Polyomavirus DNA is damaged in target cells during cytotoxic T-lymphocyte-mediated killing

The distribution and kinetics of polyomavirus in lungs of intranasally infected newborn mice

The primary cell types that sustain polyomavirus (Py) replication following intranasal infection as well as the nature of the host cellular response to Py were unknown. As this is an essential and specific site for virus entry, it seems likely that viral gene function must be adapted to these mucosal tissues. Using immunohistochemistry and in situ hybridization, we determined the cell types in the lung that support Py gene expression and replication following intranasal inoculation of newborn mice within 24 h of birth. Lungs were collected daily from days 1 to 10 postinfection for Py DNA and early T antigen analysis and for histological examination by H&E staining, using methods that preserve the delicate newborn lung architecture. Viral DNA was present in increasing quantities from 2 to 6 dpi in a subset of the Clara cells lining the inner lumen of the bronchi and bronchioles, while T antigen expression was present in a majority of the cells in the bronchi and bronchiole lumen. A distinct and transient pattern of hyperplasia was observed among the cells expressing T antigen and was present from 3 through 6 dpi. Py DNA-containing cells exfoliated into the bronchiole lumen and alveolar ducts, but Py T antigen was not detected in these cells. Py DNA was first detected at 2 dpi, increased through 6 dpi, and abruptly declined through 9 dpi at which time there was no sign of viral DNA in the lungs by in situ hybridization. An unusual infiltration of neutrophils began before the presence of exfoliated cells or Py replication and continued for 2-3 days and was followed by a lymphocytic infiltration at 8-10 dpi lasting 2-3 days. Neither the hyperplasia nor the neutrophil infiltration occurred following infection with the MOP1033 MT-Ag or RB1 LT-Ag mutants of Py. In addition, both the neutrophil infiltration and the transient hyperplasia are in stark contrast to the heavy macrophage infiltration that follows infection of lungs with mouse adenovirus. Thus it appears that Py elicits a distinct host response pattern not seen with other DNA viral infections.

Fas-mediated cytotoxicity induces degradation of vesicular stomatitis virus RNA transcripts and reduces viral titer

Several investigators have recently examined the effect of Fas (CD95)-mediated apoptotic cell death on target cells (TC). The effect of Fas-mediated death on viral RNA within the TC, however, has not been explored. In this study, we investigated the ability of the Fas pathway to mediate pre-lytic degradation of vesicular stomatitis virus (VSV) RNA and TC RNA. We show that engagement of Fas antigen on VSV-infected Jurkat cells induces pre-lytic degradation of VSV RNA transcripts, whereas full-length VSV genome RNA, known to be tightly associated with viral proteins, is not degraded. Cellular RNA, including beta-actin and glyceraldehyde-3-phosphate-dehydrogenase mRNAs, is also degraded by Fas-mediated cytotoxicity. In addition, Fas-mediated cytotoxicity reduced the yield of VSV plaque-forming units (PFU) from Jurkat by an average of 82.0%. An anti-Fas blocking Ab inhibited the RNA degradation and restored the number of VSV PFU to near control levels. These data indicate that the Fas lytic pathway could play a role in the elimination of viruses through degradation of intracellular viral RNA. reserved

Granzyme B independently of perforin mediates noncytolytic intracellular inactivation of vesicular stomatitis virus

Cytotoxic cells provide a crucial defense against DNA and RNA viral infections. Here we describe an in vitro model to study the fate of vesicular stomatitis virus (VSV) RNA in cells undergoing apoptosis. Using the [3H]uridine release assay, we show that human LAK cells induce the degradation of RNA in infected U937 cells in addition to inhibiting the production of infectious virions. LAK cell-mediated RNA degradation was blocked by the serine protease inhibitor, 3,4-dichloroisocoumarin. Purified human granzyme B but not inactivated granzyme B, granzyme A, or perforin rapidly induced degradation of RNA in VSV-infected U937 cells in a dose- and time-dependent manner without lysing the cells and suppressed viral production. Northern analysis of RNA extracted from infected cells with a VSV full-length cDNA probe confirmed that levels of viral transcripts were reduced by treatment with granzyme B. Nevertheless, the amount of host beta-actin mRNA was also reduced in infected cells, suggesting that treatment with granzyme B induced apoptosis. Consistent with this notion, infected cells exposed to granzyme B rapidly developed DNA strand breakage. Taken together, the data suggest that granzyme B in the absence of perforin reduced VSV production by activating a mechanism that degraded viral transcripts in infected U937 cells.

