The Syndrome of Sea Cucumber (Apostichopus japonicus) Infected by Virus and Bacteria

A outbreak of disease with symptoms of evisceration and skin ulteration led to mass mortality in sea cucumber Apostichopus japonicus cultivated in indoor ponds near the Dalian coast from December 2004 to April 2005. Spherical virus particles with a diameter of 75-200 nm were found in the cytoplasm of cells in the water-system, the alimentary canal and in the respiratory trees of the diseased and dying sea cucumber individuals by electron microscopic observation of ultrathin sections. Examination by negative stained samples revealed that all the diseased sea cucumbers were infected by the virus, while the healthly ones cultivated outside the contagious area were not. Two bacterial strains were also isolated from the diseased animals. When exposed to a medium containing the virus particles, regardless of whether the bacterial suspension was added,healthy sea cucumbers exhibited identical disease symptoms as the ones in the indoor ponds, and had a mortality of 90%-100%. However, when exposed to a medium in which there was only one of the two bacterial strains, 30%-80% of the sea cucumbers were infected and nearly 20% died. Negative staining showed that the viral particles were detected only in the bodies of the tested animals that were exposed to the viral medium. Histopathologically, the diseased sea cucumbers are characterized by karyopycnosis, and disintegration of the endoplasmic reticula and mitochondria in the epithelial cells in the water-system, the respiratory tree and the alimentary canal.     

The syndrome of sea cucumber (Apostichopus japonicus) infected by virus and bacteria

A outbreak of disease with symptoms of evisceration and skin ulteration led to mass mortality in sea cucumber Apostichopus japonicus cultivated in indoor ponds near the Dalian coast from December 2004 to April 2005. Spherical virus particles with a diameter of 75–200 nm were found in the cytoplasm of cells in the water-system, the alimentary canal and in the respiratory trees of the diseased and dying sea cucumber individuals by electron microscopic observation of ultrathin sections. Examination by negative stained samples revealed that all the diseased sea cucumbers were infected by the virus, while the healthly ones cultivated outside the contagious area were not. Two bacterial strains were also isolated from the diseased animals. When exposed to a medium containing the virus particles, regardless of whether the bacterial suspension was added, healthy sea cucumbers exhibited identical disease symptoms as the ones in the indoor ponds, and had a mortality of 90%–100%. However, when exposed to a medium in which there was only one of the two bacterial strains, 30%–80% of the sea cucumbers were infected and nearly 20% died. Negative staining showed that the viral particles were detected only in the bodies of the tested animals that were exposed to the viral medium. Histopathologically, the diseased sea cucumbers are characterized by karyopycnosis, and disintegration of the endoplasmic reticula and mitochondria in the epithelial cells in the water-system, the respiratory tree and the alimentary canal. Fundation items: Supported by commonweal program of State Oceanic Administration of China (200705007), Science and Technology Project of Liaoning Province (2004203001) and Science and Technology Project of Dlian (2004 BINS030).     

The Syndrome of Sea Cucumber （Apostichopusjaponicus） Infected by Virus and Bacteria

A outbreak of disease with symptoms of evisceration and skin ulteration led to mass mortality in sea cucumber Apostichopus japonicus cultivated in indoor ponds near the Dalian coast from December 2004 to April 2005. Spherical virus particles with a diameter of 75-200 nm were found in the cytoplasm of cells in the water-system, the alimentary canal and in the respiratory trees of the diseased and dying sea cucumber individuals by electron microscopic observation of ultrathin sections. Examination by negative stained samples revealed that all the diseased sea cucumbers were infected by the virus, while the healthly ones cultivated outside the contagious area were not. Two bacterial strains were also isolated from the diseased animals. When exposed to a medium containing the virus particles, regardless of whether the bacterial suspension was added, healthy sea cucumbers exhibited identical disease symptoms as the ones in the indoor ponds, and had a mortality of 90%-100%. However, when exposed to a medium in which there was only one of the two bacterial strains, 30%-80% of the sea cucumbers were infected and nearly 20% died. Negative staining showed that the viral particles were detected only in the bodies of the tested animals that were exposed to the viral medium Histopathologically, the diseased sea cucumbers are characterized by karyopycnosis, and disintegration of the endoplasmic reticula and mitochondria in the epithelial cells in the water-system, the respiratory tree and the alimentary canal.     

