FIG. 4.

Growth curves in MBM cell cultures of molecularly cloned FIV variants Lys-Ser (♦), Asn-Ser (•), Lys-Asn (▴), and Asn-Asn (▪). The cultures were supplemented with 2% NCS (A) or immune sera (B to D) during the entire duration of the experiment. Immune sera were obtained from cat 275 when the infecting virus had not yet developed changes at position 481 and 557 (month 3 p.i.) (B), after Asn had become fixed at position 481 (month 15 p.i.) (C), and after position 557 had also become fixed with Asn (month 55 p.i.) (D). Virus growth is expressed as levels of RT activity in the supernatants. OD, optical density.