Enhanced macrophage resistance to Pseudomonas exotoxin A is correlated with decreased expression of the low-density lipoprotein receptor-related protein

The low density receptor-related protein 1 plays a significant role in ricin-mediated intoxication of lung cells

Ricin, a highly lethal plant-derived toxin, is a potential biological threat agent due to its high availability, ease of production and the lack of approved medical countermeasures for post-exposure treatment. To date, no specific ricin receptors were identified. Here we show for the first time, that the low density lipoprotein receptor-related protein-1 (LRP1) is a major target molecule for binding of ricin. Pretreating HEK293 acetylcholinesterase-producer cells with either anti-LRP1 antibodies or with Receptor-Associated Protein (a natural LRP1 antagonist), or using siRNA to knock-down LRP1 expression resulted in a marked reduction in their sensitivity towards ricin. Binding assays further demonstrated that ricin bound exclusively to the cluster II binding domain of LRP1, via the ricin B subunit. Ricin binding to the cluster II binding domain of LRP1 was significantly reduced by an anti-ricin monoclonal antibody, which confers high-level protection to ricin pulmonary-exposed mice. Finally, we tested the contribution of LRP1 receptor to ricin intoxication of lung cells derived from mice. Treating these cells with anti-LRP1 antibody prior to ricin exposure, prevented their intoxication. Taken together, our findings clearly demonstrate that the LRP1 receptor plays an important role in ricin-induced pulmonary intoxications.

Differences in Genetic Background Contribute to Pseudomonas Exotoxin A-Induced Hepatotoxicity in Rats

Pseudomonas aeruginosa exotoxin A (PEA) causes severe hepatotoxicity in experimental animals and is useful in investigations of immune-mediated liver injury. However, strain differences in the sensitivity to PEA-induced hepatotoxicity in rats remains be elucidated. In this study, we determined the severity of PEA-induced hepatotoxicity in six genetically different rat strains. Male LE (Long Evans), Wistar, F344, WKY, BN/SsN and LEW rats were administered a single intravenous injection of PEA (20 μg/kg). Significantly elevated serum ALT and AST levels, massive necrosis and hemorrhage, and numerous TUNEL-positive hepatocytes were observed in BN/SsN rats. In contrast, low levels of ALT and AST as well as mild changes in liver histopathology were observed in Wistar and F344 rats. Moderate levels of hepatic injuries were observed in LE, WKY, and LEW rats. Pro-inflammatory cytokines including TNF-α, IL-2 and IL-6 serum levels were markedly increased in BN/SsN rats compared to Wistar and F344 rats. However, the hepatic levels of low density lipoprotein receptor-related protein (LRP), which functions as the PEA receptor, were not significantly different in each strain. Taken together, we suggest that BN/SsN is the most sensitive rat strain, whereas Wistar and F344 were the most resistant rat strains to PEA-induced liver damage. The different genetic background of rat strains plays an important role in the susceptibility to PEA-induced epatotoxicity that may depend on immune-regulation but not LRP receptor levels.

Pretreatment with lipopolysaccharide ameliorates Pseudomonas exotoxin A-induced hepatotoxicity in rats

CONTEXT

Liver injury can be induced by various hepatotoxicants, including Pseudomonas aeruginosa exotoxin A (PEA). Our previous study indicated that PEA-induced rat hepatotoxicity was T cells and Kupffer cells dependent. Several reports have demonstrated that non-toxic doses of bacterial lipopolysaccharide (LPS) can protect liver against the chemicals-induced toxicity such as acetaminophen and concanavalin-A.

OBJECTIVE

This study aimed to investigate the protecting mechanisms of LPS on PEA-induced hepatotoxicity.

RESULTS

Rats pretreated with LPS (40 μg/kg, 12 h before PEA admission) significantly decreased animal mortality, serum enzyme (ALT, AST and T-bil) activities, histopathological changes and hepatocytes apoptosis following challenge with PEA. The concentrations of tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ) and interleukin-2 (IL-2) were reduced, but IL-6 and IL-10 were increased in the serum. In addition, prior treatment of these LPS-pretreated rats with gadolinium chloride (GdCl3), a selective Kupffer cell depletion agent, markedly enhanced liver injury after PEA administration. In contrast, the pretreatment of LPS to T-cell deficient athymic nude rats still display significant attenuation of PEA-induced liver injury. This observation further confirmed our hypothesis that LPS ameliorate PEA-hepatotoxicity was through Kupffer cells but not T cells. Moreover, LPS-induced hepatoprotection ability was neutralized by co-treatment with anti-TNF-α antibodies, but not with anti-IFN-γ antibodies. Finally, replacement of LPS with RS-LPS (Rhodobacter sphaeroides LPS), a Toll like receptor-4 (TLR-4) antagonist, resulted in severe hepatotoxicity.

