Role of vector in activation of T cell subsets in immune responses againstthe secreted transgene product factor IX

Defining immune responses against the secreted transgene product in a gene therapy setting is critical for treatment of genetic diseases such as hemop hilia B (coagulation factor IX deficiency). We have previously shown that i ntramuscular administration of an adeno-associated viral (AAV) vector resul ts in stable expression of therapeutic levels of factor IX (F.IX) and may b e associated with humoral immune responses against F.IX. This study demonst rates that intramuscular injection of an AAV vector expressing F.IX fails t o activate F.IX-specific cytotoxic T lymphocytes (CTLs) in hemostatically n ormal or in hemophilia B mice, so that there is an absence of cellular immu ne responses against F.IX. However, transgene-derived F.IX can cause B cell responses characterized by production of T helper cell-dependent antibodie s (predominantly IgG1, but also IgG2 subclasses) resulting from activation of CD4(+) T helper cells primarily of the Th2 subset. In contrast, administ ration of an adenoviral vector efficiently activated F.IX-specific CTLs and T helper cells of both Th1 and Th2 subsets, leading to inflammation and de struction of transduced muscle tissue and activation of B cells as well. Th erefore, vector sequences fundamentally influence T cell responses against transgene-encoded F.IX. In conclusion, activation of the immune system in A AV-mediated gene transfer is restricted to pathways mediated by F.IX antige n presentation through MHC class II determinants resulting in T and B cell responses that are more comparable to responses in the setting of protein i nfusion rather than of viral infection/gene transfer.