Blood-Borne Immune Checkpoint Proteins Linked to Kidney Cancer Risk in New Case-Control Study

A team of researchers in Hangzhou, China, has uncovered fresh evidence that the immune system’s own braking machinery, measured in a simple blood sample, may help reveal who is susceptible to renal cell cancer, one of the most immunologically responsive tumors known to medicine. In a case-control study published in BMC Cancer, investigators from the Affiliated Sir Run Run Shaw Hospital of Zhejiang University School of Medicine and the university’s School of Public Health report that circulating levels of several soluble immune checkpoint proteins differ markedly between patients with renal cell cancer and healthy individuals, and that a composite score built from three of these proteins can distinguish cases from controls with moderate accuracy. The findings, while exploratory, point toward a new class of blood-based biomarkers for a disease that is often diagnosed late and for which early detection options remain frustratingly limited.

Immune checkpoints are the molecular gatekeepers of the immune system. Receptors such as PD-1, CTLA-4, LAG-3 and TIM-3 sit on the surface of T cells and, when engaged by their binding partners, dampen immune activation to prevent autoimmunity. Tumors exploit this system, hijacking checkpoint signaling to cloak themselves from immune destruction, which is precisely why checkpoint inhibitor drugs such as anti-PD-1 and anti-CTLA-4 antibodies have transformed the treatment landscape for renal cell carcinoma and many other cancers. What is less well appreciated is that many of these checkpoint molecules also circulate in a soluble form, shed from cell surfaces or released as free-floating variants, and can be measured in plasma. These soluble versions may not merely be passive byproducts; they can actively bind ligands, modulate immune responses, and potentially serve as accessible readouts of the immune state that would otherwise require tissue biopsies to observe.

To probe whether these circulating proteins carry information about kidney cancer susceptibility, the research team recruited 40 patients with renal cell cancer and 40 healthy controls. Using a multiplex Luminex assay, a bead-based platform capable of quantifying multiple proteins simultaneously from a small sample volume, they measured the plasma concentrations of 14 soluble immune checkpoint proteins: TIM-3, CD28, CD80, CD137, CD27, CTLA-4, BTLA, GITR, HVEM, IDO, LAG-3, PD-1, PD-L1 and PD-L2. The multiplex approach is technically demanding, requiring careful calibration and quality control, but it offers a panoramic snapshot of the checkpoint axis in a single measurement, something conventional single-analyte assays cannot achieve efficiently.

The results were striking. Compared with healthy controls, the median circulating levels of six checkpoint proteins were significantly elevated in the cancer patients: soluble CTLA-4, soluble LAG-3, soluble BTLA, soluble GITR, soluble CD80 and soluble PD-1. Each of these molecules plays a distinct role in immune regulation. Soluble CTLA-4, for example, is associated with regulatory T cell activity and broad immunosuppression, while LAG-3 and BTLA mark exhausted or inhibited T cell states that tumors frequently induce. GITR, by contrast, is generally considered a co-stimulatory receptor, and its elevated soluble form may reflect intense immune activation or dysregulation within the tumor microenvironment. The breadth of the signal suggests that renal cell cancer is accompanied by a systemic shift in checkpoint biology, not merely a localized change within the tumor itself.

To move from association to a more rigorous assessment of risk, the researchers turned to multivariate logistic regression, a statistical technique that estimates the independent contribution of each variable while accounting for the others. After adjustment, higher levels of soluble CTLA-4 were associated with an increased risk of renal cell cancer, with an odds ratio of 1.91 per unit increase and a 95 percent confidence interval spanning 1.18 to 3.09, a statistically significant finding with a P value of 0.008. Higher soluble HVEM levels also carried elevated risk, with an odds ratio of 1.22 and a confidence interval of 1.01 to 1.48, reaching significance at P equals 0.048. HVEM, encoded by the gene TNFRSF14, is a tumor necrosis factor receptor family member with complex dual roles, capable of transmitting both activating and inhibitory signals depending on the ligand it engages. In contrast, higher soluble CD28 levels were associated with a decreased risk of renal cell cancer, with an odds ratio of 0.93 per unit increase and a confidence interval of 0.88 to 0.98, significant at P equals 0.011. CD28 is the prototypical co-stimulatory receptor required for robust T cell activation, so its protective association hints that preserved co-stimulatory capacity in the circulation may reflect an immune system better positioned to resist tumor emergence.

