Cause of death and neoplasia in mice continuously exposed to very low dose rates of gamma rays.

Four thousand 8-week-old SPF B6C3F1 mice (2000 of each sex) were divided into four groups, one nonirradiated (control) and three irradiated. The irradiated groups were exposed to (137)Cs gamma rays at dose rates of 21, 1.1 and 0.05 mGy day(-1) for approximately 400 days with total doses equivalent to 8000, 400 and 20 mGy, respectively. All mice were kept until natural death, and pathological examination was performed to determine the cause of death. Neoplasms accounted for >86.7% of all deaths. Compared to the nonirradiated controls, the frequency of myeloid leukemia in males, soft tissue neoplasms and malignant granulosa cell tumors in females, and hemangiosarcoma in both sexes exposed to 21 mGy day(-1) were significantly increased. The number of multiple primary neoplasms per mouse was significantly increased in mice irradiated at 21 mGy day(-1). Significant increases in body weights were observed from 32 to 60 weeks of age in males and females exposed to 1.1 mGy day(-1) and 21 mGy day(-1), respectively. Our results suggest that life shortening (Tanaka et al., Radiat. Res. 160, 376-379, 2003) in mice continuously exposed to low-dose-rate gamma rays is due to early death from a variety of neoplasms and not from increased incidence of specific neoplasms.

Morphologic and functional heterogeneity among rat alveolar macrophage fractions isolated by centrifugation on density gradients.

Lavaged rat bronchoalveolar cells were separated into four different density fractions (I-IV) by centrifugation through a discontinuous Percoll gradient. Maximum cell size was found in the lowest density fraction (I) and minimal cell size in the highest density fraction (IV), showing an inverse correlation with cell density. These fractions contained alveolar macrophages (AM) in the proportion of 97% or more by morphologic criteria, while phagocytic AM was approximately 80% in each fraction. Although the proportion of Ia antigen-positive AM was low in each fraction, it was elevated after incubation with supernatants from concanavalin A (Con A) stimulated spleen cell cultures, with greater Ia expression in higher density fractions. Differences in these fractions were also noted in several functions, including Fc receptor activity, chemotactic migration, tumoricidal activity, and interleukin 1 (IL-1) production, all of which were greater in higher density fractions (III and IV). Con A-induced T cell proliferation was, however, suppressed by these higher density fractions, whereas only an intermediate density fraction (II) enhanced T cell responses. These results indicate morphologic and functional heterogeneity among rat AM.

Exudation of proliferative macrophages in local inflammation in the peritoneum.

Thioglycollate (TG)-elicited peritoneal macrophages (m phi s) were highly proliferative and formed m phi colonies in vitro in the presence of m phi colony-stimulating factor (M-CSF), while resident peritoneal m phi s did not. To determine whether such proliferative m phi s are immigrant or locally activated resident m phi s, mice depleted of bone marrow cells and circulating monocytes by bone-seeking radiostrontium (89Sr) were injected intraperitoneally with TG. For control (88Sr) and splenectomized (Spx) mice, more than 4 x 10(4) m phi colony-forming cells (M-CFCs) per mouse were recovered in the peritoneal lavage fluid 5 days after TG injection. 89Sr-treated mice, on the other hand, had only 20% of those in the control mice. Splenectomized and 89Sr-treated (Spx/89Sr) mice showed further depletion of bone marrow cells and monocytes and, as expected, total numbers of peritoneal M-CFCs were severely depressed to less than 1% of those in the control mice. The results suggest that levels of peritoneal M-CFCs are strongly dependent on the presence of radiosensitive bone marrow cells and circulating monocytes, and resident peritoneal m phi s activated locally by inflammatory stimuli do not form m phi colonies under the defined conditions.

Differential induction of prostaglandin E2-dependent and -independent immune suppressor cells by tumor-derived GM-CSF and M-CSF.

