The application of new protocols for gene therapy against monogenic diseases requires the development of safer therapeutic vectors, particularly in the case of diseases in which expression of the mutated gene is subject to fine regulation, as it is with
CD40L (CD154).
CD40L, the gene mutated in the X-linked hyper-
immunoglobulin M syndrome (
HIGM1), is tightly regulated to allow surface expression of its product only on T cells stimulated by
antigen encounter. Previous studies in an
HIGM1 animal model showed that transduction of progenitor cells corrected the syndrome but caused a thymic lymphoproliferative disease because of the unregulated expression of the transgene by constitutive vectors. To develop a tissue-specific, activation-inducible, lentiviral vector (LV) for gene therapy to counter
HIGM1, we have constructed two self-inactivating LVs, pCD40L-eGFP and pCD40L-CD40L, regulated by a 1.3 kb fragment of the human
CD40L proximal promoter. The expression of pCD40L-eGFP LV is restricted to cells in which
mRNA transcripts of the endogenous
CD40L gene can be detected. Moreover, the expression of the reporter gene in primary T lymphocytes depends on the activation state of the cells. Remarkably, primary
HIGM1 lymphocytes transduced with pCD40L-CD40L LV expressed
CD40L only after T-cell stimulation. Therefore, the CD40L-promoter-driven vectors are able to achieve a near-physiological expression pattern that follows very closely that of the endogenous
CD40L gene.