CONCLUSION

These results suggested that Kupffer cells, TNF-α and TLR-4 play central mediator roles during the hepatoprotection against PEA-induced hepatotoxicity conferred by LPS.

Influence of age on susceptibility to Pseudomonas aeruginosa exotoxin A-induced hepatotoxicity in Long-Evans rats

Epidemiological investigations suggest that increased age is associated with susceptibility to infection. Pseudomonas aeruginosa (P. aeruginosa) infection and associated exotoxin A (PEA) toxicity have been reported in hospitalized elderly patients and young children with cystic fibrosis. The present study investigated age-related differences in PEA-induced hepatotoxicity in post weaning (PW, 3 weeks), young adult (YA, 12 weeks), and mature adult (MA, 60-64 weeks) rats. PEA (20 microg/kg) was injected intraveneously and mortality, clinical chemistry, hepatic histopathology, TUNEL (Terminal deoxynucleotidyl transferase-mediated dUTP nick end-labelling) and PCNA (Proliferating cell nuclear antigen) staining, and serum cytokine levels were assessed at specific time points, up to 72 hr post-exposure (HPE). Mortality in MA rats was 100% at less than 48 HPE. Serum ALT levels in MA rats were approximately 5-fold greater than levels in PW and YA rats at 36 HPE. MA rat liver histological sections showed diffuse hepatocellular necrosis. In contrast, hepatocellular apoptosis, demonstrable by the TUNEL method, was noted simply in the periportal and midzonal regions from 36 to 48 HPE. Increased morphological mitoses and PCNA-positive hepatocytes were seen in PW and YA rats at 72 HPE. These parameters were correlated with age-dependent significant increases in TNF-alpha, IL-2, IL-6, and IL-18 levels. These data suggest that inflammatory cytokines play an important role in age-related differences in PEA-induced hepatotoxicity. Moreover, these cytokines might correlate with different patterns histopathologic features at various ages.

Elevated furin levels in human cystic fibrosis cells result in hypersusceptibility to exotoxin A-induced cytotoxicity

Progressive pulmonary disease and infections with Pseudomonas aeruginosa remain an intractable problem in cystic fibrosis (CF). At the cellular level, CF is characterized by organellar hyperacidification, which results in altered protein and lipid glycosylation. Altered pH of the trans-Golgi network (TGN) may further disrupt the protein processing and packaging that occurs in this organelle. Here we measured activity of the major TGN endoprotease furin and demonstrated a marked upregulation in human CF cells. Increased furin activity was linked to elevated production in CF of the immunosuppressive and tissue remodeling cytokine TGF-beta and its downstream effects, including macrophage deactivation and augmented collagen secretion by epithelial cells. As furin is responsible for the proteolytic processing of a range of endogenous and exogenous substrates including growth factors and bacterial toxins, we determined that elevated furin-dependent activation of exotoxin A caused increased cell death in CF respiratory epithelial cells compared with genetically matched CF transmembrane conductance regulator-corrected cells. Thus elevated furin levels in CF respiratory epithelial cells contributes to bacterial toxin-induced cell death, fibrosis, and local immunosuppression. These data suggest that the use of furin inhibitors may represent a strategy for pharmacotherapy in CF.

Activated α2-macroglobulin induces cell proliferation and mitogen-activated protein kinase activation by LRP-1 in the J774 macrophage-derived cell line

The low-density lipoprotein receptor-related protein-1 (LRP-1) is an endocytic receptor of activated forms of the proteinase inhibitor alpha(2)-macroglobulin (alpha(2)M*). It has been proposed that alpha(2)M* and LRP-1 modulate diverse cellular processes, including cell adhesion, proliferation, and migration, which are involved in inflammation and tumor progression. However, relatively little is known about the role of alpha(2)M*/LRP-1 interaction on these processes. In this work, we demonstrate that alpha(2)M* binding to LRP-1 induces cell proliferation and MAPK activation in the J774 macrophage-derived cell line, which were blocked by RAP, an antagonist of LRP-1-binding ligands, and by PD980059, a specific inhibitor for the Mek1-ERK1/2 pathway. In addition, we demonstrate that LPS, a bacterial product that it is known to down-regulate the LRP-1 expression on macrophage, abrogated the signaling activity triggered by alpha(2)M* on LPS-treated J774 cells. These results suggest that alpha(2)M*/LRP-1 interaction constitutes a key role in the macrophage functioning during inflammation and cancer.