Both prostaglandin E2 (PGE2)-dependent (indomethacin-sensitive) and PGE2-independent (indomethacin-insensitive) suppressor cell activities that inhibited mitogenic T cell blastogenesis appeared in the bone marrow and spleen of mice on days 20 to 30 following transplantation of NFSA fibrosarcoma molecularly expressing mRNA for both macrophage (M) and granulocyte-macrophage (GM) colony-stimulating factors (CSFs). The present study was done to characterize the two different suppressor cells isolated from NFSA tumor-bearing mice and to verify a role of CSFs in the induction of suppressor cells in vitro. Whereas PGE2-releasing suppressor cells were found in bone marrow and spleen cells isolated from tumor-bearing mice, indomethacin-insensitive suppressor cells in both tissues were localized predominantly in adherent cell fractions. An increase in Mac-1+ and Mac-2+ spleen cell populations with two to three times larger cell volumes was observed, and both showed strong PGE2-releasing capacity and indomethacin-sensitive suppressor cell activity. However, after elimination of Mac-1+ or Mac-2+ cells, bone marrow cells still showed higher PGE2-releasing capacity and indomethacin-sensitive suppressor activity. The in vitro cultures of normal bone marrow and spleen cells with NFSA cell conditioned medium (NFSA-CM) induced heterogeneous mixtures of indomethacin-sensitive and - insensitive suppressor cells like those observed in cultures with the combination of M-CSF and GM-CSF. However, cultures with either GM-CSF or M-CSF resulted in the induction of indomethacin-sensitive suppressor cells by GM-CSF and of indomethacin-insensitive suppressor cells by M-CSF. In addition, NFSA-CM pretreated with anti-GM-CSF antibody induced indomethacin-insensitive suppressor cells in in vitro cultures of bone marrow and spleen cells. These results suggest that two distinctly different suppressor cells developed under hemopoiesis of myelomonocytic lineage cells are regulated differentially by the two macrophage growth factors, M-CSF and GM-CSF.

Regulation of phospholipase A2 activation and arachidonic acid metabolism in an interleukin-3-dependent macrophage-like cell line.

An interleukin 3 (IL-3)-dependent macrophage-like cell line, 11-1-B3, was newly established from CBA/J mouse bone marrow cell cultures. Assay of eicosanoids in the culture supernatants of the intact and [3H]arachidonic acid (AA)-prelabeled cells showed that, after stimulation with the Ca2+ ionophore A23187, the 11-1-B3 cells synthesized and released relatively large amounts of prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) but not LTC4. In addition, 11-1-B3 cells showed Ca(2+)-dependent and alkaline pH-optimal phospholipase A2 (PLA2) activity that preferentially hydrolyzed cleavage of sn-2-arachidonyl- but not sn-2-oleoylphosphatidylcholine. The cellular enzyme was distributed with 90% of the activity in the cytosol and 10% in the membrane fraction. Treatment of cells with A23187 for 5-10 min resulted in five- to sevenfold increases in the membrane-associated PLA2 but activity in the cytosol was unchanged. This increase in membrane-associated enzyme activity was transient, returning to the pretreatment distribution after 30 min. In sharp contrast, phorbol myristate acetate (PMA) stimulation failed to induce either eicosanoid release or PLA2 activation, although PMA induced translocation of protein kinase C (PKC) to the membrane fraction within 10 min. The data suggest that increases in cellular Ca2+ directly activate membrane-associated PLA2 and consequently initiate AA metabolism; PKC activation by PMA requires additional steps to activate PLA2, a mechanism that is apparently deficient in the IL-3-dependent M phi-like cells.

Effects of instilled fibrogenic particles on the clonal growth of murine pulmonary alveolar macrophages.

Murine pulmonary alveolar macrophages (PAM) form macrophage colonies in vitro with colony-stimulating factors, which stimulate the clonal growth of radioresistant alveolar colony-forming cells (AL-CFC). The toxic effects of fibrogenic mineral dust particles on AL-CFC were investigated after intratracheal instillation into mice. Exposure to either crocidolite asbestos or silica (Min-u-sil) induced a significant depletion of AL-CFC as well as a decrease in PAM recovery compared to either untreated or titanium dioxide-exposed animals. Such effects were also noted with different doses (50-200 micrograms/animal) of instilled particles. The plating efficiency of AL-CFC was depleted in PAM exposed to fibrogenic particles in vitro, but not when exposed to nonfibrogenic titanium dioxide particles. These results indicate the toxic effects of fibrogenic dust particles on the clonal growth of PAM, cells which play a role in the clearance of inhaled particles from the lung and in subsequent pathologic processes.

Strain differences in carcinogenic and hematopoietic responses of mice after injection of plutonium citrate.

