Antigen-presenting cells serve as the central bridge between innate and adaptive immunity. Dendritic cells and tumor-associated macrophages patrol tissues, engulf cellular debris and tumor fragments, chop those fragments into peptides, and display the resulting epitopes on major histocompatibility complex molecules at their surface. This display is what trains CD8-positive T cells to recognize and kill cancer cells. Yet within the immunosuppressive tumor microenvironment, these antigen-presenting cells often become functionally impaired or tolerized, a phenomenon the review identifies as one of the core mechanisms of tumor immune evasion. The enzymes responsible for cutting antigens into presentable peptides are not passive housekeepers; their activity directly determines the efficiency, diversity, and ultimate immunogenicity of the peptide repertoire that reaches the cell surface. When that activity is misregulated, the immune system may simply never see the tumor as a threat.

The review’s central conceptual contribution is the description of cysteine peptidases as molecular scissors operating within the lysosomal compartments of antigen-presenting cells. Antigen processing is a delicate balancing act: peptides must be cleaved to the right length and with the right termini to bind MHC molecules, but excessive degradation destroys the very epitopes that would provoke a protective T cell response. The authors synthesize recent advances showing that cysteine peptidases regulate antigen processing, MHC molecule loading, cross-presentation, and immune regulatory signaling pathways. Cross-presentation, the process by which dendritic cells display antigens from tumor cells on MHC class I molecules to activate cytotoxic T cells, is especially sensitive to lysosomal protease activity. In this sense, the enzymatic milieu of a single intracellular compartment can tip the balance between an immunogenic response that eliminates a tumor and a tolerogenic one that permits its growth.

Perhaps the most striking finding is the identification of two core pathways through which cysteine peptidases actively drive tumor immune evasion. The first involves excessive antigen degradation within tumor-associated macrophages. When cathepsin activity is abnormally elevated, phagocytosed tumor antigens are over-degraded, destroying key immunogenic epitopes before they can bind to MHC molecules. The macrophage effectively shreds the evidence, presenting nothing useful to T cells and fostering tolerance rather than immunity. The second pathway centers on PD-L1 regulation, the molecular brake exploited by many tumors. Cathepsin S modulates PD-L1 expression on tumor cells and macrophages, directly suppressing T cell activity through the PD-1/PD-L1 axis. This links lysosomal protease biology to the checkpoint pathway that has become the backbone of modern cancer immunotherapy, suggesting that peptidase inhibition could sensitize tumors to existing checkpoint blockade.

The review also reveals a subtler layer of regulation: cysteine peptidases exert dose-dependent dual control over Toll-like receptor signaling pathways. Moderate peptidase activity promotes immune activation, helping innate immune receptors generate the inflammatory signals needed to launch adaptive responses, while excessive activity causes premature signal attenuation, damping down inflammation before an effective response can consolidate. This biphasic behavior carries an important therapeutic warning. Simply blocking these enzymes everywhere and always may not be beneficial; the goal is to restore a functional activity window, not to eliminate peptidase function altogether. It also helps explain why broad-spectrum inhibition has produced mixed results in some contexts, and why the field is moving toward selective, precisely dosed interventions guided by measurements of enzymatic activity rather than gene expression alone.

Across cancer types, the expression and functional patterns of cysteine peptidases display remarkable specificity, and the review maps these differences in detail. In colorectal cancer, cathepsin S is consistently upregulated in tumor-associated macrophages and correlates positively with PD-L1 expression and poor response to anti-PD-1 therapy, making it a candidate biomarker for checkpoint resistance. In melanoma, single-cell proteomics has identified elevated cathepsin L and B activity in a subset of tumor-educated dendritic cells associated with regulatory T cell induction and immune tolerance. In pancreatic ductal adenocarcinoma, one of the most lethal malignancies, cathepsins B and L are overexpressed in cancer-associated fibroblasts and tumor cells, where they promote metastasis, whereas cathepsin S activity remains relatively low, indicating that this tumor type employs a distinct immune evasion strategy. These findings underscore that therapeutic targeting of cysteine peptidases must be cancer-type-specific and stratified by patient, based on enzymatic activity profiling rather than transcript levels alone.