Viral Membrane Fusion and Nucleocapsid Delivery into the Cytoplasm are Distinct Events in Some Flaviviruses

Flaviviruses deliver their genome into the cell by fusing the viral lipid membrane to an endosomal membrane. The sequence and kinetics of the steps required for nucleocapsid delivery into the cytoplasm remain unclear. Here we dissect the cell entry pathway of virions and virus-like particles from two flaviviruses using single-particle tracking in live cells, a biochemical membrane fusion assay and virus infectivity assays. We show that the virus particles fuse with a small endosomal compartment in which the nucleocapsid remains trapped for several minutes. Endosomal maturation inhibitors inhibit infectivity but not membrane fusion. We propose a flavivirus cell entry mechanism in which the virus particles fuse preferentially with small endosomal carrier vesicles and depend on back-fusion of the vesicles with the late endosomal membrane to deliver the nucleocapsid into the cytoplasm. Virus entry modulates intracellular calcium release and phosphatidylinositol-3-phosphate kinase signaling. Moreover, the broadly cross-reactive therapeutic antibody scFv11 binds to virus-like particles and inhibits fusion.     

Immunocytochemical analysis of Uukuniemi virus budding compartments: role of the intermediate compartment and the Golgi stack in virus maturation.

Previous studies have suggested that Uukuniemi virus, a bunyavirus, matures at the membranes of the Golgi complex. In this study we have employed immunocytochemical techniques to analyze in detail the budding compartment(s) of the virus. Electron microscopy of infected BHK-21 cells showed that virus particles are found in the cisternae throughout the Golgi stack. Within the cisternae, the virus particles were located preferentially in the dilated rims. This would suggest that virus budding may begin at or before the cis Golgi membranes. The virus budding compartment was studied further by immunoelectron microscopy with a pre-Golgi intermediate compartment marker, p58, and a Golgi stack marker protein, mannosidase II (ManII). Virus particles and budding virus were detected in ManII-positive Golgi stack membranes and, interestingly, in both juxtanuclear and peripheral p58-positive elements of the intermediate compartment. In cells incubated at 15 degrees C the nucleocapsid and virus envelope proteins were seen to accumulate in the intermediate compartment. Immunoelectron microscopy demonstrated that at 15 degrees C the nucleocapsid is associated with membranes that show a characteristic distribution and tubulo-vesicular morphology of the pre-Golgi intermediate compartment. These membranes contained virus particles in the lumen. The results indicate that the first site of formation of Uukuniemi virus particles is the pre-Golgi intermediate compartment and that virus budding continues in the Golgi stack. The results raise questions about the intracellular transport pathway of the virus particles, which are 100 to 120 nm in diameter and are therefore too large to be transported in the 60-nm-diameter vesicles postulated to function in the intra-Golgi transport. The distribution of the virus in the Golgi stack may imply that the cisternae themselves have a role in the vectorial transport of virus particles.     

Morphogenesis of Sindbis virus in cultured Aedes albopictus cells.

Cultured mosquito cells were found to produce Sindbis virus nearly as efficiently as BHK-21 cells at 28 C. In virtually all of the cells observed in the electron microscope, virus morphogenesis was found to occur within complex vesicular structures which developed after viral infection. Viral nucleocapsids were first seen in these vesicles and appeared to be enveloped within these structures. The process of envelopment within these inclusions differed in some respects from the process previously described for the envelopment of nucleocapsids at the plasma membrane of vertebrae cells. Free nucleocapsids were only rarely seen in the cytoplasm of infected mosquito cells, and budding of virus from the cell surface was detected so infrequently that this process of virus production could not account for the amount of virus produced by the infected cells. The vast majority of extracellular virus was produced by the fusion of the virus-containing vesicles with the plasma membrane releasing mature virions and membrane nucleocapsid complexes in various stages of development.     

Effects of potential probiotic Bacillus subtilis T13 on growth, immunity and disease resistance against Vibrio splendidus infection in juvenile sea cucumber Apostichopus japonicus

