CAR VII
Here I return to the possibility of devloiping off the shelf killer T-cells with chimeric antigen receptors (CARs). Off the shelf means that the CAR cell does not have to be custom made for each patient using the patient's killer T-cells. I presenting my (nonexpert) thoughts on how to make them
here. Now I note an article which explains how off the shelf CAR T-cells were made Matsuzaki et al (2024) Blood volume 144 supplement 1 page 4811.
Recall the problem is that the immune system (mainly the host' killerT-cells) reacts violently to foreing HLA A HLA B and HLA C and while Natural Killer (NK) cells kill cells which don't display HLA proteins. My thought was to delete the genes for HLA A,B and C and also insert an HLA E gene with a strong promoter. HLA E is a don't kil me signal which prevents NK cell killing by bindiong to the receptors NKG2A and NKG2B.
From Matsuzaki et al, I infer that this doesn't work, and that HLA E is not enough to stop NK cells. Matsuzaki et al take another approach, they delete the bene for Beta 2 microglobulin preventing the expression of HLA A, B, C and E (this is actually an idea I heard from my late father who was a very emminent immunologist and pioneer of the immunotherapy of cancer). The HLA proteins have 3 subunits which form a shape like a capital gamma (or a gallows). Beta 2 microglobulin is a separate protein about the size of one of the subunits which nests in they place of a man hanged by the gallows. It is required for expression of the HLA on the cell surface. But it is not key to the functioning of HLA A B and C. Instead beta 2 microglobulin size pieces of all the proteins in the cell replace the beta 2 microglogulin and are displayed where Killer T-cell receptors can bind to them and recognize the HLA antigen bit of another protein complex. If presented by a properuy activated antigen presenting cell (usually a dendritic cell which has spikes maximizing surface area and looks like a sea urchin) the Killer cells goes off and kills all cells which display the HLA bit of another protein antigen. This is very useful if the other protein is a part of a virus and also if the other protein is strrange because it is mutated because the cell making it is cancerous (cancerous cells usually make such "neoantigens" especially if they have a defect in a system which detects missmatched DNA strands are replaces the abnormal DNA with normal DNA from the homologous chromasome gotten from the other parent (this is not just a digression is is very important below).
Matsuzaki et al them make the cells display HLA E and HLA A by making chimeric proteins with normal HLA E and then the protein sequenc continuing with beta 2 microglobulin all attached as one protein and by making chimeric proteins with normal HLA A then the continuing with beta2 microgolbulin. I understand that the HLA A is needed in addition to the HLA E to convince the NK cells not refrain from killing.
This seemed very odd to me. In the end the cells display HLA A and E but not HLA B and C. It seemed much simpler to delete HLA B and C rather than deleting Beta 2 microglobulin (so the cells don't display A,B,C or E) then engineer a modified HLA A and HLA E which don't need beta 2 microglobulin as they have their own attached.
Then I understood. the beta 2 microglobulin covalently attached to the HLA A will not be replaced by the bits of other proteins. Normal HLA A would desplay all of the donor's proteins and the recipients immune system would react (not as ferouciously as to foreign HLA but still react). This is why the immune system must be suppressed with transplants even from HLA matched donors.
The modified HLA A which just won't let go of the beta 2 microglobulin does not display bits of other proteins. Ah yes. brilliant.
There still are, in principle 2 problems. HLA A from different people is different and HLA E from different people is different. In fact there are about 7000 alleles of the HLA E gene (that is 700 kinds of HLA E in different people). This is not a terrible problme because Killer T-cells receptors do not deal with HLA E. It remains true that the donor and recipient must have the same HLA A. A problem but not a huge one. The problem of HLA matching is that (the number of HLA A alleles)(the number of HLA B alleles)(the number of HLA C alleles) is very large. Just the number of HLA A alleles is not so huge.
Consider the analogy implied by "off the shelf" not all off the shelf pants are identical - they come in different sizes, Whjat matters is that the number of sizes (or HLA A alleles) is not too huge.
Matsuzaki et al discuss their off the shelf CAR T-cells in a very specific context (which I don't really understand) reactivating exhausted killer T-cells. THat is very exciting too, but the off the shelf technology seems to me to have very general applications.