The 90kb plasmid resident in Salmonella typhimurium confers increased virulence in mice by promoting the spread of infection after invasion of the intestinal epithelium. The nucleotide sequence of a 13.9kb segment of this plasmid known to encode an outer membrane protein related in sequence to components of fimbrial biosynthesis in enteric bacteria was determined. This cloned segment between the repB and repC replicon regions programmed expression of abundant surface fimbriae in Escherichia coli and S. typhimurium cells. A 7kb region contained seven open reading frames, the protein products of five of which were related in sequence to regulatory, structural, and assembly proteins of adherence fimbriae/pili, such as the P and K88 pili. These five genes and two adjacent ones which were not markedly related to proteins in the data bases comprise the pef (plasmid-encoded fimbriae) locus. Transposon TnphoA insertions in four genes in the pef locus (pefA, pefC, orf5 and orf6) resulted in active PhoA fusions and blocked or reduced the surface presentation of fimbriae, indicating that the proteins encoded by these four genes are translocated at least across the cytoplasmic membrane and contribute to formation of the fimbrial structure. The differences in genetic organization and protein sequence relatedness from other fimbrial gene clusters suggest that the pef locus might encode a novel type of fimbria. Between the pef and the repB loci, there were five open reading frames, one of which (orf8) gave rise to active PhoA fusions but was not necessary for fimbrial expression. Two of the other proteins were homologous to transcription regulatory proteins and a third was the rck gene, which encodes an outer membrane protein that confers complement resistance to serum-sensitive hosts.

Although several in vitro lines of evidence support the potential power of antibody-dependent cell-mediated cytotoxicity (ADCC) in controlling HIV infection, the role of ADCC in the pathogenesis of HIV infection in vivo remains uncertain. There are few studies to date that longitudinally determine the plasma ADCC activity in HIV-infected subjects. We sought to establish an SIV/macaque model to perform such a longitudinal study. In the rhesus macaque cohort studied here, three of five macaques (designated Group 1) maintained higher plasma ADCC activity for at least 1 year after inoculation with SIV/17E-Br. The ADCC activity of the two remaining macaques (Group 2) fell 12 weeks after inoculation. There were also differences in longitudinal measurements of anti-SIV envelope IgG titers and CD4 counts. Group 1 macaques maintained higher antienvelope IgG titers and higher CD4(+) T cell numbers as late as 60 weeks postinoculation, while Group 2 macaques had significantly lower titers at 1 year postinoculation and lower CD4(+) T cell counts by 30 weeks postinoculation. Our study shows a correlation between humoral response, ADCC activity, and disease progression (as measured by CD4(+) T cell counts). In these animals, ADCC activity is associated with delayed progression to AIDS. Further studies are underway to determine if ADCC is a protective immune response in SIV infection or if ADCC is a marker of intact cellular and humoral immune responses.

Antigenic variation is a characteristic feature of lentiviral infection. The SIV/macaque model of AIDS provides an ideal system in which to investigate the molecular basis of antigenic variation. The purpose of this study was to genetically map the nucleotide changes in env that alter the neutralization phenotype of SIV. Serum taken from an SIVmac239-infected macaque (2D) at 30 weeks postinoculation was found to neutralize the input virus (SIVmac239) and an isolate, P9, obtained at 10 weeks p.i., but did not neutralize two other isolates, P13 and P23, obtained at 20 and 52 weeks, respectively. Sequence analysis of these virus variants revealed clustered amino acid changes in V1 and single base pair changes in V2-V4 of P13 and P23. Infectious recombinant viruses in which the V1 and V1-V3 sequences of SIVmac239 were replaced with those of P13 or P23 retained the neutralization profile of SIVmac239; both were neutralized by macaque 2D serum. Recombinants containing the entire surface glycoprotein (gp120) (V1-V5) and the 5' portion of gp41 of P13 and P23 and those containing gp120 sequences from V4 through the 5' portion of the transmembrane glycoprotein (gp41) were not neutralized by 2D serum. Using a panel of monoclonal antibodies in radioimmunoprecipitation assays, P23 and recombinants containing V4 and V5 of P23 were shown to be antigenically distinct from P13 and SIVmac239. The majority of the amino acid changes in the antigenically distinct viruses were clustered in V4 (amino acids 413-418) and these changes created new potential N-linked glycosylation sites. This study demonstrates that a small number of specific amino acid changes (amino acids 412 to 418 in the env gene) in the V4 region of the SIV envelope glycoprotein can alter antibody recognition and neutralization and that these phenotypic changes may be associated with altered glycosylation of the envelope.