We count one passage as one split and generally we split when the cell concentration approaches 1e6/ml. Generally around 8-9x10e5. This works out to be about two splits per week, depending on the seed concentration and other conditions.
While we have only worked with relatively young THP1 cell lines, I would not be surprised to find out that the phenotype drifts over time. How old are your cells?
Counting the splits the cells are around passage 48. But the stock that I received from a friend lab was in 30. Most of articles only reported until 30. Probably mine are to old to I see any positive results...
I have never seen a papers on exome or proteome changes in THP-1 cells, but from experience I have seen my HEK293 cells start to have abnormal signaling events beyond passage 35-40. For this reason I try to keep my THP-1 cells below passage 25 for functional assays as they tend to be a more finicky cell line. I would suggest either asking your friend if there are lower passage plugs (and freeze your own stocks off of it before doing any other experiments) or buy fresh from ATCC.
I also forgot to mention that we count passages similar to Christopher. I count THP-1 cells daily and split at 7-9 x 10^5 cells/ml and dilute in prewarmed complete RPMI medium to 3 x 10^5 cells/ml (each time I count that as a passage). We end up splitting ours around 3 times a week, and have found that the more surface area you give THP-1 cells the faster they grow (e.g. place non-culture treated flasks flat instead of upright). Hope this helps.
Thanks Bryan and Christopher, yes I'm culturing the cells in lying non-treated flasks and split it once they reach 9*105 cells/ml and count that as a passage. It seem that they are becoming old I will search for younger ones. As my frozen stocks are passage 30, they maybe are a little bit seniors to the experiments. Thanks for the confirmation about how to count the passages.
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