[The French mesothelioma network from 1998 to 2013]. / Mésothéliome: les dispositifs en place en France « le réseau mésothéliome ¼ 1998-2013.

Mesothelioma is a rare disease less than 0.3% of cancers in France, very aggressive and resistant to the majority of conventional therapies. Asbestos exposure is nearly the only recognized cause of mesothelioma in men observed in 80% of case. In 1990, the projections based on mortality predicted a raise of incidence in mesothelioma for the next three decades. Nowadays, the diagnosis of this cancer is based on pathology, but the histological presentation frequently heterogeneous, is responsible for numerous pitfalls and major problems of early detection toward effective therapy. Facing such a diagnostic, epidemiological and medico-legal context, a national and international multidisciplinary network has been progressively set up in order to answer to epidemiological survey, translational or academic research questions. Moreover, in response to the action of the French Cancer Program (action 23.1) a network of pathologists was organized for expert pathological second opinion using a standardized procedure of certification for mesothelioma diagnosis. We describe the network organization and show the results during this last 15years period of time from 1998-2013. These results show the major impact on patient's management, and confirm the interest of this second opinion to provide accuracy of epidemiological data, quality of medico-legal acknowledgement and accuracy of clinical diagnostic for the benefit of patients. We also show the impact of these collaborative efforts for creating a high quality clinicobiological, epidemiological and therapeutic data collection for improvement of the knowledge of this dramatic disease.

Eighth international mesothelioma interest group.

The eighth International Mesothelioma Interest Group (IMIG) meeting was held in Chicago, IL, United States, in 19-22 October 2006 to discuss mesothelioma - the cancer often linked to asbestos exposure. It is a very aggressive malignancy with a median survival of less than 1 year from diagnosis. Millions of people have been exposed worldwide to asbestos, especially during the second half of the twentieth century when asbestos use increased significantly. The tons of asbestos utilized in the past remain a health hazard for current and future generations because asbestos is difficult to be disposed off. This makes asbestos and mesothelioma research a public health issue in addition to a medical problem. Moreover, the very high costs of asbestos litigation have a significant impact on the whole economy. In the United States, up until 2001, defendant companies had paid 54 billion dollars in claims and estimated future liabilities ranged from 145 to 210 billion. Therefore, asbestos research is of great interest to a large audience that includes patients, millions of asbestos-exposed individuals, scientists, physicians, public health officials, politicians, unions of asbestos workers, lawyers and the public at large. During the past few years, there has been significant progress in understanding the process of mineral fiber carcinogenesis and mesothelioma pathogenesis. With improved understanding of the pathogenesis of mesothelioma, new diagnostic, preventive and therapeutic options are being developed. A total of 247 papers were presented at the IMIG: the abstracts of these presentations were published in Lung Cancer, Supplement 1, October 2006. Here, experts in different disciplines critically review some of the most exciting presentations of the IMIG meeting. The result is a comprehensive review of the research field of asbestos carcinogenesis and mesothelioma, and of the progress that has been made in recent years in both basic and clinical sciences.

Pleural carcinogenic potency of mineral fibers (asbestos, attapulgite) and their cytotoxicity on cultured cells.

The carcinogenicity of several samples of mineral fibers was tested following injection of 20 mg in the pleural cavity of noninbred Sprague-Dawley rats. Three samples of chrysotile asbestos (mean length: 3.2, 2.1, and 1.2 micron) induced mesotheliomas at a rate of 48, 52, and 19%, respectively. The first sample was acid leached prior to intrapleural injection; in that group, the percentage of mesotheliomas was reduced to 25%. Treatment with amosite and crocidolite resulted in the occurrence of 57 and 56% of mesotheliomas. Acid-treatment of amphiboles did not significantly modify the percentage of mesotheliomas. When the Stanton's fiber dimensions were taken into consideration to correlate with mesothelioma incidence, the observed number of mesotheliomas in the chrysotile-treated animals was much lower than that expected, suggesting that other fiber parameters (chemistry, physicochemistry) play a role in the carcinogenicity. Attapulgite fibers (mean length: 0.77 micron) did not induce tumor, and the mean survival time was of the same order as that observed in the control groups. The injection of quartz resulted in no mesothelioma but did result in 6 malignant histiocytic lymphomas (17%) and 2 malignant schwannomas (6%). In vitro experiments did not show strong correlation between cytotoxicity and the carcinogenic potency of these minerals, but the qualitative cellular responses might give some indications on the fiber's potency. In addition, the in vitro effects of the fibers seem to be modulated by their size.

