Scientists Discover CD55 Is the Hidden Switch That Powers Natural Killer Cells Against Cancer

Natural killer cells have long been celebrated as the swift, unsung assassins of the immune system, capable of detecting and destroying tumor cells without the elaborate priming that T cells require. Yet oncologists and immunologists have also known that these potent cells routinely falter once they enter the hostile terrain of a solid tumor, a phenomenon that has constrained decades of effort to weaponize them against cancer. Now a study published in Cell Research offers an unexpectedly elegant explanation for both the power and the fragility of NK cells, and it centers on a molecule that immunologists thought they already understood: CD55.

CD55, also known as decay-accelerating factor, has historically been classified as a complement regulatory protein, a guard that prevents the body’s own complement cascade from shredding healthy tissue. It has also been implicated, in T cells, as a co-stimulatory receptor that fine-tunes adaptive immune responses. The new work, led by Lingyu Li, Zhaozhi Li, Yang Liu and colleagues under the supervision of Yufeng Wang and Jianhua Yu, reveals that on natural killer cells CD55 performs a role that is far more fundamental than previously appreciated. Rather than merely assisting activation, CD55 acts as a self-sufficient primary signal initiator, the molecular ignition switch that allows NK cells to launch their cytotoxic program upon first contact with a tumor.

The researchers found that when an NK cell first encounters a tumor, the activating receptor NKG2D engages its ligands on the malignant cell and triggers a signaling cascade that activates the transcription factor p65, a component of the NF-κB family. This NKG2D–p65 axis drives a rapid upregulation of CD55 on the NK cell surface. In other words, the very act of meeting a tumor instructs the killer cell to equip itself with the molecule it needs to kill. This inducible, self-reinforcing architecture ensures that CD55 appears exactly when and where it is most needed, at the immunological synapse where killer and target cells meet.

What happens next is the mechanistic heart of the study. Unlike its role in T cells, CD55 on NK cells directly binds CD97, a receptor that is abundantly expressed on tumor cells, in what immunologists call a trans interaction, meaning the two molecules engage each other across the junction between two different cells. This CD55–CD97 engagement triggers the aggregation of lipid rafts, the cholesterol-rich, ordered microdomains that float within the cell membrane and serve as organizing platforms for signaling machinery. Within these coalescing rafts sits LCK, the Src-family tyrosine kinase that stands at the apex of the lymphocyte activation cascade. By concentrating lipid rafts, CD55 brings LCK molecules together, permitting their phosphorylation and activation, which in turn amplifies the downstream signaling that commands the release of cytotoxic granules.

The technical elegance of this discovery lies in its demonstration that NK cells do not depend on external co-stimulation to fire their killing apparatus. Where T cells typically require multiple reinforcing signals before committing to attack, the CD55–CD97 lipid raft mechanism provides a single, autonomous activation pathway. The researchers confirmed this through careful loss-of-function experiments: NK cells engineered to lack CD55 showed impaired raft aggregation, diminished LCK activation and weakened cytotoxicity, and these defects could be traced specifically to the raft–kinase axis rather than to CD55’s classical complement-regulating duties. Methyl-beta-cyclodextrin disruption of lipid rafts abrogated the activation induced by CD55 agonists, and the functional defects in CD55-deficient NK cells proved independent of the complement pathway altogether, underscoring that this is a signaling role, not a complement role.

The story darkens when the investigators followed NK cells into the tumor microenvironment. Upon prolonged exposure to tumor cells, CD55 expression on the NK cell surface progressively declines, tracking closely with the well-known downregulation of NKG2D that occurs under chronic stimulation. This erosion of CD55 is not a benign byproduct of exhaustion. Within the tumor microenvironment, the loss of CD55 causally impairs NK cell function, collapsing the raft–LCK signaling platform that the cells depend on and leaving them unable to mount effective attacks. The very chronicity of tumor exposure, which initially instructs NK cells to upregulate CD55, ultimately strips them of the molecule and with it their killer instinct.

