PHENOTYPIC SWITCH IN A SPANISH HIV TYPE-1 ISOLATE ON SERIAL PASSAGE ON MT-4 CELLS

A biological clone (F0) of a syncytium-inducing (ST) isolate (S61) was unable to produce syncytia in MT-4 cells, On serial passage on MT-4 c ells this virus [F15(-3)] became capable of inducing syncytia (Sanchez -Palomino S, et al.: J Virol 1993;67:2938), After sequencing different regions of the env gene including V1-V2, V3, and the fusion domain of both viruses, we have found only an asparagine (N)-to-isoleucine (I) change in position 7 of the V3 loop, By mutagenesis and in vitro recom bination, using infectious molecular clones, we have identified this a mino acid change as the only one responsible for the syncytial phenoty pic switch, However, this cytopathic change was not accompanied by a c hange in the replication rate, indicating that these two properties ar e not linked genotypic traits, Thus serial passaging of an HIV-1 isola te on MT-4 cells has produced a nonsyncytial-to-syncytial switch throu gh a point mutation in position 7 of the V3 loop.