Liposomes are attractive, as by themselves liposomes appear to be nonimmunogenic. However, when used in gene therapy (ie, with plasmids inside), they generate a significant immune response, probably triggered by the CpG sequences in the vector, and while repeated delivery is feasible, the overall efficiency of transgene expression is low.- Newer attempts to improve nonviral transfer systems have included the use of glycoconjugates and polyplexes, targeting serpin-enzyme complex receptors, poly-ethyleneimine and nanoparticle formulations, and the modulation of CpG motifs in the plasmids themselves.” A recent approach combined the directability of liposome preparations, such as dexamethasone spermine and dioleoylphosphatidyleth-anolamine, with the content and efficiency of viral vectors to shepherd the viral vector to the appropriate site in the lung and to help entry to the cell, accompanied by reduced inflammation with the presence of a steroid. plan B birth control
The major advantages of adenovectors are their excellent efficiency in gene transfer, which has been demonstrated in numerous systems. However, gene expression is transient, and the immunogenicity of the human adenovirus (serotypes Ad2 and Ad5) prevents efficient readministration. Extended expression can be seen using a fully deleted helper-dependent vector, the so-called gutless adenovirus, which also reduces the extent of the immune response. However, the preparation is technically demanding and thus limited in scope, although this approach was recently used to allow the readministration of adenovirus vectors to mouse lung, suggesting that such strategies may be useful for humans.” A more useful approach involves the development of nonhuman adenovirus vectors, such as serotypes from primates and other animals, which allow administration to humans who are immune to the human serotypes and thus readministration when needed. However, transfer efficiency limitations persist, as the attachment/entry receptors reside primarily on the basolateral surfaces of the epithelial cells (Fig 2), and efforts must be made to redirect the specificity of vector-cell interaction or retargeting of the vector (eg, through alteration to the capsid coat of the vector). AAV vectors have shown broad specificity of infection and prolonged expression in lung tissue. The vectors are thought to exhibit less inflammatory and immune reactions than the adenovirus. However, the small cloning capacity, the difficulty in achieving high titers during manufacturing, and the lack of efficiency in human trials still limits this system for lung gene transfer, although success with safe repeat aerosol administration to humans is a welcome development.
Figure 2. Pseudotyping of lentiviral vectors for gene transfer to the lung epithelium. Pseudotyped lentiviral vectors are produced by the triple transfection of 293T cells with a transfer vector, a packaging plasmid, and an envelope (env) plasmid. The transfer vector contains the transgene, while the packaging plasmid provides viral proteins in trans. Because the envelope gene has been deleted in the packaging plasmid, the tropism of the particles is determined by the viral envelope used for pseudotyping. The receptors required for cell entry by the Ebo Zaire (EboZ) virus are present on the apical side of the airway cells. Lentiviral vectors that have been pseudotyped with the EboZ envelope can therefore transduce airway cells on the apical side directly. In contrast, transduction with adenovectors or VSV-G-pseudotyped lentivectors requires the disruption of tight junctions to access cell entry receptors, which are present in the basolateral membrane. P-gal = P-galactosidase; CMV = cytomegalovirus; GP = glycoprotein; LTR = long terminal repeat; orf2 = open reading frame 2; pA = polyadenylation site; rev = regulation of virus expression; vif = viral infectivity factor; rev = regulator of virus expression; wPRE = woodchuck posttransitional regulatory element.