The carcinogenicity of injected (239)Pu citrate was compared in female mice of the C3H, C57BL/6 and BC3F(1) hybrid strains with different spectra of spontaneous or radiation-induced tumors. A significant reduction in survival due to early death caused particularly by the induction of osteosarcomas was noted in each strain after injection of 500 Bq or more. The dose response of osteosarcomas appeared to have a similar pattern in each strain except for the differences in the skeletal dose ranges for the maximum induction. While the incidence of lymphoid tumors decreased as that of osteosarcomas increased sharply to the maximum at higher doses, their histological phenotypes were predominantly non-thymic, pre-B-cell leukemic lymphomas compared to the controls in each strain. Myeloid leukemias were not highly induced in any of the control and (239)Pu-injected mice, and solid tumors involving the other organs were reduced in each strain after injection of 500 Bq or more. To follow up the hematological kinetics related to alpha-particle irradiation of bone marrow stem cells, sequential examinations were done in mice of each strain within 1 year after injection of 5000 Bq. The numbers of peripheral white blood cells and bone marrow cells were consistently reduced in each strain from 90 days on, while spleen cells increased from 180 days on. Granulocyte-macrophage and macrophage colony-forming cells were also consistently reduced in the bone marrow, with a compensatory increase in the spleen from 90 days on. These findings indicate that the carcinogenic and hematopoietic responses were specific to alpha-particle irradiation and were independent of mouse strain after injection with (239)Pu citrate.

Pathogenetic process of lung tumors induced by inhalation exposures of rats to plutonium dioxide aerosols.

Sequential examinations were done on the pulmonary cytokinetics and pulmonary lesions in rats after inhalation exposure to (239)PuO(2) aerosols to investigate the pathogenesis of lung tumors. Total cell yields of lavaged bronchoalveolar cells as well as the estimated numbers of pulmonary alveolar macrophages were significantly reduced from 1 to 3 months after exposure but recovered thereafter to the control levels. The proportions of multinucleated or micronucleated pulmonary alveolar macrophages increased significantly in lavaged cells from 1 month, and the increase was sustained up to 18 months after exposure. Both tumor necrosis factor and nitric oxide were shown to be differentially released from stimulated cultures of pulmonary alveolar macrophages during the period from 6 to 18 months after exposure. The labeling indices of alveolar and bronchiolar epithelial cells treated with 5-bromo-2'-deoxyuridine increased significantly in lungs from 3 months and were sustained up to 18 months after exposure. Histopathological examinations revealed that after the early inflammation, hyperplasia and metaplasia of the lining of the bronchioloalveolar epithelium were predominant from 3 to 6 months, while adenomatous or adenocarcinomatous lesions appeared and developed from 12 months after exposure. The appearance of primary lung tumors, almost all of which were adenomas and adenocarcinomas, was found in the dose range of 1 to 2 Gy from 12 months after exposures. These results indicate that the pathogenetic process initiated by early cellular damage and alterations associated with inflammation is followed by the proliferative and metaplastic lesions of pulmonary epithelium, leading to the appearance and development of pulmonary neoplasms from 1 year after the inhalation exposures in rats that received a minimum lung dose of more than 1 Gy.

Carcinogenesis in mice after injection of soluble plutonium citrate.

The carcinogenicity of injected soluble plutonium ((239)Pu) citrate was investigated in life-span animal experiments using mice of three strains, C3H, C57BL/6 and their hybrid BC3F(1), that have different spectra of spontaneous and radiation-induced tumors. Bone tumors, mostly osteosarcomas, were induced at skeletal doses of 0.6 to 0.7 Gy, and the incidence increased markedly at doses of 2 to 4 Gy in all strains, while lymphoid tumors appeared to decrease at higher doses; the incidences of bone tumors remained higher at higher doses, suggesting a differential and competitive dose response between bone and lymphoid tumors. Among the histological phenotypes of lymphoid tumors, nonthymic, pre-B-cell type leukemic lymphomas were induced preferentially and early, while thymic lymphomas and myeloid leukemias were rarely or never observed in any of the strains after injection of (239)Pu. These findings indicate a specificity of (239)Pu-induced carcinogenesis in mice that is different from that of external low-LET irradiations.

Mutations in Tp53 gene sequences from lung tumors in rats that inhaled plutonium dioxide.

We investigated mutations of the Tp53 tumor suppressor gene (formerly known as p53) in the lung tumors induced in rats after inhalation of plutonium dioxide ((239)PuO(2)) aerosols. Exons 5, 6, 7 and 8 of the Tp53 gene were examined for mutations by single-strand conformation polymorphism (SSCP) analysis of polymerase chain reaction (PCR)-amplified fragments and direct sequencing analysis. Almost all the mutations were guanine (G) to adenine (A) transitions and were distributed in exons 5 and 6. The Tp53 mutations occurred in lung tumors of various phenotypes and levels of immunohistochemical staining of Tp53 nuclear protein. These results indicate that the Tp53 mutations are not associated with tumor phenotype and nuclear accumulation of Tp53 protein, and that the G to A transition could be a common point mutation in the lung tumors seen after the inhalation of plutonium dioxide. The point mutations in the Tp53 gene seem to play a role in the development of lung tumors in rats after inhalation exposures to plutonium dioxide.

