In order prevent rejection of the transplant, you give patients immunosuppressive drugs. But part of the immune system's function has always been to clean up any nascent tumerous or pre-cancerous cells. So you are winning on one side and losing on the other. The same co-stimulatory pathways which are getting attention for their effect in waking up the immune system against cancers (PD-L1, CTLA-4, etc) are the pathways which are being studied for the purpose of 'weakening the immune system', or inducing tolerance. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6519766/ https://www.cancerhealth.com/article/immune-system-fight-can...

The key will likely include the ability to induce tolerance or an immune response to a specific antigen and possibly of a specific immune subtype, so as not to derange the entire system. Also, the ability to target medications to specific regions of the body so that the entire immune system is not affected.

Sufficiently fine control of the body's immune system such that we can prevent it from attacking a donor organ is likely to also be able to be used to make it attack cells that are reproducing out of control (i.e. cancer).

Xenotransplation is translating animal organs (which are easy to get) into humans. If you can do this then you can aggressively cut out whole organs and replace them with new cancer free organs from animals.