Macrophage development: III. Transformation of pulmonary macrophages from precursors in fetal lungs and their later maturation in organ culture

Ontogeny of Tissue-Resident Macrophages

The origin of tissue-resident macrophages, crucial for homeostasis and immunity, has remained controversial until recently. Originally described as part of the mononuclear phagocyte system, macrophages were long thought to derive solely from adult blood circulating monocytes. However, accumulating evidence now shows that certain macrophage populations are in fact independent from monocyte and even from adult bone marrow hematopoiesis. These tissue-resident macrophages derive from sequential seeding of tissues by two precursors during embryonic development. Primitive macrophages generated in the yolk sac (YS) from early erythro-myeloid progenitors (EMPs), independently of the transcription factor c-Myb and bypassing monocytic intermediates, first give rise to microglia. Later, fetal monocytes, generated from c-Myb⁺ EMPs that initially seed the fetal liver (FL), then give rise to the majority of other adult macrophages. Thus, hematopoietic stem cell-independent embryonic precursors transiently present in the YS and the FL give rise to long-lasting self-renewing macrophage populations.

Group B Streptococcus induces a caspase-dependent apoptosis in fetal rat lung interstitium

Group B Streptococcus (GBS) is an important pathogen and is associated with sepsis and meningitis in neonates and infants. An ex vivo model that facilitates observations of GBS interactions with multiple host cell types over time was used to study its pathogenicity. GBS infections were associated with profound reductions in fetal lung; explant size, and airway branching. Elevated levels of apoptosis subsequent to GBS infections were observed by whole-mount confocal immunofluorescence using activated-caspase-3-antibodies and terminal deoxynucleotidyl transferase dUTP nick end-labeling (TUNEL) assays. The caspase inhibitor Z-VAD-FMK abolished the increase in TUNEL-positive cells associated with GBS infections, indicating that the GBS-induced apoptosis was caspase-dependent. Digital image analyses revealed that both GBS and the active form of caspase-3 were distributed primarily within the lung interstitium, suggesting that these tissues are important targets for GBS. Antibodies to the active form of caspase-3 colocalized with both macrophage- and erythroblast-markers, suggesting that these hematopoietic cells are vulnerable to GBS-mediated pathogenesis. These studies suggest that GBS infections profoundly alter lung morphology and caspase-dependent hematopoietic cell apoptosis within the lung interstitium play roles in GBS pathophysiology in this model.

Factors influencing fetal macrophage development: III. Immunocytochemical localization of cytokines and time-resolved expression of differentiation markers in organ-cultured rat lungs

BACKGROUND

Exogenous TNF alpha, IL-1 beta, M-CSF, and GM-CSF all stimulate growth of macrophages arising in explanted fetal rat lungs. The present study examines the intrinsic availability of these factors in intact and organ-cultured lungs and utilizes expression of cytokines and marker proteins to explore the differentiation pathway followed by phagocytes in vitro.

METHODS

Factors and markers were localized immunocytochemically in paraffin sections of 14- and 15-day fetal rat lungs and lungs organ-cultured up to 7 days on serum-containing medium solidified with agar. Western analyses for the cytokines were performed on lysates of whole 15-day lungs, and in situ hybridization of M-CSF receptor mRNA was carried out in sections of 14 + 2 day cultured lung.

RESULTS

IL-1 beta, M-CSF, and GM-CSF were demonstrated in the stroma of intact and cultured lungs by immunostaining, results confirmed by Western blotting. TNF alpha appeared to be absent. A few precursors (angular cells) expressed the macrophage lineage marker RM-1 as early as day 14, and immunostaining became stronger and more widespread as the population matured and expanded in cultures. The OX-6 antibody to Ia antigen first reacted with macrophages in 14 + 1 day explants, and within a week 50% of cells were positive. M-CSF and mRNA for its receptor were present at 14 + 2 days, as was PDGF, which had been demonstrated in the stroma and epithelium prior to explantation. Definite reactivity for IL-1 beta and GM-CSF followed at 14 + 4 and 14 + 5 days.

CONCLUSIONS

M-CSF, GM-CSF, and IL-1 beta, but not TNF alpha, are available to replicating angular cells before and during their conversion to phagocytes. Fetal lungs thus qualify as a hematopoietic tissue supportive of macrophages. The path of differentiation pursued in organ cultures involves early expression of structural elements (RM-1, Ia antigen) followed by synthesis of cytokines of the TNF alpha cascade. Immunostaining for both RM-1 and OX-6 suggests that fetal lung macrophages share a common heritage with antigen-presenting pulmonary dendritic cells.

