Staphylococcal Protein Reprograms Liver Tumor Macrophages to Boost Immunotherapy

One of the most stubborn obstacles in cancer immunotherapy is not the tumor cell itself but the cellular entourage that surrounds it. In hepatocellular carcinoma, the most common form of primary liver cancer, immune checkpoint inhibitors have transformed outcomes for some patients, yet the majority either fail to respond or eventually relapse. A study published in the Journal of Experimental & Clinical Cancer Research by Gan Liu, Xiaoling Ma, Shanshan Zhang and colleagues at the First Affiliated Hospital of the University of Science and Technology of China now points to a surprising ally in the fight against this resistance: a protein component derived from Staphylococcus aureus, the bacterium better known for causing skin infections and pneumonia. The molecule, called LukS-PV, is the S component of Panton-Valentine leukocidin, a pore-forming toxin long studied by microbiologists. Rather than acting as a poison, however, LukS-PV functions in this context as a precision ligand, binding selectively to a receptor called C5aR1 that is abundantly displayed on tumor-associated macrophages within liver tumors.

Tumor-associated macrophages, or TAMs, are among the most numerous immune cells in the hepatocellular carcinoma microenvironment, and their prevailing M2-like state is a principal reason why checkpoint inhibitors so often fall short. These macrophages secrete immunosuppressive cytokines such as interleukin-10 and transforming growth factor-beta, suppress cytotoxic T lymphocytes, and actively construct a barrier against immune attack. C5aR1, a G-protein-coupled receptor normally engaged by the complement fragment C5a during inflammatory responses, is highly expressed on these TAMs, making it an attractive point of intervention. Despite this biological logic, no C5aR1-targeted cancer therapy has ever been approved, largely because complement signaling is deeply woven into normal immune regulation and systemic blockade carries risks. The Chinese team hypothesized that LukS-PV, as a natural ligand for the human form of C5aR1, might offer a way to engage the receptor selectively on macrophages inside tumors and flip them from suppressive to inflammatory.

To test this idea rigorously, the researchers built an experimental system that could distinguish human C5aR1-dependent effects from murine biology. They established both subcutaneous tumor models and a hydrodynamic tail vein injection model that produces spontaneous liver tumors resembling immunotherapy-refractory hepatocellular carcinoma, using mice engineered to carry the human C5AR1 gene alongside wild-type controls. This design proved critical. When the animals were treated with LukS-PV, tumor growth was significantly suppressed in the humanized C5AR1 mice but not in the wild-type animals, a result that supports a mechanism specifically dependent on the human receptor. Macrophage depletion experiments further demonstrated that macrophages were essential mediators of the antitumor activity: when these cells were removed, the therapeutic benefit of LukS-PV largely disappeared, confirming that the drug’s effect runs through the macrophage compartment rather than through a direct action on tumor cells.

Single-cell RNA sequencing provided a high-resolution view of what LukS-PV actually does inside the tumor. The technique, which profiles gene expression in thousands of individual cells simultaneously, revealed that treatment reshaped the entire immune landscape of the tumor. Most strikingly, TAMs shifted away from their immunosuppressive M2-like phenotype toward inflammatory M1-like states, a transcriptional transformation accompanied by changes in the broader immune ecosystem. Flow cytometry and immunofluorescence staining of tumor tissue validated these findings in vivo, showing that the reprogrammed macrophage population was accompanied by increased infiltration of CD8-positive T cells, the cytotoxic lymphocytes responsible for killing cancer cells. Those T cells displayed enhanced effector activation and reduced markers of exhaustion, indicating that the therapy did not merely bring more immune cells into the tumor but restored their functional capacity to attack it.

The mechanistic core of the study lies in the signaling cascades that LukS-PV triggers downstream of C5aR1. Using transcriptomic and molecular analyses, the team showed that the bacterial protein inhibits C5aR1-dependent PI3K/AKT signaling within macrophages. This pathway, when active, supports the suppressive macrophage program, and its inhibition set off a cascade of downstream consequences. The researchers found that LukS-PV suppressed the GSK3β/CREB/IL-10 axis, a signaling route that drives production of interleukin-10, one of the most potent immunosuppressive cytokines in the tumor microenvironment. At the same time, treatment promoted NF-κB-associated inflammatory activation, steering macrophages toward a program characterized by pro-inflammatory gene expression. Chromatin immunoprecipitation followed by quantitative PCR was used to confirm the transcriptional wiring of this switch, linking receptor engagement to changes in the activity of specific transcription factors.

