Monoclonal Antibody Analysis ofNeutralization and Antibody-Dependent Enhancement ofFeline Infectious Peritonitis Virus

enhancement (ADE)ofFIPVinfection invitro. The MAbswerefound tobespecific foroneofthree structural proteins ofFIPV.A total of47MAbswerespecific forthe205-kDa spike protein (S), 3MAbswerespecific forthe45-kDa nucleocapsid protein (N), and4MAbs werespecific forthe26-to28-kDa membrane protein (M).TheS-specific MAbsshowed various degrees of cross-reactivity withstrains ofFIPV,feline enteric coronavirus, canine coronavirus, andporcine transmissible gastroenteritis virus. Nineteen S-specific MAbsneutralized FIPV.A total of 15 oftheneutralizing MAbs induced ADE,andali but1wereoftheimmunoglobulin G2asubclass. Theremaining fourneutralizing MAbs that didnotinduce ADEwereoftheimmunoglobulin Glsubclass. TwoS-specific MAbsinduced ADEbutwere nonneutralizing. NoneoftheN-orM-specific MAbswasneutralizing orinduced ADE.Onthebasis ofthe reactivity patterns oftheMAbswithFIPVandrelated coronaviruses, itwasconcluded that there isaminimum offive neutralizing sites onS.Inmostinstances, neutralizing MAbswereabletoinduce ADE,demonstrating adirect relationship between neutralization andenhancement. Thedifference inimmunoglobulin subclass between neutralizing MAbsthat induced ADEandthose that didnotinduce ADEsuggests that there maybe arestriction intheimmunoglobulin subclasses capable ofmediating ADE.