When most people think about the immune system fighting cancer, they picture T cells - the aggressive soldiers that hunt down and destroy tumor cells. But a growing body of research is revealing that B cells, another major immune cell type traditionally associated with antibody production, also play important roles in the tumor environment - sometimes helping, and sometimes hindering, the immune response.
In colorectal cancer (CRC), the tumor microenvironment is often described as immune-suppressive - meaning the tumor creates conditions that prevent immune cells from doing their job effectively. This is one major reason why immunotherapy drugs, which are designed to unleash the immune system against cancer, only work in a small fraction of colorectal cancer patients - roughly 15% of cases with specific genetic profiles.
This study investigated a specific B cell subtype called germinal center B cells (Bgc) that are found inside colorectal tumors. Using cutting-edge single-cell RNA sequencing technology, the researchers analyzed thousands of individual immune cells from patient tumor samples and discovered that Bgc cells communicate intensively with CD8+ T cells - the main cancer-killing immune cells - and that this communication is much stronger in patients who respond well to immunotherapy.
The central finding is that Bgc cells can enter a special cellular state, called a senescence-like state, which paradoxically boosts their ability to help T cells. In this state, the B cells secrete a signaling molecule called Interleukin-7 (IL-7) that keeps T cells active and prevents them from becoming exhausted. This discovery opens a new therapeutic avenue for making more colorectal cancer patients responsive to immunotherapy.
Inside a colorectal tumor, the immune environment is hostile to the very cells meant to destroy it. CD8+ cytotoxic T cells - the main cell type responsible for killing cancer cells - are often found inside tumors in a dysfunctional state called exhaustion. Exhausted T cells are still alive, but they have significantly reduced ability to kill tumor cells or produce the immune molecules needed to coordinate an attack.
Exhausted T cells are characterized by high expression of inhibitory receptors like PD-1, LAG3, TIM3, and TIGIT. These molecular brakes prevent the T cells from functioning normally. Immune checkpoint blockade (ICB) therapy, such as anti-PD-1 or anti-PD-L1 drugs, works by removing these brakes - but only in patients whose tumors have specific features like DNA mismatch repair deficiency (dMMR) or high microsatellite instability (MSI-H).
For the 85% of CRC patients without these features, checkpoint inhibitor drugs provide little benefit. Understanding why some patients respond and others do not - and finding ways to expand the responding population - is one of the most important challenges in colorectal cancer treatment today.
B cells within the tumor microenvironment include multiple subtypes: naive, memory, follicular, and germinal center B cells. Previous studies had focused mainly on tumor cells themselves as drivers of immune suppression, but this work was among the first to systematically investigate how B cell subtypes specifically modulate anti-PD-L1 immunotherapy response in CRC.
The research team used a combination of computational and experimental approaches to uncover how Bgc cells influence the immune response in colorectal cancer. The starting point was publicly available single-cell RNA sequencing (scRNA-seq) data from CRC patients who received immune checkpoint inhibitor therapy, comparing patients who achieved a complete response to treatment (pCR group) versus those who did not respond (non-pCR group).
Using a computational tool called CellChat analysis, the team mapped the communication signals between different immune cell types in the tumor. They found that signaling between Bgc cells and CD8+ T cells was dramatically stronger in patients who responded to immunotherapy. This was visually confirmed using multiplex immunofluorescence (mIF) imaging, which stained tumor tissue sections with multiple antibodies at once to show how closely different cell types were interacting physically.
To confirm that Bgc cells specifically were responsible for T cell support - and not other B cell types - the researchers performed co-culture experiments, mixing Bgc cells with activated CD8+ T cells in laboratory dishes. Flow cytometry measurements showed that Bgc cells specifically reduced T cell exhaustion markers and boosted T cell function, while naive, memory, and follicular B cells had no such effect.
The most powerful validation came from adoptive transfer experiments in B cell-deficient mice (muMt mice). These mice cannot make B cells naturally. When given colorectal cancer tumors, they grew tumors faster than normal mice. But when Bgc cells were transferred into these mice, tumor growth slowed significantly - proving that Bgc cells play a real anti-tumor role. Critically, depleting CD8+ T cells reversed this benefit, confirming that Bgc cells act through T cells.
Having established that Bgc cells are critical for immune control in colorectal cancer, the researchers next asked: what determines whether Bgc cells enter their powerful anti-tumor state? The answer came from a molecular analysis of what genes were different in Bgc cells from responding versus non-responding patients.
