Removal of transgene-expressing cells by a specific immune response induced by sustained transgene expression

Induction of the immune response to transgene products is a serious concern in gene therapy, and is generally known to be influenced by the transgene expression profile, as well as the types of cells that express the transgene. However, the exact nature of the association between the transgene expression profile and immune induction following gene transfer is unclear.

In the present study, plasmids, pCpG-fLuc or pCMV-fLuc, used for driving long- or short-term expression of firefly luciferase, respectively, were injected into mice by hydrodynamic injections along with a reporter plasmid expressing Gaussia luciferase (pROSA-gLuc) to evaluate the transgene expression profile in the liver. Single pROSA-gLuc administration resulted in stable gLuc activity in serum for more than 1 year; thus, gLuc activity was used for monitoring immune responses to the liver cells expressing both gLuc and fLuc after co-injection.

A significant reduction in gLuc activity was observed 2 weeks after co-injection of pROSA-gLuc with pCpG-fLuc, whereas stable gLuc activity was observed when pROSA-gLuc was co-injected with pCMV-fLuc. A high level of fLuc-specific immunoglobulin G was detectable in pCpG-fLuc-injected mice; furthermore, histological analysis of the liver sections of these mice indicated CD8(+) cell infiltration, implying that the transgene-expressing hepatocytes were removed by the infiltrating cells.

Our results demonstrate that sustained transgene expression in hepatocytes triggers antigen-specific immune responses, although short-term expression of the same transgene product elicits little, if any, immune response.