Differential CD91 dependence for calreticulin and Pseudomonas exotoxin-A endocytosis

Calreticulin (CRT) interaction with cell-surface receptors is integral to its function in escorting associated peptides into the antigen-presenting cell (APC) antigen presentation pathway. Additionally, extracellular CRT is proposed to be required for lung APC interaction with collectins. In both cases, CD91 has been proposed to act as the APC cell-surface receptor requisite for mediating these processes. However, the evidence for a CRT interaction with CD91 is indirect, predicated on partial competition of cellular binding by gp96, of which CD91 has been proposed as the unique endocytic receptor, and by the CD91 ligand alpha2-macroglobulin. Here, we directly investigate the function of CD91 in binding and trafficking CRT. We find that the ability of CRT to interact with APC does not correlate with cellular CD91 expression or function. Additionally, in the first genetic test of CD91 function regarding CRT, CD91 expression neither conferred CRT association nor did CD91-deficient (CD91-/-) and CD91-expressing cells differ in their ability to traffic CRT. Finally, cellular CRT trafficking did not parallel that of Pseudomonas exotoxin-A, an obligate CD91 ligand, by the criteria of CD91 dependence, cell-type specificity and endocytic itinerary. These data identify that CRT trafficking is not, as previously hypothesized, CD91 dependent and indicate usage of alternative cellular trafficking pathways.

A quantitative and highly sensitive luciferase-based assay for bacterial toxins that inhibit protein synthesis

Inhibition of protein synthesis is a common mechanism by which bacterial and plant toxins injure human cells. Examples of toxins that inhibit protein synthesis include shiga toxins of Escherichia coli, diphtheria toxin, Pseudomonas exotoxin A and the plant toxin ricin. In order to facilitate studies on toxin pathogenesis and to enable screening for inhibitors of toxin action, a quantitative and highly sensitive assay for the action of these toxins on mammalian cells was developed. The cDNA encoding destabilized luciferase was cloned into an adenoviral expression plasmid and a high-titre viral stock was prepared. Following transduction of Vero cells, luciferase expression was found to be linear with respect to viral multiplicity of infection. Luciferase expression by as few as 10 cells was readily detected. Treatment of transduced cells with either cycloheximide or shiga toxin resulted in a decrease in luciferase activity, with a half-life ranging from 1 to 2 h. Inhibition of luciferase expression was evident at toxin concentrations as low as 1 pg ml(-1). The assay was adapted for use in 24-, 96- and 384-well plates, enabling rapid processing of large numbers of samples. Using this approach, susceptibility of Vero, Hep2, Chang, A549, COS-1 and HeLa cells to three different toxins was determined. These results demonstrate that the luciferase-based assay is applicable to the study of numerous cell types, is quantitative, highly sensitive and reproducible. These features will facilitate studies on pathophysiology of toxin-mediated diseases and allow high-throughput screening for inhibitors of cytotoxicity.

Genetic disorders of the LRP5-Wnt signalling pathway affecting the skeleton

Osteoporosis is a common, increasingly prevalent and potentially debilitating condition of men and women. Genetic factors are major determinants of bone mass and the risk of fracture, but few genes have been definitively demonstrated to be involved. The identification of these factors will provide novel insights into the processes of bone formation and loss and thus the pathogenesis of osteoporosis, enabling the rational development of novel therapies. In this article, we present the extensive genetic and functional data indicating that the LRP5 gene and the Wnt signalling pathway are key players in bone formation and the risk of osteoporosis, and that LRP5 signalling is essential for normal morphology, developmental processes and bone health.