Human malignant mesothelioma cell growth: activation of janus kinase 2 and signal transducer and activator of transcription 1alpha for inhibition by interferon-gamma.

Intrapleural injections of recombinant human IFN-gamma have shown some efficacy in reducing tumor growth in early stages of diffuse malignant mesothelioma (DMM). Here, we have addressed the potential therapeutic effect of IFN-gamma in DMM by investigating the activation of the JAK/STAT signaling pathway in seven human mesothelioma cell lines (HMCLs) that were differentially responsive to the antiproliferative activity of IFN-gamma. We showed that janus kinase 2 (JAK2) and signal transducer and activator of transcription 1 (STAT1) were phosphorylated on tyrosine residues within 15 min in all the HMCLs in which IFN-gamma (500 units/ml) inhibited proliferation. In addition, STAT1 binding activity to the gamma-activated sites DNA sequence was detected within 15 min in electrophoretic mobility-shift assay analysis, and IFN regulatory factor-1 RNA expression was observed within 6 h in the more responsive cells (72.7-95.2% inhibition of DNA synthesis after 72 h of treatment). Conversely, in several HMCLs, absent or limited growth suppressive effect (less than 22% inhibition of DNA synthesis) was associated with alterations in expression or activation of JAK2 or STAT1 or, downstream, with low induction of IFN regulatory factor-1 RNA expression and/or STAT1 protein expression following IFN-gamma treatment. These data suggest that at least part of the IFN-gamma effect on proliferation of HMCLs is mediated directly through activation of the JAK/STAT1 signaling pathway, and it could account for the antitumoral activity reported in DMM patients treated with IFN-gamma.

Intrapleural production of interleukin 6 during mesothelioma and its modulation by gamma-interferon treatment.

In vivo production of monokines was analyzed in 17 human malignant pleural mesotheliomas. High concentrations of interleukin 6 (IL-6) were detected in pleural effusions, contrasting with low levels of IL-1 beta and tumor necrosis factor alpha. This production arose from malignant cells, as shown by immunochemical analysis of pleural cells and by production of IL-6 by mesothelial cell lines. Intrapleural administration of recombinant human gamma-interferon to six patients led to a marked decrease in intrapleural IL-6 concentrations in all cases. This treatment was associated with in situ activation of macrophages and cytotoxic T-lymphocytes, as indicated by increased intrapleural neopterin and soluble CD8 concentrations. In vitro gamma-interferon had no effect on the production of IL-6 by mesothelial cell lines but decreased the growth of 3 of 6 mesothelioma cell lines. These results indicate that systemic manifestations of malignant mesothelioma, including fever, cachexia, and thrombocytosis may be related to the production of IL-6 by malignant cells, and that local gamma-interferon infusion may reduce this production by stimulating antitumoral immunity and/or by directly decreasing the proliferation of malignant cells.

Control of cell cycle progression in human mesothelioma cells treated with gamma interferon.

Recombinant human interferon gamma (r-hu-IFNgamma) exerts both antitumoral activity in the early stages of human malignant mesothelioma and a cytostatic effect in human mesothelioma (HM) cell lines in vitro. The antiproliferative effect of interferons (IFNs) reported in a variety of cells has been attributed to several mechanisms. In order to progress in the understanding of HM cell growth modulation by r-hu-IFNgamma, modifications of cell cycle progression and expression of key cell cycle regulator proteins in response to r-hu-IFNgamma were examined. Nine HM cell lines were studied, including one resistant to the antiproliferative effect of r-hu-IFNgamma. Except in the resistant cell line r-hu-IFNgamma produced an arrest in the G1 and G2-M phases of the cell cycle, associated with a reduction in both cyclin A and cyclin dependent kinase inhibitors (CDKIs) expression. Moreover cyclin B1/cdc2 activity was decreased. The present study provides the first evidence of a G2-arrest in r-hu-IFNgamma-treated HM cell lines and indicates that HM cell lines, despite their tumorigenic origin still support cell cycle control. The cell cycle arrest induced by r-hu-IFNgamma seems to depend on cyclin regulation through p21(WAF1/CIP1)- and p27(Kip1)-independent mechanisms and is not directly related to the induced DNA damage.

