Immunophenotypic and ultrastructural heterogeneity of macrophage differentiation in bone marrow and fetal hematopoiesis of mouse in vitro and in vivo

Pre-Existing Maternal Antibodies Cause Rapid Prenatal Rejection of Allotransplants in the Mouse Model of In Utero Hematopoietic Cell Transplantation

In utero hematopoietic cell transplantation (IUHCT) is a nonmyeloablative nonimmunosuppressive alternative to postnatal hematopoietic stem cell transplantation for the treatment of congenital hemoglobinopathies. Anti-HLA donor-specific Abs (DSA) are associated with a high incidence of graft rejection following postnatal hematopoietic stem cell transplantation. We determine if DSA present in the mother can similarly cause graft rejection in the fetus following IUHCT. Ten million C57BL/6 (B6, H2k^b) bone marrow cells were transplanted in utero into gestational day 14 BALB/c (H2k^d) fetuses. The pregnant BALB/c dams carrying these fetuses either had been previously sensitized to B6 Ag or were injected on gestational days 13-15 with serum from B6-sensitized BALB/c females. Maternal-fetal Ab transmission, Ab opsonization of donor cells, chimerism, and frequency of macrochimeric engraftment (chimerism >1%) were assessed by flow cytometry. Maternal IgG was transmitted to the fetus and rapidly opsonized donor cells following IUHCT. Donor cell rejection was observed as early as 4 h after IUHCT in B6-sensitized dams and 24 h after IUHCT in dams injected with B6-sensitized serum. Efficient opsonization was strongly correlated with decreased chimerism. No IUHCT recipients born to B6-sensitized dams or dams injected with B6-sensitized serum demonstrated macrochimeric engraftment at birth compared with 100% of IUHCT recipients born to naive dams or dams injected with naive serum (p < 0.001). In summary, maternal donor-specific IgG causes rapid, complete graft rejection in the fetus following IUHCT. When a third-party donor must be used for clinical IUHCT, the maternal serum should be screened for DSA to optimize the chance for successful engraftment.

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.

In situ proliferation and differentiation of macrophages in dental pulp

The presence of macrophages in dental pulp is well known. However, whether these macrophages proliferate and differentiate in the dental pulp in situ, or whether they constantly migrate from the blood stream into the dental pulp remains unknown. We have examined and compared the development of dental pulp macrophages in an organ culture system with in vivo tooth organs to clarify the developmental mechanism of these macrophages. The first mandibular molar tooth organs from ICR mice aged between 16 days of gestation (E16) to 5 days postnatally were used for in vivo experiments. Those from E16 were cultured for up to 14 days with or without 10% fetal bovine serum. Dental pulp tissues were analyzed with immunohistochemistry to detect the macrophages and with reverse transcription and the polymerase chain reaction (RT-PCR) for the detection of factors related to macrophage development. The growth curves for the in vivo and in vitro cultured cells revealed similar numbers of F4/80-positive macrophages in the dental pulp. RT-PCR analysis indicated the constant expression of myeloid colony-stimulating factor (M-CSF) in both in-vivo- and in-vitro-cultured dental pulp tissues. Anti-M-CSF antibodies significantly inhibited the increase in the number of macrophages in the dental pulp. These results suggest that (1) most of the dental pulp macrophages proliferate and differentiate in the dental pulp without a supply of precursor cells from the blood stream, (2) M-CSF might be a candidate molecule for dental pulp macrophage development, and (3) serum factors might not directly affect the development of macrophages.

Endogenous oncogenic Nras mutation promotes aberrant GM-CSF signaling in granulocytic/monocytic precursors in a murine model of chronic myelomonocytic leukemia

Oncogenic NRAS mutations are frequently identified in myeloid diseases involving monocyte lineage. However, its role in the genesis of these diseases remains elusive. We report a mouse bone marrow transplantation model harboring an oncogenic G12D mutation in the Nras locus. Approximately 95% of recipient mice develop a myeloproliferative disease resembling the myeloproliferative variant of chronic myelomonocytic leukemia (CMML), with a prolonged latency and acquisition of multiple genetic alterations, including uniparental disomy of oncogenic Nras allele. Based on single-cell profiling of phospho-proteins, a novel population of CMML cells is identified to display aberrant granulocyte-macrophage colony stimulating factor (GM-CSF) signaling in both the extracellular signal-regulated kinase (ERK) 1/2 and signal transducer and activator of transcription 5 (Stat5) pathways. This abnormal signaling is acquired during CMML development. Further study suggests that aberrant Ras/ERK signaling leads to expansion of granulocytic/monocytic precursors, which are highly responsive to GM-CSF. Hyperactivation of Stat5 in CMML cells is mainly through expansion of these precursors rather than up-regulation of surface expression of GM-CSF receptors. Our results provide insights into the aberrant cytokine signaling in oncogenic NRAS-associated myeloid diseases.