Regulation of transforming growth factor-beta 1-mediated apoptosis in head and neck squamous cell carcinoma

The apoptotic cell death in Cal-27 cells induced by exposure to transforming growth factor-beta 1 was inhibited by the endonuclease inhibitor aurintricarboxylic acid (ATA) in a concentration-dependent fashion. In vitro studies of cytotoxicity, DNA fragmentation, and protein synthesis by Cal-27 cell lines were performed. Inhibition of cytotoxicity as well as endonucleolytic DNA cleavage was detected. ATA did not inhibit cytotoxicity either via transforming growth factor cell-surface-receptor alteration or by inhibition of macromolecular synthesis. ATA-sensitive events occurred late during treatment. These data suggest that endonucleolytic DNA cleavage is a mandatory event leading to cell death in this system.

Inhibition of Head and Neck Squamous Cell Carcinoma Cell Lines by Transforming Growth Factor-β

Transforming growth factor-β is known to be a potent autocrine growth inhibitor produced by a wide variety of cells, including cells of the immune system. Other investigators have noted that the growth of nontransformed keratinocytes is inhibited by transforming growth factor-β, whereas various carcinoma cell lines are resistant to these effects. Head and neck squamous cell carcinoma cells are known to have surface receptors for this cytokine. We thus assessed the effect of transforming growth factor-β on the growth of head and neck squamous cell carcinoma cell lines. Four head and neck squamous cell carcinoma cell lines were incubated with varying concentrations of transforming growth factor-β, and cytotoxicity was evaluated with a methylene blue colorimetric assay. After culturing in transforming growth factor-β for 4 days, inhibition of growth was detected in CAL-27 (maximal inhibition at 5.0 ng/ml), UMSCC-1, and UMSCC-19 (maximal inhibition at 50 ng/ml) cell lines. One other cell line, UMSCC-8 was found resistant to the inhibitory effects of transforming growth factor-β. Kinetics analysis experiments revealed minimal inhibition before day 2 of incubation, at which time inhibition increased linearly to day 4. Assessment of double-stranded DNA fragmentation suggested that DNA fragmentation occurs before significant cytotoxicity. Electron microscopic analysis and gel electrophoresis of extracted DNA revealed morphologic features consistent with apoptotic cell death. Our findings indicate that transforming growth factor-β significantly inhibits the growth of head and neck squamous cell carcinoma cell lines by inducing apoptotic cell death.

Episomal amplification or chromosomal integration of the viral genome: alternative pathways in hamster polyomavirus-induced lymphomas

The state and expression of the hamster polyomavirus genome in a large panel of virus-induced lymphomas have been investigated. The viral genome is present within tumor cells either as abundant nonrandomly deleted extrachromosomal copies or as a single copy integrated into cellular DNA. We show that these two physical states are likely to be functionally equivalent: first, deletion and integration of the viral genome both inactivate the late coding region; second, the amount of viral early RNAs yielded by a single integrated copy appears to be very similar to that associated with several thousands of extrachromosomal copies of the viral genome. These data underline two essential requisites for hamster polyomavirus to become lymphomagenous: suppression of the late coding functions of the viral genome and expression of the viral oncogenes above a threshold level.

Viruses, virulence and pathogenicity

Pathogenicity is a complex process with stringent requirements of both the host cell and the infecting virion. Among these requirements are a port of entry into host cells, a means of replication for the virus, and a means by which infection damages host cells. Damage to the host can result from multiple mechanisms including transformation, suppression of cellular metabolism, apoptosis, autoimmune responses directed against infected or uninfected tissues, or by molecular mimicry. In the attempt to identify new associations between viral infection and disease, investigators should be mindful that variable host factors as well as viral infection may be required for pathogenesis. Efforts to associate specific viral infections with specific diseases may be obscured by final common pathways through which multiple agents damage host cells in similar ways.

Atypical apoptotic cell death induced in L929 targets by exposure to tumor necrosis factor