Gene expression analysis identified the top ten genes that were downregulated in Bgc cells from responding patients. Comparing these genes against a database of mitochondria-associated genes (MITOCARTA3.0) revealed that two stood out: PRELID1 (a mitochondrial phospholipid transport protein) and VDAC3 (a channel protein on the outer mitochondrial membrane). Both were significantly lower in Bgc cells from patients who responded to immunotherapy.
Laboratory experiments using gene knockdown (reducing gene activity) and overexpression (increasing gene activity) confirmed these proteins' roles. Knocking down PRELID1 in Bgc cells caused them to become more activated - mimicking the state seen in treatment-responding patients. Overexpressing VDAC3 reversed this effect. Conversely, knocking down VDAC3 also promoted Bgc activation. Crucially, knocking down a third mitochondrial gene, BAX, had no such effect - confirming the specificity of the PRELID1-VDAC3 regulatory relationship.
Spatial transcriptomics analysis of actual colorectal cancer tissue sections showed that PRELID1 and VDAC3 are co-expressed across the tumor. Immunohistochemistry (IHC) confirmed that PRELID1 protein levels were lower specifically in Bgc cells within tumors from responding patients, providing direct tissue-level evidence that reduced PRELID1 corresponds to better immunotherapy outcomes.
To understand how two mitochondrial proteins control the immune activity of Bgc cells, the researchers investigated the interaction between mitochondria and lysosomes - two critical organelles inside every cell. Mitochondria are the cell's energy generators, while lysosomes are the cell's recycling centers that break down damaged components. These two organelles physically contact each other at specialized sites, and disruption of this contact is linked to cellular aging processes.
Using ultra-high-resolution 3D structured illumination microscopy (3D-SIM), the team visualized the subcellular locations of PRELID1, VDAC3, and a lysosomal protein called TRPML1. They found that all three proteins co-localize at the junction between mitochondria and lysosomes, suggesting they form a physical complex that regulates this organelle interaction.
When PRELID1 was knocked down in Bgc cells, the number of mitochondria-lysosome contact points dropped dramatically. This was confirmed by both fluorescence imaging and electron microscopy. Strikingly, the mitophagy process - the cellular mechanism that removes damaged mitochondria - was also impaired when PRELID1 was reduced. This meant that damaged mitochondria accumulated in the cells, which normally triggers cellular stress responses.
Importantly, overexpression of either VDAC3 or TRPML1 could restore the mitochondria-lysosome connections that were lost when PRELID1 was removed. This reveals a compensatory mechanism: VDAC3 can partially substitute for PRELID1 in maintaining organelle contacts. The PRELID1-VDAC3-TRPML1 axis therefore functions as an integrated system that regulates the health and function of both mitochondria and lysosomes in Bgc cells.
Cellular senescence is typically associated with aging - cells that have stopped dividing and can no longer replicate. In most contexts, senescence is considered a negative phenomenon because senescent cells can release inflammatory molecules that damage surrounding tissue. But this study found something unexpected: in Bgc cells within colorectal tumors, a senescence-like state is actually beneficial.
GSEA (Gene Set Enrichment Analysis) of Bgc cells showed that genes related to mitochondria, lysosomes, and cellular senescence were significantly enriched in cells from patients who responded to immunotherapy. Comparing across patient groups, Bgc cells from the immunotherapy-responding group showed higher senescence scores, higher expression of the senescence markers P16 and P21, and higher activity of beta-galactosidase - a classic senescence indicator.
A key distinction is that this is called a senescence-like state rather than true senescence. In classic senescence, cells permanently stop dividing and can promote inflammation. In Bgc cells, the researchers found no evidence of irreversible cell cycle arrest or increased apoptosis (programmed cell death). Instead, the cells showed a stress-adaptive, metabolically reprogrammed state with enhanced immune-stimulatory function - a positive outcome.
When PRELID1 was knocked down, Bgc cells showed increased P16 and P21 expression, confirming that loss of PRELID1 pushes Bgc cells toward this beneficial senescence-like state. Conversely, overexpressing PRELID1 suppressed senescence markers. Knockdown of VDAC3 also promoted senescence, while knockdown of an unrelated mitochondrial gene (BAX) did not - again confirming the specificity of this regulatory axis.