Factors influencing fetal macrophage development: I. Reactions of the tumor necrosis factor-alpha cascade and their inhibitors

BACKGROUND

When fetal rat lungs are explanted to organ culture, precursor angular cells soon convert to nascent macrophages that multiply rapidly as they mature into efficient phagocytes. The present study examines the influence of proinflammatory early cytokines of the tumor necrosis factor-alpha (TNF alpha) cascade on this initial expression of the macrophage phenotype.

METHODS

Fourteen- and 15-day fetal rat lungs were grown for varying periods on an agar-solidified medium with and without test factors added singly or in combination. Growth of the macrophage population was followed daily by light microscopy and quantified by measuring the area of coronas formed as cells emerged from explants.

RESULTS

TNF alpha interleukin-1 beta (IL-1 beta) stimulated growth of the macrophage population, as had macrophage- and granulocyte-macrophage colony-stimulating factors (M- and GM-CSFs) in prior studies. Inhibition was obtained by exposure to IL-1 receptor antagonist and antibodies neutralizing the CSFs. Only the effects of TNF alpha were sufficiently delayed to discount possible influence on conversion and growth of nascent macrophages. Two transcription blockers, dexamethasone and pyrrolidine dithiocarbamate (PDTC), an inhibitor of nuclear factor NF-kappa B, both profoundly suppressed macrophage growth without preventing conversion of precursors. Effects of dexamethasone were significantly ameliorated by IL-1 beta alone and combined with GM-CSF; those of PDTC were mitigated by M-CSF and a combination of IL-1 beta and TNF alpha but not by GM-CSF.

CONCLUSIONS

IL-1 beta, M-CSF, and GM-CSF all promote growth of the young macrophage population. TNF alpha is effective only later on, likely because early-stage cells lack its receptors which normally use intracellular signalling pathways similar to those for IL-1. The severity of PDTC inhibition to population growth indicates that NF-kappa B is important for transmitting proliferative signals in these cells.

Factors influencing fetal macrophage development: II. Effects of the PDGF subfamily of protein-tyrosine kinase receptor ligands as studied in organ-cultured rat lungs

BACKGROUND

Macrophage precursors in pseudoglandular rat lungs rapidly differentiate into phagocytes in organ culture, although this occurs only gradually in vivo. Macrophage colony-stimulating factor is vital for the process, but the possible importance of other ligands in the platelet-derived growth factor (PDGF) subfamily is scarcely appreciated.

METHODS

Macrophage development was compared in 15-day fetal rat lungs cultured on solid, serum-containing media with and without added stem cell factor (SCF) (100 ng/mL) or antibodies to PDGF-AA and -BB (10-15 micrograms/mL each). In addition, organ cultures and intact lungs were immunostained for PDGF-AA and -BB to confirm their presence in the tissues. Macrophage population growth was measured by coronal area assay.

RESULTS

SCF initially stimulated macrophage production. Thereafter, results varied depending on baseline production by control cultures: where this was vigorous, SCF-exposed explants performed similarly; where this was moderate, the SCF explants outperformed them 1.5-2.6 times over (P < 0.01-0.001). Inhibition of macrophage production by pyrrolidine dithiocarbamate (100 microM) was not significantly diminished in the presence of SCF (10 ng/mL). Immunoreactivity for PDGF-AA and -BB was prevalent in cells of the airway epithelium and stroma during the period macrophage precursors were converting, and both isoforms were detected in differentiating macrophages as early as 2 days in vitro. Nonetheless, exposure of cultures to anti-PDGFs had no significant effect on macrophage population growth.

CONCLUSIONS

Ligands of the PDGF subfamily differ greatly in their influence over development of fetal macrophages. Whereas the PDGFs are ineffective, SCF stimulates growth of macrophage precursors and early differentiating forms and enhances survival of older cells. It appears to act mainly in synergy with other growth factors present in fetal lungs. Furthermore, in the hierarchy of hematopoietic progenitors, the macrophage precursors may be ranked on a par with burst-forming units in the red cell lineage.