A feeding experiment was conducted to determine influences of potential probiotic Bacillus subtilis T13 (isolated from intestine of healthy sea cucumbers) on growth, immunity and disease resistance against Vibrio splendidus infection in juvenile sea cucumbers Apostichopus japonicus. Animals were fed with diets containing B. subtilis T13 at 0, 10(5), 10(7) and 10(9) CFU/g for 30 days, respectively. At the end of the growth trial, fifteen sea cucumbers from each aquarium were sampled for immune indices measurement. Then twenty sea cucumbers from each replicate were challenged with V. splendidus. Results showed that administration of B. subtilis T13 had significant effect on the specific growth rates (SGR) of sea cucumbers (P < 0.05). Phagocytosis, respiratory burst activity and total nitric oxide synthase (T-NOS) activity were significantly improved in coelomocytes of sea cucumbers fed with T13 at 10(9) CFU/g diet (P < 0.05). The highest values of the total coelomocytes counts (TCC) and superoxide dismutase (SOD) activity were found in sea cucumbers fed diet containing T13 at 10(9) CFU/g. The cumulative mortality after V. splendidus challenge decreased significantly in sea cucumbers fed with T13 at dose of 10(9) CFU/g (P < 0.05). The present study confirmed the potential beneficial effects of B. subtilis T13 as dietary probiotic in juvenile A. japonicus.     

Brefeldin A arrests the maturation and egress of herpes simplex virus particles during infection.

Herpes simplex virus (HSV) requires the host cell secretory apparatus for transport and processing of membrane glycoproteins during the course of virus assembly. Brefeldin A (BFA) has been reported to induce retrograde movement of molecules from the Golgi to the endoplasmic reticulum and to cause disassembly of the Golgi complex. We examined the effects of BFA on propagation of HSV type 1. Release of virions into the extracellular medium was blocked by as little as 0.3 microgram of BFA per ml when present from 2 h postinfection. Characterization of infected cells revealed that BFA inhibited infectious viral particle formation without affecting nucleocapsid formation. Electron microscopic analyses of BFA-treated and untreated cells (as in control cells) demonstrated that viral particles were enveloped at the inner nuclear membrane in BFA-treated cells and accumulated aberrantly in this region. Most of the progeny virus particles observed in the cytoplasm of control cells, but not that of BFA-treated cells, were enveloped and contained within membrane vesicles, whereas many unenveloped nucleocapsids were detected in the cytoplasm of BFA-treated cells. This suggests that BFA prevents the transport of enveloped particles from the perinuclear space to the cytoplasmic vesicles. These findings indicate that BFA-induced retrograde movement of molecules from the Golgi complex to the endoplasmic reticulum early in infection arrests the ability of host cells to support maturation and egress of enveloped viral particles. Furthermore, we demonstrate that the effects of BFA on HSV propagation are not fully reversible, indicating that maturation and egress of HSV type 1 particles relies on a series of events which cannot be easily reconstituted after the block to secretion is relieved.     

N-terminal amino acid sequence,immunohistochemical localization and tissue distribution of a plasma membrane protein (Prot17) of rat enterocytes

Summary Prot17, a protein of the basolateral membrane of rat small intestine with a mol.wt. of 17 kDa, can be isolated using a previously described method (Schiechl 1988). It occurs in the membrane as an oligomer with a mol.wt. of 90 kDa. In the present study a polyclonal antibody specific for Prot17 was used to explore by immunohistochemical techniques the tissue distribution of Prot17 and its ultrastructural localization within the cells. Furthermore the amino acid sequence of the N-terminal part of this molecule up to position 17 could be analyzed. The results are summarized as follows: Prot17 is a membrane anchored protein. Its partial amino acid sequence suggests that it is neither identical nor related to other known proteins. Immunofluorescence studies revealed, that it occurs only in epithelial cells. It is mainly found in the absorptive and goblet cells of the intestine and the acinar cells of the pancreas. Smaller quantities are found also in the bile duct epithelium of the liver, in the proximal tubule cells of the kidney and in the cells of the respiratory epithelium. Ultrastructural localization of Prot17 was possible in the intestinal epithelium and pancreas acinar cells. In both cell types it was found in the basolateral and microvillous membrane. In pancreas, Prot17 was also detected in the membrane of the zymogen granules. In the absorptive cells of the intestine Prot17 was found in both the membrane and the contents of subluminal vesicles. Furthermore, in apical granules of secretory cells of the respiratory epithelium binding of Prot17 specific antibody was found in the granular content, the membrane being negative.     