Macrophage diversity in renal injury and repair

Monocyte-derived macrophages can determine the outcome of the immune response and whether this response contributes to tissue repair or mediates tissue destruction. In addition to their important role in immune-mediated renal disease and host defense, macrophages play a fundamental role in tissue remodeling during embryonic development, acquired kidney disease, and renal allograft responses. This review summarizes macrophage phenotype and function in the orchestration of kidney repair and replacement of specialized renal cells following injury. Recent advances in our understanding of macrophage heterogeneity in response to their microenvironment raise new and exciting therapeutic possibilities to attenuate or conceivably reverse progressive renal disease in the context of fibrosis. Furthermore, parallels with pathological processes in many other organs also exist.

Src-family kinases play an essential role in differentiation signaling downstream of macrophage colony-stimulating factor receptors mediating persistent phosphorylation of phospholipase C-gamma2 and MAP kinases ERK1 and ERK2

Macrophage colony-stimulating factor (M-CSF) has been found to be involved in multiple developmental processes, especially production of cells belonging to the mononuclear phagocyte system. The decision of myeloid progenitor cells to commit to differentiation depends on activation levels of the mitogen-activated protein kinases (MAPK), ERK1 and ERK2. Using the murine myeloid progenitor cell line FD-Fms, we show here that persistent activity of Src-family kinases (SFK) is necessary for FD-Fms cell differentiation to macrophages in response to M-CSF. Chemical inhibition of SFK blocked FD-Fms cell differentiation while it caused strong inhibition of the late phosphorylation of phospholipase C (PLC)-gamma2 and MAPK. The PLC inhibitor U73122, previously shown to block M-CSF-induced differentiation, strongly decreased long-term MAPK phosphorylation. Interestingly, inhibiting SFK with SU6656 or the MAPK kinases MEK with U0126 significantly impaired development of mononuclear phagocytes in cultures of mouse bone marrow cells stimulated with M-CSF. Collectively, results support a model in which SFK are required for sustained PLC activity and MAPK activation above threshold required for commitment of myeloid progenitors to macrophage differentiation.

Phenotypic comparison of multiple monocyte-related populations in murine peripheral blood and bone marrow

BACKGROUND

Monocyte subsets are not well defined in murine peripheral blood (PB). Monocyte-related populations could also be located in bone marrow (BM), but studies correlating monocyte populations found in these two tissues are lacking. This study simultaneously analyzed PB and BM to phenotypically define multiple monocyte-related subsets in each location.

METHODS

Murine PB and BM cells were simultaneously stained for monocyte-related populations, using five-color flow cytometry. Relevant subsets were defined on the basis of Ly-6C, CD11b, and wheat germ agglutinin phenotype in addition to light-scatter characteristics. These populations were extensively characterized for the expression of other myeloid and dendritic cell markers, adhesion molecules, chemokine receptors, and growth factor receptors.

RESULTS

Six monocyte-related populations were defined, three each in BM and PB. No identical populations were found between the two tissues. Two BM populations and one PB population have heterogeneous expression of many markers, suggesting additional complexity among monocyte-related subsets.

CONCLUSIONS

The murine monocytic series comprises multiple subsets, differing between PB and BM. This study defines and extensively phenotypes six of these populations, providing preliminary information about possible developmental relationships and migratory capacities of these cells.

