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Multiple foci of secondary demyelination were observed in the cerebellum of nude mice given two inoculations of avirulent Semliki Forest virus compared to single lesions seen in similarly treated Swiss A2G mice. The increased number of demyelinative plaques were attributed to brain viral persistence for 35 days in the nude mice with correspondingly low serum antibody titres. No neurological signs were observed in any of the mice.
Three types of viral sensitivity were observed in various mouse strains upon MHV3 infection: resistance, full susceptibility, and semisusceptibility. In the latter type, seen in several inbred strains including C3H, approximately 50% of the adult injected animals resisted to the acute disease. Most of the surviving mice, however, developed a chronic disease with a wasting syndrome and occurrence of paralysis. The chronic period of the disease was characterized by a persistent viral infection, since MHV3 virus was recovered from brain, liver, spleen, and lymph nodes throughout the evolution in most of the animals. In addition, a correlation was observed between the clinical evolution and the titer of virus tested 4 days after infection.
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Murine leukemia virus particles assembled in actinomycin D-treated cells were detected by determination of reverse transcriptase [RNA-dependent DNA polymerase (nucleotidyltransferase)] activity and by radioimmunoassay of the major virion protein, p30. The levels of enzyme activity and p30 protein were both 30-40% relative to the control over an 8 hr period, whereas after 3 or 4 hr infectivity was reduced by 95%. Thus, virions produced in the absence of RNA synthesis represent a fairly homogeneous population of defective particles. Although RNA synthesis is not necessary for virus assembly, protein synthesis is required. Treatment of cells with 10 mug/ml of cycloheximide reduced virus production by 80-85% within 2 hr, and by greater than 95% at later times. As might be expected from this finding, viral protein synthesis accompanies virus assembly in actinomycin D-treated cells. Newly synthesized proteins associated with the defective particles were identical with those found in standard virions and were present in the correct proportions. The results demonstrate that viral mRNA persists in cells in which RNA synthesis is blocked and continues to direct viral protein synthesis with a functional half-life of approximately 6-8 hr. Since viral mRNA is not packaged in virions even when viral RNA synthesis is shut off [Levin et al. (1974) J. Virol. 14, 152-161], we propose that murine leukemia virus-infected cells contain two nonequilibrating pools of intracellular viral RNA molecules, one associated with polyribosomes and one which is encapsidated into extracellular particles.
Acute viral hepatitis has several identifiable morphologic components but the major categories are (1) cytopathic, (2) inflammatory, and (3) regenerative. Each category has independently variable characteristics. Extreme alterations related to severity of disease, alteration of immune response, or pre-existing liver disease may result in diagnostic difficulties for the pathologist. In contrast to the usual concept, patients who survive fulminant viral hepatitis rarely, if ever, develop cirrhosis and those who have severe hepatic necrosis from hepatitis also do not usually develop serious sequelae of that disease except in the older age group where the difficulty is in impaired regeneration (IR). The usual criteria for the diagnosis of chronic active hepatitis or chronic aggressive hepatitis need a thorough review since many of the variations of acute viral hepatitis result in histologic patterns that might be considered to be chronic aggressive hepatitis using the previous definitions; yet such patients recover without developing chronic liver disease. Chronic active hepatitis, a progressive hepatic disorder, is characterized by changes in the distribution of necrosis and regeneration within the lobule from that usually observed in acute viral hepatitis. Persistent viral hepatitis, a development in 10 to 12 per cent of adult patients after icteric acute disease, is characterized by a "cobblestone" hepatocellular change that resembles continued regeneration, focal hepatocytolysis, and often portal lymphoid hyperplasia. Apparently with time, these histologic features fade and the incidence, in type B PVH, of "ground glass" HBs Ag laden cells increases. This may reflect a continued adaptation of host and virus to one another.
Approximately 2% of cultured alveolar macrophages (AM), originally lavaged from the lungs of parainfluenza type 3 virus (PI-3V)-infected calves, were observed to contain viral antigen (by fluorescent antibody method) or viral nucleocapsids (by electron microscopy). Plaque assays, however, indicated that virus titers were generally low when cultures were incubated at 37 degrees C for 10 days. AM, obtained from "in vivo infected" and "noninfected" calves, were found to be equally susceptible to further in vitro PI-3V infection when cultures were incubated at 37 degrees C. AM that were obtained from the lungs of normal calves, cultured at 37 degrees C, and inoculated with PI-3V were observed to produce relatively high virus titers when the incubation temperature was shifted down to 32 degrees C. Results from hemagglutinin assays showed that considerable amounts of hemagglutinin were detected when AM cultures were incubated at 32 degrees C, but only limited amounts were detected at 37 degrees C. Results from electron microscopic examinations at both temperatures substantiated the results of plaque and hemagglutinin assays. The PI-3V, isolated from AM cultures incubated at 32 degrees C, grew well in Madin-Darby bovine kidney cells at 32 degrees C, but little virus was produced at 37 degrees C. In contrast, parent PI-3V grew equally well at both temperatures. The results are discussed in terms of host susceptibility, temperature-sensitivity and virus maturation, and surface viral antigens and persistent viral infection.
