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Comment by cabalamat | Hacker News Reader

> If it senses another (somewhat famous) protein, PD-L1, the immune cell self destructs

So why don't pathogens evolve to display PD-L1 on their surfaces, to make immune cells harmless against them? Or do they?

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PD-L1 already exists in human cells, for cancers leverage it, they just need to overexpress it, which can be a minor genetic change.

Pathogens, like bacteria, don't have PD-L1 and it's a very large transmembrane protein. Rather than a small genetic change to over express it, pathogens would need to generate the entire protein by scratch. Possible, but highly unlikely.

couldn't a retrovirus accidentally copy it during replication?

Retroviruses don't have cellular membranes. They could over-express PD-L1 in a cell it infects, but they don't need the cell to live, they want it to lyse and release more viruses (though I guess you could argue it wants it to survive immune attack long enough to do so).

Bacteria could potentially do it, but also bacteria cell walls are very different than human. I'm not sure an expressed transmembrane protein could actually be embedded in a bacterial cell wall and still remain functional.

What a horrifying thought

A glance at Wikipedia confirms that some pathogens are aware of PD-L1 and like to mess with proteins that regulate the immune system. Epstein-Barr Virus (EBV), the most common human virus, the herpes that 90% of people have had and a common cause of cancer, apparently has a protein that induces expression of PD-L1. Some other viruses in mice have been observed to do it.

What a mess. I love immunology.

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