The researchers next investigated the molecular messenger that allows Bgc cells in their senescence-like state to help T cells. Analyzing gene expression in CD8+ T cells from immunotherapy-responding patients, they found that IL-7R (the receptor for a protein called Interleukin-7) was one of the most upregulated genes - meaning the T cells were actively seeking IL-7 signaling.
IL-7 is a cytokine (signaling protein) known to support T cell survival, proliferation, and memory formation. Experiments confirmed that Bgc cells produce significantly more IL-7 than other B cell subtypes, and that this production increases further when Bgc cells are in their senescence-like state. Knocking down IL-7 in Bgc cells reduced their ability to support CD8+ T cells in co-culture experiments, while adding back recombinant IL-7 protein partially restored T cell function.
In mouse tumor models, transferring PRELID1-knockdown Bgc cells (which are in the senescence-like state and secrete more IL-7) into tumors led to increased IL-7 protein detectable specifically in intratumoral B cells. More importantly, blocking the IL-7 receptor with an antibody reversed the tumor-suppressive effects of PRELID1-knockdown Bgc cells - proving that IL-7 is the essential messenger mediating Bgc cells' anti-tumor activity.
The connection to immunotherapy was made explicit in experiments combining Bgc cells with anti-PD-1 antibody treatment. The combination of Bgc cells plus anti-PD-1 worked better than either alone at slowing tumor growth in mice. But blocking IL-7R reversed the benefit - confirming that IL-7 signaling is essential for Bgc cells to cooperate with immune checkpoint therapy. This establishes the PRELID1-VDAC3-senescence-IL-7 cascade as a complete mechanistic pathway from organelle regulation to immunotherapy response.
Currently, immune checkpoint blockade therapy - which includes drugs targeting PD-1 and PD-L1 - only helps about 15% of colorectal cancer patients, specifically those with DNA mismatch repair deficiency. The remaining 85% of CRC patients receive minimal benefit from these powerful drugs. Finding strategies to overcome this immunotherapy resistance is one of the most urgent challenges in colorectal cancer treatment.
This research suggests a promising new strategy: modulating the PRELID1-VDAC3-IL-7 axis in germinal center B cells. By reducing PRELID1 activity - for example, using targeted gene therapy, small molecule inhibitors, or RNA-based approaches - it may be possible to push more Bgc cells into their senescence-like, IL-7-producing state within the tumor microenvironment. This could potentially awaken otherwise exhausted T cells and restore sensitivity to anti-PD-L1 immunotherapy in patients who would not otherwise respond.
Alternatively, delivering recombinant IL-7 or using therapies that mimic IL-7 signaling could achieve similar effects by directly supporting CD8+ T cell survival and function. IL-7 is already being investigated in clinical trials for other cancer types, and this research provides additional rationale for exploring its role specifically in colorectal cancer immune therapy.
The study also suggests that PRELID1 expression levels and IL-7 production in Bgc cells could serve as new biomarkers to predict which patients are most likely to respond to immunotherapy. Patients whose tumors show low PRELID1 and high IL-7 in B cells may already have an immune microenvironment primed for a response to checkpoint inhibitors, potentially helping oncologists personalize treatment decisions.
This research makes a conceptually important contribution beyond its specific findings about PRELID1 and VDAC3. It demonstrates for the first time that organelle-level regulation - specifically the physical communication between mitochondria and lysosomes - inside B cells can determine the immune state of an entire tumor microenvironment. This level of regulation had previously been overlooked in the context of anti-tumor immunity.
The study shows that a chain of events beginning inside a single organelle (mitochondria-lysosome interaction) propagates outward to control cell state (senescence-like phenotype), then to intercellular communication (IL-7 secretion), then to T cell function, and ultimately to tumor growth and immunotherapy response. This multi-level cascade spans from nanometer-scale molecular interactions to whole-body treatment outcomes.
The findings also suggest that similar mitochondria-lysosome crosstalk may operate in T cells and potentially other immune cell types within the tumor. If organelle-level regulation is a general immunometabolic checkpoint across immune cells, then therapies targeting this pathway could have broad applications across multiple cancer types beyond colorectal cancer.
In conclusion, this study identifies the PRELID1-VDAC3-IL-7 axis in senescence-like germinal center B cells as a promising therapeutic target that holds significant potential to overcome immune resistance, reshape the tumor microenvironment, and enhance the efficacy of current immunotherapies in colorectal cancer. Future clinical validation of these biomarkers and translational testing of PRELID1 inhibition strategies will be important next steps.