Absence of erythroblast macrophage protein (Emp) leads to failure of erythroblast nuclear extrusion

In mammals, the functional unit for definitive erythropoiesis is the erythroblastic island, a multicellular structure composed of a central macrophage surrounded by developing erythroblasts. Erythroblast-macrophage interactions play a central role in the terminal maturation of erythroblasts, including enucleation. One possible mediator of this cell-cell interaction is the protein Emp (erythroblast macrophage protein). We used targeted gene inactivation to define the function of Emp during hematopoiesis. Emp null embryos die perinatally and show profound alterations in the hematopoietic system. A dramatic increase in the number of nucleated, immature erythrocytes is seen in the peripheral blood of Emp null fetuses. In the fetal liver virtually no erythroblastic islands are observed, and the number of F4/80-positive macrophages is substantially reduced. Those present lack cytoplasmic projections and are unable to interact with erythroblasts. Interestingly, wild type macrophages can bind Emp-deficient erythroblasts, but these erythroblasts do not extrude their nuclei, suggesting that Emp impacts enucleation in a cell autonomous fashion. Previous studies have implicated the actin cytoskeleton and its reorganization in both erythroblast enucleation as well as in macrophage development. We demonstrate that Emp associates with F-actin and that this interaction is important in the normal distribution of F-actin in both erythroblasts and macrophages. Thus, Emp appears to be required for erythroblast enucleation and in the development of the mature macrophages. The availability of an Emp null model provides a unique experimental system to study the enucleation process and to evaluate the function of macrophages in definitive erythropoiesis.

PERIPHERAL HEMOPHAGOCYTOSIS

Peripheral hemophagocytosis (PHP) is seen in patients with hemophagocytic syndrome (HPS), a clinical status in which activated macrophages play a role in its pathogenesis. The inflammatory state, systemic inflammatory response syndrome (SIRS), is also associated with activated macrophages. However, the link between HPS and SIRS and the clinical implications of PHP remain to be determined. In the present work, we examined the clinical utility and impact of the detection of PHP and the link between HPS and SIRS. We studied the clinical and laboratory profiles of 322 SIRS patients (174 men; mean age, 68 +/- 22 years; range, 16-99 years) who visited an urban hospital specializing in respiratory, cardiovascular, digestive, renal diseases, general surgery, and orthopedics in Japan. Peripheral hemophagocytosis was detected in 40 (23 men; mean age, 81.3 +/- 8.7 years; range, 63-98 years) of 322 patients on 3 +/- 2 days after SIRS diagnosis as determined with a "blunt-edged-smear" method differing from the conventional "feather-edged smear" method. The incidence of advanced SIRS and ensuing death in the SIRS+ PHP- group (37 and 21 of 40, respectively) was significantly greater than in the SIRS+ PHP- group (82 and 17 of 282) (P < 0.01). The duration from SIRS diagnosis to recovery in 19 SIRS+ PHP- surviving patients (26 +/- 18 days) was longer than that in 19 age-matched SIRS+ PHP- surviving patients who initially presented comparable clinical profiles (6 +/- 3 days) (P < 0.001). Bone marrow analysis in all 7 patients having PHP and SIRS showed no HPS initially (<3% hemophagocytes), but all subsequently developed HPS at 7 +/- 1 days after the diagnosis, confirmed by the presence of 9% +/- 13% hemophagocytes in the bone marrow. Electron microscopic and immunohistochemical analyses revealed that PHP was derived from hemophagocytes in the bone marrow. The present data strongly suggest that PHP detection could serve as an early indicator for advanced SIRS and/or HPS and that the use of the blunt-edged method is preferable for PHP detection.

Retinoblastoma promotes definitive erythropoiesis by repressing Id2 in fetal liver macrophages

In mammals, the fetal liver is the first site of definitive erythropoiesis-the generation of mature, enucleated red cells. The functional unit for definitive erythropoiesis is the erythroblastic island, a multicellular structure composed of a central macrophage surrounded by erythroblasts at various stages of differentiation. Targeted disruption of the retinoblastoma (Rb) tumour suppressor gene in the mouse leads to embryonic death caused by failure of erythroblasts to enucleate. The erythroid defect has been attributed to loss of Rb in cells that support erythropoiesis, but the identity of these cells is unknown. Here we show that Rb-deficient embryos carry profound abnormalities of fetal liver macrophages that prevent physical interactions with erythroblasts. In contrast, wild-type macrophages bind Rb-deficient erythroblasts and lead them to terminal differentiation and enucleation. Loss of Id2, a helix-loop-helix protein that mediates the lethality of Rb-deficient embryos, rescues the defects of Rb-deficient fetal liver macrophages. Rb promotes differentiation of macrophages by opposing the inhibitory functions of Id2 on the transcription factor PU.1, a master regulator of macrophage differentiation. Thus, Rb has a cell autonomous function in fetal liver macrophages, and restrains Id2 in these cells in order to implement definitive erythropoiesis.

