Genetically controlled resistance to flavivirus and lactate-dehydrogenase-elevating virus-induced disease

Biological transmission of arboviruses: reexamination of and new insights into components, mechanisms, and unique traits as well as their evolutionary trends

Among animal viruses, arboviruses are unique in that they depend on arthropod vectors for transmission. Field research and laboratory investigations related to the three components of this unique mode of transmission, virus, vector, and vertebrate host, have produced an enormous amount of valuable information that may be found in numerous publications. However, despite many reviews on specific viruses, diseases, or interests, a systematic approach to organizing the available information on all facets of biological transmission and then to interpret it in the context of the evolutionary process has not been attempted before. Such an attempt in this review clearly demonstrates tremendous progress made worldwide to characterize the viruses, to comprehend disease transmission and pathogenesis, and to understand the biology of vectors and their role in transmission. The rapid progress in molecular biologic techniques also helped resolve many virologic puzzles and yielded highly valuable data hitherto unavailable, such as characterization of virus receptors, the genetic basis of vertebrate resistance to viral infection, and phylogenetic evidence of the history of host range shifts in arboviruses. However, glaring gaps in knowledge of many critical subjects, such as the mechanism of viral persistence and the existence of vertebrate reservoirs, are still evident. Furthermore, with the accumulated data, new questions were raised, such as evolutionary directions of virus virulence and of host range. Although many fundamental questions on the evolution of this unique mode of transmission remained unresolved in the absence of a fossil record, available observations for arboviruses and the information derived from studies in other fields of the biological sciences suggested convergent evolution as a plausible process. Overall, discussion of the diverse range of theories proposed and observations made by many investigators was found to be highly valuable for sorting out the possible mechanism(s) of the emergence of arboviral diseases.

Enhancement of encephalomyeloradiculitis in mice sensitized with spinal cord tissue and infected with lactate dehydrogenase-elevating virus

C57BR/cdJ mice develop encephalomyeloradiculitis following peripheral inoculation of the C strain of lactate dehydrogenase-elevating virus (LDV-C). We investigated the effect of subcutaneous administration of syngeneic spinal cord homogenate mixed with phosphate buffered saline or emulsified in complete or incomplete Freund's adjuvant 1 week before or after inoculation of LDV-C on the incidence and severity of central nervous system lesions. C57BR/cdJ mice developed an acute histological allergic encephalomyelitis when given 2 weekly sensitizing injections of homogenate in complete Freund's adjuvant. When peripheral inoculation of LDV-C was substituted for one of the two sensitizing injections, a higher percentage of mice developed lesions, and the lesions were more severe and persisted for much longer periods of time. This same lesion-enhancing effect was not observed if the mice were sensitized with homogenate suspended in buffer or emulsified in incomplete Freund's adjuvant. Interestingly, mice sensitized with complete Freund's adjuvant alone before or after infection with LDV-C also developed intense central nervous system lesions, suggesting that the mycobacterial component of the adjuvant was the critical element in enhancing the lesions.

Genetic aspects of macrophage involvement in natural resistance to virus infections

Macrophages are thought to constitute an important element in the body's natural defense against invasion and dissemination of viruses. Possible antiviral mechanisms of macrophages are defined and referred to as intrinsic, i.e. the ability of macrophages to serve as a nonpermissive barrier between the virus and susceptible cells and extrinsic, i.e. the ability of macrophages to affect the virus or virus replication in surrounding cells. Most studies on the role of macrophages in natural resistance to virus infections have been performed in animal models. An interesting aspect of many viral infections in animals is the finding of a genetically determined variation in natural resistance. Because of the availability of numerous inbred and congenic strains most studies on genetically determined resistance have been performed in mice. The classical examples are resistance to flaviviruses and susceptibility to mouse hepatitis virus, both of which are inherited as dominant, monogenic traits. With these viruses macrophage intrinsic restriction of virus replication has been found to express at the cellular level the genetics of resistance/susceptibility seen in the intact animal. Other examples, where macrophages have been implicated in genetically determined resistance include herpes simplex virus and influenza virus. The involvement of macrophages in natural resistance to these viruses is discussed in relation to other putative resistance determinants like interferon production and sensitivity and natural killer cell activity.