The reduction in TAM-derived interleukin-10 emerged as a pivotal consequence of this reprogramming. Interleukin-10 is a master regulator of immune suppression, and its abundance in liver tumors helps explain why CD8-positive T cells in hepatocellular carcinoma so often arrive exhausted and dysfunctional. When LukS-PV lowered IL-10 production by macrophages, the suppressive brake on T cells was released, allowing effector function to recover. The team confirmed this causally in vitro through macrophage-polarization assays and experiments using recombinant interleukin-10, which could partially restore the suppressive environment even after LukS-PV treatment. Enzyme-linked immunosorbent assays quantified the cytokine shift, and conditioned-medium experiments showed that the secreted products of reprogrammed macrophages were sufficient to influence T-cell behavior, establishing a paracrine mechanism by which macrophage reprogramming translates into T-cell revival.

Perhaps the most clinically consequential finding is that LukS-PV synergizes with immune checkpoint blockade. When the researchers combined LukS-PV with PD-1 antibodies in their models, the antitumor efficacy of the checkpoint inhibitor was substantially enhanced. This makes mechanistic sense: PD-1 blockade releases the inhibitory checkpoint on T cells, but it cannot help T cells that never become activated or that remain trapped in a suppressive microenvironment dominated by M2-like macrophages and interleukin-10. By converting the macrophage population from a suppressive to an inflammatory state, LukS-PV prepares the immunological ground on which checkpoint inhibitors operate. The combination addresses two complementary layers of immune dysfunction, the extrinsic suppression imposed by the tumor microenvironment and the intrinsic checkpoints operating on T cells themselves, offering a rationale for testing this pairing in patients whose tumors have proven refractory to immunotherapy alone.

The choice of LukS-PV as the targeting agent reflects an intriguing convergence of microbiology and cancer immunology. Panton-Valentine leukocidin is a bipartite toxin whose S component, LukS-PV, naturally recognizes human C5aR1 with high specificity. Because the receptor is a G-protein-coupled receptor expressed on myeloid cells, and because the human and mouse versions differ sufficiently that the ligand discriminates between them, the humanized mouse model was essential for demonstrating activity. The species specificity that complicates preclinical work also underscores a translational consideration: the mechanism demonstrated here is explicitly human C5aR1-dependent, which strengthens the argument for clinical relevance even as it highlights the limitations of standard murine models for evaluating such agents. The study’s use of both subcutaneous and orthotopic spontaneous models, including one designed to mimic immunotherapy-refractory disease, adds weight to the conclusion that the approach can work in a tumor microenvironment that closely resembles treatment-resistant human liver cancer.

Several questions will need answers before LukS-PV reaches the clinic. The dose, schedule, and safety profile of a molecule derived from a bacterial toxin require careful evaluation, particularly regarding off-tumor effects on myeloid cells that also express C5aR1 in inflamed tissues. The study was conducted under institutional ethics approval with rigorous monitoring of animal welfare, and the authors declare no competing interests, but the path from a humanized mouse model to a phase I trial in hepatocellular carcinoma will demand formal toxicology, pharmacokinetic characterization, and manufacturing development. Nevertheless, the conceptual advance is significant. The work identifies C5aR1 targeting as a promising translational strategy for overcoming immunotherapy resistance, and it demonstrates that a naturally occurring bacterial ligand can be repurposed as a macrophage-reprogramming agent rather than a cytotoxic one.

For patients with hepatocellular carcinoma, a disease that remains one of the leading causes of cancer mortality worldwide and for which immunotherapy benefits only a subset of those treated, the study offers a new molecular handle on the problem of resistance. By showing that the fate of tumor-associated macrophages can be deliberately redirected through a single receptor, and that this redirection unlocks the power of CD8-positive T cells and potentiates PD-1 blockade, Liu, Ma, Zhang and their colleagues have added a compelling entry to the growing list of strategies aimed at remodeling the tumor microenvironment. The idea that a component of one of medicine’s oldest adversaries, Staphylococcus aureus, might help convert cold liver tumors into hot ones is a reminder that the boundary between pathogen and medicine is often thinner than it appears, and that some of immunotherapy’s most useful tools may come from the most unexpected corners of biology.

Subject of Research: Targeting C5aR1 with LukS-PV to reprogram tumor-associated macrophages and improve immunotherapy in hepatocellular carcinoma

Article Title: LukS-PV targets human C5aR1 to reprogram tumor-associated macrophages and potentiate immunotherapy in hepatocellular carcinoma

Article References: Liu, G., Shi, L., Wei, Y., Yang, Z., Ding, P., Chang, W., Dai, Y., Nie, Z., Lu, B., Liu, X., Ma, X., & Zhang, S. (2026). LukS-PV targets human C5aR1 to reprogram tumor-associated macrophages and potentiate immunotherapy in hepatocellular carcinoma. Journal of Experimental & Clinical Cancer Research. https://doi.org/10.1186/s13046-026-03837-w

Image Credits: AI Generated

DOI: 10.1186/s13046-026-03837-w

Keywords: hepatocellular carcinoma, LukS-PV, C5aR1, tumor-associated macrophages, macrophage reprogramming, PD-1 blockade, immunotherapy resistance, IL-10, PI3K/AKT signaling, NF-kappaB, tumor microenvironment, liver cancer