Theiler's virus infection of primary cultures of bone marrow-derived monocytes/macrophages

Theiler's virus, a murine picornavirus, causes a persistent infection of macrophage/microglial cells in the central nervous systems of SJL/J mice. Viral replication is restricted in the majority of infected cells, whereas a minority of them contain large amounts of viral RNA and antigens. For the present work, we infected primary cultures of bone marrow monocytes/macrophages from SJL/J mice with Theiler's virus. During the first 10 h postinfection (p.i.), infected monocytes/macrophages were round and covered with filopodia and contained large amounts of viral antigens throughout their cytoplasm. Later on, they were large, flat, and devoid of filopodia and they contained only small amounts of viral antigens distributed in discrete inclusions. These two types of infected cells were very reminiscent of the two types of infected macrophages found in the spinal cords of SJL/J mice. At the peak of virus production, the viral yield per cell was approximately 200 times lower than that for BHK-21 cells. Cell death occurred in the culture during the first 24 h p.i. but not thereafter. No infected cells could be detected after 4 days p.i., and the infection never spread to 100% of the cells. This restriction was unchanged by treating the medium at pH 2 but was abolished by treating it with a neutralizing alpha/beta interferon antiserum, indicating a role for this cytokine in limiting virus expression in monocyte/macrophage cultures. The role of alpha/beta interferon was confirmed by the observation that monocytes/macrophages from IFNA/BR(-/-) mice were fully permissive.

Constitutive retinal CD200 expression regulates resident microglia and activation state of inflammatory cells during experimental autoimmune uveoretinitis

Recent evidence supports the notion that tissue OX2 (CD200) constitutively provides down-regulatory signals to myeloid-lineage cells via CD200-receptor (CD200R). Thus, mice lacking CD200 (CD200(-/-)) show increased susceptibility to and accelerated onset of tissue-specific autoimmunity. In the retina there is extensive expression of CD200 on neurons and retinal vascular endothelium. We show here that retinal microglia in CD200(-/-) mice display normal morphology, but unlike microglia from wild-type CD200(+/+) mice are present in increased numbers and most significantly, express inducible nitric oxide synthase (NOS2), a macrophage activation marker. Onset and severity of uveitogenic peptide (1-20) of interphotoreceptor retinoid-binding protein-induced experimental autoimmune uveoretinitis is accelerated in CD200(-/-) mice and although tissue destruction appears no greater than seen in CD200(+/+) mice, there is continued increased ganglion and photoreceptor cell apoptosis. Myeloid cell infiltrate was increased in CD200(-/-) mice during experimental autoimmune uveoretinitis, although NOS2 expression was not heightened. The results indicate that the CD200:CD200R axis regulates retinal microglial activation. In CD200(-/-) mice the release of suppression of tonic macrophage activation, supported by increased NOS2 expression in the CD200(-/-) steady state accelerates disease onset but without any demonstration of increased target organ/tissue destruction.

Flow cytometric analysis of PKH26-labeled goldfish kidney-derived macrophages

We recently demonstrated that a goldfish macrophage cell line (GMCL) and primary in vitro-derived kidney macrophage (IVDKM) cultures contain three distinct macrophage subpopulations. Morphological, cytochemical, functional, and flow cytometric characterization of these sub-populations suggested that they may represent cells of the macrophage lineage temporally arrested at distinct differentiation junctures of fish macrophage development (putative early progenitors, monocytes, and macrophages). In this study, we examined the proliferation and differentiation events leading to the generation of mature macrophage-like cells from goldfish kidney hematopoietic tissues. The flow cytometric studies were done after labeling macrophages with PKH26 fluorescent dye and analysis of the data using the MODFIT software. Our results showed that IVDKM cultures proliferated non-synchronously, suggesting the presence of a temporal control mechanism regulating the number of cells entering the paths towards maturation. Such control is most evident during early progenitor proliferation and differentiation events. Our results showed that proliferation may not be a requirement for differentiation of early progenitors to putative monocyte and macrophage subsets. Detailed observation of the mature macrophage-like subpopulation indicated that: 1) they appear to develop from both, the differentiation of monocyte-like cells, and direct differentiation of early progenitors in the absence of a monocyte-like stage; and (2) mature macrophage-like cells appeared to be capable of self-proliferation. Our results suggest the presence of alternate pathways of fish macrophage development other than the classical hematopoietic pathway.