Effect of simian virus large T antigen expression on cell cycle control and apoptosis in rat pleural mesothelial cells exposed to DNA damaging agents.

Cell cycle progression and apoptosis are controlled by regulatory proteins, including p53, of which functional alterations are linked to carcinogenesis. Recently, malignant mesothelioma (MM), a primary tumour related to asbestos exposure, alternatively to post therapeutic radiations, has proven to be an important problem in oncogenesis. The p53 protein does not seem mutated or deleted in MM but a possible inactivation by binding to other proteins [mdm2; SV40 large T antigen (Tag)] has been suggested. The present work investigated cell cycle regulation in normal rat pleural mesothelial cells (RPMC) and in RPMC expressing Tag (RPMC-TSV40), under exposure to asbestos and radiations. In RPMC, these agents induced activation of cell cycle checkpoints located at G1/S and G2/M and/or mitosis but a lack of control at G1/S was found in RPMC-TSV40. A loss of G2/M control may account for the formation of micronuclei observed after exposure of RPMC-TSV40 to radiations. In RPMC-TSV40 the enhancement of abnormal mitoses and apoptosis after asbestos exposure, in comparison with RPMC, suggests a loss of mitotic control and a p53-independent mechanism of apoptosis. Thus Tag expression in mesothelial cells might have both adverse and beneficial effects by impairing the control of DNA integrity and enhancing apoptosis respectively.

Inhibition of acid sialidase by inorganic sulfate.

Sulfated glycosaminoglycans are known to inhibit mammalian acid-active sialidase. Although the inhibition depends clearly on the presence of sulfate groups on these macromolecules, there was no information on the intrinsic inhibitory potency of inorganic sulfate. In this study, we demonstrate that inorganic sulfates inhibit acid-active Mu-Neu5Ac sialidase of U937 cells. This inhibition was found to be reversible and it appeared to be of the mixed competitive type. Sulfate-induced inhibition was also observed in other cells as well as with other substrates such as sialyl lactose and bovine mixed brain gangliosides. We conclude that the intrinsic inhibitory potency of sulfate groups may be significantly involved in the inhibition of acid-active sialidase by sulfated glycosaminoglycans. In addition, inorganic sulfate by its apparent potency to selectively inhibit acid sialidases might constitute an interesting tool for the characterisation of the minor forms of sialidases occurring in mammalian cells.

Transfer into a mesothelioma cell line of tumor suppressor gene p16 by cholesterol-based cationic lipids.

In this work, the tumor suppressor gene p16 was efficiently transferred into FR cells isolated from a patient with malignant mesothelioma using cationic liposomes prepared from trimethyl aminoethane carbamoyl cholesterol (TMAEC-Chol) and triethyl aminopropane carbamoyl cholesterol (TEAPC-Chol). This transfer was performed after preliminary assays were undertaken to find the optimal transfection conditions. Results showed that an efficient transfer of plasmids containing the reporter gene pCMV-beta galactosidase vectorized by TMAEC-Chol/DOPE and TEAPC-Chol/DOPE liposomes into mesothelioma FR cells was obtained as assessed by luminometric measurements of beta-galactosidase activity. Cytotoxicity studied by MTT test showed that at concentrations used for this study, the cationic liposomes have no effect on cell growth. Transfer into mesothelioma FR cells of a plasmid construct containing the tumor suppressor gene p16 was carried out with these liposomes. Western blotting and immunofluorescence showed the presence of p16 in treated cells. An inhibition of cell growth was observed, indicating that efficient tumor suppressor gene transfer can be performed by using cationic liposomes.

Quantitation of elastin in human urine and rat pleural mesothelial cell matrix by a sensitive avidin-biotin ELISA for desmosine.

