Dear Kristian,

I have copied some paragraphs from the publication of Kohn et al. entitled " In Vivo Transduction by Intravenous Injection of a Lentiviral Vector Expressing Human ADA into Neonatal ADA Gene Knockout Mice: A Novel Form of Enzyme Replacement Therapy for ADA Deficiency" published in Molecular Therapy (2006) 13, 1110–1120; doi: 10.1016/j.ymthe.2006.02.013 that might shed light on the mechanism by which intravenous lentivirus delivery occurs:

We observed the highest levels of integrated vector sequences and expressed ADA enzymatic activity in the liver, with the lung having the second highest levels of vector from among the organs studied. This biodistribution pattern is what is typically seen for most viral vectors administered into the bloodstream of mice, for example, lentiviral 25,27, gamma-retroviral 26, and adeno-associated virus 28. It may reflect the patterns of blood flow, with the lung capillary bed being the first traversed after injection into a peripheral vein, and the liver parenchyma being more accessible to vector particles because the endothelial fenestrae of the sinusoids allow direct access from blood to hepatocytes.

The levels of ADA enzyme in the liver of the treated mice exceeded those in the liver from wild-type mice, with 2 to 10 times greater than normal levels of ADA enzyme activity seen in the liver at 2 months, when there were 1 to 10 copies of vector per cell on average. This high level of ADA may be responsible for the beneficial effects of survival and multisystem correction, including immune function, either as a source of enzyme that can be distributed systemically or as a "sink" where adenosine and deoxyadenosine nucleosides can be catabolized. The pattern of liver cells expressing ADA protein, as revealed by immunohistochemical staining showed focal clusters of positive cells scattered between areas with no detectable immunoreactive ADA protein. This pattern suggests that either single cells were transduced by the vector and multiplied in situ to form local colonies of transduced cells, or focal groups of cells were transduced by the vector as a result of nonuniform access of the liver cells to the vector. The frequency and density of ADA-expressing foci correlated with the average vector copy per cell and the ADA enzyme activity measured from the same livers, in each of the five animals studied.

The lung had the second highest vector copy numbers per cell and ADA enzyme activity of the organs examined. Immunoreactive ADA protein was detected in cells in the alveoli but not the airways. It is not known whether transduction of pulmonary cells and local production of ADA is responsible, in part, for the prolonged survival and protection from the otherwise fatal pulmonary complications described in ADA-deficient mice 17. Potentially, the ADA expressed in the liver is sufficient to prevent the pulmonary complications.

These surprising findings may indicate that the lymphoid system is rescued in trans by metabolic effects of ADA produced in other organs analogous to the effects of administered PEG-ADA enzyme replacement, rather than in cis by selective survival of gene-corrected cells. The presence of detectable enzyme activity in the plasma of the mice injected with the lentiviral vector suggests that some of the ADA enzyme is leaving the cells where it is expressed and being distributed systemically. This trans-rescue would be unique to the ADA-deficient form of SCID, in which the critical missing protein can have a systemic effect, either by circulating as a source of ADA enzyme activity or by acting as a metabolic sink, breaking down deoxyadenosine and adenosine nucleosides and lowering total body pools. Other forms of SCID would not be expected to benefit from systemic administration of the relevant gene, because the encoded proteins are cell intrinsic (e.g., common cytokine receptor gamma, Jak3, Rag1/2).

Taken together, these results show that neonatal intravenous injection of a lentiviral vector expressing human ADA in ADA-/- mice is sufficient to correct laboratory parameters of immune function, despite the low level of proviral marking and enzyme activity in the thymus and spleen. Uncorrected ADA-/- pups suffer multisystem failure, all of which were corrected after a single injection of 1.0 108 TU after birth, suggesting the ADA enzyme being produced and detected in the liver, and to a lesser extent the lung, is correcting these systems,in trans. A better understanding of the mechanisms of action of this therapeutic approach may allow greater efficacy to be achieved, with obtainable amounts of vector. The relative merits of alternative vector systems for achieving in vivoADA enzyme production, especially AAV serotype 8, remain to be studied in this model, compared with lentiviral vectors 29,30. 

Regarding the efficiency: "Despite these potential limitations, the use of intravenous lentiviral vector delivery of the ADA gene may provide a novel form of long-lasting in vivo ERT".

http://www.nature.com/mt/journal/v13/n6/full/mt2006133a.html

Hoping this will contribute to the discussion on this topic.

Rafik

Dear Kristian,

It depends upon several factors. Firstly, the pseudotype of the lentivirus vector (VSV-G, gp64, rabies etc). Secondly, the venous route - i.e tail vein means first pass for the vector is pulmonary vasculature. Hepatic portal vein - obviously hepatic portal system. Third factor is age - efficiency of delivery is hugely increased by injecting in very young rodents. 

Depends which tissues you're looking to target, and for what purpose.

BW, Simon