From bedside to bench: linking autoimmunity-associated gene variants to immune function

Autoimmunity happens when the immune system — normally tasked with defending against infections and cancer — attacks the body’s own tissues. There are over 80 clinically distinct autoimmune diseases. They differ in which tissues they target and which therapies work best.

• Rheumatoid arthritis (RA) and inflammatory bowel disease (IBD) destroy joints and the intestinal tract. These diseases respond well to agents like adalimumab (Humira) and etanercept (Enbrel), which block TNF-alpha, a cytokine that promotes inflammation.
• During multiple sclerosis (MS), the immune system attacks the central nervous system, causing progressive neurologic damage. Despite being an inflammatory disease, MS actually gets worse with anti-TNF-alpha therapies.

Finding the root dysfunction behind an autoimmune disease would help clinicians choose the best available therapy, or develop new ones. Advances in genome sequencing technology have let researchers build a growing list of genetic differences found in people with various autoimmune diseases compared to healthy people. Several of these disease-linked gene variants have known roles in the immune system. But how they contribute to specific autoimmune processes is still largely unknown. Researchers need to test these gene variants directly, to see how they alter immunity and to prioritize them as drug targets.

Studying a Genetic Variant Linked to Multiple Autoimmune Diseases

Dr. David Rawlings’ group at Seattle Children’s Hospital took on this challenge in a paper published in the May 2013 issue of the Journal of Clinical Investigation. The group, led by first author Dr. Xuezhi Dai, studied a genetic variant of protein tyrosine phosphatase non-receptor 22 (PTPN22). This gene had already been linked to several autoimmune diseases, including type 1 diabetes (T1D), RA, Graves’ Disease, and systemic lupus erythematosus (SLE).

PTPN22 encodes an enzyme called LYP, a protein tyrosine phosphatase that generally tunes the strength of certain signals in cellular signaling networks. The disease-linked variant swaps one amino acid — arginine for tryptophan — at position 620 (LYP-R620W). How LYP or LYP-R620W affect immune activity isn’t fully understood.

To learn more about the role of LYP-R620W in autoimmune patients, Dai et al. built a genetically engineered mouse with the same arginine-to-tryptophan swap in the mouse version of LYP (called PEP-R619W). These “knock-in” mice were viable, but had somewhat shorter lifespans than mice with normal PEP. As the engineered mice aged, they developed signs of autoimmunity, including:

• Inflamed lung tissue and blood vessels
• Signs of chronic kidney damage
• Higher susceptibility to an experimental form of type 1 diabetes
• Numerous auto-antibodies — a hallmark of certain autoimmune diseases

How the Gene Variant Affects Immune Cells

The PEP-R619W knock-in mouse let the researchers study this gene variant’s effect on specific immune cell populations in detail.

• Knock-in mice had more activated/memory T cells than normal mice, showing a chronically active immune system.
• T cells from knock-in mice were hyper-responsive to stimulation of their antigen receptors. This means the internal signals that drive T cell activation were amplified.
• Knock-in mice also had larger populations of specific B cells that appear during active immune states.
• B cells from knock-in mice proliferated more than normal B cells after stimulation, and were more easily triggered to secrete antibody.

These findings led the researchers to conclude that expressing PEP-R619W lowers the activation threshold for both T and B cells, contributing to the autoimmune phenotype. Notably, expressing the disease-linked variant in B cells alone was enough to produce mice with signs of autoimmunity.

What This Means for Future Research

Dai et al. offer a great example of how bench science tools can deepen the knowledge gained from studying patient samples. Further work to define PEP/LYP’s substrate specificity, and how its phosphatase activity changes during lymphocyte activation, could reveal targets for highly selective immune suppressants.

The autoimmune phenotype in this knock-in mouse is also fairly mild. It would be worth testing how other disease-linked gene variants might combine with PEP-R619W to produce either a more aggressive disease, or one that resembles a specific autoimmune syndrome.

Finally, since B-cell-specific PEP-R619W expression alone can trigger autoimmunity, B cells appear to be a critical part of the autoimmune process in patients with this genetic variant. This model gives researchers a way to compare different treatment approaches in the PEP-R619W background — for example, B cell depletion versus anti-TNF agents. Such studies could eventually help predict how patients will respond to autoimmune therapies based on their genotype.

References:

A disease-associated PTPN22 variant promotes systemic autoimmunity in murine models. Dai X, James RG, Habib T, Singh S, Jackson S, Khim S, Moon RT, Liggitt D, Wolf-Yadlin A, Buckner JH, Rawlings DJ. J Clin Invest. 2013 May 1;123(5):2024-36. doi: 10.1172/JCI66963. Epub 2013 Apr 24.