A specific enzyme-linked immunosorbent assay (ELISA) was developed for the determination of desmosine, a cross-linked amino acid specific to fibrous elastin. Competition between solid phase-bound desmosine-protein conjugate and free desmosine for binding to monospecific anti-desmosine antiserum constituted the underlying principle of the assay. The conjugation of desmosine to different protein carriers was carried out with the 1-ethyl-3-(dimethylamino-propyl)carbodiimide (ECDI); rabbits were immunized with desmosine-bovine serum albumin and micro-titer plates were coated with desmosine-egg albumin. An avidin-biotin peroxidase system was used to reveal anti-desmosine antibodies bound to the desmosine-protein conjugate. As both conjugates revealed new non-specific common epitopes on the carrier proteins, prior absorption of the anti-desmosine antiserum on rabbit albumin polymerized with ECDI was required to remove the antibodies directed against these neo-antigens. The absorption procedure resulted in an increased specificity and sensitivity. Values ranging from 0.07 to 4 ng of desmosine/well could be detected and this sensitivity was greater than that obtained in previous immunoassays for desmosine. In order to assess the specificity of the test, samples containing aminoacids and urine hydrolysates were included in an assay. Some cross-reactivity was observed with the desmosine precursor lysinonorleucine and the desmosine isomer isodesmosine but, in contrast the very low cross-reactivity observed with collagen hydrolysate was similar to that exhibited by albumin hydrolysate. Analysis of urine samples from 118 normal male volunteers showed, firstly, that urinary creatinine measurement was a good indicator of the amount of urine which could be safely introduced in the assay without risk of non-specific interference by other organic compounds and, secondly, that the desmosine/creatinine ratio was a reliable index for an in vivo assessment of degraded elastin excretion. The assay also allowed quantitation of elastin fiber biosynthesis in the connective tissue matrix of cultured rat pleural mesothelial cells. This ELISA for demosine is a simple technique which should be useful for further in vivo or in vitro investigations of fibrous elastin tissue metabolism.

Plasma protein immunocytochemistry and polysaccharide cytochemistry at the surface of alveolar and endothelial cells in the rat lung.

Ultrastructural immunocytochemistry of plasma proteins and cytochemistry of polysaccharides with ruthenium red and concanavalin A were combined with different types of fixation with or without prior vascular or airway washing, to study the surface of capillary endothelial and alveolar epithelial cells of the blood-air barrier in the rat lung. The endothelial and epithelial cell surface layers were found to have two components: a moveable part belonging to the cellular microenvironement made of plasma proteins and a deeper glucidic or anionic fixed part bound to the plasma membranes (cell coat).

Mechanisms of fiber-induced genotoxicity.

The mechanisms of particle-induced genotoxicity have been investigated mainly with asbestos fibers. The results are summarized and discussed in this paper. DNA damage can be produced by oxidoreduction processes generated by fibers. The extent of damage yield depends on experimental conditions: if iron is present, either on fibers or in the medium, damage is increased. However, iron reactivity does not explain all the results obtained in cell-free systems, as breakage of plasmid DNA was not directly associated with the amount of iron released by the fibers. The proximity of DNA to the site of generation of reactive oxygen species (ROS) is important because these species have an extremely short half-life. Damage to cellular DNA can be produced by oxidoreduction processes that originate from cells during phagocytosis. Secondary molecules that are more stable than ROS are probably involved in DNA damage. Oxidoreduction reactions originating from cells can induce mutations. Genotoxicity is also demonstrated by chromosomal damage associated with impaired mitosis, as evidenced by chromosome missegregation, spindle changes, alteration of cell cycle progression, formation of aneuploid and polyploid cells, and nuclear disruption. In some of these processes, the particle state and fiber dimensions are considered important parameters in the generation of genotoxic effects.

In vitro assessment of biopersistence using mammalian cell systems.

Biopersistence of fibers in the respiratory airways is a concept including both the physical durability of the fibers and their chemical stability. Physical durability results from several events of diverse origins: fiber epuration by the lung clearance mechanisms, internalization by scavenger cells and fiber splitting. Fibers residing in the lung milieu will be attacked and modified chemically, structurally, and physically (size and shape). Fiber toxicity, which is very likely to be dependent on physical fiber characteristics, will also be dependent on the duration of the fiber's stay in the tissue. Biopersistence, therefore, will be a key issue in determining fiber toxicity. So far, few in vitro systems have been used to study parameters involved in biopersistence. However, examples exist of investigations of fiber phagocytosis by mammalian cells in culture, either by macrophages, or epithelial or mesothelial cells, and studies have also been reported of the fate of internalized fibers in relation to fiber dimensions and chemical stability, especially within macrophages and mesothelial cells. The methods will be presented and discussed to determine to what extent the development of in vitro biophysical models could help in determining those parameters, known or thought to be relevant to fiber persistence.

