg/mL v 176 +/- 19 pg/mL). In conclusion, the presented results of serum concentrations of G-CSF in relation to TNC counts and various diseases suggests an important role of G-CSF in the regulation of granulopoiesis during the neonatal period.

Serum concentrations of granulocyte colony-stimulating factor in healthy term and preterm neonates and in those with various diseases including bacterial infections

The neonate is uniquely susceptible to severe and overwhelming bacterial infections. One of the most important deficits in the neonatal host defense system seems to be a quantitative and qualitative deficiency of the myeloid and the phagocytic system. Future optimal therapy of neonatal sepsis may include the use of adjuvant immunologic therapy. Granulocyte colony-stimulating factor (G-CSF) has been shown to induce neutrophilia and to enhance mature effector neutrophil function. To evaluate the role of G-CSF with respect to infection, we examined serum levels of G-CSF in term and preterm neonates, using an enzyme-linked immunosorbent assay method. G-CSF levels in healthy neonates showed peak levels up to 7 hours after birth, followed by an increase in total neutrophil cell (TNC) counts. Both G-CSF levels determined between 4 and 7 hours after birth and peak TNC counts correlated with the gestational age of the neonates. The state of nutrition, maternal treatment with glucocorticoids, maternal infection and hypertension, and the mode of delivery influenced peak G-CSF levels. Neonates with signs of infection between 4 and 7 hours after birth had higher levels of G-CSF than did healthy neonates (1,312 +/- 396 pg/mL v 176 +/- 19 pg/mL). In conclusion, the presented results of serum concentrations of G-CSF in relation to TNC counts and various diseases suggests an important role of G-CSF in the regulation of granulopoiesis during the neonatal period.

Drug-induced agranulocytosis: bone marrow granulocytic progenitor cells

Bone marrow CFU-GM and cluster forming cells were studied in ten patients in different stages of drug-induced agranulocytosis using a methylcellulose cell culture technique in vitro. In the aplastic phase of the disease (A), the number of both CFU-GM and cluster forming cells was decreased in comparison to normal values. In the regenerative phase of the disease (R), the number of both granulocytic progenitors increased but did not reach normal values. In patients considered to be recovered from acute agranulocytosis (Rec), a decreased number of progenitors persisted indicating residual damage at this granulocytic cell level. It is suggested that agranulocytosis is due to isolated damage of granulocytic cells and predictable cascade of events within different cell compartments could be used as an in vivo model for investigation of the regulation of granulopoiesis.

Endogenous inhibitors of human granulocyte/macrophage colony forming cells in vitro are inactivated by free radical scavengers.

Granulocyte/macrophage colony forming cells (GM-CFC) require colony stimulating factors (CSF) for growth in vitro. Crude preparations of CSF also contain inhibitors. To investigate whether this inhibition involves free radicals, the following scavenging agents were tested on human GM-CFC grown in agar: cysteine, reduced glutathione, dithiothreitol, sodium selenite, alpha-tocopherol, superoxide dismutase, catalase and ascorbic acid. All except ascorbic acid and catalase had enhancing and stabilising effects on colony formation. The colonies also contained more cells but no changes in cell composition were detected. Cysteine, the most extensively tested substance, was active at concentrations present in body fluids and enhanced colony formation without an exogenous source of CSF. Our results are consistent with an imbalance between the production and removal of free radicals generated in the human GM-CFC assay in vitro. This hampers investigation of regulators of granulopoiesis and results in underestimation of GM-CFC, but can be alleviated by the addition of free radical scavengers.

The role of eosinophil in regulation of granulopoiesis.

An association between eosinophilia and neutropenia has been observed in a number of clinical conditions. To probe the role of eosinophils in granulopoiesis, marrow and peritoneal eosinophils, obtained from Schistosoma mansoni-infected mice, were separated and purified. Normal bone marrow cells were cultured in semi-solid culture medium in concentrations ranging from 5 X 10(4) to 5 X 10(5), with and without added eosinophils. To examine whether high prostaglandin E (PGE) content of eosinophils affects granulopoiesis, indomethacin was added to duplicate marrow cultures containing eosinophils. The addition of eosinophils to normal syngeneic marrow culture caused a significant inhibition of granulocyte-macrophage colony formation (CFU-GM) in culture. This suppressive effect was reversible upon addition of indomethacin. These findings suggest that eosinophils, in vitro, are capable of inhibition of granulopoiesis. The reversal of this effect by indomethacin indicates that this suppression may be prostaglandin mediated.

Regulation of granulopoiesis by T cells and T-cell subsets.

Regulation of granulopoiesis by T cells and T-cell subsets.

Regulation of Normal Human Blood Neutrophilic, Macrophagic, and Eosinophilic Committed Stem Cell Proliferation by Autologous Blood T Lymphocyte Subsets

