Seroswitch Vectors

The induction of neutralizing antibodies is presumably one of the barriers against successful readministration of the same Ad gene transfer vector. In experimental animals, this has been observed for intravenous (111), intratracheal (71,112), and intraperitoneal (34) administration where the vector is exposed to antibodies prior to contact with the tissue. Effective readministration of the same vector has also been demonstrated to be impossible using direct tissue injection (113).

Because prior infection by one Ad does not protect against infection by a different serotype (71), using different serotypes of gene therapy vector should allow readministration of the same transgene. Extensive testing of this concept is difficult because existing vectors are only of serotypes 2 and 5, as the E1 functions of these two viruses can be efficiently complemented by the 293 cell line. For example (Fig. 5), when rats were administered intratracheally with Ad of serotype 2-ex-pressing ^-glucuronidase, there was expression that peaked at 3 days and declined to undetectable levels by 14 days. Readministration of the same vector resulted in a very low level of gene expression due to the immune response to the first vector administration. However, a second administration of a vector expressing the same transgene but of serotype 5 resulted in a level of gene expression at day 1 comparable to that seen in a naive rat. The decline to baseline was faster than in naive animals, probably due to the elimination of infected cells by the cellular immune system using epitopes conserved between serotypes 2 and 5 (71).

These data posed the question as to which epitopes are responsible for preventing readministration of the same serotype. Vectors have been constructed with the capsid of serotype 5 but with the fiber gene of serotype 7a. Fortunately, the fiber protein from serotype 7a interacts with the penton base of serotype 5, allowing the assembly of serotype 7a/5 chimeric capsids. In vivo experiments show that the fiber switch from 5 to 7a does not facilitate readministration, suggesting that the immune response to fiber is not the barrier that prevents readministration of the same serotype (114). This is consistent with the concept that the primary humoral immune response is directed to the external loops of the hexon protein. Vectors have been constructed in which the hexon gene of Ad5 has been replaced by that of Ad serotype 2 (115). Even though the level of serological cross-reaction between the pure Ad5 capsid and the variant with hexon from Ad2 was low, the hexon switch did not allow successful readministration in vivo. This illustrates the importance of other arms of the immune system and the diversity of epitopes that are involved in immune response to gene therapy vectors.

It is not clear whether seroswitching is a viable strategy for long-term gene therapy. First, the difficulty of making efficient complementing cell lines for viruses of different subgroups is unknown. Second, the delivery of a therapeutic gene to the target tissue and its subsequent expression would not necessarily be the same among serotypes. Finally, even if this could be achieved, the therapeutic advantage of expressing the transgene once for a few days rather than several times for a few days might not be all that great in terms of genetic disease where persistent gene therapy is needed.

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