N-terminal amino acid sequence, immunohistochemical localization and tissue distribution of a plasma membrane protein (Prot17) of rat enterocytes

Prot17, a protein of the basolateral membrane of rat small intestine with a mol.wt. of 17 kDa, can be isolated using a previously described method (Schiechl 1988). It occurs in the membrane as an oligomer with a mol.wt. of 90 kDa. In the present study a polyclonal antibody specific for Prot17 was used to explore by immunohistochemical techniques the tissue distribution of Prot17 and its ultrastructural localization within the cells. Furthermore the amino acid sequence of the N-terminal part of this molecule up to position 17 could be analyzed. The results are summarized as follows: Prot17 is a membrane anchored protein. Its partial amino acid sequence suggests that it is neither identical nor related to other known proteins. Immunofluorescence studies revealed, that it occurs only in epithelial cells. It is mainly found in the absorptive and goblet cells of the intestine and the acinar cells of the pancreas. Smaller quantities are found also in the bile duct epithelium of the liver, in the proximal tubule cells of the kidney and in the cells of the respiratory epithelium. Ultrastructural localization of Prot17 was possible in the intestinal epithelium and pancreas acinar cells. In both cell types it was found in the basolateral and microvillous membrane. In pancreas, Prot17 was also detected in the membrane of the zymogen granules. In the absorptive cells of the intestine Prot17 was found in both the membrane and the contents of subluminal vesicles. Furthermore, in apical granules of secretory cells of the respiratory epithelium binding of Prot17 specific antibody was found in the granular content, the membrane being negative.     

Nucleocapsid-independent assembly of coronavirus-like particles by co-expression of viral envelope protein genes.

Budding of enveloped viruses has been shown to be driven by interactions between a nucleocapsid and a proteolipid membrane. By contrast, we here describe the assembly of viral envelopes independent of a nucleocapsid. Membrane particles containing coronaviral envelope proteins were assembled in and released from animal cells co-expressing these proteins' genes from transfected plasmids. Of the three viral membrane proteins only two were required for particle formation, the membrane glycoprotein (M) and the small envelope protein (E). The spike (S) protein was dispensable but was incorporated when present. Importantly, the nucleocapsid protein (N) was neither required not taken into the particles when present. The E protein, recently recognized to be a structural protein, was shown to be an integral membrane protein. The envelope vesicles were found by immunogold labelling and electron microscopy to form a homogeneous population of spherical particles indistinguishable from authentic coronavirions in size (approximately 100 nm in diameter) and shape. They were less dense than virions and sedimented slightly slower than virions in sucrose velocity gradients. The nucleocapsid-independent formation of apparently bona fide viral envelopes represents a novel mode of virus assembly.     

Production of CCHF Virus-Like Particle by a Baculovirus-Insect Cell Expression System

Crimean-Congo Haemorrhagic Fever Virus (CCHFV) is a tick-born virus of the Nairovirus genus within the Bunyaviridae family,which is widespread and causes high fatality.The nucleocapsid of CCHFV is comprised of N proteins that are encoded by the S segment.In this research,the N protein of CCHFV was expressed in insect cells using a recombinant baculovirus.Under an electron microscope,Virus-Like Particles (VLPs) with various size and morphology were observed in cytoplasmic vesicles in the infected cells.Sucro...     

Role of the parasporal body in causing toxicity of Bacillus thuringiensis toward Aedes aegypti larvae

A unique type of microorganism has been found causing an unusual disease in larvae of the clover cutworm, Scotogramma trifolii (Lepidoptera; Noctuidae). The organism contains DNA and reproduces exclusively by self-assembly forming enveloped reniform/bacilliform particles which measure 170 × 450 nm in negatively stained preparations. During initial stages of development, the organism apparently reproduces primarily within vesicles in the cytoplasm of a variety of cell types including hemocytes, and epidermal, fat body, and tracheal matrix cells. Most reproduction, however, occurs in vesicles that circulate in the hemolymph. These vesicles, most of which are derived from host cells, measure 2–10 μm in diameter, are highly refractile, reach populations as high as 10⁸/ml of hemolymph, and are diagnostic for the disease. The pathology caused by this organism, its shape and ultrastructure, and reproduction within vesicles indicate it is either a peculiar type of rickettsia, possibly related to those of the genus Rickettsiella, or a new type of invertebrate virus. Among its unusual features are its ability to induce the formation of reproductive vesicles from host cell components, and its apparent control of de novo ribosome and membrane synthesis within these vesicles as it develops. The possible relationship of this organism to baculoviruses and rickettsia is discussed.     

