Study Maps How CARD11 Mutations Disable Immune Signaling in T and B Cells

On Feb. 18, researchers at Johns Hopkins Medicine published a paper in iScience describing what goes wrong inside the immune cells of people born with a rare set of inherited disorders. The work centers on a gene called CARD11, active in both B cells and T cells, and on a single step in the chain of molecular events that tells those cells a pathogen has arrived.

The team studied three inherited conditions classified as primary immunodeficiency syndromes, each caused by mutations in CARD11. People with these syndromes cannot mount effective defenses against invading organisms. They are prone to life-threatening fungal infections, pneumonia and upper respiratory infections, along with food and environmental allergies.

In a healthy cell, CARD11 carries instructions for building a cluster of proteins known as an oligomer. When the gene is altered, the cell produces an abnormal form of that cluster, and the signaling pathway it normally switches on fails to activate. That pathway is what spurs the immune system to recognize pathogens and launch a response. It is also the pathway most vaccines rely on.

What makes these mutations unusual is that a healthy second copy of the gene does not rescue the cell. With many genes, one normal copy offers some protection. Here, some mutations badly impair the oligomer whether one or both copies are affected. Joel Pomerantz, Ph.D., an associate professor of biological chemistry at the Johns Hopkins University School of Medicine, explained that such clusters sometimes need every subunit to work properly, so in certain CARD11-related syndromes a single bad copy can disrupt the entire cluster.

Pomerantz and Jacquelyn Bedsaul, a Johns Hopkins graduate student and the paper's first author, set out to find which step in the signaling cascade demands that all the subunits be functional. They worked with laboratory-grown T cells carrying either working or mutated CARD11, tracking protein levels and the cells' ability to become activated and signal other immune cells. Their results showed that the mutations mainly interfere with how the protein cluster opens itself so it can bind other proteins in the chain reactions that alert T cells to a pathogen. The mutated version keeps the cluster from opening at all, and a closed cluster cannot pass the signal along.

They then asked whether opening was the only broken step. To test it, they used genetically engineered T cells whose CARD11 proteins are held permanently open. Even then, the mutation blocked the signaling pathway. Pomerantz said the mutation also appears to impair the subunits' ability to interact with other signaling partners and to function normally.

The most severe forms of these conditions are rare in humans, and Pomerantz hopes scientists may eventually develop gene editing techniques to correct CARD11 mutations in the immune cells of affected patients. The researchers also believe the findings extend beyond these syndromes. Milder genetic variants in CARD11 and related genes are present across human populations, and Pomerantz suggests they may help explain why people respond so differently to infections, including SARS-CoV-2, and why some face a higher risk of bad outcomes.

The work was done in cultured cells, so it describes mechanism rather than treatment. Pomerantz's stated next horizon is understanding how genetic variation in immune-related genes produces different immune outcomes, with gene editing as a longer-term hope.