Heterogeneity of the radiosensitivity and origins of tissue macrophage colony-forming cells.

Previous studies suggest that the radiosensitivity and origin of tissue macrophage precursors differ from those of hemopoietic macrophage colony-forming units (CFU-Ms) committed to macrophage-lineage cells. We assessed the origins of tissue macrophage colony-forming cells (M-CFCs) in mice by comparing their kinetics and radiosensitivities in the normal steady state and under the conditions of bone marrow depletion by 89Sr-administration and/or splenectomy. The results indicate that the radiosensitive peritoneal M-CFCs elicited by thioglycollate are derived from bone marrow macrophage precursors; where as alveolar M-CFCs, which are radioresistant, are self-sustained locally and independent of hemopoietic macrophage precursors. In contrast, highly radiosensitive liver M-CFCs are probably derived from CFU-Ms that appear to be propagated in the spleen in association with hemopoietic responses.

High frequency of leukemic lymphomas with osteosarcomas but no myeloid leukemias in C3H mice after 239Pu citrate injection.

Female C3H strain mice, which do not spontaneously exhibit frequent bone tumors and myeloid leukemias, were injected intraperitoneally with various doses of 10 to 12,000 Bq/animal of monomeric 239Pu citrate to clarify lifetime carcinogenesis caused by alpha-particles distributed mainly in the skeleton. Survival time was reduced significantly at mean skeletal doses of more than 2.93 Gy and was accompanied by marked life-shortening as compared to the controls due to an earlier increase in neoplastic or non-neoplastic death. Bone tumors, almost all of which were osteosarcomas, were not found in the controls, whereas their incidence increased sharply to a maximum of 96% at 6.92 Gy, then dropped at higher doses (20% at 42.4 Gy). Although lymphoid tumors were present in 20% of the control animals, their incidence dropped to zero at 6.92 Gy, then increased at higher doses (36% at 25.5 Gy). Non-thymic, mostly pre-B cell type, leukemic lymphomas mainly occurred at early time after 239Pu-injection; whereas, in the controls thymic, lymphocytic or histiocytic lymphomas occurred only at a later time. Of the other soft tissue tumors, neither myeloid leukemias nor myelogenous neoplasms were found in the controls or the 239Pu-injected animals. Tumors affecting the lungs, liver, ovaries, and skin were much fewer or not found at mean doses of more than 2.93 Gy. These results indicate dose-dependent, differential tumor induction resulting from bone and lymphoid tumor competition after an injection of plutonium.

Differential induction of bone and hematopoietic tumors in C3H mice after the injection of 239Pu citrate.

Although alpha-emitting plutonium is easily distributed in the skeleton via circulation and subsequently induces bone tumors, there is little evidence that hematopoietic neoplasias are highly induced even though bone marrow stem cells are irradiated internally by alpha particles. We injected groups of female C3H strain mice with doses of 239Pu citrate from 500 to 10000 Bq to investigate the dose-related spectrum of tumor types induced during a lifetime. Survival time was reduced strikingly in all the injected mice due to much earlier induction of bone and lymphoid tumors as compared to the control animals that showed a variety of soft tissue tumors after a longer period of survival. Induction of osterosarcomas was dose-dependent, being maximal in 70% of the animals that received a mean skeletal dose of 10 Gy or less, but was 48% or less at 20 Gy or more. Non-thymic lymphomas accompanied by lymphocytic leukemia were observed in only 4-6% of the animals that received a dose of 10 Gy or less whereas it was maximal in 17-19% at 20 Gy or more. In contrast, there were no bone tumors in the control animals, rather thymic lymphomas or histiocytic lymphomas were found very late in 20% and other soft tissue tumors, including lung, liver and ovary tumors, were noted in 60%. Neither myeloid leukemia nor other myelogenous neoplasias were found in the control and 239Pu-injected animals that received a mean skeletal dose of 3 Gy or more. These results indicate that the differential induction of bone tumors and hematopoietic tumors in mice depends on the dose range and the time after the injection of plutonium.

Radiosensitivity of macrophage colony-forming cells-implications for their heterogeneity.

