Modification of vaccinia virus penetration proteins analyzed by monoclonal antibodies.

Modifications induced in structural vaccinia virus proteins that elicit the high infectious state by virus activating treatments involving trypsin and phosphatidylserine were analyzed using antivaccinia monoclonal antibodies (MABs). MABs reactive against each of the five outer layer proteins (VP54K, 34K, 32K, 29K, and 17K-25K) neutralized infectivity. VP54K possesses at least two neutralizing epitopes. Treatment with trypsin or with isolated plasma membrane cleaved VP54K into TVP41K carrying epitope A and removed a fragment containing epitope B from the virus. MABs against either of the epitopes could neutralize the virus. The exposure of epitope A concomitantly activated virus infectivity, and it was an essential step of penetration. MABs against VP17K-25K reacted more efficiently with trypsin-treated virus than with untreated virus, but the size of VP17K-25K was not affected by trypsin; this finding indicated that trypsin treatment rendered the VP17K-25K epitopes more accessible to antibody and hence to neutralization. MABs against VP32K and VP29K neutralized infectivity to the same extent irrespective of the state of activation. Virus treated with phosphatidylserine (PS) was neutralized more efficiently by MAB against VP34K than untreated virus, but the amount of antibody that reacted with the virus was the same before and after treatment with PS. Phosphatidylserine did not modify epitope structure itself, but it activated the function of VP34K. It was concluded that blocking of the functions attributed to any of the five proteins resulted in neutralization of virus infectivity, and treatment with trypsin and phosphatidylserine activates infectivity of vaccinia virus by modifying three of them (VP54K, VP34K, VP17K-25K) with characteristic behavior for each protein.