On the therapeutic front, the review systematically evaluates the strategies now in development. Small-molecule inhibitors remain the workhorse approach, ranging from the broad-spectrum inhibitor E64 to the selective cathepsin S inhibitor LHVS. Beyond conventional pharmacology, intelligent nanocarriers such as Ft-E64/Hf@Lipo have been engineered to co-deliver cysteine protease inhibitors and radiosensitizers, remodeling the antigen-presenting function of tumor-associated macrophages while simultaneously sensitizing tumor cells to radiation. These delivery platforms address one of the persistent problems in the field: getting inhibitors to the right cells, in the right compartment, at the right time, without systemic toxicity. The convergence of nanotechnology and protease inhibition illustrates how lysosomal biology, long considered an obscure corner of cell biology, is becoming an actionable target for precision oncology.

The most clinically consequential results come from combination strategies. Preclinical models demonstrate that combining cathepsin B inhibition with PD-1 antibodies markedly increases T cell tumor infiltration and cytotoxic capacity, producing strong synergistic antitumor effects in melanoma, lung cancer, and colorectal cancer models. For immunologically cold tumors, those lacking meaningful T cell infiltration and therefore unresponsive to checkpoint blockade, this combination can convert them into hot tumors by enhancing T cell infiltration and activation, improving response rates to immune checkpoint blockade. Because many of the most common and aggressive cancers fall into the cold category, a strategy that mechanically reprograms the antigen-presenting machinery to restore immune visibility could be transformative. The synergy is mechanistically coherent: removing the enzymatic shredding of antigens increases the supply of presentable epitopes, while checkpoint blockade removes the inhibitory brakes on the T cells that recognize them.

The authors are candid about the obstacles that stand between these findings and routine clinical practice. Prolonged exposure to broad-spectrum inhibitors can trigger compensatory resistance mechanisms, in which alternative cathepsins are upregulated through TFEB-mediated lysosomal stress responses, effectively rerouting the degradation machinery around the blocked enzyme. Overcoming translational bottlenecks in spatiotemporal dynamics, delivery specificity, and patient stratification, the review argues, will require integrated multi-omics approaches, gene-editing tools, and artificial intelligence-driven methods to identify which patients, which tumors, and which peptidase targets should be pursued in each case. Activity-based probes and single-cell technologies are expected to play a growing role in building the enzymatic profiles that guide treatment decisions.

What emerges from the review is a picture of cancer immunotherapy expanding downward, from the level of immune cell interactions to the level of the lysosomal enzymes that make those interactions possible. Cysteine peptidases, once studied primarily as housekeeping proteases or as mediators of tumor invasion, now sit at a control point where antigen fate, checkpoint signaling, and innate immune calibration converge. If the challenges of specificity, resistance, and stratification can be solved, peptidase-targeted therapies hold the potential to revolutionize cancer immunotherapy by reactivating robust antitumor immune responses in patients whose tumors have so far remained invisible to their own immune systems. For a field searching for ways to turn non-responders into responders, the molecular scissors of the lysosome have become an unexpectedly promising place to look.

Subject of Research: The role of cysteine peptidases in regulating antigen-presenting cell function and antitumor immunity

Article Title: Cysteine peptidases emerge as key regulators of antitumor immunity and promising therapeutic targets

Article References: Cysteine peptidases emerge as key regulators of antitumor immunity and promising therapeutic targets. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cysteine peptidases, cathepsins, antigen-presenting cells, tumor-associated macrophages, dendritic cells, PD-L1, immune checkpoint blockade, tumor microenvironment, cross-presentation, cathepsin S inhibitors, cancer immunotherapy, lysosomal proteolysis