The mechanism by which tumor necrosis factor (TNF) induces cytotoxicity of murine fibroblasts was investigated. Electrophoresis of DNA extracted from TNF-treated L929 targets showed fragmentation of DNA into a ladder-like pattern, typical of cells dying by apoptosis. Morphologic analysis also indicated apoptotic cell death, demonstrating clumping and crescentic condensation of chromatin. In contrast, chromatin condensation and ladder-like DNA fragmentation were not detected in L929 targets dying by necrosis from exposure to heat, repeated cycles of freeze-thaw, and sodium azide. Chromatin condensation was an early event, detected as early as 6 h of incubation. However, DNA fragmentation (assayed by double-stranded fragmentation assay and gel electrophoresis), as well as the apoptotic changes detected by Hoechst fluorescence, both occurred later and did not precede TNF cytotoxicity (membrane permeabilization detected by trypan blue or propidium iodide staining). This atypical pattern of apoptosis was a characteristic of L929 target cells rather than a generalized cytotoxic response to TNF because TNF-treated squamous cancer cells showed typical features of apoptosis (DNA fragmentation before cytotoxicity) and etoposide-treated L929 cells demonstrated the same atypical kinetics as TNF-treated cells. Zinc significantly inhibited TNF cytotoxicity as well as DNA fragmentation of L929. However, because DNA fragmentation occurred belatedly in TNF-treated targets, lagging behind cytotoxicity, the protection by zinc against TNF appears mediated by events that occur before the ultimate endonuclease-induced cleavage of DNA into small fragments.

Immunohistochemical detection of Fas antigen in liver tissue of patients with chronic hepatitis C

Apoptosis is a type of cell death that occurs in acute or chronic hepatitis. It has been suggested to be mediated through Fas antigen. To evaluate the role of apoptosis on liver injury of chronic hepatitis C, we studied the expressions of Fas antigen and hepatitis C virus antigen (core antigen) immunohistochemically. Forty liver biopsy samples from patients with type C chronic liver disease were immunostained for Fas antigen and hepatitis C virus antigen. Expression of Fas antigen was found mainly in the cytoplasm of hepatocytes, and these positive cells were found particularly among infiltrating lymphocytes at the advancing edges of "piecemeal necrosis." The histological activity index showed inflammation of both portal and periportal areas to be more severe in the Fas antigen-positive samples than in the Fas antigen-negative ones (p < 0.05 and p < 0.001, respectively). Furthermore, semiquantitative analysis revealed more expression of Fas antigen in the liver tissues with active inflammation than in those without it (p < 0.01). The prevalence of Fas antigen expression in the hepatitis C virus antigen-positive group was higher than that in the hepatitis C virus antigen-negative group (p < 0.05). Our findings suggest that Fas antigen expression (apoptosis) plays an important role in inflammation in the hepatitis C virus-infected liver, particularly in the active inflammation of chronic hepatitis C.

Site-specific mutagenesis of the 35-kilodalton protein gene encoded by Autographa californica nuclear polyhedrosis virus: cell line-specific effects on virus replication

The gene encoding the 35-kDa protein (35k gene) located within the EcoRI-S genome fragment of Autographa californica nuclear polyhedrosis virus (AcMNPV) is transcribed early in infection. To examine its function(s) with respect to virus multiplication, we introduced specific mutations of this early gene into the AcMNPV genome. In Spodoptera frugiperda (SF21) culture, deletion of the 35K gene reduced yields of extracellular, budded virus from 200- to 15,000-fold, depending on input multiplicity. Mutant replication was characterized by dramatically diminished levels of late and very late (occlusion-specific) virus gene expression and premature cell lysis. In contrast, 35K gene inactivation had no effect on virus growth in cultured Trichoplusia ni (TN368) cells. Insertion of the 35K gene and its promoter at an alternate site (polyhedrin locus) restored virus replication to wild-type levels in SF21 culture. Subsequent insertion of 4 bp after codon 81 generated a frameshift mutant that exhibited a virus phenotype indistinguishable from that of 35K deletion mutants and demonstrated that the 35K gene product (p35) was required for wild-type replication in SF21 cells. Mutagenesis also indicated that the C terminus of p35, including the last 12 residues, was required for function. In complementation assays, wild-type virus bearing a functional 35K gene allele stimulated all aspects of 35K null mutant replication and suppressed early cell lysis. These findings indicated that p35 is a trans-dominant factor that facilitates AcMNPV growth in a cell line-specific manner.

Genome digestion is a dispensable consequence of physiological cell death mediated by cytotoxic T lymphocytes

We examined virally transformed murine fibroblast clones as targets for cytotoxic T lymphocyte (CTL)-triggered lysis and genome digestion. Strikingly, while all clones were essentially equivalent in the ability to be lysed, one clone, SV3T3-B2.1, failed to exhibit genome digestion associated with CTL attack. Other aspects of the physiological cell death process, including loss of adhesion and nuclear envelope breakdown (lamin phosphorylation and solubilization), were not altered in this clone. The absence of genome digestion associated with CTL-induced cell death correlated with the absence of endodeoxyribonuclease activity in the nuclei of that clone. Characterization of the activity affected identifies a calcium-dependent, DNase I-like endonuclease of approximately 40 kDa, normally present constitutively in all cell nuclei, as the enzyme responsible for genome digestion associated with CTL-mediated cell death. These observations indicate that neither genome digestion per se nor its consequences [such as activation of poly(ADP-ribose) polymerase] are essential for cell death resulting from the triggering of this cell suicide process.