Precursors of macrophages in embryonic rat lungs fail to exhibit granulocyte-forming potential

BACKGROUND

Mesenchyme-like macrophage (M) precursors called angular cells are present in rat lungs on the thirteenth day of gestation and by then can differentiate into outright macrophages. Based on studies of bone marrow-derived cells, it is widely believed that the macrophage line necessarily proceeds from a colony-forming unit with dual granulocyte-macrophage potential (CFU-GM). In embryos this seems doubtful since macrophages are already scattered throughout the body before the first granulocytes appear. We examined the question in organ cultured 14 day prenatal rat lungs after having shown earlier that the macrophage population developed in explants is increased by exposure to M- and GM-colony-stimulating factors (CSFs) but is unaffected by multi (IL-3)- or granulocyte (G)-CSF. Reportedly retinoic acid (RA) shifts CFU-GM strongly towards granulocytic differentiation and inhibits mitosis of unipotential macrophage precursors but not differentiated cells. Transforming growth factor beta 1 (TGF) inhibits multipotential blood progenitors but allows proliferation of committed precursors, and TGF together with GM-CSF induces granulocytopoiesis from CFU-GM.

METHODS

Lung pairs were grown on a serum-containing medium or one supplemented either by RA, TGF, or TGF/GM-CSF to form a control and three experimental groups. A fourth experiment compared responses to M-CSF exposure and M-CSF/TGF. Macrophage population growth was estimated by measuring the areas of coronas formed by macrophages emerged from the explants. F-actin was stained with fluorescein-labeled phalloidin.

RESULTS

In all experiments macrophages were produced unmixed with granulocytes. By +8 days they had largely emerged to form coronas about the lungs. In cultures exposed to RA, macrophages were less intensely stained for actin and slower to emerge than controls. At +8 days, however, coronal areas were not significantly different from controls, as was also true for the TGF group. In contrast, coronal areas of cultures grown with TGF/GM-CSF were much larger. At +17 days, mean coronal area of TGF cultures was about half that of controls (P < 0.05), whereas mean coronal area of the TGF/GM-CSF group was 5.4 times greater (P < 0.001). Macrophages from control and TGF-exposed cultures responded to M-CSF by an increase in coronal area which was greater among cultures given M-CSF alone than those given TGF + M-CSF (both P < 0.005).

CONCLUSIONS

Macrophage precursors in embryonic lungs are distinct from CFU-GM.

Exogenous cytokines enhance survival of macrophages from organ cultured embryonic rat tissues

BACKGROUND

Macrophage precursors are present in embryonic rats shortly after the onset of hematopoiesis. During organogenesis they soon establish residency in many parts of the body and become convertible into phagocytes, at first gaining morphological characteristics of macrophages and later a range of surface antigens used to characterize subpopulations in adults. Nonetheless, it is uncertain whether representatives of this fetal lineage continue to exist past birth. We investigated the question indirectly by seeing if such cells can be made to survive in vitro to an age equivalent to adulthood and by examining underlying conditions that favor this outcome.

METHODS

Fourteen-day embryonic lungs, hearts, and limb buds were organ cultured on a firm serum-containing medium. Fetal macrophages developed within all explants and then migrated out to form a corona of cells surrounding each explant. The lung cultures were selected for subsequent work which mainly used coronal area as the measure of macrophage population size in experimental and control groups. Baseline growth and survival of macrophages were established for cultures grown on standard medium, then effects of the following were examined: indomethacin (10(-6) M) as it influences initial production of macrophages from precursors and later survival of differentiated cells; and macrophage colony-stimulating factor (M-CSF), used alone at moderate dosage (50-100 U), and combined with granulocyte-macrophage CSF (both 200 U), for its importance to long-term survival of the population. Mitogenic influence of M-CSF on differentiated macrophages was demonstrated by uptake of 5-bromo-2'-deoxyuridine.

RESULTS

Indomethacin inhibited the formation of macrophages from precursors but enhanced the survival of differentiated cells. M-CSF increased BrdU uptake of differentiated macrophages and permitted coronal growth to continue long past the approximately 30 day limit of controls. Beyond this interval, M-CSF was essential for macrophage survival, since coronas quickly shrank after the cytokine was withdrawn. Administration of the M-CSF/GM-CSF mixture to the 2 oldest M-CSF-exposed cultures between 98 and 127 days in vitro resulted in an increase in the number of coronal macrophages (P < 0.001); withdrawal between 129 and 140 days led to a decrease (P < 0.005). Ultimately a few cells were still surviving at 183 days.

