Cancer cells have the ability to take advantage of the body's mechanism to prevent the immune system from attacking healthy cells, essentially signalling immune T cells to turn off using what's called immune checkpoint proteins. Essentially a cold tumor is one that has successfully shut off the immune response in this way, whereas a hot tumor is one that hasn't. This can be because of the innate behaviour of the tumor, or due to an intervention to turn it hot—like the one described by the OP, or more traditionally, using immune checkpoint inhibitors.

Is this the same mechanism that the eye and testes use to prevent being attacked by the immune system? I recall the eye has its own "immune system"(tears) since the usual one is deactivated in that region.

Would it be possible to engineer a disease(virus, bacteria) that can't be defeated by the immune system because it has this signal?

PD-1 is just one of many cell immunity expressions: https://en.wikipedia.org/wiki/Immunoglobulin_superfamily

A quick search, if I'm reading this correctly, says that HIV expresses PD-1: https://retrovirology.biomedcentral.com/articles/10.1186/s12...

Let's agree as a species not to try and find out.

If the answer is "Yes, there's a protein on the cell membrane or a hormone that acts as the signal", then we already know the answer.

Can you explain where the uncertainty is coming from?

Read your question again and then realize you just wondered if it would be possible to engineer a virus to kill all of humanity, with no possible immunity.

See the problem?

This is why people hate (software) engineers, they are oh-so-dumb on the "empathizing with humanity" front... pie-in-the-sky ambitions that are more likely to be shitty solutions replacing ones which worked well enough, to blow up, potentially catastrophically, or cause various humanity-ending apocalypses, than to actually solve the problem in most cases...and what do they have to say for themselves?

"Oh I didn't think about that".

-_-

The notion is quaint that there exists Knowledge Man Is Not Meant To Know. If it's something humans can find out, it's better that it be right out there in the open so that the danger can be met head on.