Human MxA protein inhibits the replication of Crimean-Congo hemorrhagic fever virus

Crimean-Congo hemorrhagic fever virus (CCHFV) belongs to the genus Nairovirus within the family Bunyaviridae and is the causative agent of severe hemorrhagic fever. Despite increasing knowledge about hemorrhagic fever viruses, the factors determining their pathogenicity are still poorly understood. The interferon-induced MxA protein has been shown to have an inhibitory effect on several members of the Bunyaviridae family, but the effect of MxA against CCHFV has not previously been studied. Here, we report that human MxA has antiviral activity against CCHFV. The yield of progeny virus in cells constitutively expressing MxA was reduced up to 1,000-fold compared with control cells, and accumulation of viral genomes was blocked. Confocal microscopy revealed that MxA colocalizes with the nucleocapsid protein (NP) of CCHFV in the perinuclear regions of infected cells. Furthermore, we found that MxA interacted with NP by using a coimmunoprecipitation assay. We also found that an amino acid substitution (E645R) within the C-terminal domain of MxA resulted in a loss of MxA antiviral activity and, concomitantly, in the capacity to interact with CCHFV NP. These results suggest that MxA, by interacting with a component of the nucleocapsid, prevents replication of CCHFV viral RNA and thereby inhibits the production of new infectious virus particles.     

Ultrastructural changes in salivary glands of tsetse, Glossina morsitans morsitans, infected with virus and rickettsia-like organisms

Electron microscope observations on enlarged hypertrophied salivary glands dissected from adult laboratory-reared male Glossina morsitans morsitans show a concurrent infection of the salivary gland tissue with rod-shaped virus particles and intracellular rickettsia-like organisms. The latter are found intracellular in the epithelium and in the gland lumen enclosed within lytic zones. The virus particles are found within the degenerating cytoplasm, nuclei, and lumen of the cell where they are especially numerous. Stratified epithelium and gland enlargement are a prominent feature of the infection. These observations suggest that biological associations between salivary gland tissue and diverse microbes may be more common than formerly recognized. The microbes appear to cause damage to salivary gland cells, causing hyperplasia which assumes pathologic proportions.     

Hepatitis B virus nucleocapsid particles do not cross the hepatocyte nuclear membrane in transgenic mice.

Transgenic mice that express the hepatitis B virus core protein were used to examine factors that influence the intracellular localization of nucleocapsid particles in the primary hepatocyte in vivo. In this model, viral nucleocapsid particles are strictly localized to the nucleus of the hepatocyte except when the nuclear membrane dissolves during cell division, at which time they enter the cytoplasm. The cytoplasmic nucleocapsid particles do not reenter the nucleus, however, when the nuclear membrane re-forms after cell division. The data support the notion that nucleocapsid particles can form de novo within the nucleus, and they suggest that performed nucleocapsid particles cannot be transported across the intact nuclear membrane in either direction. The results imply that nucleocapsid disassembly is probably required for entry of the hepadnaviral genome into the nucleus, and they question the role of the intranuclear viral nucleocapsid particle during the viral life cycle.     

Association of sindbis virus capsid protein with phospholipid membranes and the E2 glycoprotein: implications for alphavirus assembly

A late stage in assembly of alphaviruses within infected cells is thought to be directed by interactions between the nucleocapsid and the cytoplasmic domain of the E2 protein, a component of the viral E1/E2 glycoprotein complex that is embedded in the plasma membrane. Recognition between the nucleocapsid protein and the E2 protein was explored in solution using NMR spectroscopy, as well as in binding assays using a model phospholipid membrane system that incorporated a variety of Sindbis virus E2 cytoplasmic domain (cdE2) and capsid protein constructs. In these binding assays, synthetic cdE2 peptides were reconstituted into phospholipid vesicles to simulate the presentation of cdE2 on the inner leaflet of the plasma membrane. Results from these binding assays showed a direct interaction between a peptide containing the C-terminal 16 amino acids of the cdE2 sequence and a Sindbis virus capsid protein construct containing amino acids 19-264. Additional experiments that probed the sequence specificity of this cdE2-capsid interaction are also described. Further binding assays demonstrated an interaction between the 19-264 capsid protein and artificial vesicles containing neutral or negatively charged phospholipids, while capsid protein constructs with N-terminal truncations displayed either little or no affinity for such vesicles. The membrane-binding property of the capsid protein suggests that the membrane may play an active role in alphavirus assembly. The results are consistent with an assembly process involving an initial membrane association, whereby an association with E2 glycoprotein further enhances capsid binding to facilitate membrane envelopment of the nucleocapsid for budding. Collectively, these experiments elucidate certain requirements for the binding of Sindbis virus capsid protein to the cytoplasmic domain of the E2 glycoprotein, a critical event in the alphavirus maturation pathway.     

