Evidence for nonspecific adsorption of targeted retrovirus vector particles to cells

The ability to specifically target a cell-type is important for the development of vectors for in vivo gene therapy. In order to produce retrovirus vectors targeting ovarian cancer cells, which specifically overexpress alpha folate receptor (alphaFR), a single chain antibody was fused as an N-terminal extension of the ecotropic and amphotropic murine leukemia virus (MLV) envelope glycoproteins. Vector particles bearing the modified glycoproteins were produced and analysed. Although conventional FACS studies indicated that viral particles bearing the modified Env could bind to ovarian cancer cells, targeted infection was not achieved. The initial step of virus-cell interaction was further studied using an immunofluorescence technique, which allows visualisation of single retrovirus particles. Vectors bearing chimeric or wild-type glycoproteins bound equally well to cells with or without the targeted receptor, although soluble chimeric glycoproteins bound specifically to FBP. Our results indicate that the incorporation of specific ligands to the virus envelope does not necessarily result in significant enhancement of vector particle binding. A similar interaction was also observed using Env-defective virus particles, suggesting that cellular factors incorporated into the lipid envelope play a dominant role in promoting initial adsorption of virus particles to cells. Significant implications arise from these observations on the interpretation of previous reports on 'targeted' vectors, and for the development of vectors for in vivo gene therapy protocols.

Increase of trimethoprim resistance among Shigella species, 1975-1988: analysis of resistance mechanisms

Trimethoprim (TMP) resistance among Shigella species isolated from Finnish travelers increased from 3.0% in 1975-1982 to 42.0%-43.8% in 1987-1988. Of the 317 TMP-resistant Shigella isolates identified during 1975-1988, 175 (55%) collected in 1985-1987 and in 1988 were tested further. Almost all (98%) were highly resistant to TMP, suggesting a plasmid-mediated origin. The type I dihydrofolate reductase (DHFR) gene was detected in 85% of the isolates studied. Twenty-three percent of the type I DHFR-positive isolates failed to hybridize with a probe detecting only Tn7-derived sequences, suggesting that the type I DHFR gene may occur independently of transposon Tn7. Four of the five Shigella species isolated from travelers to Sri Lanka hybridized with the probe for type V DHFR gene, implying a local distribution of the type V DHFR gene. The type II and type III DHFR genes were not found among the isolates studied. Only 12% of the TMP-resistant Shigella isolates failed to hybridize with any of the DHFR gene probes used.

Increasing resistance to trimethoprim-sulfamethoxazole among isolates of Escherichia coli in developing countries

Resistance of Escherichia coli to trimethoprim (TMP)-sulfamethoxazole remains at 3%-8% at many medical centers within the United States. In this study a 44% resistance rate was observed among E. coli isolated at a pediatric hospital in Santiago, Chile, and a 40% resistance rate at a general teaching hospital in Bangkok, Thailand. Most isolates were from urinary tract infections and showed high-level resistance (minimal inhibitory concentration of TMP greater than 1,000 micrograms/ml). Nineteen of 35 isolates tested transferred resistance to TMP; most cotransferred resistance to streptomycin and sulfonamides. Dihydrofolate reductase type I was detected by gene probing in 14 of 35 strains. Subsequent investigations in Brazil, Honduras, and Costa Rica revealed that this high rate of resistance was not an isolated phenomenon.