Genome digestion is a dispensable consequence of physiological cell death mediated by cytotoxic T lymphocytes

We examined virally transformed murine fibroblast clones as targets for cytotoxic T lymphocyte (CTL)-triggered lysis and genome digestion. Strikingly, while all clones were essentially equivalent in the ability to be lysed, one clone, SV3T3-B2.1, failed to exhibit genome digestion associated with CTL attack. Other aspects of the physiological cell death process, including loss of adhesion and nuclear envelope breakdown (lamin phosphorylation and solubilization), were not altered in this clone. The absence of genome digestion associated with CTL-induced cell death correlated with the absence of endodeoxyribonuclease activity in the nuclei of that clone. Characterization of the activity affected identifies a calcium-dependent, DNase I-like endonuclease of approximately 40 kDa, normally present constitutively in all cell nuclei, as the enzyme responsible for genome digestion associated with CTL-mediated cell death. These observations indicate that neither genome digestion per se nor its consequences [such as activation of poly(ADP-ribose) polymerase] are essential for cell death resulting from the triggering of this cell suicide process.

How do CTL control virus infections? Evidence for prelytic halt of herpes simplex

Cytotoxic T lymphocytes (CTL) induce in target cells a rapid, prelytic fragmentation of target cell DNA, accompanied by apoptosis. In contrast, complement and (with a few exceptions) chemical and physical means of inducing cytolysis induce necrosis, without DNA fragmentation. The function of the unusual DNA fragmentation induced by CTL remains to be elucidated. The major recognized function of CTL is in halting virus infections. Earlier, we proposed that CTL might halt virus infections prelytically, by fragmenting viral and cellular nucleic acids, and that in this case, cytolysis per se might be a less important function of CTL. We report here experiments designed to detect prelytic halt of virus replication. We employed in vivo-like conditions: fibroblast targets (difficult to lyse) were infected with herpes simplex virus (HSV), then incubated at low E/T cell ratios overnight. At the highest E/T ratios which produced less than 10% CTL-induced lysis, plaque-forming unit yield was reduced by about 50%. At higher E/T ratios which lysed 1/6 to 1/3 of the infected target cells, 3/4 to 9/10 of the virus production was prevented. The discrepancy between the level of lysis and the reduction in virus yield is evidence for significant CTL-induced prelytic halt of HSV replication. At present, it is unclear whether the antiviral effect observed involves an activity of CTL distinct from their lytic ability, such as their DNA fragmenting ability.

Cell death mechanisms and the immune system

The immune system provides good models for cell death, a phenomenon now recognized to be of fundamental importance in many fields of biology. Cell death is strikingly polymorphic: it can proceed via necrosis (as in complement-mediated cell death) or apoptosis, but the latter displays different patterns (in the receptor-mediated death of some thymocytes, in cell death mediated by TNF alpha or by cytotoxic T cells), perhaps reflecting different pathways of control of a common core mechanism. Even though there are differences in the morphological and metabolic changes associated with the different patterns of apoptosis, some recurrent sequences of events are observed in almost all dying cells. The metabolic state of a cell often seems to play a major role in determining if and how this cell will die in given external circumstances. The nature of molecules causally involved in the dying cell can now be approached in some systems.

Epstein-Barr virus (EBV) antigens processed and presented by B cells, B blasts, and macrophages trigger T-cell-mediated inhibition of EBV-induced B-cell transformation

The ability of B cells, B blasts, and macrophages to present Epstein-Barr virion antigens to autologous T cells and trigger their capacity to inhibit Epstein-Barr virus-induced B-cell transformation was tested. Macrophages were as efficient as B cells and B blasts in presenting the virus to T lymphocytes. This function required antigen processing, because it was inhibited by chloroquine treatment and by fixation of the antigen-presenting cells immediately after viral exposure but not 18 h later. T cells exposed to the purified Epstein-Barr virus envelope antigen gp350 coupled to immunostimulating complexes also showed inhibitory function. These results suggest that recognition of processed virion antigens elicits the generation of T-cell-mediated inhibition of Epstein-Barr virus-induced B-cell transformation.