The influence of T lymphocyte (T cell) subsets on the proliferation of the blood neutrophilic, macrophagic, and eosinophilic cluster and colony (aggregate) forming cells (CFC) was evaluated. The T cells and the CFC-enriched B-null cells (mononuclear cells depleted of monocytes and T cells) and null cells (B-null) cells depleted of B lymphocytes) were separated from the blood of normal individuals. The population enriched for OKT4 (T4 cells) monoclonal antibody-binding T helper/inducer cells and for OKT8 (T8 cells) monoclonal antibody-binding T suppressor/cytotoxic cells were separated from the T cells or mononuclear cells depleted of monocytes by negative enrichment using an immunoadherence "panning" method. These separated cell populations were cultured separately, and the B-null or null cells were cocultured with the T, T4, and T8 cells in agar medium in the presence and absence of an exogenous source of colony-stimulating activity (CSA). The cultures were evaluated at day 14. The B-null and null cells, but not the T cells or subsets, contained CFC. The number of CFC proliferated from the B-null or null cells increased significantly (p less than 0.001) in cocultures with unseparated T and T8 cells in a dose-dependent manner. The T4 cells neither promoted nor inhibited the CFC growth. The T8 population was a better promoter (p less than 0.01) of CFC growth than the unseparated T cells. This suggests that the CFC promoting effect of the unseparated T cells is probably due to the influence of the T8 subset present within the T cells. The proportion of the neutrophilic, macrophagic, and eosinophilic CFC that proliferated in these cultures was comparable under the influence of the T and T8 cells. The results of these studies indicate that the T8 population, which is the suppressor in the classical immune system, promotes the growth of the blood CFC. We speculate that the T cells are involved in the regulation of granulopoiesis in vivo.

Regulation of normal human blood neutrophilic, macrophagic, and eosinophilic committed stem cell proliferation by autologous blood T lymphocyte subsets

The influence of T lymphocyte (T cell) subsets on the proliferation of the blood neutrophilic, macrophagic, and eosinophilic cluster and colony (aggregate) forming cells (CFC) was evaluated. The T cells and the CFC-enriched B-null cells (mononuclear cells depleted of monocytes and T cells) and null cells (B-null) cells depleted of B lymphocytes) were separated from the blood of normal individuals. The population enriched for OKT4 (T4 cells) monoclonal antibody-binding T helper/inducer cells and for OKT8 (T8 cells) monoclonal antibody- binding T suppressor/cytotoxic cells were separated from the T cells or mononuclear cells depleted of monocytes by negative enrichment using an immunoadherence “panning” method. These separated cell populations were cultured separately, and the B-null or null cells were cocultured with the T, T4, and T8 cells in agar medium in the presence and absence of an exogenous source of colony-stimulating activity (CSA). The cultures were evaluated at day 14. The B-null and null cells, but not the T cells or subsets, contained CFC. The number of CFC proliferated from the B-null or null cells increased significantly (p less than 0.001) in cocultures with unseparated T and T8 cells in a dose-dependent manner. The T4 cells neither promoted nor inhibited the CFC growth. The T8 population was a better promoter (p less than 0.01) of CFC growth than the unseparated T cells. This suggests that the CFC promoting effect of the unseparated T cells is probably due to the influence of the T8 subset present within the T cells. The proportion of the neutrophilic, macrophagic, and eosinophilic CFC that proliferated in these cultures was comparable under the influence of the T and T8 cells. The results of these studies indicate that the T8 population, which is the suppressor in the classical immune system, promotes the growth of the blood CFC. We speculate that the T cells are involved in the regulation of granulopoiesis in vivo.

Abnormal regulation of granulopoiesis in myeloproliferative disorders

Abnormal regulation of granulopoiesis in myeloproliferative disorders

A Monokine Regulates Colony-Stimulating Activity Production by Vascular Endothelial Cells

Human umbilical vein endothelial cells were cultured in supernatants of peripheral blood monocytes that had been cultured for 3 days with and without lactoferrin. Colony-stimulating activity (CSA) was measured in supernatants of the endothelial cell cultures and appropriate control cultures using normal, T-lymphocyte-depleted, phagocyte-depleted, low-density bone marrow cells in colony growth (CFU-GM) assays. Monocyte-conditioned medium contained a nondialyzable, heat labile factor that enhanced 4-15--fold the production of CSA by endothelial cells. The addition of lactoferrin to monocyte cultures reduced the activity of this monokine by 69%. Lactoferrin did not inhibit CSA production by monokine-stimulated endothelial cells. Therefore, vascular endothelial cells are potent sources of CSA, the production of CSA by these cells is regulated by a stimulatory monokine, and the production and/or release of the monokine is inhibited by lactoferrin, a neutrophil-derived putative feedback inhibitor of granulopoiesis. Inasmuch as a similar monokine is known to stimulate CSA production by fibroblasts and T lymphocytes, we suggest that mononuclear phagocytes play a pivotal role in the regulation of granulopoiesis by recruiting a variety of cell types to produce CSA.

A monokine regulates colony-stimulating activity production by vascular endothelial cells

Human umbilical vein endothelial cells were cultured in supernatants of peripheral blood monocytes that had been cultured for 3 days with and without lactoferrin. Colony-stimulating activity (CSA) was measured in supernatants of the endothelial cell cultures and appropriate control cultures using normal, T-lymphocyte-depleted, phagocyte-depleted, low- density bone marrow cells in colony growth (CFU-GM) assays. Monocyte- conditioned medium contained a nondialyzable, heat labile factor that enhanced 4–15--fold the production of CSA by endothelial cells. The addition of lactoferrin to monocyte cultures reduced the activity of this monokine by 69%. Lactoferrin did not inhibit CSA production by monokine-stimulated endothelial cells. Therefore, vascular endothelial cells are potent sources of CSA, the production of CSA by these cells is regulated by a stimulatory monokine, and the production and/or release of the monokine is inhibited by lactoferrin, a neutrophil- derived putative feedback inhibitor of granulopoiesis. Inasmuch as a similar monokine is known to stimulate CSA production by fibroblasts and T lymphocytes, we suggest that mononuclear phagocytes play a pivotal role in the regulation of granulopoiesis by recruiting a variety of cell types to produce CSA.