CONCLUSIONS

Intrinsic factors promote early formation of macrophages within the explants, but the availability of factors is lessened by the anti-inflammatory action of indomethacin. Its later promotion of macrophage survival may be based on suppression of autogenous prostaglandin (PGE2) synthesis. M-CSF greatly promotes macrophage survival; in context this is sufficient to show that the fetal macrophage line has a clear potential to survive well into adulthood.

Early development of macrophages in intact and organ cultured hearts of rat embryos

BACKGROUND

Macrophage precursors are present early in embryonic life, being demonstrable in placental and embryonic connective tissues of rats at the neurula stage and as potential macrophages in the brain, liver, and lungs near the onset of organogenesis. We examined the development of macrophages in the heart and the possibility that they initially appear at sites of programmed cell death (apoptosis).

METHODS

Precursors were recognized by the binding of peroxidase-coupled Griffonia simplicifolia isolectin B4 (GSA) on the cell membrane. Their capacity for conversion into macrophages was assayed in organ cultures; confirmation of the progeny as bona fide macrophages was obtained from their responses to particle exposure and macrophage colony-stimulating factor (M-CSF).

RESULTS

GSA+cells were first seen on gestational day 12 (4 mm embryos) as 2-3 cycling, nonvacuolated cells located in cardiac tissue outside the blood vessels. This population increased to approximately 12 cells by day 14 (9 mm embryos). Two-thirds were distributed along the bulbus cordis in the jellylike endocardium and a more densely cellular connective tissue closer to the aortic arches where apoptotic sites are expected to develop. Such sites were not found in serial glycol methacrylate sections through our 14-day specimens, although in whole heart explants of this age an area of necrosis developed along the prospective line of bulbar endocardial fusion on the second day of organ culturing, and by then macrophages were fairly abundant. Organ culturing of 13-day embryonic hearts also yielded large, highly vacuolated, GSA+mononuclear phagocytes. After a few days in culture most of the macrophages migrated onto the medium where they formed a tight corona of cells about the explants. They readily ingested iron oxide particles and concentrated supravitally administered neutral red in their vacuoles. Macrophages from 14-day cultures exposed to M-CSF developed significantly larger coronas than macrophages from explants grown in serum-rich control medium (p < 0.001). In the presence of cytokines, moreover, these cardiac macrophages survived as many as 100 (92 "postnatal") days.

CONCLUSIONS

Macrophage precursors first appear in embryonic rat hearts well before they are needed to clear debris generated by programmed cell death and are capable of rapid conversion into outright phagocytic cells as early as the 13th prenatal day.

Macrophage development: IV. Effects of blood factors on macrophages from prenatal rat lung cultures

Effects of colony-stimulating factors M-CSF, GM-CSF, G-CSF, and IL-3 were assessed on cells of macrophage lineage present in organ cultured 14-day prenatal rat lungs. Treatment groups were compared between one another and against control lungs grown on standard medium containing 40% fetal bovine serum without added factors, where a monoculture of macrophages rapidly develops from precursors present at explantation, leading to appearance of a large mature population on the pleural surface outside the lungs. Studies were carried out in living cultures and by light and electron microscopy using peroxidase-coupled isolectin B4 of Griffonia simplicifolia to identify macrophages and their precursors. In the first experiment, 14-day prenatal lung explants (14 + 0 days) containing macrophage precursors but not matured cells were exposed to individual CSFs for 7 days in an attempt to determine whether precursors are committed irrevocably to the macrophage line or can be altered by exposure to factors promoting significant granulocyte development. In succeeding experiments, 4- and 7-day-old cultures (14 + 4, 14 + 7 days) containing matured macrophages were targeted to see whether macrophage survival can be extended beyond expectations in controls and whether mitotic activity is stimulated. Recombinant CSFs were used at dosage levels known to promote colony formation in vitro (200-1,000 CFU/ml). Cultures exposed from prenatal day 14 to M-, GM-, G-CSF, or IL-3 yielded a monoculture of macrophages without exception. Populations developed in the presence of M- or GM-CSF were much larger than in controls or cultures grown with the other blood factors. GM-CSF-exposed cultures produced by far the largest macrophages, among them many multinucleate giant cells. Macrophages developed in the presence of G-CSF were also significantly larger than controls. Growth of the mature macrophage population was greatly stimulated by exposure to M-CSF or GM-CSF but not by IL-3 or G-CSF. Mitotic figures were noted in the coronas of emerged cells surrounding stimulated cultures, compared to none in the controls. Ultrastructurally, macrophages stimulated by M-CSF retained a mature appearance like macrophages in control, IL-3, and G-CSF treatment groups, whereas many in the GM-CSF group became less differentiated. As to long-term survival, a single 14-day explant was grown for 8 days on standard medium (the equivalent date for birth), then placed in a soft agar medium containing M-CSF.(ABSTRACT TRUNCATED AT 400 WORDS)

