Dear Leonhard,

If you are looking for the "correct" way to determine working concentration, then what you have to do is titrate your virus after production. Most people use 293T (also the preferred cells for virus production). You can find protocols online. Once you have determined the infectious units/ml on 293T then you can try different dilutions of this defined concentrations on your T cells and determine your MOI. As you probably guess your T cells will be much more difficult to transduce so by knowing how much more virus you need to transduce the same number of T cells (as 293T) you will know the working concentration.

Btw for both retro- and lentiviral transduction of T cells we always concentrate the virus in order to increase the transduction efficiency. 

Up to here are the things you can control. Then it depends on your viral production protocol, the quality of the produced virus, the lentiviral vectors/backbones and packaging plasmids used (very important!), the fitness of the virus production cell line and your T cells, the activation of T cells etc etc. 

I am sure it will be easy to find good protocols online.

Also, the virus can be frozen. We usually snap freeze and store at -80C in aliquots for up to a year. Whenever you need, you thaw and use.

Hi Leonhard,

I titrate my virus on 293 cells. For T cell infections, I have used an MOI of around 50. I store my virus in aliquots at -80 but never refreeze after thawing. Hope that helps!

Dear Evangelos and Dessislava,

Thank you very much for your fast answer and help.

I will use 293T cells for production and titration and also

concentrate the virus with PEG-it Virus Pecipitation Solution (5x).

As lentiviral vectors/backbones and packaging plasmids I use VsVg, Delta 89. For activation of T cells I will give anti CD3/28/2 beads and was planning, as you suggest to freeze the Virus in -80C in aliquots.

Thank you very much,

Leonhard