Effects of lipopolysaccharide on the appearance of macrophage populations and fibrogenesis in cisplatin-induced rat renal injury

Macrophages play an important role in renal interstitial fibrosis via production of transforming growth factor-beta1 (TGF-beta1) and tumor necrosis factor-alpha (TNF-alpha); these fibrogenic factors mediate induction of myofibroblastic cells capable of producing extracellular matrices. We investigated the effects of lipopolysaccharide (LPS), a macrophage activator, on the appearance of macrophage populations and subsequent fibrogenesis in cisplatin (CDDP)-induced rat renal lesions. In keeping with the progression of interstitial fibrosis, alpha-smooth muscle actin (alpha-SMA)-immunopositive myofibroblastic cell number began to increase on day 4 and continued gradually until day 16 after CDDP injection. Cells immunoreactive for ED1 (for exudate macrophages), ED2 (for resident macrophages) and ED3 (for activated resident macrophages) showed the highest number on day 4 or day 7, and thereafter, the numbers were gradually decreased up to day 16. On the other hand, the number of cells immunoreactive for OX6 (rat MHC class II-recognizing antibody) was increased on day 7 and remained elevated up to day 16. LPS was injected on day 7 after CDDP injection when the greatest number of ED1-positive macrophages were present. In CDDP/LPS-injected rats, the numbers of macrophages reacting to ED1, ED2, ED3, and OX6 were higher than those in CDDP-injected rats during the observation period between days 7 and 16; ED3- and OX6-positive cells were more prominently increased than ED1- and ED2-postive cells. By RT-PCR analysis, the expression of TGF-beta1 and TNF-alpha mRNAs in CDDP/LPS-injected rats on day 7 was markedly increased in contrast to those in CDDP-injected rats. These findings indicate that LPS treatment enhanced the macrophage expression of fibrogenic factors. However, there was no marked difference in the fibrogenesis between CDDP/LPS- and CDDP-injected rats. These findings suggest that the macrophage populations appearing in CDDP-induced rat renal lesions should be investigated further, to address the complicated pathogenesis of renal interstitial fibrosis.

Induction of apoptosis in macrophage cell line, J774, by the cell-free supernatant from Pseudomonas aeruginosa

Pseudomonas aeruginosa is able to secrete many virulence factors that are cytotoxic towards eukaryotic cells. To investigate the effect of the bacterium on macrophages, we obtained cell-free supernatants from P. aeruginosa (Pa) IID1117 (elastase-positive and protease-positive) and Pa IID1130 (elastase-positive and protease-negative). After 6 hr of incubation with the cell-free supernatant from the Pa IID1117 strain, the viability of J774 macrophages was shown to be significantly reduced (47.5+/-11%), but not Pa IID1130 (96.4+/-1.6%) at a concentration of 10% (v/v) compared to control J774 macrophages without any supernatant (97.2+/-1.7%) by the detection of trypan blue dye exclusion. The death of cells was further demonstrated to be due to apoptosis characterized by chromatin condensation and apoptotic bodies by Hoechst 33258 staining, DNA fragmentation by agarose gel electrophoresis and terminal deoxynucleotidyl transferase-mediated d-UTP nick end labeling (TUNEL). An activated subunit was found to be released from procaspase-3 in cell lysate. But in the presence of protease inhibitor, the apoptosis was completely blocked. The findings indicate that the Pa IID1117 strain is capable of inducing apoptosis in J774 macrophages. The apoptosis induced by the cell-free supernatant from Pa IID1117 strain is suggested to be dependent on protease, but not elastase.

Multiple receptors mediate apoJ-dependent clearance of cellular debris into nonprofessional phagocytes

Phagocytosis of apoptotic, senescent, and dying cells by macrophages is a well characterized process. More recently it has been shown that in addition to macrophages vital neighboring cells in the affected tissue participate in the cellular clearance. While scavenger receptors have been shown to mediate uptake into macrophages, it is poorly understood how cellular debris is internalized by nonprofessional phagocytes. We here analyze the endocytic activity of vital fibroblasts and epithelial cells exposed to cellular debris and membrane remnants. We show a mutual stimulation in the endocytosis of debris and apolipoproteinJ (clusterin) in these cells. Experiments using RAP (receptor-associated protein) to block ligand binding to LRP and megalin as well as studies in LRP- and megalin-deficient cells suggest that the uptake of apoJ and cellular debris is mediated by megalin, LRP, and yet unidentified internalization mechanisms.

