The line is an embryonic kidney derivative carrying a viral large T antigen, and that one addition is why it packages vectors: plasmids with the matching origin replicate episomally inside it, so transfected DNA reaches a copy number an ordinary line never sees. Everything else about using it follows from that, including the habits that quietly cost titre.
Lentiviral vector production, and what the lentiviral vector actually is at harvest
Three or four plasmids are transfected together: the transfer construct carrying your insert between the long terminal repeats, the packaging functions, and the envelope. What comes out is a supernatant containing enveloped particles, unpackaged plasmid, host protein and a good deal of debris, and the useful fraction is smaller than the volume implies. Harvest at the right time rather than the convenient one, because particles are unstable at culture temperature and a late harvest loses what an early one would have kept.
Lentivirus vector production at scale, and where the process changes
What works in a dish does not scale linearly. Transfection efficiency falls as the vessel grows unless the reagent and the DNA-to-reagent ratio are re-optimised at volume; the medium change that is trivial in a flask becomes a process step; and concentration by ultracentrifugation stops being practical and is replaced by tangential flow. Plan the concentration method before the production method, because it sets the volume and the vessel rather than the other way round.
A lentiviral expression vector, and choosing the promoter before the titre
The construct decides more of the outcome than the packaging does. A strong ubiquitous promoter gives the highest expression and the most silencing over time; a cellular promoter is weaker and more stable; a tissue-restricted one trades titre for specificity, because a larger insert packages less well. Insert size is the constraint people meet late: past a few kilobases titre falls sharply, and a construct trimmed early is worth more than any optimisation of the transfection.
Lentivirus packaging, and the generation you are using
Splitting the packaging functions across more plasmids reduces the chance of recombination producing something replication-competent, which is why later systems use more of them and remove accessory genes. The practical consequences are that you cannot mix components between systems, that your institution's biosafety approval names a system rather than a technique, and that a transfer construct designed for one generation may not be packaged properly by another.
Lentiviral particles, and what a lentivirus titer measures
There are two numbers and they are not the same. A physical count of particles by capsid protein is quick and includes everything, functional or not; a titre measured by transducing a reporter into a stated cell line is the number that predicts an experiment. The ratio between them is the quality of the preparation, and it moves with harvest timing, freeze and thaw cycles and the transfer construct. Report which number you measured, in which cells, and at what multiplicity, because a titre without a cell line attached is not comparable to anything.
An mRNA transfection reagent against a DNA one
The two cargoes need different chemistry and behave differently in this line. Nucleic acid without a nucleus to reach expresses within hours, peaks early and disappears, which suits a transient effect and avoids integration entirely; plasmid takes longer, lasts longer and depends on the cells dividing. Reagents optimised for one are usually poor for the other, so match the reagent to the cargo rather than to the cell line, and re-titrate the ratio whenever either changes.
Questions people ask about 293t cells
Why is this line used for packaging rather than the parent?
Because of the large T antigen it carries: plasmids with the matching origin replicate episomally inside it, reaching a copy number the parent line cannot, which is what makes transient production practical.
Physical particles or functional titre?
Functional, measured by transduction into a stated cell line at a stated multiplicity, if you want a number that predicts an experiment. The physical count is faster and includes everything, and the ratio between the two is the quality of the preparation.
Why did my titre fall when I made the insert bigger?
Packaging efficiency falls with genome size past a few kilobases. Trim the construct, choose a smaller promoter, or accept the lower titre and concentrate; optimising the transfection will not recover it.