Oncogenic Marek's disease herpesvirus in avian encephalitis (temporary paralysis)

Identification of a neurovirulence factor from Marek's disease virus

In addition to tumors, Marek's disease (MD) virus (MDV) can induce a variety of syndromes linked to the central nervous system. In fact, early descriptions of MD suggested that it was a condition affecting mainly the nervous system. Cytokines and other immune-related genes have been suggested to play a crucial role in MDV-mediated neuropathology, but the mechanisms behind the viral-induced neurologic dysfunction are still poorly understood. In the present study we have used reverse genetic strategies to show that pp14 is not involved in the oncogenic phenotype of MDV1 and is not required for viral replication; however, we provide evidence indicating that the absence of pp14 expression is correlated with increased survival of MDV1-infected chickens, and that its expression is associated with enhanced viral neurovirulence. Our data identify for the first time pp14 as a neurovirulence factor from MDV1 and open the possibility to investigate the molecular mechanisms by which pp14 mediates the damage to the avian nervous system.

Contribution of a single host genetic locus to mouse adenovirus type 1 infection and encephalitis

Susceptibility to mouse adenovirus type 1 (MAV-1) is mouse strain dependent; susceptible mice die from hemorrhagic encephalomyelitis. The MAV-1 susceptibility quantitative trait locus Msq1 accounts for ~40% of the phenotypic (brain viral load) variance that occurs between resistant BALB/c and susceptible SJL mice after MAV-1 infection. Using an interval-specific congenic mouse strain (C.SJL-Msq1^SJL), in which the SJL-derived allele Msq1^SJL is present in a BALB/c background, we demonstrate that Msq1^SJL controls the development of high brain viral titers in response to MAV-1 infection, yet does not account for the total extent of brain pathology or mortality in SJL mice. C.SJL-Msq1^SJL mice had disruption of the blood-brain barrier and increased brain water content after MAV-1 infection, but these effects occurred later and were not as severe, respectively, as those noted in infected SJL mice. As expected, BALB/c mice showed minimal pathology in these assays. Infection of SJL- and C.SJL-Msq1^SJL-derived primary mouse brain endothelial cells resulted in loss of barrier properties, whereas BALB/c-derived cells retained their barrier properties despite being equally capable of supporting MAV-1 infection. Finally, we provide evidence that organ pathology and inflammatory cell recruitment to the brain following MAV-1 infection were both influenced by Msq1. These results validate Msq1 as an important host factor in MAV-1 infection and refine the major role of the locus in development of MAV-1 encephalitis. They further suggest that additional host factors or gene interactions are involved in the mechanism of pathogenesis in MAV-1-infected SJL mice.

A successful viral infection requires both host and viral factors; identification of host components involved in viral replication and pathogenesis is important for development of therapeutic interventions. A genetic locus (Msq1) controlling mouse adenovirus type 1 (MAV-1) brain infection was previously identified. Genes in Msq1 belong to the same family of genes associated with susceptibility to other encephalitic viruses, HIV-1 and West Nile virus. We constructed an interval-specific congenic mouse strain to examine the contribution of Msq1 to MAV-1 susceptibility and brain morbidity. We compared infected resistant, susceptible, and congenic mice regarding known MAV-1 disease manifestations in the brain (survival, viral loads, blood-brain barrier disruption, edema, mouse brain endothelial cell barrier properties, pathology, and inflammatory cell recruitment) to determine the extent to which Msq1 influences MAV-1 infection outcome. Our results showed that Msq1 is a critical host genetic factor that controls many aspects of MAV-1 infection.

Marek's disease virus-induced transient paralysis in chickens. 1. Time course association between clinical signs and histological brain lesions

Association between Marek's disease virus (MDV)-induced clinical signs of transient paralysis (TP) and brain. histological lesions (vasogenic oedema and perivascular mononuclear cell cuffs) were evaluated in TP-susceptible line G-B2 chickens in a time sequence study. The most consistent histological lesions were seen in the cerebellum. Leakage of albumin and vacuolation were parallel in development with clinical signs, but preceded the clinical signs by 6 to 12 h. During resolution of signs, a parallel decline in vacuolation, but not in extra-vascular albumin content was observed. The extravascular albumin shifted from an extracellular to intracellular location. No association was seen between the IgG leakage and vasogenic oedema or clinical signs. Perivascular mononuclear cell cuffing was statistically associated with clinical signs, but evaluation of plotted data indicated the slope for the cuffs was less than the slope for corresponding clinical signs. In addition, cuffing began 2 days prior to clinical signs. Thus, perivascular mononuclear cell cuffing was not causally associated with the TP syndrome.

