We have been trying to develop 4T1 breast tumor model in balb/c mice. We have been injected 10^6 4T1 cells suspended in 100 uL PBS into the mammary fad pad region, subcutaneously. After 10 days we see mass, in site of injection. The size of mass is approximately 5mm ×5 mm. After 20-days mass is completely regressed in some mice and in some other the size of mass aggressively diminished. in pathological examination, some of the small mass have tumor properties and are malignant. if these mass are tumor what are the causes of restricted or abolished growth of them? why these cell line can induce small tumors in some balb/c mice but not in all of them?
This happens with 4T1 model that tumors start to regress after certain time due to host immunity. Although, 4T1 cells are derived from BALB/c mice and one may expect that this should not be rejected by the same strain. But, in my opinion the rejection occurs because the variation in 4T1 cell line arises due to long time cultures, and also variation in mice strain.
To avoid this problem to a certain extent, I would recommend you to give booster injection of cells after 2-3 days of first injection. Remember that the second injection should be given at the same site where you injected first time. I hope this will help you.
Alternatively, you can inject the cells in 200-300 uL of matrigel instead of PBS.
I would also suggest you to harvest the cells at 70-80% confluency, and check the cell viability before injection.
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