Review of animal/in vitro data on biological effects of man-made fibers.

This paper reviews the investigations with man-made fibers (MMF). Insulation woods: glasswool (GW), rockwool (RW), slagwool (SW), glass microfibers (GMF), glass filaments (GFiI), and refractory ceramic fibers (RCF) have been used in experimental animals and in in vitro cell systems. A large heterogeneous number of fibers, methods of fiber preparation, size selection, aerosolization, fiber size, and fiber burden measurement were noted, rendering difficult a comparison between results. By inhalation, RCF and asbestos used as positive controls produced a significant tumor increase. In some studies, a low tumor yield was found after inhalation of insulation wools; when all inhalation data were gathered, a significant tumor increase was found with GW. However, it is difficult to draw definitive conclusions on the potential of other fiber types because, in addition to the different compositions of the fibers, differences in fiber number and sizes existed, especially in comparison with asbestos. Moreover, experiments using inoculation, especially by the intraperitoneal route revealed a carcinogenic potential of all fibers types but GFiI and SW. In these two groups a small number of animals has been investigated and the fiber characteristics were sometimes irrelevant. So far, a relationship between the carcinogenic potency and fiber dimensions has been established. Other fiber parameters may be of importance (surface chemistry, biopersistence, fiber structure, for example) but further investigations are necessary to determine the correlations between these parameters and tumor incidence. In vitro experiments have emphasized the fiber characteristics identified in vivo as playing a role in the carcinogenic potency and should be developed as a better approach of the mechanistic effects of MMF.

Biological in vitro and in vivo responses of chrysotile versus amphiboles.

Although all commercial forms of asbestos have been demonstrated to be carcinogenic in animals, so far epidemiological data are controversial concerning what asbestos types are the most carcinogenic and fibrogenic in humans. In order to understand the early cellular events induced by fibrous particles, different in vitro studies (hemolysis, release of enzymes by macrophages, assays on cell culture systems) have been carried out in several laboratories; most of these studies have shown that cell and subcellular in vitro responses were different depending on fiber types: chrysotile versus amphiboles. This presentation compares the results of different laboratories with our data obtained by using a model which modifies the chemistry of the fibers by acid treatment. The acid-leached chrysotile and acid-treated amphibole fibers showed different biological responses in several in vitro systems used in comparison to unleached fibers. These differences in the in vitro reactivity were related to the chemical state of the fibers and might explain the differences in their effects in animals after intrapleural injection as assessed by the percentage of mesothelioma, the latency period, the survival time and the degree of pleural fibrosis. The carcinogenic effect of the fibers is discussed in relation of their in vitro inflammatory or cytotoxic responses.

A method to obtain a well-defined fraction of respirable para-aramid fibers.

We developed a preparation method to obtain respirable-sized fractions of para-aramid fibers. The procedure, based on floatability, consists of stirring and subsequent settling of p-aramid pulp in distilled water. Two distinct phases are obtained, with small fibers in the upper part of the suspension, which represents about 33% of the total volume. Optimal results were obtained when 2.0 g pulp was stirred for 15 hr in 800 ml distilled water containing 0.125% ethanol and settled for 5 hr. The mass yield ranged between 0.4 and 0.6%, more than 90% of the particles had an aspect ratio > or = 3:1. The mean fiber length was about 6 microns, and the mean fiber diameter was about 0.4 microns as determined by transmission and scanning electron microscopy. The number of fibers obtained was 4 x 10(6) fibers/micrograms under our standard conditions.

Microfiltration method for quantitative study of fibrous particles in biological specimens.

Counting coated and uncoated inorganic fibers in sputum has been used to investigate the level of environmental or occupational asbestos exposure and the concentration of fibrous dusts in human lung. Inorganic fibers in sputum were counted by light microscopy after chemical digestion and microfiltration processing. The same method was used for processing gastric juice and lung tissue. There were no ferruginous bodies (FB) in sputum from 49 patients without any asbestos exposure. The study of sputum from 125 patients with various asbestos exposure pointed out a high correlation between the number of FB in sputum and the level of asbestos exposure. These 125 patients were classified into three groups according to the type of their asbestos occupational hazard: group I, raw asbestos workers; group II, workers manufacturing asbestos products; group III, workers with mixed industrial dust exposure. For these three groups, the mean number of FB in sputum was 100, 10, and 1, respectively. The comparison of the FB content of sputum and lung parenchyma showed the absence of FB in sputum when the concentration of FB in lung parenchyma was under 1000/cm(3) of lung parenchyma; above this concentration the number of FB in sputum was in good correlation with fiber concentration in lung parenchyma. A preliminary study with the use of gastric juice showed that gastric juice is a less sensitive sample for evaluating fiber concentration in lung. The microfiltration method for the counting of uncoated fibers gave results as accurate as those in the centrifugation method.