Origins and functions of phagocytes in the embryo

To review the data on the origins, phenotype, and function of embryonic phagocytes that has accumulated over past decade. Most of the relevant articles were selected based on the PubMed database entries. In additional, the Interactive Fly database (http://sdb.bio. purdue.edu/fly/aimain/1aahome.htm), FlyBase (http://flybase.bio. indiana.edu:82/), and TBase (http://tbase.jax.org/) were used to search for relevant information and articles. Phagocytes in a vertebrate embryo develop in two sites (yolk sac and liver) and contribute to organogenesis in part through their ability to recognize and clear apoptotic cells. Yolk sac-derived phagocytes differ in differentiation pathway and marker gene expression from macrophages produced via classic hematopoietic progenitors in the liver. We argue that yolk sac-derived phagocytes constitute a separate cell lineage. This conclusion raises the question of whether primitive phagocytes persist into the adulthood.

Fibroblast-Secreted Macrophage Colony-Stimulating Factor Is Responsible for Generation of Biphenotypic B/Macrophage Cells from a Subset of Mouse B Lymphocytes

Normal and malignant CD5+ B lymphocytes can develop macrophage-like characteristics. One stimulus of this phenotypic shift is culture of normal mouse splenic B lymphocytes with splenic fibroblasts or their conditioned media. These biphenotypic B/macrophage (B/Mφ) cells simultaneously display macrophage characteristics, such as phagocytosis and F4/80 expression, while retaining B cell features, including expression of surface Ig, CD5, B220, and rearranged Ig genes. The present study investigated the fibroblast-secreted factor that promotes this phenotypic change from B cell to B/Mφ cell. RT-PCR analysis demonstrated that mRNA for M-CSF is produced by splenic fibroblasts. Recombinant M-CSF (CSF-1) could replace fibroblast-conditioned medium to elicit the development and survival of B/Mφ cells from splenic B lymphocytes. In addition, neutralization of fibroblast-secreted M-CSF with specific mAbs abrogated the ability of conditioned supernatants to promote outgrowth of B/Mφ cells. The transition from B lymphocyte to B/Mφ cell was marked by the kinetic appearance of mRNA for the M-CSF receptor, c-fms, at day 3 following culture initiation. These results demonstrate that M-CSF is important in the development and physiology of mouse B/Mφ cells and potentially in the growth of human biphenotypic hematological malignancies. Interestingly, the presence of IFN-γ in splenic B lymphocyte cultures abrogated the effect of fibroblast-conditioned medium or M-CSF on outgrowth of B/Mφ cells. Furthermore, these findings suggest that a Th1 microenvironment favored by typical macrophages is detrimental to the outgrowth of B/Mφ cells.

Differential activity of the -2.7 kb chicken lysozyme enhancer in macrophages of different ontogenic origins is regulated by C/EBP and PU.1 transcription factors

Expression of the chicken lysozyme gene is upregulated during macrophage maturation. Recently, an additional regulatory feature was discovered: the gene is differentially expressed in macrophages of embryonic/fetal and adult origin. The lysozyme gene is only weakly expressed in mature embryo-derived macrophages, whereas there is a high level of expression in macrophages derived from adult animals. This finding provided a molecular tool to investigate the heretofore ill-defined differences between embryonic/fetal- and adult-type macrophages. We showed that the low expression in the embryo is associated with reduced activity of the myeloid-specific -2.7 kb lysozyme enhancer. Our protein-binding analyses and transfection studies demonstrated that this enhancer, in order to be fully active in activated macrophages, requires the combined action of C/EBPs, PU.1, and a third, as yet unidentified, protein binding to an AP-1-like site. Of these three, PU.1 and C/EBPs display significantly reduced nuclear DNA-binding activities in embryo-derived macrophages compared with adult-type cells. These results point to different roles of C/EBPs and PU.1 in embryonic/fetal and adult myelopoiesis.