We compared the radiosensitivity of macrophage (M) colony-forming cell (CFC) in and outside the hemopoietic bone marrow. Murine bone marrow cells (BMC) are stimulated by either macrophage colony-stimulating factor (CSF-1) or murine recombinant granulocyte-macrophage colony-stimulating factor (rGM-CSF) to form M- and granulocyte- macrophage (GM) colonies on soft agarose medium, whereas both peritoneal exudate cells (PEC) and pulmonary alveolar macrophages (PAM) also have a capacity to make only M- colonies either by rGM-CSF or CSF-1. The clonal growth of peritoneal exudate CFC (PE-CFC) and alveolar macrophage CFC (AL-CFC) was more effectively achieved with rGM-CSF, and their cloning efficiencies were much higher than bone marrow CFC (BM-CFC). Following in vitro exposures to gamma-irradiation (1Gy/min), the dose-survival response of each M-CFC grown by cultures with either rGM-CSF or CSF-1 indicates that the radiosensitivity was the highest in BM-CFC, whereas Al-CFC was more radioresistant than PE-CFC. Surface antigen expression, such as macrophage-specific F4/80, on these CFCs, was invariable before or after irradiation except that it was diminished on irradiated PE-CFC. These results indicate the heterogeneity of tissue M-CFCs in their radiosensitivities as well as in responsiveness to CSFs.

Differential dose responses of pulmonary tumor types in the rat after inhalation of plutonium dioxide aerosols.

Dose responses were compared among primary lung tumors and their histological types induced by a single inhalation exposure of female Wistar strain rats to submicron-size and polydispersed aerosols of plutonium dioxide (239PuO2). While the primary lung tumors were found only in 2.3% of the unexposed control animals, the frequency of all the primary lung tumors in the exposed animals was 44% at the mean lung dose of 0.71 Gy, and increased sharply at the doses of 1.5 Gy or more, reaching the maximum of 97% at 5.4 Gy, and the dose responses around at 1.0 Gy were different between benign and malignant lung tumors. Almost all the pulmonary tumors in the exposed animals were classified into epithelial types such as adenomas, adenocarcinomas, adenosquamous carcinomas, and squamous cell carcinomas. The dose responses were different between these tumor types as shown by the peak incidence of adenomas at 0.71 Gy, adenocarcinomas at 2.9 Gy, adenosquamous and squamous cell carcinomas at 5.4-8.5 Gy, respectively. As the magnitudes of neoplastic lesions in pulmonary carcinomas were expressed by histological scores, metaplasias and adenomatous lesions most frequently appeared at doses of 1.5 Gy, while the appearance and increase of carcinomatous lesions differed in the dose ranges as shown by the peak incidence of adenocarcinomatous lesions at 2.9 Gy, and adenosquamous or squamous lesions at 5.4-6.6 Gy. These results indicate a differential dose response of pulmonary carcinogenesis in which metaplasias and benign adenomas were induced at lower doses (< 1.0 Gy), whereas malignant carcinomas were induced at relatively higher doses (> 1.5 Gy). Together with the increase of carcinomatous lesions at higher doses, the intranuclear p53 protein accumulation was detectable, but only in a few percentages of malignant carcinomas.

The distribution and retention of 89Sr in mice after a single intravenous injection of high and low doses.

Female C3H/He mice, aged 12-17 weeks, were injected intravenously with 89SrCl2 at doses of 0.74kBq g-1 body weight (low dose) or 74.0kBq g-1 (high dose). During the first week after injection, approximately 45% of the injected 89Sr was excreted in the urine and feces in both dose groups. The excretion of 89Sr decreased thereafter. The total excretion up to 56 days after injection was 63.0% and 64.5% of the injected dose in the high and low dose groups, respectively. There was no significant difference in the excretion of 89Sr in the urine between the two groups, while the fecal excretion in the low dose group was slightly higher than in the high dose group. The retention of 89Sr in the right and left femurs was 1.30 and 1.46% of the injected dose on day 7, which decreased to 0.64 and 0.70% on day 56, in the high and low dose groups, respectively. No significant difference was observed in bone retention of 89Sr between the two groups. The amount of 89Sr distributed to the soft tissues was very small and became undetectable by 42 days after injection.

Lung retention of Pu following inhalation of PuO2 in rats measured using a whole body counter.