Bidirectional virus secretion and nonciliated cell tropism following Andes virus infection of primary airway epithelial cell cultures

Hantavirus pulmonary syndrome (HPS) is an acute disease resulting from infection with any one of a number of New World hantaviruses. HPS has a mortality rate of 40% and, unlike many other severe respiratory diseases, often occurs in young, healthy adults. Infection is usually initiated after inhalation of rodent excreta containing virus particles, but human-to-human transmission has been documented. Postmortem tissue samples show high levels of viral antigen within the respiratory endothelium, but it is not clear how the virus can traverse the respiratory epithelium in order to initiate infection in the microvasculature. We have utilized Andes virus infection of primary, differentiated airway epithelial cells to investigate the ability of the virus to interact with and cross the respiratory epithelium. Andes virus infects the Clara and goblet cell populations but not the ciliated cells, and this infection pattern corresponds to the expression of beta(3) integrin, the viral receptor. The virus can infect via the apical or basolateral membrane, and progeny virus particles are secreted bidirectionally. There is no obvious cytopathology associated with infection, and beta(3) integrins do not appear to be critical for respiratory epithelial cell monolayer integrity. Our data suggest that hantavirus infection of the respiratory epithelium may play an important role in the early or prodrome phase of disease as well as serving as a source of virus involved in transmission.     

Electron microscope observations on intracellular virus-like particles associated with the cells of the Lucké renal adenocarcinoma

The common renal adenocarcinoma of the leopard frog was studied in thin sections with the electron microscope. Approximately a third of the tumors examined were found to contain spheroidal bodies of uniform size and distinctive morphology that are believed to be virus particles. These consist of hollow spheres (90 to 100 mmicro) having a thick capsule and a dense inner body (35 to 40 mmicro) that is eccentrically placed within the central cavity (70 to 80 mmicro). Virus particles of this kind occur principally in the cytoplasm but occasionally they are also found in the nucleus and in the extracellular spaces of the tumor. The intranuclear inclusion bodies that are visible with the light microscope are largely comprised of hollow, spherical vesicles with thin limiting membranes. These are embedded in a finely granular matrix. A few of the thin walled vesicles contain a dense inner body like that of the cytoplasmic virus particles. This suggests that they may be immature virus particles. The inclusion bodies are believed to be formed in the course of virus multiplication but they usually contain very few mature virus particles. Bundles of dense filaments and peculiar vacuolar inclusions also occur in the cytoplasm of the tumor cells. These seem to be related in some way to the presence of virus but their origin and significance remain obscure. These findings are discussed in relation to previous work suggesting that the Lucké adenocarcinoma is caused by an organ-specific filtrable agent. It is concluded that the "virus particles" found in electron micrographs of the tumor cells may be the postulated tumor agent. On the other hand, the possibility remains that the particles described here are not those that are causally related to the tumors.     

ELECTRON MICROSCOPE OBSERVATIONS ON INTRACELLULAR VIRUS-LIKE PARTICLES ASSOCIATED WITH THE CELLS OF THE LUCKÉ RENAL ADENOCARCINOMA

The common renal adenocarcinoma of the leopard frog was studied in thin sections with the electron microscope. Approximately a third of the tumors examined were found to contain spheroidal bodies of uniform size and distinctive morphology that are believed to be virus particles. These consist of hollow spheres (90 to 100 mµ) having a thick capsule and a dense inner body (35 to 40 mµ) that is eccentrically placed within the central cavity (70 to 80 mµ). Virus particles of this kind occur principally in the cytoplasm but occasionally they are also found in the nucleus and in the extracellular spaces of the tumor. The intranuclear inclusion bodies that are visible with the light microscope are largely comprised of hollow, spherical vesicles with thin limiting membranes. These are embedded in a finely granular matrix. A few of the thin walled vesicles contain a dense inner body like that of the cytoplasmic virus particles. This suggests that they may be immature virus particles. The inclusion bodies are believed to be formed in the course of virus multiplication but they usually contain very few mature virus particles. Bundles of dense filaments and peculiar vacuolar inclusions also occur in the cytoplasm of the tumor cells. These seem to be related in some way to the presence of virus but their origin and significance remain obscure. These findings are discussed in relation to previous work suggesting that the Lucké adenocarcinoma is caused by an organ-specific filtrable agent. It is concluded that the "virus particles" found in electron micrographs of the tumor cells may be the postulated tumor agent. On the other hand, the possibility remains that the particles described here are not those that are causally related to the tumors.