Building on these three independent signals, the team constructed a composite metric they call the soluble immune checkpoint, or SIC, score, integrating sCTLA-4, sHVEM and sCD28 into a single risk index. The SIC score was itself significantly associated with increased renal cell cancer risk, with an odds ratio of 1.55 and a 95 percent confidence interval of 1.18 to 2.16. When the researchers combined the SIC score with clinical variables in a multivariable diagnostic model and evaluated its performance using receiver operating characteristic curves, the model achieved an area under the curve of 0.86, with a 95 percent confidence interval of 0.78 to 0.94. In diagnostic terms, an AUC of 0.86 represents moderate to good discriminative ability, meaning the model correctly ranks a randomly chosen cancer patient above a randomly chosen healthy control in roughly 86 percent of paired comparisons. That level of performance from a blood-based panel is noteworthy, although the authors are careful to frame it as an exploratory result requiring validation.

To understand the biology behind the statistical associations, the investigators turned to large public genomics resources, analyzing gene expression data from The Cancer Genome Atlas pan-kidney cohort and the Gene Expression Omnibus. This computational arm of the study revealed that upregulation of IDO1, the gene encoding indoleamine 2,3-dioxygenase, and of TNFRSF14, the gene encoding HVEM, was associated with tumorigenesis and disease development in both datasets. IDO1 is a well-known immunosuppressive enzyme that catabolizes tryptophan in the tumor microenvironment, starving cytotoxic T cells of a nutrient they need and generating metabolites that promote regulatory T cell activity. The concordance between the gene-level findings and the protein-level associations strengthens the plausibility of the overall picture. Additionally, upregulation of PDCD1LG2, the gene encoding PD-L2, was correlated with the development of renal cell cancer, adding another layer to the evidence that checkpoint pathway dysregulation accompanies kidney tumor progression.

The significance of this work lies in the context of renal cell cancer’s clinical trajectory. Kidney cancers, of which renal cell carcinoma accounts for the vast majority, are frequently discovered incidentally or only after symptoms arise, at which point the disease may have advanced. There is no widely adopted screening test for the general population, and while imaging has improved detection, blood-based markers that could flag susceptibility or early disease would fill a genuine unmet need. The idea that soluble checkpoint proteins, measurable with existing laboratory platforms, could contribute to such a tool is appealing because it leverages the same molecular axis that has already proven therapeutically targetable. If soluble checkpoint profiles reflect the immune conditions that permit tumor emergence, they could eventually complement imaging and clinical risk factors in identifying individuals who warrant closer surveillance.

Nevertheless, important caveats temper the enthusiasm. The study involved a modest sample of 40 cases and 40 controls, and its case-control design captures a snapshot at a single point in time rather than tracking whether elevated checkpoint levels precede cancer onset. Reverse causation remains possible, meaning the tumors themselves might drive the elevated protein levels rather than the other way around. Confounding by factors such as age, smoking status, body mass index and comorbidities must be fully excluded in larger cohorts, and the moderate discriminative performance, while promising, falls short of what would be required for standalone screening. The authors themselves emphasize that their findings are exploratory and highlight the need for future prospective studies to determine whether soluble immune checkpoint proteins can genuinely serve in disease screening for renal cell cancer.

Even with those limitations, the study adds a compelling piece to the rapidly evolving puzzle of cancer immunosurveillance. It demonstrates that the checkpoint molecules made famous by immunotherapy drugs are not confined to the tumor microenvironment but circulate systemically in patterns that track with cancer susceptibility. As multiplex proteomic technologies become faster and cheaper, and as prospective cohorts accumulate serial blood samples over years, the vision of a blood test that reads the immune system’s checkpoint chatter and flags kidney cancer risk before tumors announce themselves moves from speculation toward testable reality. For now, the Hangzhou team’s SIC score stands as an early but intriguing signal that the immune system’s brakes, measured in a vial of blood, may one day help catch one of the world’s most common urologic cancers at its most vulnerable stage.

Subject of Research: Association between soluble immune checkpoint proteins and susceptibility to renal cell cancer

Article Title: Soluble immune checkpoints proteins are significantly associated with the susceptibility of renal cell cancer, a case–control study

Article References: Chao, Y., Ye, S., Chen, L., Li, W., Jin, Z., Wei, G., Yu, H., Yu, Y., & Wang, Q. (2026). Soluble immune checkpoints proteins are significantly associated with the susceptibility of renal cell cancer, a case–control study. BMC Cancer. https://doi.org/10.1186/s12885-026-17008-9

Image Credits: AI Generated

DOI: 10.1186/s12885-026-17008-9

Keywords: renal cell cancer, soluble immune checkpoint proteins, CTLA-4, HVEM, CD28, biomarkers, case-control study, cancer immunotherapy, Luminex assay, TCGA, early detection, kidney cancer