Differentiation of the Mononuclear Phagocyte System During Mouse Embryogenesis: The Role of Transcription Factor PU.1

During mouse embryogenesis, macrophage-like cells arise first in the yolk sac and are produced subsequently in the liver. The onset of liver hematopoiesis is associated with the transition from primitive to definitive erythrocyte production. This report addresses the hypothesis that a similar transition in phenotype occurs in myelopoiesis. We have used whole mount in situ hybridization to detect macrophage-specific genes expressed during mouse development. The mouse c-fms mRNA, encoding the receptor for macrophage colony-stimulating factor (CSF-1), was expressed on phagocytic cells in the yolk sac and throughout the embryo before the onset of liver hematopoiesis. Similar cells were detected using the mannose receptor, the complement receptor (CR3), or the Microphthalmia transcription factor (MITF) as mRNA markers. By contrast, other markers including the F4/80 antigen, the macrophage scavenger receptor, the S-100 proteins, S100A8 and S100A9, and the secretory product lysozyme appeared later in development and appeared restricted to only a subset of c-fms–positive cells. Two-color immunolabeling on disaggregated cells confirmed that CR3 and c-fmsproteins are expressed on the same cells. Among the genes appearing later in development was the macrophage-restricted transcription factor, PU.1, which has been shown to be required for normal adult myelopoiesis. Mice with null mutations in PU.1 had normal numbers of c-fms–positive phagocytes at 11.5dpc. PU.1(−/−) embryonic stem cells were able to give rise to macrophage-like cells after cultivation in vitro. The results support previous evidence that yolk sac–derived fetal phagocytes are functionally distinct from those arising in the liver and develop via a different pathway.

Commitment to the Monocytic Lineage Occurs in the Absence of the Transcription Factor PU.1

Mice homozygous for the disruption of the PU.1 (Spi-1) gene do not produce mature macrophages. In determining the role of PU.1 in macrophage differentiation, the present study investigated whether or not there was commitment to the monocytic lineage in the absence of PU.1. Early PU.1−/− myeloid colonies were generated from neonate liver under conditions that promote primarily macrophage and granulocyte/macrophage colonies. These PU.1−/− colonies were found to contain cells with monocytic characteristics as determined by nonspecific esterase stain and the use of monoclonal antibodies that recognize early monocyte precursors, including Moma-2, ER-MP12, ER-MP20, and ER-MP58. In addition, early myeloid cells could be grown from PU.1−/− fetal liver cultures in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF). Similar to the PU.1 null colonies, the GM-CSF–dependent cells also possessed early monocytic characteristics, including the ability to phagocytize latex beads. The ability of PU.1−/− progenitors to commit to the monocytic lineage was also verified in vivo by flow cytometry and cytochemical analysis of primary neonate liver cells. The combined data shows that PU.1 is absolutely required for macrophage development after commitment to this lineage.

Commitment to the Monocytic Lineage Occurs in the Absence of the Transcription Factor PU.1

Mice homozygous for the disruption of the PU.1 (Spi-1) gene do not produce mature macrophages. In determining the role of PU.1 in macrophage differentiation, the present study investigated whether or not there was commitment to the monocytic lineage in the absence of PU.1. Early PU.1−/− myeloid colonies were generated from neonate liver under conditions that promote primarily macrophage and granulocyte/macrophage colonies. These PU.1−/− colonies were found to contain cells with monocytic characteristics as determined by nonspecific esterase stain and the use of monoclonal antibodies that recognize early monocyte precursors, including Moma-2, ER-MP12, ER-MP20, and ER-MP58. In addition, early myeloid cells could be grown from PU.1−/− fetal liver cultures in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF). Similar to the PU.1 null colonies, the GM-CSF–dependent cells also possessed early monocytic characteristics, including the ability to phagocytize latex beads. The ability of PU.1−/− progenitors to commit to the monocytic lineage was also verified in vivo by flow cytometry and cytochemical analysis of primary neonate liver cells. The combined data shows that PU.1 is absolutely required for macrophage development after commitment to this lineage.

