Virulence attenuation of Dengue virus due to augmented glycosaminoglycan-binding affinity and restriction in extraneural dissemination

To gain insight into the role of cell surface glycosaminoglycans (GAG) in dengue virus (DEN) cell tropism and virulence, DEN-2 mouse brain-adapted vaccine candidate, neurovirulent prototype strain (NGC) and low-passage strain, PUO-218, were passaged in BHK-21 and SW13 cells to isolate variants with high affinity for GAG. Sequence comparisons of parent and passage variants revealed five GAG-binding determinants, which all cluster in a surface-exposed region in domain II of the three-dimensional structure of the DEN envelope protein. Using an infectious cDNA clone of NGC and an NGC/PUO-218 prM-E chimeric clone, it was demonstrated that the GAG-binding determinants augment the specific infectivity for BHK-21 and/or SW13 cells by 10- to 170-fold and in some cases marginally reduce that for Vero cells. This altered cell tropism was due to a greater dependence of the variants on cell surface GAG for attachment/entry, given their increased susceptibility to heparin inhibition. The effect of the GAG-binding determinants on virulence was examined in mice deficient in alpha/beta/gamma interferon responses. High GAG affinity strongly correlated with low neuroinvasiveness due to rapid virus clearance from the blood. It was speculated that this mechanism accounts for the attenuation in primates of some DEN vaccine candidates. Interestingly, the GAG-binding variants did not display marked attenuation of neurovirulence and the opposing effect of enhanced neurovirulence was associated with one determinant (Lys¹²⁶) already present in mouse brain-adapted NGC. This discrepancy of attenuated neuroinvasiveness and augmented neurovirulence may be reconciled by the existence of different mechanisms of virus dissemination in the brain and in extraneural tissues.