A lung retention function on the amount of PuO2 in rats was determined. Five rats were exposed to polydisperse aerosols of PuO2 having a size of 0.47 micron activity median aerodynamic diameter. The initial lung burden was between 1990 Bq and 2960 Bq. Instead of serial-sacrifice study, in vivo counting of low energy L X-rays with thin NaI(Tl) scintillation detectors was used to follow the lung retention of Pu at various intervals up to 468 days after inhalation. The calibration of this counting system was made by measuring lung activity of rats sacrificed for other experimental purposes. It was confirmed that the potential skin contamination and the Pu translocated to the other organs was not counted at the present counting geometry. Our results showed that 77% of PuO2 deposited deep in the lung was cleared with a half-time of 53 days, whereas the residual 23% stayed there for a longer period (a half-time of 795 days).

High incidence of malignant lung carcinomas in rats after inhalation of 239PuO2 aerosol.

Female Wistar strain rats were exposed to a single inhalation of a submicron-size aerosol of high-fired 239PuO2 to investigate pulmonary carcinogenesis during lifespan periods. The absorbed lung doses of the exposed animals ranged from 0.6 to 12 Gy and were well correlated with the initial lung deposition (ILD) of 0.1 to 2.3 kBq. Survival and induction of primary lung tumors in 116 exposed rats were compared with those in 56 untreated control rats in respect to lung doses received. Mean survival time was greatly reduced, and the cumulative incidence of total lung tumors was markedly increased to 90-100% in rats that received more than 4 Gy, whereas of the controls only one animal (1.8%) died of primary lung tumors. Primary but benign adenomas were present in exposed animals given 1.0 Gy or less, and the incidence of adenomas was 22-25% at 4-5 Gy, but decreased sharply to 3-5% at 6-8 Gy. In contrast, no malignant carcinomas, including adenocarcinomas, adenosquamous carcinomas and squamous cell carcinomas, developed at a dose of less than 1.0 Gy, whereas they were present in 75% or more of the rats given 4-10 Gy, but only in 55% at 12 Gy. Although there were no clear differences in the dose and time required for induction among the carcinoma types, all tended to develop in earlier periods after inhalation than adenomas. Despite the limited number of exposed animals that received lower doses, results suggest that malignant lung carcinomas are highly and early induced and have a different dose-effect relationship than benign adenomas at doses of more than 1 Gy after inhalation exposure to 239PuO2.

Time-dependent changes of microscopic localization of intravenously administered colloidal carbon particles in mouse lymph nodes.

Time-dependent changes of microscopic localization of intravenously administered colloidal carbon particles were studied in mouse lymph nodes. Carbon particles were preferentially trapped by postcapillary venules (PCV) immediately after injection, and migrated easily outside of PCV either through intercellular space of the PCV endothelium or by phagocytic process during the 1 hr after injection. Particles were thereafter up-taken by pericytes and macrophages around PCV during the 24 hr, and consequently distributed throughout the cortex and medulla. Finally, they migrated to the medullary lymphatic sinuses, and phagocytosed by endothelial cells. Redistribution process of particles via the lymphatic sinuses from the regional lymph was observed during 30 minutes or 10 to 14 days after injection in different lymph nodes.

Pulmonary deposition and effects of inhaled silica particles after short-term exposures in the rat.

The present study was designed to evaluate the pulmonary deposition and the effects of inhaled silica particles in the rat model. Wistar (W/M strain) rats were exposed to silica aerosols generated from a fluidized bed dust generator for 1 hr a day, intermittently for 6 days, using a "nose-only" inhalation chamber. After the cessation of the exposures, analysis of lavaged bronchoalveolar cells (BAC) and histological examinations of lungs and tracheobronchial lymph nodes (TBLN) were performed during a period of 6 months. Total cell yields and the proportions of alveolar macrophages (AM) in BAC were not altered, whereas the proportions of lymphocytes in BAC were significantly increased in the exposed animals. Although the proportions of silica-laden AM in BAC were gradually decreased during the 6 months, particle-laden AM were predominantly and persistently observed in the alveoli under light microscopy. Silica particles were also identified in macrophages of granulomatous nodules in pulmonary peribronchial lymphoid tissues(PBLT) and TBLN, indicating the translocation of particles via the lymph. Associated with pulmonary particle deposition, some characteristic histopathological features were evident, including thickening of alveolar duct bifurcations and lymphocyte infiltrations both in the alveolar sacs and around the interstitial blood vessels. At later months after the exposures, the alveolar interstitium was thickened with the increase of fibroblasts and collagen. These results implicate that short-term exposures of silica particles in the rat can evoke histopathologic changes in the lungs and lymphatic tissues, associated with their deposition and translocation.