Comparative toxicities of different forms of asbestos on rat pleural mesothelial cells.

The effects of UICC crocidolite and chrysotile A, either oxalic acid-leached or unleached, on the viability, morphology and growth characteristics of rat pleural mesothelial cells (PMC) were examined; DQ12 quartz particles were also used. When asbestos fibers were added for 48 hr at the beginning of exponential growth, 20 or 50 micrograms/mL of chrysotile fibers were cytotoxic and no growth occurred; with 5 or 10 micrograms/mL a latent period was observed, and the mean population doubling time was increased. Chrysotile ingestion was associated with morphological changes (spreading, intense vacuolation); moreover, a large proportion of the cells was binucleated (more than 30% with 10 micrograms/mL). The oxalic acid-leached chrysotile inhibited growth at a concentration of 50 micrograms/mL; with 5 or 10 micrograms/mL, no spreading occurred, but a shrinkage of some cells was observed. A few large vacuoles were seen in the cytoplasm of the cells; there were fewer binucleated cells. Addition of 5 or 10 micrograms/mL of crocidolite, either leached or unleached, did not significantly change the growth rate, in spite of the presence of a large number of fibers inside the cells which persisted when the cells reached confluency. With 20 or 50 micrograms/mL, the mean population doubling time was increased in a dose-dependent manner. A slight vacuolation of the cells occurred. The sample of quartz did not modify the parameters studied in this report. The results confirm the different in vitro reactivities of the two kinds of unleached asbestos fibers. Leaching of chrysotile fibers decreased their reactivity; alternatively, leaching of crocidolite increased the effects on PMC.

Study of the stability of a paramagnetic label linked to mesoporous silica surface in contact with rat mesothelial cells in culture.

Stable radicals detectable by electron paramagnetic resonance (EPR) may be use in the investigation of early events in cell-particle toxicity. Piperidine-N-oxyl derivatives (nitroxides), covalently linked to the surface of a high surface area silica (used as model solid for the technique), served as probes in the investigation of the effects of incubation of silica particles with mesothelial cells. A mesoporous silica (MCM-41), prepared by precipitation from a micellar solution, was the most appropriate silica-based particle for this purpose, as its channels allow direct contact with small molecules but not with macromolecules. The cytotoxicity of this amorphous silica is very low, allowing relatively high particle loading in the cell cultures. Both the high surface area of the sample and the large amount of inorganic material extracted from the cell culture provide enough material to run reasonably intense EPR spectra. Computer-aided analysis of the EPR spectra of silica-bound nitroxides provided information on the sensitivity of the labeled silica monitoring different environments, e.g., to follow the path of particles in a mammalian cell culture. Upon contact of the particles with mesothelial cells, the mean distance among the labels at the silica surface decreased as a consequence of the release of oxidizing and/or radical moieties from the cells.

Studies on in vitro chrysotile-pleural mesothelial cell interaction: morphological aspects and metabolism of benzo-3,4-pyrene.

Cultures of rat pleural mesothelial cells (PMC) were exposed to nonlethal doses of UICC chrysotile A. The morphology was studied by optical and electron microscopy. The consequences of chrysotile ingestion on the rate of pinocytosis of horseradish peroxidase (HPR) metabolism and benzo-3-4-pyrene (BP) were studied. Nonlethal doses of chrysotile (5 micrograms/mL) induced a time-dependent vacuolation of PMC; a dose-dependent inhibition of the vacuolation was observed when PMC were pretreated with DMSO. The origin of the vacuoles is not clear, but some features of autophagy and lysosomal storage were observed. Chrysotile fibers did not modify the rate of pinocytosis of HRP. Similarly, the metabolism of BP was unchanged when BP and chrysotile were both added to the culture medium or when PMC were preincubated with the fibers 24 hr prior to the addition of BP.