Background
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The therapeutic use of small interfering RNAs (siRNA) to prevent or treat HIV infection requires an effective means for *in vivo*delivery into susceptible target cells. Transfection of lymphocytes is especially difficult, even *in vitro*. Aptamers, which are small structured nucleic acid sequences that bind with high specificity to individual proteins, provide an attractive approach for cell-specific targeting.

Methods
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We designed a chimeric RNA, which was transcribed *in vitro*. It was composed of a CD4-specific aptamer fused to the 21 nucleotide passenger siRNA strand, and then complexed with the complementary 21 nucleotide active siRNA strand. We hypothesized that the partially-duplexed RNA would be selectively internalized into CD4+ cells following receptor binding to the aptamer, and would be subsequently processed by Dicer into an active siRNA capable of knocking down target genes.

Results
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Specific delivery and knockdown was first evaluated by comparing lamin expression, measured by RT-PCR and Western blot, in HeLa cells stably transfected to express CD4 or control CD4- HeLa cells treated with CD4 aptamer-lamin siRNA chimeras. Lamin gene silencing was observed in CD4+ HeLa cells, but not in CD4- HeLa cells, and required both the CD4-aptamer and the lamin siRNA. Similarly, lamin expression was knocked down in primary CD4+ T-cells and macrophages. To investigate whether this system could be used to suppress HIV infection, CD4-aptamer chimeras were designed to encode siRNAs targeting the viral genes gag, vif and the HIV co-receptor, CCR5. Anti-HIV RNAs, alone and in combination, inhibited HIV infection, as monitored by intracellular p24 staining and p24 ELISA, in primary macrophages and CD4+ T-cells by 70--90%. Preliminary data also suggest efficient inhibition of HIV transmission to polarized human cervical explants.

Conclusion
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These findings suggest that siRNA-aptamer chimeric RNAs could be an effective, cell-type specific therapeutic gene silencing approach to prevent HIV transmission or treat HIV infection.
