The target in question is C4d, a small fragment of the complement system, the branch of innate immunity that normally helps antibodies destroy microbes and damaged cells. When a therapeutic antibody binds to its antigen on a tumor cell surface, it can trigger the classical complement cascade. Complement component C4 is cleaved, and the active product C4b forms a covalent bond with nearby molecules on the cell membrane. That covalent attachment is crucial, because it means the fragment stays glued to the cell even after the cascade fizzles out. C4b is then progressively inactivated and cleaved by complement regulators, ultimately leaving C4d as the terminal degradation product, still anchored in place.

Here lies the central insight of the study. Tumor cells that survive an antibody assault do so largely because they are protected by complement inhibitors, both membrane-bound proteins such as CD55 and CD59 and soluble regulators circulating in the blood. These inhibitors shut down the cascade before the membrane attack complex, the pore-forming assembly that punches lethal holes in cell membranes, can fully form. But the shutdown process itself leaves C4d behind, covalently deposited on the membrane. In other words, every cell that has been challenged by a complement-activating antibody and lived to tell the tale carries a permanent footprint of that encounter. Cells that were never touched by the first antibody carry no such mark.

This makes C4d a molecular signature of prior antibody exposure, and, more provocatively, a target for a follow-up attack. The research team engineered a candidate anti-C4d antibody as a chimeric human-rabbit construct, a design that combines the specificity of the rabbit-derived binding region with a human antibody framework intended to reduce immunogenicity. Critically, they introduced amino acid substitutions that enhance oligomerization, meaning the antibody molecules cluster more readily once bound to C4d on the cell surface. That clustering matters because complement activation depends on the density of antibody Fc regions; a tightly packed array of Fc domains recruits and assembles the C1 complex far more efficiently than scattered individual antibodies, and therefore ignites a stronger classical pathway response.

To test the concept, the researchers pretreated human lymphoma and breast cancer cells with standard immunotherapeutics and then challenged the surviving cells with the anti-C4d antibody in the presence of normal human serum, which supplies the full repertoire of complement proteins. The results were striking. Applying the anti-C4d antibody significantly increased the killing of lymphoma cells that had previously been treated with the anti-CD20 antibodies rituximab or obinutuzumab, or with the anti-CD38 antibody daratumumab, compared with simply re-treating the cells with the same antibody a second time. The resistant cells, marked by their C4d deposits, became preferential targets of a fresh complement attack.

The effect extended to the hardest cases. The team worked with lymphoma sublines that had been selected for resistance to rituximab, models that represent the clinical nightmare of relapse after anti-CD20 therapy. Even in these sublines, the anti-C4d approach boosted complement-mediated cytotoxicity, suggesting that resistance to the primary antibody does not necessarily abolish the footprint it leaves, nor the ability of a second antibody to exploit it. The strategy also worked in a setting where the first antibody barely activates complement at all: a human breast cancer cell line treated with trastuzumab, the anti-HER2 therapeutic that by itself induced no measurable complement-dependent cytotoxicity in these experiments. Even that weak, sublethal complement activation was enough to deposit C4d, and the anti-C4d antibody converted those sparse deposits into a robust secondary wave of killing.

From a mechanistic standpoint, the approach can be understood as a two-stage amplifier. The first antibody, whether it is rituximab, obinutuzumab, daratumumab or trastuzumab, performs a dual function: it kills some cells directly through complement and immune effector mechanisms, and it tags the survivors with C4d. The second antibody then reads that tag and recruits the complement system again, this time against cells that have already demonstrated they can withstand the first round. Because C4d is covalently bound and resistant to further enzymatic removal, the tag persists on the membrane, giving the second antibody a stable epitope that does not depend on the tumor cell continuing to express the original antigen at high levels, a common escape route in antibody therapy resistance.

The concept also sidesteps several problems that plague conventional second-line strategies. Re-treating patients with the same antibody often fails because antigen expression has dropped or complement regulators have been upregulated, and switching to a different antigen-targeting antibody requires that the new antigen be present and accessible. C4d, by contrast, is deposited as a direct consequence of the first treatment itself, so its presence is essentially guaranteed on cells that experienced complement activation. The approach is therefore not tied to any particular tumor type or antigen; in principle, any complement-activating first-line antibody creates the substrate for the second-line anti-C4d antibody. The authors describe such antibodies as immunotherapeutic amplifiers, capable of converting limited complement activation induced by a primary antibody into a powerful secondary wave of complement-mediated killing.

There are important caveats. This is an in vitro study, conducted with cell lines and normal human serum rather than in patients, and the transition from cell culture to the clinic involves formidable challenges, including the pharmacokinetics of a second antibody, the accessibility of C4d deposits within solid tumors, and the risk of off-target effects on healthy tissues that might carry complement deposits from inflammation. The team also has commercial interests in the technology: Anna Blom and Marcin Okrój are named inventors on a European patent covering antibodies specific for C4d, and a further patent application covers the use of anti-C4d antibodies as second-line immunotherapeutics for patients resistant to first-line therapy. Budka and Okrój receive royalties from a company that commercialized the earlier invention. These disclosures do not diminish the scientific interest of the work, but they signal that the researchers see a clear path toward translation.

Still, the conceptual shift is what makes the study noteworthy. For decades, complement has been viewed mainly as an effector arm that therapeutic antibodies recruit, and resistance has been viewed as a property of the tumor to be overcome with new targets or new drugs. This work reframes the complement fragments left on surviving cells as an opportunity rather than a byproduct, turning the debris of a failed attack into the aiming point for the next one. If the strategy can be validated in animal models and eventually in clinical trials, patients who relapse after rituximab, daratumumab or trastuzumab might one day receive a follow-up antibody that does not need to find a new molecular target at all, because the first treatment has already marked the enemy.

Subject of Research: Targeting the complement C4d fragment to overcome resistance to antibody-based cancer immunotherapy

Article Title: Targeting C4d to overcome resistance to antibody-based immunotherapy: an in vitro study

Article References: Budka, D., Stasiłojć, G., Stasiłojć, M., Kusowska, A., Bieńkowski, M., Bobrowicz, M., Blom, A. M., Winiarska, M., & Okrój, M. (2026). Targeting C4d to overcome resistance to antibody-based immunotherapy: an in vitro study. Cancer Immunology, Immunotherapy. https://doi.org/10.1007/s00262-026-04567-w

Image Credits: AI Generated

DOI: 10.1007/s00262-026-04567-w

Keywords: C4d, complement system, monoclonal antibodies, immunotherapy resistance, rituximab, obinutuzumab, daratumumab, trastuzumab, lymphoma, breast cancer, complement-dependent cytotoxicity, cancer immunotherapy