In this study we have investigated the generation of
African swine fever (ASF) virus variants resistant to
neutralizing antibodies after cell culture propagation. All highly passaged ASF viruses analyzed were resistant to neutralization by
antisera from convalescent pigs or
antibodies generated against individual
viral proteins which neutralized low-passage viruses. A molecular analysis of neutralizable and nonneutralizable virus isolates by sequencing of the genes encoding for neutralizing
proteins revealed that the absence of neutralization of high-passage viruses is not due to antigenic variability of critical
epitopes. A comparative analysis of
phospholipid composition of viral membranes between low- and high-passage viruses revealed differences in the relative amount of
phosphatidylinositol in these two groups of viruses, independent of the cells in which the viruses were grown. Further purification of low- and high-passage viruses by
Percoll sedimentation showed differences in the
phospholipid composition identical to those found with the partially purified viruses and confirmed the susceptibility of these viruses to neutralization. The incorporation of
phosphatidylinositol into membranes of high-passage viruses rendered a similar neutralization susceptibility to low-passage viruses, in which this is a major
phospholipid. In contrast, other
phospholipids did not interfere with high-passage virus neutralization, suggesting that
phosphatidylinositol is essential for a correct
epitope presentation to
neutralizing antibodies. Additionally, the removal of
phosphatidylinositol form a low-passage virus by a specific
lipase transformed this virus from neutralizable to nonneutralizable. These data constitute clear evidence of the importance of the
lipid composition of the viral membranes for the
protein recognition by
antibodies and may account in part for the past difficulties in reproducibly demonstrating ASF virus-
neutralizing antibodies by using high-passage viruses.