IntroductionChemokine receptor CCR5 is an attractive therapeutic target for inhibiting HIV-1, as it serves as a HIV-1 coreceptor and is essential for CCR5 tropic HIV-1 infection. [1][2][3][4] Blocking CCR5 expression should prevent HIV-1 infection at the initial stage of the viral life cycle. Individuals with a ⌬32/⌬32 homozygous mutation in the CCR5 gene do not express CCR5, are highly protected from HIV-1, and are apparently normal. [5][6][7] Recently, an HIV ϩ acute myelogenous leukemia patient was treated for leukemia and HIV infection by bone marrow transplantation using donated CCR5 ⌬32/⌬32 marrow. After the transplantation, nearly 100% of the patient's blood cells were replaced with donor cells. HIV DNA and RNA were undetectable at 20 months, even after the discontinuation of highly active antiretroviral therapy. 8 This evidence supports that long-term and stable reduction of CCR5 is a promising strategy for treating HIV-infected patients. The major limitation of this strategy is the difficulty of identifying human leukocyte antigen-matched CCR5 ⌬32/⌬32 homozygous donors as the mutation exists in approximately 1% of white populations and is rare in other ethnic populations. 9 Small interfering RNAs (siRNAs) induce sequence-specific degradation of mRNAs by RNA interference. 10 Many forms of siRNA have been used to inhibit HIV coreceptors and HIV-1 gene expression in in vitro and in vivo experimental settings. [11][12][13][14][15][16][17][18] To stably inhibit HIV replication, we and others developed lentiviral vectors that are capable of stably delivering short hairpin RNA (shRNA) in mammalian cells. [19][20][21][22][23][24][25] We demonstrated that expression of CCR5-specific shRNA in human primary T lymphocytes results in efficient CCR5-knockdown and protection of cells from HIV-1 infection in vitro. 22 However, we and others recognized that a high level of sustained shRNA expression may be toxic to cells because of competition with endogenous micro-RNA biogenesis, induction of interferon responses, and/or off-targeting effects. 23,[26][27][28][29][30][31][32][33] To stably reduce CCR5 expression without cytotoxicity, we identified a highly efficient shRNA (shRNA 1005) directed to human CCR5 mRNA using the enzymatic production of RNAi libraries (EPRIL) screening technique. 21,34 We expressed shRNA 1005 using the transcriptionally weak H1 promoter to stably reduce CCR5 expression without inducing cytotoxicity in human primary peripheral blood lymphocytes in vitro. 21,34 To test stable CCR5 reduction in vivo, we used a nonhuman primate hematopoietic stem cell transplantation model in which we were able to demonstrate stable reduction of CCR5 expression in peripheral blood lymphocytes in shRNA-transduced CD34 ϩ cell-transplanted rhesus macaques. 21 Because of a single nucleotide mismatch in the shRNA 1005 target sequence between human and rhesus macaque CCR5 mRNA, we mutated the human CCR5 shRNA 1005 so that it would be 100% homologous to the corresponding rhesus macaque CCR5 mRNA target sequence. This rhes...