Breaking the Exhaustion Barrier: A New Target for Cancer Immunotherapy – CD47 & Thrombospondin-1
For years,cancer immunotherapy has held immense promise,harnessing the power of the body’s own immune system to fight disease. While checkpoint inhibitors like those targeting PD-1 have revolutionized treatment for cancers like melanoma, many patients still don’t respond, or develop resistance. Now, groundbreaking research from Weill Cornell Medicine is illuminating a critical mechanism driving T cell exhaustion – and a potential new pathway to overcome it. This discovery centers on the interplay between CD47, a molecule on both cancer and immune cells, and thrombospondin-1 (TSP-1), a protein secreted by aggressive tumors. Understanding this interaction could unlock more durable and effective cancer immunotherapies.
The Challenge of T Cell Exhaustion
T cells are the frontline soldiers of the immune system, tasked with identifying and destroying cancerous cells. Though, tumors aren’t passive targets. They actively suppress immune responses, and a key tactic is inducing “exhaustion” in T cells.Exhausted T cells lose thier ability to effectively kill cancer cells, becoming functionally inert.
Previous research has focused on PD-1, a protein on T cells that acts as an “off switch” for the immune response. Checkpoint inhibitors block PD-1, effectively removing the brake and allowing T cells to regain their anti-cancer activity. However,PD-1 isn’t the whole story. Researchers have long suspected other mechanisms contribute to T cell exhaustion, and the latest findings point strongly to CD47 as a meaningful player.
CD47: A Dual Role in Immune Evasion
CD47 has been known for its role in helping cancer cells evade destruction by macrophages – immune cells that engulf and eliminate threats. Cancer cells display CD47 as a “don’t eat me” signal, preventing macrophages from recognizing them as targets. However, the surprising revelation from Dr. Taha Merghoub’s team is that T cells themselves express CD47, and its levels dramatically increase as they become exhausted.
“When T cells are activated, they express CD47. And when they get exhausted,they increase CD47 to very high levels,” explains Dr. Merghoub.This suggests CD47 isn’t just a shield for cancer cells, but an intrinsic marker and driver of T cell dysfunction.
Experiments in mice confirmed this hypothesis. Mice lacking CD47 exhibited slower tumor growth, demonstrating that CD47 on immune cells, rather than cancer cells, was primarily responsible for the observed exhaustion effect. Furthermore, T cells genetically engineered to lack CD47 were substantially more effective at eliminating melanoma tumors.
The TSP-1 Connection: A Key to Manipulating Exhaustion
The research team then sought to understand how cancer cells might be exploiting CD47 to induce T cell exhaustion. Their examination led them to thrombospondin-1 (TSP-1), a protein produced by metastatic cancer cells. TSP-1 binds directly to CD47 on T cells, triggering a cascade of events that ultimately leads to exhaustion.
“That was the real eureka moment,” says Dr. Merghoub. “It showed us that CD47 and thrombospondin are clearly key players because eliminating either one gives you the same effect.” Mice engineered to lack TSP-1 showed reduced T cell exhaustion, mirroring the results seen in CD47-deficient mice. This established a clear link: TSP-1 binding to CD47 actively contributes to T cell dysfunction.
TAX2: A Proof-of-Concept for Disrupting the Signal
To validate this interaction and demonstrate its therapeutic potential, the researchers developed TAX2, a peptide designed to specifically block the connection between CD47 and TSP-1. The results were compelling. TAX2 treatment in mice with melanoma or colorectal cancer maintained T cell activity, boosted the release of immune-stimulating cytokines, and improved T cell infiltration into tumors.
Importantly, TAX2 also enhanced the effectiveness of existing PD-1 immunotherapy in colorectal cancer models. This suggests that targeting the CD47-TSP-1 pathway could overcome resistance to PD-1 blockade, offering a powerful combination therapy.
“We used the TAX2 peptide as a proof-of-concept to confirm that disrupting the crosstalk between TSP-1 and CD47 prevents T cell exhaustion in mice with tumors,” explains Dr.Chien-Huan (Gil) Weng, the study’s lead author. “Next, we plan to study both upstream and downstream modulators that regulate the TSP-1:CD47
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