Divergent Pseudomonas exotoxin A sensitivity in normal and transformed liver cells is correlated with low-density lipoprotein receptor-related protein expression

Pseudomonas exotoxin A (PEA) is an extracellular virulence factor produced by the opportunistic human pathogen Pseudomonas aerguinosa. PEA intoxification begins when PEA binds to the low-density lipoprotein receptor-related protein (LRP). The liver is the primary target of systemic PEA, due largely to the high levels of functional LRP expressed by liver cells. Using a 3H-leucine incorporation assay to measure inhibition of protein synthesis we have demonstrated that normal (BNL CL.2) and transformed (BNL 1ME A7R.1) liver cells exhibit divergent PEA sensitivity; with BNL 1ME A7R.1 cells demonstrating greater PEA sensitivity than their non-transformed counterparts. The receptor-associated protein, a LRP antagonist, decreased PEA toxicity in BNL 1ME A7R.1 cells, confirming the importance of the LRP in PEA intoxification in this cell type. Increased PEA sensitivity in BNL 1ME A7R.1 cells was associated with increased functional cell surface LRP expression, as measured by alpha2-macroglobulin binding and internalization studies, and increased LRP mRNA levels, as determined by Northern blot analysis. Interestingly, BNL CL.2 cells were more sensitive than BNL 1ME A7R.1 cells to conjugate and mutant PEA toxins that do not utilize the LRP for cellular entry. These data demonstrate that increased LRP expression is an important mechanism by which PEA sensitivity is increased in BNL 1ME A7R.1 transformed liver cells.

Differential effects of transforming growth factor-beta1, a fibrogenic factor, on macrophage-like cells (HS-P) and myofibroblastic cells (MT-9) in vitro

Transforming growth factor-beta1 (TGF-beta1) produced by infiltrating macrophages plays a role in fibrotic disorders through the induction of myofibroblasts. To explore possible mechanisms by which TGF-beta1 may act in this context, we investigated effects of TGF-beta1 on macrophage-like (HS-P) and myofibroblastic (MT-9) cells, two novel cell lines developed by us. Immunocytochemically, the addition of TGF-beta1 (0, 0.1, 0.5, and 1.0 ng/ml) dose-dependently suppressed the expressions of antigens recognized by macrophage/histiocyte-specific antibodies (ED1 and ED2) in HS-P cells, whereas the addition concomitantly increased the number of anti-alpha-smooth muscle actin antibody-positive myofibroblastic cells, suggesting a possible phenotypical modulation of macrophages into myofibroblasts in the fibrotic lesions. By contrast, MT-9 cells did not show such immunophenotypical changes following TGF-beta1 addition. DNA synthesis, measured by tritiated thymidine-incorporation, was inhibited in a dose-dependent manner in MT-9 cells by TGF-beta1 addition (0, 0.1, 0.2, 0.5, 1.0, 5, and 10 ng/ml), but that in HS-P cells was unchanged. Northern blot analysis revealed that expressions of cell cycle-related early genes, c-jun and c-myc, were increased in HS-P cells after TGF-beta1 (1 ng/ml) addition, with c-jun showing peak expression prior to c-myc. By contrast, the peak expressions of c-jun and c-myc were delayed in TGF-beta1 (1 ng/ml)-added MT-9 cells, and their levels were less in MT-9 cells than in HS-P cells. Furthermore, TGF-beta1 (1 and 10 ng/ml) induced DNA laddering in MT-9 cells, but did not in HS-P cells. Based on these findings, it was speculated that TGF-beta1 could have induced G1 arrest in cell cycle and apoptosis in MT-9 cells. The present study showed that there were significant differences in the effects of TGF-beta1 between macrophage-like HS-P cells and myofibroblastic MT-9 cells, presumably depending on divergent susceptibilities to TGF-beta1 between both cell types. Because such cell types are key cells in the fibrogenesis, HS-P and MT-9 might be useful models for investigating the pathogenesis of fibrosis in vitro.

Effects of lipopolysaccharide on a macrophage-like cell line (HS-P) from a rat histiocytic sarcoma

Lipopolysaccharide (LPS) is a major modulator of macrophage functions. To characterize a newly established rat histiocytic sarcoma-derived cell line (HS-P), immunophenotypic changes and cellular growth responses of HS-P cells exposed to LPS were investigated and compared with those of MT-9 cells isolated from a rat malignant fibrous histiocytoma. MT-9 cells have somewhat histiocytic features, because occasional cells react to rat macrophage-specific antibodies. Addition of LPS to cultured HS-P cells increased the numbers of cells immunopositive to ED1 (rat macrophage-specific antibody) and ED2 (rat histiocyte-specific antibody) and stimulated the phagocytosis of latex beads, whereas LPS-treated MT-9 cells did not show such immunophenotypic changes. LPS-treated HS-P cells showed enhanced immunolabelling of alpha-smooth muscle actin, suggesting a possible modulation of macrophages towards myofibroblastic cells. To evaluate cellular growth after the addition of LPS or fetal bovine serum, DNA synthesis was examined by measuring tritiated thymidine incorporation, and the mRNA expression of c- jun and c- myc (immediate early genes in the cell cycle) was examined by Northern blot analysis. In HS-P cells, the addition of serum greatly increased DNA synthesis and induced high expression of c- jun and c- myc; in contrast, LPS markedly depressed DNA synthesis and reduced the expression of c- jun and c- myc. HS-P cells were more sensitive than MT-9 cells to the growth-promoting effect of serum and the growth-inhibiting effect of LPS. The study demonstrated that HS-P cells are highly LPS-responsive, indicating that they would be useful for studies of macrophage functions.