Development, differentiation, and maturation of Kupffer cells

Primitive macrophages first develop in the murine and human yolk sac and then differentiate into fetal macrophages. Primitive or fetal macrophages enter the blood stream and migrate into the fetal liver. Fetal macrophages possess a high proliferative capacity and express antigens and peroxidase activity of resident macrophages with the progress of gestation; they become mature and then transform into Kupffer cells. In contrast, myelopoiesis and monocytopoiesis are not active in yolk sac hematopoiesis and in the early stages of hepatic hematopoiesis. Precursor cells of primitive or fetal macrophages exist and granulocyte/macrophage colony-forming cells develop in the yolk sac and in the early stages of fetal liver development, whereas macrophage colony-forming cells emerge and increase later in fetal liver development. In vitro, similar colonies were formed from each fetal hematopoietic cell in the presence of different macrophage growth factors. During culturing of the yolk sac cells and hepatic hematopoietic cells on a monolayer of mouse stromal cell line, ST2, primitive or fetal macrophage colonies developed before the formation of monocyte colonies, suggesting the existence of a direct pathway of differentiation from primitive macrophages into fetal macrophages during ontogeny. In severely monocytopenic mice induced by the administration of strontium-89, Kupffer cells have a proliferative capacity and are maintained by self-renewal. In macrophage colony-stimulating factor (M-CSF)-deficient (op/op) mice, the number of Kupffer cells is reduced, and they are characterized by immature morphology and a proliferative potential similar to that of primitive or fetal macrophages during ontogeny. Immediately after the administration of M-CSF to op/op mice, Kupffer cells start proliferating and become mature. This finding indicates that M-CSF plays an important role in the differentiation and proliferation of Kupffer cells.

In vitro differentiation of embryonic stem cells: immunophenotypic analysis of cultured embryoid bodies

The process of in vitro embryonic stem cell differentiation and embryoid body development was monitored using a panel of antibodies against surface markers traditionally associated with embryonic tissue (Forssman, SSEA-1) and hematopoietic progenitor cells (Fall-3, HSA, Sca-1, Thy-1.2, ER-MP12, CD45, AA4.1, and c-kit). All markers with the exception of CD45 and AA4.1 were initially detected in cultures of undifferentiated ES cells. During the first 11 days of differentiation, distinct and reproducible patterns of surface expression were observed for each marker. Using the kinetic display of surface markers as a gauge of differentiation, perturbations in embryoid body development were detected in cultures supplemented with interleukin-11, a gp130-activating cytokine thought to affect embryonic stem cell differentiation. In the absence of exogenous cytokines, microbead immunoselected day 7 c-kit, ER-MP12, and CD45-positive embryoid body cells were enriched for hematopoietic progenitors as detected by methylcellulose colony assays, while no significant enrichment of hematopoietic progenitors was observed with Sca-1, Thy-1.2, Fall-3, and Forssman-immunoselected cells. These results indicate that the process of early embryoid body development is associated with a programmed sequence of cell surface marker display, concomitant with the development of phenotypically definable embryonic cell lineages.

Structural identification of the hematopoietic progenitor antigen ER-MP12 as the vascular endothelial adhesion molecule PECAM-1 (CD31)

The monoclonal antibody ER-MP12 was recently described to recognize an antigen present on cell subpopulations of adult mouse bone marrow, including pluripotent hematopoietic stem cells. In an effort to understand the function of ER-MP12 antigen in hematopoiesis, we used biochemical and physical methods to determine its identity. ER-MP12 antigen was isolated by immunoprecipitation from FDCP-1 cell membrane proteins, yielding a glycosylated 113-kDa band upon analysis by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Thirteen peptides derived from trypsinized ER-MP12 antigen were analyzed by electrospray ionization mass spectrometry and compared to a protein sequence database. The search revealed the identity of the ER-MP12 antigen as platelet endothelial cell adhesion molecule-1, CD31 (PECAM-1). This result was subsequently confirmed by Edman sequencing of a single ER-MP12 peptide fragment followed by comparison with PECAM-1 sequence. In addition, flow cytometric analysis of bone marrow and embryonic stem cells revealed highly similar profiles between ER-MP12 and CD31 (MEC 13.3 antibody)-stained cells. The presence of PECAM-1 on primitive hematopoietic stem cells supports the theory for the interaction of hematopoietic progenitor and stem cells with bone marrow stroma and transendothelial migration.

