An electron-microscope study of the trachea of the fowl infected with avian infectious bronchitis virus

Infectious bronchitis virus generates spherules from zippered endoplasmic reticulum membranes

Replication of positive-sense RNA viruses is associated with the rearrangement of cellular membranes. Previous work on the infection of tissue culture cell lines with the betacoronaviruses mouse hepatitis virus and severe acute respiratory syndrome coronavirus (SARS-CoV) showed that they generate double-membrane vesicles (DMVs) and convoluted membranes as part of a reticular membrane network. Here we describe a detailed study of the membrane rearrangements induced by the avian gammacoronavirus infectious bronchitis virus (IBV) in a mammalian cell line but also in primary avian cells and in epithelial cells of ex vivo tracheal organ cultures. In all cell types, structures novel to IBV infection were identified that we have termed zippered endoplasmic reticulum (ER) and spherules. Zippered ER lacked luminal space, suggesting zippering of ER cisternae, while spherules appeared as uniform invaginations of zippered ER. Electron tomography showed that IBV-induced spherules are tethered to the zippered ER and that there is a channel connecting the interior of the spherule with the cytoplasm, a feature thought to be necessary for sites of RNA synthesis but not seen previously for membrane rearrangements induced by coronaviruses. We also identified DMVs in IBV-infected cells that were observed as single individual DMVs or were connected to the ER via their outer membrane but not to the zippered ER. Interestingly, IBV-induced spherules strongly resemble confirmed sites of RNA synthesis for alphaviruses, nodaviruses, and bromoviruses, which may indicate similar strategies of IBV and these diverse viruses for the assembly of RNA replication complexes.

IMPORTANCE All positive-sense single-stranded RNA viruses induce rearranged cellular membranes, providing a platform for viral replication complex assembly and protecting viral RNA from cellular defenses. We have studied the membrane rearrangements induced by an important poultry pathogen, the gammacoronavirus infectious bronchitis virus (IBV). Previous work studying closely related betacoronaviruses identified double-membrane vesicles (DMVs) and convoluted membranes (CMs) derived from the endoplasmic reticulum (ER) in infected cells. However, the role of DMVs and CMs in viral RNA synthesis remains unclear because these sealed vesicles lack a means of delivering viral RNA to the cytoplasm. Here, we characterized structures novel to IBV infection: zippered ER and small vesicles tethered to the zippered ER termed spherules. Significantly, spherules contain a channel connecting their interior to the cytoplasm and strongly resemble confirmed sites of RNA synthesis for other positive-sense RNA viruses, making them ideal candidates for the site of IBV RNA synthesis.

All positive-sense single-stranded RNA viruses induce rearranged cellular membranes, providing a platform for viral replication complex assembly and protecting viral RNA from cellular defenses. We have studied the membrane rearrangements induced by an important poultry pathogen, the gammacoronavirus infectious bronchitis virus (IBV). Previous work studying closely related betacoronaviruses identified double-membrane vesicles (DMVs) and convoluted membranes (CMs) derived from the endoplasmic reticulum (ER) in infected cells. However, the role of DMVs and CMs in viral RNA synthesis remains unclear because these sealed vesicles lack a means of delivering viral RNA to the cytoplasm. Here, we characterized structures novel to IBV infection: zippered ER and small vesicles tethered to the zippered ER termed spherules. Significantly, spherules contain a channel connecting their interior to the cytoplasm and strongly resemble confirmed sites of RNA synthesis for other positive-sense RNA viruses, making them ideal candidates for the site of IBV RNA synthesis.

Cytopathology of chick renal epithelial cells experimentally infected with avian infectious bronchitis virus

The renal ducto-tubular epithelial cells of chicks infected with the MA-87 strain of avian infectious bronchitis virus (IBV) were examined ultrastructurally. Infected epithelial cells containing IBV particles were more numerous in the collecting ducts, collecting tubules, distal convoluted tubules and Henle's loops than in the proximal convoluted tubules. Virus particles invaded host cells through endocytotic vesicles. Cytopathologic changes in the infected epithelial cells were manifested by a variety of organlle alterations including swelling of mitochondria, dilation of Golgi vesicles and an increase in the amount of rough endoplasmic reticulum (RER). Virus particles were produced by budding into RER and, rarely, toward the perinuclear space. As virus replication progressed, virus particles were enclosed mainly in the dilated RER, cytoplasmic vesicles or virus-containing electron-dense bodies. Virus particles were also found in vesicles of Golgi complex, the dilated perinuclear space, in some autophagic vacuoles or free in the cytoplasm. Virus particles were released by exocytosis through cytoplasmic vesicles, or appeared to be discharged through disrupted cell membranes. It was concluded that epithelial cells of lower nephron and ducts are the primary target cells in IBV-infected kidneys.

