Replication competition between lactate dehydrogenase-elevating virus quasispecies in mice. Implications for quasispecies selection and evolution

The common quasispecies of lactate dehydrogenase-elevating virus (LDV), LDV -P and LDV-vx, are highly resistant to the humoral host immune response bec ause the single neutralization epitope on the ectodomain of the primary env elope glycoprotein, VP-3P, carries three large N-glycans. Two laboratory mu tants, LDV-C and LDV-v, have lost two of the N-glycans on the VP-3P ectodom ain, thereby gaining neuropathogenicity for AKR/C58 mice but at the same ti me, becoming susceptible to the humoral immune response of the host. In att empts to further assess the origins and evolution of these LDVs we have det ermined their competitiveness by monitoring their fate in mixed infections of wild type, SCID, nude, and cyclophosphamide-treated mice by reverse tran scription/polymerase chain reaction assays that distinguish between them. I n mixed infections with LDV-P and LDV-vx, LDV-C and LDV-v became rapidly lo st even when present initially in large excess over the former. In mixed in fections of mice unable to generate neutralizing antibodies, LDV-C and LDV- v also became replaced by LDV-P and LDV-vx as predominant quasispecies but more slowly than in immunocompetent mice. The results indicate that the hum oral immune response plays an important role in the displacement of LDV-C a nd LDV-v by LDV-P and LDV-vx but that in addition, LDV-C and LDV-v possess an impaired ability to compete with LDV-P and LDV-vx in the productive infe ction of the subpopulation of macrophages that represents the host for all these LDVs. In addition, LDV-v outcompeted LDV-C in mixed infections and th e same was the case for neutralization escape mutants of LDV-v and LDV-C wh ich had regained all three N-glycosylation sites on the VP-3P ectodomain. T hus a hierarchy exists in replication fitness: LDV-P/LDV-vx > LDV-v > LDV-C , which is unrelated to the number of N-glycans on the VP-3P ectodomain. Th e implications of the results in relation to the evolution and selection of the LDV-quasispecies is discussed.