In situ transduction of stromal cells and thymocytes upon intrathymic injection of lentiviral vectors

Abstract

Background: The thymus is the primary site for T-cell development and induction of self-tolerance. Previous approaches towards manipulation of T-cell differentiation have used intrathymic injection of antigens, as proteins, cells or adenoviruses, leading to transient expression of the foreign protein. Lentiviral vectors, due to their unique ability to integrate into the genome of quiescent cells, may be best suited for long-term expression of a transgene in the thymus.

Results: Young adult mice were injected in the thymus with lentiviral vectors expressing eGFP or the hemaglutinin of the Influenza virus under the control of the ubiquitous phospho glycerate kinase promoter. Thymi were examined 5 to 90 days thereafter directly under a UV-light microscope and by flow cytometry. Intrathymic injection of lentiviral vectors predominantly results in infection of stromal cells that could be detected for at least 3 months. Importantly, hemaglutinin expression by thymic stromal cells mediated negative selection of thymocytes expressing the cognate T-cell receptor. In addition and despite the low multiplicity of infection, transduced thymocytes were also detected, even 30 days after injection.

Conclusions: Our results demonstrate that intrathymic delivery of a lentiviral vector is an efficient means for stable expression of a foreign gene in the thymus. This new method of gene delivery may prove useful for induction of tolerance to a specific antigen and for gene therapy of severe combined immunodeficiencies.

Figure 1

In vivo expression of eGFP after intrathymic injection of the LvPGK-GFP vector. D5: day 5 post-injection localisation of transduced cells around the injection site (arrow) under visible and UV-lights (upper picture). Fibroblast-shaped cells are predominantly transduced (UV-light only) (lower panel). D30: day 30 post-injection expression of eGFP in the thymus (visible + UV-light) (upper panel) and in the liver (UV-light only) (lower panel). CTRL: Thymus (upper panel) and liver (lower panel) pictures from control mice injected IT with PBS examined for background fluorescence under visible and UV-lights. Magnifications are indicated in the lower right corner of each picture.

Figure 2

Negative selection of developing thymocytes (A) TCR transgenic expression within thymic CD8SP (white) and CD4SP cells (grey) identified by the anti-clonotypic monoclonal antibody 6.5 in SFE-Tg mice six days after IT injection of 40 to 60 ng p24 of the LvPGK-GFP lentiviral vector (n = 2) or of 3.5 to 6 ng p24 of the LvPGK-HA vector (n = 3). Shown are representative profile of two independent experiments. Numbers indicate the frequency of 6.5⁺ cells (B) Absolute counts of HA-specific thymocytes six days after intra thymic injection of LvPGK-GFP or LvPGK-HA lentiviral vectors. These figures were obtained based on the percentages of total 6.5⁺ thymocytes determined by flow cytometry as shown in (A). Statistical analysis was performed using Student's t-test.

Figure 3

eGFP expression in developing thymocytes. Total thymocytes were stained with anti-CD4, anti-CD8 and anti-CD3 monoclonal antibodies. Upper panels: saline-injected control mice (CTRL) and LvPGK-GFP-injected mice at two different time points after injection (D5 and D30) are shown. Lower panels: The profile of CD4/CD8 expression is shown within gated eGFP⁺ cells.