Hello Johanna Miks,

1. The cell lines often lose their original biological characteristics such as growth rate, tumorigenicity, gene and protein expression patterns, and cellular signaling pathways through many passages, or they may be altered with increasing cell passage number. I suggest you use cells of lower passage number.

2. If you are using aged nude mice, then it is very much possible that such mice may develop small populations of extrathymic T cells also called “leaky” which are capable of rejecting tumor grafts.

3. While injecting cells into nude mice, suspend cells either in PBS or media without FBS. While the nude mice has no adaptive immune response, they still retain functional B cells and a fully intact innate immune response. So, the innate immune system may recognize components in FBS that in turn may help clear your tumor xenograft before it is fully formed.

4. Tumor regression is mainly caused by immune reponses.

5. The viability of your cells is important for tumor formation. While injecting the cells into mice ensure that minimum damage is done to the cells.

6. While injecting the cells make sure that the cell suspension is a homogeneous mixture.

7. Confirm the quality of nude mice that you are using to make sure that the mice is not the cause of the problem.

Follow the above measures, and I hope that these may be able to solve your problem.

Best.