Kinetics of lymphocytic subsets in chicken tracheal lesions infected with infectious bronchitis virus

The kinetics of T-cells (CD3 positive (+), CD4+ and CD8+ cells) and B-cells (IgG+, IgM+ and IgA+ cells) in chicken trachea were studied immunohistochemically and histopathologically following an intratracheal inoculation of infectious bronchitis virus (IBV). Viral antigen was detected in the cytoplasm of tracheal epithelium from 16 hr to 6 days post-inoculation (p.i.) with a peak on 4 days p.i. A few IgG+, IgM+ and IgA+ cells were detected in the submucosa from 8 hr p.i. Thereafter IgG+ and IgM+ cells were gradually increased in number, and dramatically increased from 3 days p.i., peaked on 4 days p.i., and gradually decreased after 5 days p.i. IgA+ cells were detected in a small number than IgG+ and IgM+ cells during the all experimental period. These B cells mainly existed in the lamina propria, and some cells were recognized in the interepithelial space. After 14 days p.i., small number of IgG+ and IgM+ cells were detected in the germinal center of lymph follicles in the lamina propria. From 24 to 60 hr p.i., a few number of CD3+, CD4+ and CD8+ cells were detected at the perivascular area in the lamina propria. After 3 or 4 days p.i., each positive T-cells increased rapidly in number, and reached on the peak at 5 days p.i. CD3+, CD4+ and CD8+ cells tend to distribute diffusely, perivascular area, and surrounding area of CD4+ cells, respectively. CD4+ cells were dramatically decreased from 7 days p.i., and CD3+ and CD8+ cells were decreased from 14 days p.i. No T-cells were detected in the lymph follicles in the lamina propria.

Epithelial cell kinetics in the inflammatory process of chicken trachea infected with infectious bronchitis virus

All stages of degeneration and regeneration in chicken tracheal epithelium were studied morphologically following an intratracheal inoculation of infectious bronchitis virus (IBV). Viral antigen was detected in the cytoplasm of tracheal epithelium from 1 to 7 days post-inoculation (d.p.i.) with a peak on 3 d.p.i. At 1 d.p.i., almost all epithelial cells were involved in the degeneration. At this time, labelling index of bromodeoxyuridine (BrdU) in the basal cells showed significantly high value compared with control. At 2 and 3 d.p.i., a great number of basal cells were recognized, but the BrdU labelling index tended to decrease. At 4 and 5 d.p.i., the BrdU labelling index of basal cells significantly decreased than 1 d.p.i., and a few number of regenerated immature ciliated epithelia appeared. At 6 to 11 d.p.i., the ciliated columnar epithelia increased rapidly in number, and returned to the normal appearance except for non-ciliated patch by 13 d.p.i. These results suggested that the tracheal epithelial cells infected with IBV degenerated within 24 hours and proliferating activity of basal cells functioned immediately, and 3 to 4 days later, these basal cells were differentiated to the ciliated epithelia.

Scanning and transmission electron microscopy of normal chicken tracheal epithelia

The ultrastructure of normal chicken tracheal epithelia was studied by scanning and transmission electron microscopy. Scanning electron microscopy revealed only two cell types on the surface of the tracheal epithelium; ciliated and nonciliated with microvilli. Three cell types were observed with the transmission electron microscope: ciliated, goblet, and basal cells. The ciliated cells were the most numerous.

Significance of lysosomes in the morphogenesis of coronaviruses

Virus-containing electron-dense membrane-bound cytoplasmic bodies are described in tracheal epithelial cells of chickens infected with Infectious Bronchitis Virus and in intestinal epithelial cells of swine infected with Porcine Epidemic Diarrhea Virus. Using silver-methenamine staining, phosphotungstic acid staining and acid phosphatase enzyme cytochemical staining of ultra-thin sections, these bodies were shown to be virus-containing secondary lysosomes and residual bodies. The accumulation of viral particles in the lysosomes is suggested to possibly represent an intracellular defense mechanism. However, no morphological alterations were found indicating a destruction of the viruses by the lytic lysosomal enzymes.

