Reduced infectivity of nervous necrosis virus due to reversible adsorption of serum albumin onto surface protrusions of viral particles

Nervous necrosis virus (NNV) belonging to the genus Betanodavirus is highly lethal to a wide range of fish species. We investigated the reactivity of purified NNV particles with serum albumin and immunoglobulin G (IgG), which are major components in serum. Infectivity of NNV suspended in high-concentration fetal bovine serum (FBS) reduced to approximately 1/60 to 1/200 but that of NNV in ≤10% FBS did not decline. On washing out the FBS, NNV regained its infectivity, suggesting that NNV infectivity was reduced by reversible adsorption of some components in the serum onto the NNV particles. On replacing FBS with bovine serum albumin, NNV infectivity gradually reduced with increasing concentration of serum albumin. Furthermore, albumin adsorbed onto the NNV particles immobilized on enzyme-linked immunosorbent assay (ELISA) plate wells. However, the adsorbed serum albumin was easily washed out by the normal ELISA washing process using phosphate-buffered saline (PBS). Thus, serum albumin reversibly adsorbed onto NNV particles. However, purified rabbit IgG non-specifically adsorbed onto NNV particles regardless of whether conformational structures of NNV surface protrusions were partially or completely denatured, suggesting that IgG is adsorbed onto areas other than the surface protrusions on NNV particles. FBS did not interfere with the adsorption of IgG onto NNV particles. Considering that pocket structures at the apex of the NNV surface protrusions function as binding sites for cellular receptors, serum albumin may get adsorbed onto or near the pocket structures. This could be one of reasons for reduction in infectivity of NNV suspended in high-concentration FBS.