Conformational Changes in the VP1-Unique Region of Native Human Parvovirus B19 Lead to Exposure of Internal Sequences That Play a Role in Virus Neutralization and Infectivity

Abstract

The unique region of the capsid protein VP1 (VP1u) of human parvovirus B19 (B19) elicits a dominant immune response and has a phospholipase A 2 (PLA 2 ) activity, which is necessary for the infection. In contrast to the rest of the parvoviruses, the VP1u of B19 is thought to occupy an external position in the virion, making this region a promising candidate for vaccine development. By using a monoclonal antibody against the most-N-terminal portion of VP1u, we revealed that this region rich in neutralizing epitopes is not accessible in native capsids. However, exposure of capsids to increasing temperatures or low pH led to its progressive accessibility without particle disassembly. Although unable to bind free virus or to block virus attachment to the cell, the anti-VP1u antibody was neutralizing, suggesting that the exposure of the epitope and the subsequent virus neutralization occur only after receptor attachment. The measurement of the VP1u-associated PLA 2 activity of B19 capsids revealed that this region is also internal but becomes exposed in heat-and in low-pH-treated particles. In sharp contrast to native virions, the VP1u of baculovirus-derived B19 capsids was readily accessible in the absence of any treatment. These results indicate that stretches of VP1u of native B19 capsids harboring neutralizing epitopes and essential functional motifs are not external to the capsid. However, a conformational change renders these regions accessible and triggers the PLA 2 potential of the virus. The results also emphasize major differences in the VP1u conformation between natural and recombinant particles.

Interaction of Parvovirus B19 with Human Erythrocytes Alters Virus Structure and Cell Membrane Integrity

“…The PLA 2 activity of VP1u is also suspected of contributing to inflammatory processes induced by the production of potent eicosanoid lipid mediators (22). All of these immunological features contrast with our recent findings indicating that the most N-terminal part of VP1u is internal and that the capsids are enzymatically inactive (30). In the present studies we have analyzed the accessibility of the PLA 2 region of VP1u, which is close to the VP1-VP2 junction, and found that this region of VP1u is also internal in native capsids.…”

“…We have previously shown that the most N-terminal part of VP1u, containing neutralizing epitopes, is not exposed on the surface of native B19V particles (30). In the present studies we also show that the PLA 2 region, distant from the N-terminal part, is also not accessible to antibodies.…”

“…We have previously found that the PLA 2 activity is barely detectable in the native B19V capsids but can be triggered upon treatment with mild heat or low pH (30). The question remained whether the lack of activity was due to an internal position within the capsid.…”

“…This mechanism of neutralization at the cell surface has already been observed for MAb 1418-1, which targets the most N-terminal part of VP1u. This antibody is highly neutralizing (17) but is not able to bind B19V in a cell-free system (30).…”

“…However, brief exposure to mild temperatures or low pH rendered this region accessible and triggered the VP1u-PLA 2 activity of the virus (30). B19V would therefore be similar to other parvoviruses in which the internal VP1u can become exposed in vitro by treatments with mild heat or low pH (3,8,14,20,34,39) and in vivo during the intracellular trafficking of the virus (24,29,33,39).…”