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

Abstract

A recent clinical trial of gene therapy for X-linked severe combined immunodeficiency (XSCID) has shown that retroviral-mediated gene correction of bone marrow stem cells can lead to the development of normal immune function. These exciting results have been preceded by successful immune reconstitution in several XSCID mouse models, all carrying null mutations of the common gamma chain (γc). One question not formally addressed by these previous studies is that of possible dominant-negative effects of the endogenous mutant γc protein on the activity of the wild-type transferred gene product. The present work was therefore undertaken to study whether corrective gene transfer was applicable to an XSCID murine model with preserved expression of a truncated γc molecule (Δγc+-XSCID). Gene correction of Δγc+-XSCID mice resulted in the reconstitution of lymphoid development, and preferential repopulation of lymphoid organs by gene-corrected cells demonstrated the selective advantage of γc-expressing cells in vivo. Newly developed B cells showed normalization of lipopolysaccharide-mediated proliferation and interleukin-4 (IL-4)–induced immunoglobulin G1 isotype switching. Splenic T cells and thymocytes of treated animals proliferated normally to mitogens and responded to the addition of IL-2, IL-4, and IL-7, indicating functional reconstitution of γc-sharing receptors. Repopulated thymi showed a clear increase of CD4−/CD8− and CD8+fractions, both dramatically reduced in untreated Δγc+-XSCID mice. These improvements were associated with the restoration of Bcl-2 expression levels and enhanced cell survival. These data indicate that residual expression of the endogenous truncated γc did not lead to dominant-negative effects in this murine model and suggest that patient selection may not be strictly necessary for gene therapy of XSCID.