In vivo and in vitro models of demyelinating diseases. V. Comparison of the assembly of mouse hepatitis virus, strain JHM, in two murine cell lines

The developmental sequence of a neurotropic strain (JHM) of mouse hepatitis virus was examined by transmission electron microscopy and immunocytology. The nucleoprotein core of this coronavirus, which contains RNA of positive polarity and is helical in configuration, becomes incorporated into enveloped particles in the same manner as the nucleocapsids of the orthomyxo- and paramyxoviruses. However, JHM virus is assembled intracellularly by budding at surfaces of smooth membranous vacuoles. A comparison of JHM virus replication in L2 and 17Cl-1 cell lines revealed that L2 cells undergo more rapid cytopathology and cease virus production much sooner than 17Cl-l cells. In L2 cells the accumulation of core material appears to continue after the abrupt cessation of virus assembly. This is evident by the massive cytoplasmic accumulation of structure resembling nucleocapsids, which react with hybridoma antibody to the nucleocapsid antigen as demonstrated by the immunoperoxidase procedure. The current findings are consistent with our previously published demonstration, using cells of neural and other deviation, of the fundamental role of the host cell type in regulating the replication and expression of coronaviruses.

Morphogenesis of avian infectious bronchitis virus in primary chick kidney cells

Primary chick kidney cells were infected with avian infectious bronchitis virus (IBV) and examined by electron microscopy. Virus particles entered the cells by viropexis and distinction could be made between engulfment by cell processes (phagocytosis) and entry by micropinocytosis in coated transport vesicles. Virus maturation occurred by budding into either the cisternae of the endoplasmic reticulum or cytoplasmic vacuoles, and evidence was obtained to suggest that the viral surface projections could be attached during the budding process. Late in infection large numbers of virus particles were present, mainly in cytoplasmic vacuoles, and the majority were released by cell lysis. Release by fusion of vacuoles with the plasma membrane was also observed, and individual virions could be transported from the endoplasmic reticulum to the surface within coated vesicles.

Electron microscope observations on the entry of avian infectious bronchitis virus into susceptible cells

Infectious bronchitis virus was observed to enter cells of chicken chorioallantoic membrane by viropexis. There was no support for the suggestion that entry took place by fusion of viral and plasma membranes. The results of electron microscopy showed that virus attachment occurred both at 4 degrees and at 37 degrees C. Viropexis was not observed until the preparations were warmed. Similar results were obtained using chicken kidney cells. Quantitative data obtained from a plaque counting system employing chicken kidney cells indicated that attachment was the same at both temperatures and that some virus particles were taken up at 4 degrees C. Virus uptake was triggered by attachment of the virus to the cell membrane and the subsequent process of virus entry visualised by E. M. appeared to proceed without the involvement of lysosomal enzymes. No intracellular virus was located by electron microscopy in warmed preparations when virus was treated with specific antiserum, either before or after adsorption to the cells.

Protein composition of coronavirus OC 43

A human coronavirus, strain OC 43, was propagated in suckling mouse brain and purified 5000-fold with, a 90% yield. Purity of the virus was confirmed by electrophoretic, ultracentrifugal, and electron microscopic procedures. Immunodiffusion and immunoelectrophoresis tests revealed one precipitin line with normal mouse brain, three with purified virus, and four with crude virus when tested against anti-pure virus or anti-crude virus animal serums. The association of a host cell antigen with the virion was confirmed by standard HI and CF tests. Polyacrylamide gel electrophoresis of solubilized purified virus revealed a minimum of six polypeptides with apparent molecular weights of 191,000 (No. 1), 104,000 (No. 2), 60,000 (No. 3), 47,000 (No. 4), 30,000 (No. 5), and 15,000 daltons (No. 6). A seventh band was occasionally found in the 165,000-dalton region of the gels. Four polypeptides contained carbohydrate and one contained lipid. Polypeptide No. 5 comprised 26% of the total viral protein and glycopolypeptide No. 3 comprised 23%. Three other components accounted for most of the remaining protein: polypeptide No. 4 (16%), glycopolypeptide No. 6 (14%), and glycolipopolypeptide No. 1 (13%). Glycopolypeptide No. 2 was 8% of the total protein. Bromelin digestion of the viral projections (spikes) removed glycopolypeptides No. 2 and No. 6. Association of the remaining polypeptides with structural components of the virion is only tentatively postulated. The buoyant density in potassium tartrate of the bromelin-treated virus was 1.15 g/cm³ and of the intact OC 43 virion was 1.18 g/cm³. By analytical ultracentrifugation the corrected sedimentation coefficient (s⁰_20w) of the OC 43 virion was determined to be 390 ± 16 S, and the apparent molecular weight (MW_a) was calculated to be 112 ± 5 × 10⁶ daltons.