Lack of dominant-negative effects of a truncated gamma c on retroviral-mediated gene correction of immunodeficient mice

A recent clinical trial of gene therapy for X-linked severe combined immuno deficiency (XSCID) has shown that retroviral-mediated gene correction of bo ne marrow stem cells can lead to the development of normal immune function. These exciting results have been preceded by successful immune reconstitut ion in several XSCID mouse models, all carrying null mutations of the commo n gamma chain (gammac). One question not formally addressed by these previo us studies is that of possible dominant-negative effects of the endogenous mutant gammac protein on the activity of the wild-type transferred gene pro duct. The present work was therefore undertaken to study whether corrective gene transfer was applicable to an XSCID murine model with preserved expre ssion of a truncated gammac molecule (Delta gammac(+)-XSCID). Gene correcti on of Delta gammac(+)-XSCID mice resulted in the reconstitution of lymphoid development, and preferential repopulation of lymphoid organs by gene-corr ected cells demonstrated the selective advantage of gammac-expressing cells in vivo. Newly developed B cells showed normalization of lipopolysaccharid e-mediated proliferation and interleukin-4 (IL-4)-induced immunoglobulin G1 isotype switching. Splenic T cells and thymocytes of treated animals proli ferated normally to mitogens and responded to the addition of IL-2, IL-4, a nd IL-7, indicating functional reconstitution of gammac-sharing receptors. Repopulated thymi showed a clear increase of CD4(-)/CD8(-) and CD8(+) fract ions, both dramatically reduced in untreated Delta gammac(+)-XSCID mice. Th ese improvements were associated with the restoration of Bcl-2 expression l evels and enhanced cell survival. These data indicate that residual express ion of the endogenous truncated ye did not lead to dominant-negative effect s in this murine model and suggest that patient selection may not be strict ly necessary for gene therapy of XSCID. (Blood, 2001;97:1618-1624). (C) 200 1 by The American Society of Hematology.