Week 7: A “Double Agent”

Hello! I’m Sidharth, and this week I’ll discuss the PD-1 Signal Transduction Pathway and the Beta 2 Microglobulin protein and their role in cancer.

The Programmed Cell Death 1 (PD-1) Signal Transduction Pathway

PD-1 is a double agent: it’s an important immuno-regulatory protein (protecting us from autoimmune diseases), however when cancer attacks our body, it weakens our immune defense system. Often, cancer releases proteins called programmed cell death ligands (PD-L1) that activate the PD-1 pathway. Thus, knocking-out the gene for PD-1 can be an effective strategy for stimulating the immune system. In 2018, Dr. James Allison and Dr. Tasuku Honjo were awarded the Nobel prize for their discovery of immune “breaks” such as PD-1. Now, there are checkpoint inhibitor immunotherapies that reverse the breaks in an attempt to cure cancer.

How is this pathway relevant to “Off-the-Shelf” CAR T-cells? Should PD-L1 proteins attach to the PD-1 receptor on the surface of CAR T-cells, CAR T-cells will be killed. Therefore, it’s critical that these sequences be deleted.

CRISPR Cas-9 Editing

CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats and consists of repeating sequences of DNA found in single-celled organisms that helps them defend against viruses and other foreign invaders, strengthening their immune system. These sequences code for a protein called Cas-9 which finds and destroys parts of a DNA strand, deactivating a certain enemy gene. Recently, scientists have adapted this mechanism to edit genes by inserting a CRISPR sequence, containing instructions on how to destroy a certain gene, into a cell.

Using CRISPR Cas-9 gene editing to delete the PD-1 gene sequence in CAR T-cells will help to reduce immune suppression by cancer cells. (Razeghian et al., 2021)

Beta 2 Microglobulin (β2M)

Beta 2 Microglobulin (β2M) is a protein that is an essential component of MHC Class I molecules. Therefore, eliminating it from the genome of “Off-the-Shelf” CAR T-cells effectively also removes MHC Class I receptors from the cell. Studies in preclinical mouse models with triple gene editing of β2M, PD-1, and the T Cell Receptor have resulted in CAR T-cells having “enhanced” capabilities against cancer. (Choi et al., 2019) All in all, these gene editing steps are a great way to reduce immune suppression / rejection of allogeneic (foreign) CAR T-cells. But will these strategies be successful in a clinical setting? What other obstacles will the immune system hurl at CAR T-cells? Are there better ways to reduce immune rejection or are these steps essential?

Be sure to read my blog next week as I continue my quest to “Off-the-Shelf” CAR T-cells. Questions? Write them in the comments below! Thanks for reading!

Sidharth