The discovery of persistent transmissible agents by veterinarians has led to striking advances in the infectious cause of neuropathies of human beings. There is evidence for persisting infection in congenital rubella and the herpes group of viruses including cytomegalovirus infections. Hepatitis types A and B are candidates for inclusion in the category of persisting viral infections. The rubeola or measles virus is established as a persistent virus which causes elevated antibodies in the serum and cerebrospinal fluid of many patients with severe demyelinating disease such as subacute sclerosing panencephalitis and multiple sclerosis. Elevated antibodies against vaccinia virus have been found in the cerebrospinal fluid of some patients with multiple sclerosis and neuromyelitis optica, a rare form of multiple sclerosis.
The "persistent" viral infections, besides "subclinical" infections, pertain to the vast group of "clinically inapparent" infections. They differ from subclinical infections by a temporally unlimited "co-existence" with the pathogen. Pathogenetically, three forms of development are possible: 1. latent infections, 2. tolerated infections, 3. occult infections. Persistent viral infections are the inexhaustible reservoir for many viruses. To the organism affected they may be of benefit (infection immunity, interference, paraimmunity) or of disadvantage (activation of the infection with conversion into a disease, cause of many chronic, slowly developing disease processes, immunopathogenic consequences), the disadvantages prevailing. To the environment, persistent infections are invisible sources of danger as they produce carriers and chronic carriers.
Despite the success of antiretroviral therapy in suppressing plasma viremia in people living with human immunodeficiency virus type-1 (HIV-1), persistent viral RNA expression in tissue reservoirs is observed and can contribute to HIV-1-induced immunopathology and comorbidities. Infection of long-lived innate immune cells, such as tissue-resident macrophages and microglia may contribute to persistent viral RNA production and chronic inflammation. We recently reported that de novo cytoplasmic expression of HIV-1 intron-containing RNA (icRNA) in macrophages and microglia leads to MDA5 and MAVS-dependent innate immune sensing and induction of type I IFN responses, demonstrating that HIV icRNA is a pathogen-associated molecular pattern (PAMP). In this report, we show that cytoplasmic expression of HIV-1 icRNA also induces NLRP1 inflammasome activation and IL-1β secretion in macrophages and microglia in an RLR- and endosomal TLR-independent manner. Infection of both macrophages and microglia with either replication-competent or single-cycle HIV-1 induced IL-1β secretion, which was attenuated when cytoplasmic expression of viral icRNA was prevented. While IL-1β secretion was blocked by treatment with caspase-1 inhibitors or knockdown of NLRP1 or caspase-1 expression in HIV-infected macrophages, overexpression of NLRP1 significantly enhanced IL-1β secretion in an HIV-icRNA-dependent manner. Immunoprecipitation analysis revealed interaction of HIV-1 icRNA, but not multiply-spliced HIV-1 RNA, with NLRP1, suggesting that HIV-1 icRNA sensing by NLRP1 is sufficient to trigger inflammasome activation. Together, these findings reveal a pathway of NLRP1 inflammasome activation induced by de novo expressed HIV icRNA in HIV-infected myeloid cells.
Inoculation of three- to four-week-old BALB/c mice with temperature-sensitive (ts) vesicular stomatitis virus (VSV) mutant G41 produced a subacute neurological disease mainly localized in the spinal cord. Meningitis and diffuse microglial infiltration of the anterior horns of the spinal cord were seen starting six days after infection when neuronal degenerative changes could be seen. Infection of neurons was demonstrated by immunofluorescence microscopy five days after infection. Two to three weeks after infection, loosening of the neuropil was evident due to neuronal dropout, and the mononuclear infiltration had become perivascularly distributed and had changed in character because of a striking increase in plasma cells. These cells together with Russell bodies became the main inflammatory cellular component about four to five weeks after viral inoculation. Starting eight days after infection, several foci of primary demyelination could be found in the anterior columns of the spinal cord. Immunological responses appeared within four days after infection when both neutralizing antibody and stimulation of specific spleen lymphocytes could be demonstrated. Serum antibody responses peaked at 21-28 days but remained elevated for up to 153 days. Stimulation of spleen lymphocyte cells peaked at 10-21 days and also remained elevated for as long as 116 days. The presence of both inflammatory changes and immunological responses to VSV mutant ts-G41 for prolonged periods is characteristic of persistent viral infections. Infection of BALB/c mice with ts-G41 thus represents the first in vivo example of persistent viral infection utilizing ts mutants of VSV.
The etiology of the immunologic abnormalities in myasthenia gravis (MG) remains unknown. Evidence for persistent viral infection was sought by determining serum antibody titers to several enveloped RNA and DNA viruses. Compared to healthy controls matched for age, sex, geography and socioeconomic status, patients with MG were more likely to have elevated titers of complement-fixing antibody to cytomegalovirus (CMV). Patients with MG not treated with thymectomy or steroids had elevated CMV titers, whereas thymectomized or steroid-treated patients did not; the differences were statistically significant (p less than 0.01). These results are consistent with the hypothesis that there is persistent viral antigenic stimulation in the myasthenic thymus, arising from viral protein incorporation into epithelioid-myoid cell surface membranes and subsequent induction of an antibody to these acetylcholine receptor (AChR)-bearing thymic cells. This antibody then cross-reacts with AChR at the neuromuscular junction.
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