Week 2: Human Leukocyte Antigens and iPS Cells

Hello! My name is Sidharth. This week I’ll be exploring induced pluripotent stem cells and human leukocyte antigens to continue laying the foundations for “off-the-shelf” CAR T-cells.

The HLA Bank

Human Leukocyte Antigens (HLA) are one of the keys to unlocking the power of “off-the-shelf” CAR T-cells. As mentioned last week, engineered CAR T-cells must match the patient’s HLA complex (located on chromosome 6) as closely as possible to avoid an immune reaction. This is far from simple for every person has a different HLA complex. (See figure below for a look at an example HLA complex.)

How can we combat this problem? Imagine if there existed a bank containing many different HLA CAR T-cell lines; doctors could choose a CAR T-cell line to administer that is best suited to the HLA complex of the patient. (Depil et al., 2020) However, there are so many HLA variations between patients that creating a bank is incredibly difficult. As a result, another research team has proposed creating only 10 cell lines using induced pluripotent stem cells that are homozygous for common HLA types which are selected from 10,000 donors. These new cell lines provided a complete HLA-A, HLA-B, and HLA-DR match for 37.7% of recipients and a beneficial match for 67.4% of the UK population in one study. (Yamanaka, 2020)

Induced Pluripotent Stem Cells

Induced pluripotent stem cells are blood or skin cells that have been reprogrammed into a pluripotent stem cell state (i.e. they can self-renew and differentiate into any cell of the body). In principle, induced pluripotent stem cells (iPSCs) can be generated from cells whose HLA characteristics are already known. There are many biobanks across the country that hold data on HLA homozygous donors. (Yamanaka, 2020) Once the iPSCs are developed, they can be differentiated into cytotoxic T cells and be edited to express the CAR protein.

How does the reprogramming work? There are four transcription factors (proteins), known as Yamanaka factors, that are responsible for the pluripotency of a human cell. In fact, these factors are extremely abundant in embryonic stem cells. In order to create these iPSCs, these four factors must be overexpressed.

Photography Credit: UCLA Broad Stem Cell Institute

CAR T-cells Lacking HLA

The HLA complex causes so many problems when it comes to attempting to avoid immune rejection. Some scientists are exploring ways to create CAR T-cells that lack the HLA complex. This course of action seems extremely dangerous – cells that lack HLA are targeted by natural killer cells. However, experiments show that there may not be many negative side effects. In a study of engineered CAR T-cells with HLA class 1, HLA class 2, and TCR deletion in mouse models, the CAR T-cells did not induce Graft v. Host disease and retained antitumor properties. (Kagoya et al., 2020) Another study explored the effect of non-HLA matched CAR T-cells on humanized immunodeficient mice, and antileukemic effects were demonstrated and the effects were sustained longer than the traditional CAR T-cells. (Georgiadis et al., 2018)

While non-HLA matched “off-the-shelf” CAR T-cells hold immense promise, they have yet to be tested on human subjects. It’s important to take the studies I referenced above with a grain of salt – they are not enough to draw any conclusions on the effectiveness of non-HLA matched cells. However, they do illustrate that NK cells behave in complex ways. There are many signals that NK cells can recognize, and potentially chemicals in the body that can suppress responses. I hope you’ll continue to follow along as I dig deeper into the mysteries of NK cells!

Conclusion

This week I explored two different ways of creating “off-the-shelf” CAR T-cells: creating an HLA CAR T-cell Bank and CAR T-cells that lack the HLA complex. I also took a deep look at iPSCs – a perfect engine (in principle) that can be used to forge CAR T-cells. This is far from the end, however. There are many complexities in the methods I discussed, and many more methods that must be unraveled. Be sure to tune in next week for more!

Sidharth