Malignant fibrous histiocytoma (MFH) cells and macrophages/histiocytes have a common antigen recognized by a monoclonal antibody risen against a rat MFH-derived cloned cell line

A monoclonal antibody (B9) was generated by using a rat malignant fibrous histiocytoma (MFH)-derived cloned cell line (MT-8) as the immunogen. Immunohistochemically, B9 reacted specifically with a cytoplasmic antigen of MT-8 cells. Furthermore, B9 immunolabeled another MFH-derived cloned cells (MT-9) and histiocytic sarcoma cells, as well as macrophages/histiocytes in normal and diseased tissues of rats. These findings suggest the presence of a common antigen recognized by B9 between MFH cells and macrophages/histiocytes. This suggests that MFH cells may express histiocytic nature.

Macrophage-like cell line (HS-P) from a rat histiocytic sarcoma

With future exploration of macrophage properties in mind, we established a novel cell line (HS-P) from a transplantable histiocytic sarcoma, derived originally from a tumour in an aged F344 rat. HS-P was subjected to 70 serial passages, in which the mean doubling time was 15.7 h. The cells, which were round, oval or polygonal in shape, were arranged in a compact sheet. They reacted to varying degrees for lysosomal enzymes (acid phosphatase and non-specific esterase) and with the following antibodies: ED1/ED2 (rat macrophage/histiocyte-specific), OX6 (rat MHC class II-specific), lysozyme antibody and alpha1-antichymotrypsin antibody. Electron microscopically, HS-P cells showed lysosomes and prominent cell projections. These findings indicated that the cultured cells were macrophage-like. Syngeneic rats inoculated subcutaneously or intraperitoneally with HS-P cells invariably developed sarcomatous tumours consisting of monomorphic mononuclear cells, which exhibited cytochemical properties similar to those of cultured HS-P cells. Bioassay and reverse transcription-polymerase chain reaction methods revealed that tumour necrosis factor-alpha increased on addition of lipopolysaccharide (LPS), indicating that HS-P cells remained LPS-responsive. HS-P cells may prove to be a useful tool for in-vitro studies of macrophage function.

Distribution of cells labelled by a monoclonal antibody (A3) against a cloned cell line derived from a rat malignant fibrous histiocytoma

To pursue the histogenesis of malignant fibrous histiocytoma (MFH), of which the cell of origin is still debated, a monoclonal antibody (A3) was produced against a rat MFH-derived cloned cell line (MT-8). Antigen recognized by A3 was around 80 kDa in molecular weight and was seen on the cytoplasmic membrane of MT-8 cells by immunoelectron microscopy. A3 reacted specifically with MT-8 cells, with another rat MFH-derived cell line (MT-9) and with their induced tumours in syngeneic rats, but not with other rat tumours such as fibrosarcoma, histiocytic sarcoma, malignant meningioma, uterine leiomyosarcoma, endometrial stromal sarcoma, mononuclear cell leukaemia and malignant schwannoma. These findings indicate that A3 has a high specificity for rat MFH cells. In fetuses on gestation days 15, 18 and 20 and in postnatal rats aged 1, 4 and 8 days, A3 reacted with primitive mesenchymal cells in visceral organs and around arteries and bronchi, as well as in the lamina propria of intestinal mucosa, renal interstitium, meninges and perineurium. There were no A3-positive connective tissue cells in organs or other sites in adult rats more than 10 weeks old. It is therefore likely that MFH cells share antigens with primitive mesenchymal cells, which may be multipotent for mesenchymal differentiation. The present study suggests that MFH consists of a population of primitive, undifferentiated mesenchymal cells. A3 also immunolabelled endothelial cells of arteries, venules and pulmonary capillaries in fetal, postnatal and adult rats; vascular endothelial cells in chemically induced hepatic and renal lesions also reacted strongly with A3. However, the significance of endothelial immunoreactivity with A3 remains to be elucidated.