POLYCLONAL BOVINE SERA BUT NOT VIRUS-NEUTRALIZING MONOCLONAL-ANTIBODIES BLOCK BOVINE LEUKEMIA-VIRUS (BLV) GP51 BINDING TO RECOMBINANT BLV RECEPTOR BLVRCP1

Bovine leukemia virus (BLV), a transactivating lymphotropic retrovirus , is the etiologic agent of enzootic lymphosarcoma or leukemia in catt le. Sera from BLV-infected animals possess high BLV-neutralizing antib ody titres. The availability of the recombinant BLV receptor candidate , BLVRcp1, allowed us to determine a mechanism of virus neutralization by polyclonal sera and monoclonal antibodies (MAbs), Bovine sera from animals naturally infected with BLV blocked gp51 binding to recombina nt BLVRcp1, In contrast, virus-neutralizing MAbs specific for gp51 F, G, and H epitopes did not prevent gp51-receptor attachment. Furthermor e, gp51 neutralization epitopes F, G, and H were accessible to antibod ies following gp51 attachment to BLVRcp1. This finding implies that vi rus neutralization by MAbs to defined BLV gp51 epitopes can occur subs equent to virus engagement of the receptor while polyclonal sera can s pecifically block virus attachment to the receptor. In conclusion, the se data suggest that cell infection by BLV is a multistep process requ iring receptor binding (inhibited by polyclonal sera) followed by a se cond, postbinding event(s) at the cell membrane (inhibited by anti-gp5 1 MAbs).