To test whether this biology matters in human disease, the team turned to single-cell RNA sequencing data from a remarkable breadth of cancers, including nasopharyngeal carcinoma, melanoma, lung cancer, breast cancer, head and neck squamous cell carcinoma, hepatocellular carcinoma, renal cancer, colorectal cancer and pancreatic ductal adenocarcinoma, drawing on datasets from the Human Tumor Atlas Network and numerous public repositories. The analysis revealed a sobering clinical correlation: in cancer patients, low CD55 expression in tumor-infiltrating NK cells is associated with poor clinical outcomes. CD55 thus emerges not only as a mechanistic linchpin but as a potential biomarker, a molecular gauge of whether a patient’s innate immune arsenal remains armed or has gone quiet.

The most therapeutically consequential finding, however, is that this dysfunctional state is reversible. When the researchers restored CD55 expression in both conventional NK cells and chimeric antigen receptor-engineered NK cells, the effects were striking. Restored CD55 augmented LCK signaling, enhanced effector function and persistence, and improved antitumor efficacy in vivo. Engineered NK cells fortified with sustained CD55 expression proved more lethal against tumors and survived longer within the tumor microenvironment, addressing two of the most stubborn limitations of NK cell immunotherapy: inadequate persistence and functional anergy after adoptive transfer. In mouse models and in the NK92 cell line used clinically, enforced CD55 expression translated into measurably better tumor control.

For the rapidly expanding field of NK cell-based cancer immunotherapy, these findings arrive at a critical moment. CAR-NK therapies have generated enormous enthusiasm because, unlike CAR-T cells, they carry a lower risk of graft-versus-host disease and cytokine storms, and they can be derived from off-the-shelf cell sources. Yet clinical results in solid tumors have lagged behind the promise, precisely because transferred NK cells dysfunction rapidly after entering tumor tissue. The CD55 discovery provides both an explanation and a solution: by engineering CD55 expression into therapeutic NK products, or by finding pharmacological means to preserve it, cell therapists may be able to keep the raft–LCK ignition switch flipped on throughout the cell’s journey into and within the tumor.

Beyond its immediate therapeutic implications, the study reshapes fundamental understanding of how innate immunity is wired. It reveals that NK cells possess an autonomous activation circuit in which tumor recognition, through NKG2D and p65, induces a membrane organizer that then amplifies signaling through lipid raft condensation and LCK activation. It also adds CD55 and CD97 to the growing roster of ligand–receptor pairs whose trans interactions at the immune synapse determine the outcome of encounters between immune cells and cancer. And it frames NK cell dysfunction not as an irreversible slide into exhaustion but as a defined, mechanistically understood state, one whose molecular signature, CD55 loss, can in principle be measured, monitored and corrected. As immunotherapy continues its migration from blood cancers toward solid tumors, the humble complement regulator once known simply as decay-accelerating factor may prove to be one of the most important switches in the killer cell’s arsenal, and flipping it back on could mark a turning point in the effort to make natural killer cells the reliable cancer weapons they have always promised to be.

Subject of Research: The role of the membrane protein CD55 in organizing lipid raft–LCK signaling that drives natural killer cell antitumor immunity.

Article Title: CD55 organizes lipid raft-LCK signaling to potentiate NK-cell antitumor immunity

Article References: Li, L., Li, Z., Liu, Y., Fan, W., Lei, Y., Tian, L., Chen, L., Qu, Z., Shi, Y., Yu, J., & Wang, Y. (2026). CD55 organizes lipid raft-LCK signaling to potentiate NK-cell antitumor immunity. Cell Research. https://doi.org/10.1038/s41422-026-01288-8

Image Credits: AI Generated

DOI: 10.1038/s41422-026-01288-8

Keywords: CD55, natural killer cells, LCK kinase, lipid rafts, NKG2D, tumor microenvironment, CAR-NK cells, immunotherapy, CD97, antitumor immunity, organizes, lipid