Marek's disease virus infection in the brain: virus replication, cellular infiltration, and major histocompatibility complex antigen expression

Marek's disease virus (MDV) infection in the brain was studied chronologically after inoculating 3-week-old chickens of two genetic lines with two strains of serotype I MDV representing two pathotypes (v and vv+). Viral replication in the brain was strongly associated with the development of lesions. Three viral antigens (pp38, gB, and meq) were detected in the brain of infected chickens. Marked differences between v and vv+ pathotypes of MDV were identified for level of virus replication, time course of brain lesions, and expression of major histocompatibility complex (MHC) antigens. Two pathologic phenomena (inflammatory and proliferative) were detected in the brain of chickens inoculated with vv+MDV, but only inflammatory lesions were observed in those inoculated with vMDV. Inflammatory lesions, mainly composed of macrophages, CD4+ T cells, and CD8+ T cells, started at 6-10 days postinoculation (dpi) and were transient. Proliferative lesions, characterized by severe infiltrates of CD4+CD8- T cells (blasts), started at 19-26 dpi and persisted. Expression of MHC antigens in endothelial cells and infiltrating cells within the brain was influenced by MDV infection. Upregulation of MHC class II antigen occurred in all treatment groups, although it was more severe in those inoculated with vv+MDV. MHC class I antigen was downregulated only in those groups inoculated with vv+MDV. These results enhance our understanding of the nature and pattern of MDV infection in the brain and help to explain the neurovirulence associated with highly virulent MDV.

Marek's disease virus-induced transient paralysis in chickens: demonstration of vasogenic brain oedema by an immunohistochemical method

The presence of vasogenic brain oedema and its distribution in Marek's disease virus (MDV)-induced transient paralysis (TP) were determined in genetically resistant and susceptible inbred White Leghorn chickens. MDV-inoculated TP-susceptible chickens with nervous signs (9 days post-inoculation) had severe vacuolation of cerebellar white matter and associated diffuse leakage of albumin and IgG. The serum protein leakage was associated morphologically with a vasculitis and intramural pseudocyst formation in the walls of blood vessels cuffed by mononuclear cells. This transient vasculitis and resulting vasogenic oedema coincided with the temporary neurological signs seen in TP-susceptible chickens. The vasculitis and vasogenic oedema were not present in brain tissue from recovered MDV-inoculated TP-susceptible chickens, MDV-inoculated TP-resistant chickens, or uninoculated control chickens from either line.

Marek's disease virus-induced transient paralysis in chickens: electron microscopic lesions

A study was made to determine the causative lesion of Marek's disease virus (MDV)-induced transient paralysis (TP) in chickens by comparing the ultrastructure of brain tissue from MDV-infected genetically susceptible and resistant birds. There were numerous intramyelinic vacuoles in the brains of TP-affected birds. Many of these vacuoles contained particulate material compatible with precipitated protein from edema. Astrocyte processes often were distended with similar particulate material. Most intramyelinic vacuoles were either adjacent to the axolemma or within inner myelin lamellae. Myelin sheaths of affected axons, while being displaced, were relatively normal with no vesiculation. Most affected axons were also otherwise normal. Cell processes adjacent to occasional affected axons were distended by degenerating mitochondria, vacuoles, and amorphous material. Some of these processes appeared to be inner loops of oligodendrocyte cytoplasm. The cell bodies of most oligodendrocytes were normal, but a few contained vacuoles similar to those seen in processes adjacent to axons. There were scattered necrotic cells. While most of these could not be specifically identified, some appeared to be oligodendrocytes. Mononuclear inflammatory cells were present both perivascularly and within the parenchyma. Although these cells occasionally contacted myelinated axons and there was myelin phagocytosis, there was no indication that they initiated demyelination. Brain tissue from virus-inoculated resistant birds had perivascular aggregates of mononuclear cells, but there were no intramyelinic vacuoles and few necrotic cells. These findings suggest that intramyelinic vacuolation contributes to the pathogenesis of transient paralysis. Potential pathophysiological mechanisms contributing to the vacuoles, including brain edema and oligodendrocyte injury, are discussed.

Marek's disease virus-induced transient paralysis in chickens: alterations in brain density

The presence of brain edema and its distribution in Marek's disease virus (MDV)-induced transient paralysis (TP) was determined in genetically resistant and susceptible inbred White Leghorn chickens. Density measurements on samples from the cerebellum, cerebral cortex, optic chiasm, and optic lobe were determined on continuous density Percoll gradients. At 9 days post inoculation of MDV, the cerebellums from the susceptible line of chickens were significantly less dense than cerebellums from the resistant line or the controls of either line. The cerebellar edema, as measured by tissue density, coincided with the transient neurological symptoms and was not present in brain tissue from clinically recovered TP birds.

Marek's disease virus-induced transient paralysis: a comparison of lesions in susceptible and resistant lines of chickens

A study was conducted to determine the causative lesion of Marek's disease virus-induced transient paralysis. Brain histologic lesions were quantitated and compared in genetically resistant and susceptible inbred chickens. Comparisons were made 11 days post inoculation (PI) of virus when clinical signs were evident in susceptible birds and on day 17 PI when these signs had remitted. Clinically-affected birds from the susceptible line on day 11 PI had consistent evidence of neuropil vacuolation compatible with brain edema; two of these birds also had vasculitis. These lesions were absent in both clinically-recovered birds from this line evaluated on day 17 PI and genetically resistant birds studied on day 11 PI. Our findings suggest that the causative lesion of transient paralysis is vasogenic brain edema.