Macrophage development: I. Rationale for using Griffonia simplicifolia isolectin B4 as a marker for the line

The isolectin B4 of Griffonia simplicifolia (GSA I-B4) binds to cell membrane glycoconjugates bearing terminal alpha-D-galactose, which macrophages possess. We have investigated the merits of its use as a marker for cells of this lineage when examining the early origin of macrophage populations in rat embryos, the stages and time scale of transformation from precursor forms to active, matured cells, and the response of precursors and macrophages to colony-stimulating blood factors, the last two studies conducted in organ cultures of prenatal lungs. In the present instance, GSA I-B4 was used either coupled with fluorescein (FITC) for light microscopy of living and fixed cells, or with peroxidase for light or electron microscopy. Control incubations of lung culture-derived macrophages proved that staining resulted from specific binding to galactosyl units on the cell membrane, since it was competitively inhibited by alpha-D-galactose. The lectin binds to few cells in 14-day prenatal lung explants but to a great many macrophages that subsequently develop in the cultures, indicating that it can be relied on for quantitative studies on population growth; however, it is important to provide reagents with good access to the cells. Apart from macrophages and their precursors, virtually no cells in prenatal lung cultures bind this lectin. Granulocytes of adult blood are GSA positive, but they are not yet present in 14-day prenatal explants and do not develop subsequent to culturing; hence they are not a source of confusion for experimental studies using this system. Precursors of granulocytes begin to appear in rat embryos around day 13 and have GSA-positive cell membranes, but like definitive granulocytes they also have conspicuous peroxidase-positive lysosomal granules which serve to distinguish them from early macrophages, particularly when cells are studied at an ultrastructural level. With these objections cleared away, GSA I-B4 emerges as a valuable means to mark cells of the macrophage line, mature or immature.

Macrophage development: II. Early ontogeny of macrophage populations in brain, liver, and lungs of rat embryos as revealed by a lectin marker

Earliest origins of macrophage populations in the central nervous system, the liver, and the lungs were studied in rat embryos aged between 10.5-11 days and 14 days of gestation, based on light and electron microscopic identification of macrophages using peroxidase-coupled isolectin B4 of Griffonia simplicifolia (GSA I-B4), which recognizes alpha-D-galactose groups on the cell membrane. During embryonic life macrophages and their precursors are GSA I-B4-positive and generally bereft of peroxidase-positive granules. At 10.5 days the yolk sac and embryonic circulations have just become joined, the brain has five vesicles but nerve cells are little differentiated, the liver exists as a diverticulum of the gut with fingerlike extensions of hepatocytes, and the lungs as a laryngotracheal groove. Macrophages and/or their precursors occurred in small numbers in embryonic mesenchyme and blood vessels but showed no special affinity for either liver or lung rudiments. The developing brain was the first organ to be colonized, beginning on prenatal day 12. The liver followed between days 12 and 13 and was succeeded by the lungs, beginning between days 13 and 14. Dividing macrophages were present in these organs at the outset of colonization and throughout the duration of the embryo series, indicating that from the beginning, replication of resident cells contributes to growth of the local population. Granulocyte precursors were first apparent in the liver around day 13; they are also GSA-positive but are distinguished from macrophages by their content of peroxidase-positive granules. Organ cultures of 13-day liver and lungs, and 14-day brain tissue, indicate that whereas isolated liver fragments support the formation of both granulocytes and macrophages, only the latter develop in brain or lung cultures. A resident population of macrophages evidently is set up very early in these organs, well before white cells colonize the spleen, bone marrow, and other future blood forming regions. The events outlined are seen as stages in an embryo-wide process that leads to establishment of macrophage populations in various organs.