Development and heterogeneity of macrophages and their related cells through their differentiation pathways

Macrophages are a heterogeneous population differing in their site of location, morphology and function. They develop from hematopoietic stem cells originating in both fetal and bone marrow hematopoiesis. In yolk sac and early hepatic hematopoiesis, primitive/fetal macrophages develop from hematopoietic stem cells, bypassing the stage of monocytic cells (monoblasts, promonocytes and monocytes), possess proliferative capacity and differentiate into resident macrophages in tissues in late ontogeny. Monocytic cells develop in hepatic hematopoiesis after the development of primitive/fetal macrophages, then move into the bone marrow in late ontogeny, forming a monocyte-derived macrophage population in tissues. Like monocytes, the monocyte-derived macrophages have no proliferative potential and are short-lived, whereas the resident macrophages are long-lived in tissue, possess proliferative capacity and can be sustained by self-renewal. In adult life, the bone marrow releases macrophage precursors (immature myeloid cells) and monocytes into peripheral blood, but normally not monoblasts or promonocyts. The myeloid precursor cells migrate into tissues and differentiate into resident macrophages or related cells in situ due to macrophage differentiation or growth factors, such as M-CSF and GM-CSF, produced in situ and/or supplied humorally. Monocytes, however, migrate into tissues in response to inflammatory stimuli and differentiate into exudate macrophages. The distinct differentiation pathways of monocyte/macrophages, resident macrophages, other macrophage subpopulations, and macrophage-related cells are reviewed together with the heterogeneity of macrophage precursor cells.

AM-3K, a novel monoclonal antibody specific for tissue macrophages and its application to pathological investigation

An anti-human macrophage monoclonal antibody, AM-3K, was produced using human alveolar macrophages as antigen. The molecular weights of the antigen recognized by AM-3K were 120 and 70 kD. Immunohistochemically, AM-3K reacted intensely with most macrophages in lymphoreticular organs and in many other organs and tissues. In the spleen, AM-3K reacted with red pulp macrophages, some white pulp macrophages, and tingible body macrophages in lymphoid follicles. In the lymph nodes, many macrophages distributed in the outer cortex, paracortical area, medulla, capsule, or within lymphoid follicles showed an intense reaction for AM-3K. Kupffer cells of the liver, macrophages in the connective tissues, and interstitial macrophages of the kidneys, pancreas, testis, and many other organs were also strongly labelled. AM-3K also reacted with macrophages in many pathological conditions. This antibody, however, did not react with dendritic cell populations, such as epidermal Langerhans cells, interdigitating cells in the paracortex of the lymph nodes, and follicular dendritic cells within the lymphoid follicles, nor with cells other than macrophages, including epithelial cells, vascular endothelial cells, lymphocytes, and granulocytes. Reaction products for AM-3K were found on the cytoplasmic membrane of macrophages by immunoelectron microscopy. In both cryostat sections and formalin-fixed paraffin sections, this monoclonal antibody recognized the antigen present on the cell surface membrane of tissue macrophages, but not monocytes or dendritic cells.

Immunohistochemical detection of macrophage-derived foam cells and macrophage colony-stimulating factor in pulmonary atherogenesis of cholesterol-fed rabbits

In order to investigate the role of monocyte/macrophages and their relationship to the expression of macrophage colony-stimulating factor (MCSF) in pulmonary atherosclerosis, lungs were excised from rabbits that had been fed for 60 and 90 days on a diet containing 0.5% cholesterol. In the lungs, fatty streaks and elevated foam cell lesions predominated in the large or medium-sized elastic pulmonary arteries, while massive accumulation of foam cells in the intima of muscular arteries produced marked luminal narrowing and nearly complete occlusion. In these lesions, most of the foam cells were reactive with RbM2, a monoclonal antibody (mAb) against rabbit macrophages, while smooth muscle cell-derived foam cells were detected by mAb against smooth muscle actin in the deeper area of elevated foam cell lesions of elastic arteries. Ultrastructural observation confirmed the presence of monocytes in the intima, their differentiation into macrophages, and their transformation into foam cells in the atherosclerotic lesions. Immunohistochemical expression of MCSF was demonstrated in the endothelial cells, smooth muscle cells and foam cells. A minor macrophage-derived foam cell population was demonstrated to possess a proliferative capacity. These data suggest that MCSF is involved in the differentiation of monocytes into macrophages, their transformation into foam cells, and their proliferation during pulmonary atherogenesis.