Common isolates of the arterivirus lactate dehydrogenase-elevating virus (e.g. LDV-P and LDV-vx) have become highly adapted to their host, the mouse. They invariably cause an asymptomatic, yet lifelong viremic infection in spite of humoral and cellular antiviral immune responses of the host. The persistent infection is maintained by continuous rounds of cytocidal replication in a renewable subpopulation of macroph-ages. However, during long term LDV passages in immunosuppressed C58 mice variants have arisen (LDV-C and LDV-v) that cause paralytic disease in these mice which is due to the additional ability of these LDVs to productively infect the anterior horn neurons of the mice. On the other hand, these neuropathogenic variants have been found to possess an impaired ability to maintain a persistent infection because of an increased sensitivity to neutralization by antibodies and an enhanced induction of such antibodies in mice. Extensive analysis of many LDV isolates including four obtained from wild house mice have shown that the distinct phenotypic properties of non-neuropathogenic and neuropathogenic LDVs quasispecies always coincide and are due to differential glycosylation of the short ectodomain of the primary envelope glycoprotein (VP- 3P) which appears to interact with host cell receptors and contains the only identified neutralization epitope of the virus. Three closely spaced polylactosaminoglycan chains are associated with the VP-3P ectodomain of the non-neuropathogenic LDVs which impede the binding of antibodies to the neutralization epitope and its immunogenicity and the interaction of the viruses with a putative receptor on anterior horn neurons of C58 mice that is recognized by the neuropathogenic LDVs. Two of these chains are lacking on the VP-3P ectodomain of the neuropathogenic LDVs which seems to allow them to interact with the neuronal receptor but at the same time renders them sensitive to antibody neutralization. LDV-v has been found to be a double genetic recombinant of LDV-vx which has specifically gained the VP-3P ectodomain of the neuropathogenic LDV-C and thus attained neuropathogenicity. Four mutants have been independently isolated from the neuropathogenic LDVs that have regained the ability of high viremic persistent infection due to increased resistance to antibody neutralization while losing neuropathogenicity. These phenotypic changes were associated in all four mutants with the regaining of the missing N-glycosylation sites on the VP-3P ectodomain. Glycosylation of critical sites on envelope glycoproteins most likely affect the tropism of other viruses and the effectiveness of antiviral humoral immune responses of their hosts.
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