What do pathogens, plant roots, and immune cells have in common? Central biological processes often proceed according to similar processes – even in organisms that are widely dissimilar in evolutionary terms. An example of this is the role of lectins: These proteins are involved in signalling processes that are crucial for cell communication and interaction. Scientists from different disciplines are collaborating closely on this topic at the University of Freiburg’s Cluster of Excellence CIBSS – Centre for Integrative Biological Signalling Studies. The understanding of common principles opens up new research approaches and innovative applications – from improved strategies against infectious diseases to more sustainable plant symbioses and new approaches to the treatment of cancer.

Lectins are present in animals, plants, and microorganisms and serve a wide range of functions. They are often attached to cell membranes and can influence the biological reactions of the membrane. As they bind mainly to sugar structures, they regulate cell processes that are crucial to immune defence, infections, and symbiotic interactions. The research on lectins at the Cluster of Excellence CIBSS shows how valuable close exchange between research groups of different disciplines is: The collaboration opens up novel perspectives that extend far beyond the possibilities of purely discipline-specific research and lead to fundamental biological insights.

What do pathogens, plants, and immune cells have in common?

The pathogen Pseudomonas aeruginosa can cause severe infections in humans. The cell biologist Prof. Dr. Winfried Römer is studying exactly how this bacterium invades host cells. He succeeded in demonstrating that Pseudomonas aeruginosa creates invaginations in the plasma membrane with the help of particular lectins to infect a cell. This finding inspired the work of the plant researcher Prof. Dr. Thomas Ott and Dr. Casandra Hernández-Reyes, postdoctoral researcher at CIBSS, who study symbiotic infections of plants by bacteria. In collaboration with Römer’s research group, Ott and his team began investigating similar mechanisms in the so-called root nodule symbiosis of certain plants with useful microorganisms such as rhizobia.

Parallel to this project, Römer’s work was integrated into a further research project at CIBSS: Under joint management and in collaboration with CLP Fund Recipient Dr. Ana Valeria Melendez, and Dr. Rubí Misol-Há Velasco Cárdenas, Römer and the immunologist Prof. Dr. Susana Minguet investigated how lectins can be used specifically to identify tumour cells. The two teams worked closely together to develop so-called lectin-based chimeric antigen receptors (CAR) on immune cells, which are capable of recognizing and destroying particular tumour cells on the basis of their binding mechanism. These projects illustrate how common molecular principles can be applied in completely different biological and medical contexts.

Proteins with diverse functions: Lectins in bacterial infections

Römer and his team have been working for many years to understand the mechanisms with which pathogens manipulate the cell membranes of their host cells to cause infections. ‘One thing we’re concentrating on is the question of how lectins influence cellular processes in bacterial infections’, says Römer. In their first studies, the researchers are focusing especially on the lectin LecA of the pathogenic bacterium Pseudomonas aeruginosa.

The researchers demonstrated that bacterial lectins not only serve as a kind of glue bacteria use to attach themselves to host cells but that LecA also functions as an invasion factor by binding specifically to the glycolipid Gb3 on the cell membrane, which leads the lipids to reorganize and congregate in the membrane – a mechanism the scientists describe as a ‘lipid zipper’. This leads to the activation of cellular signalling pathways that make it easier for the bacterium to get into the host cell. The researchers already published pioneering initial findings on this process in 2014 – providing an important basis for many more studies conducted in the context of the Cluster of Excellence CIBSS.

Understanding signalling pathways: New approaches against infections and cancer

The glycolipid Gb3 already played an integral role in the first studies conducted by Römer’s team. ‘Gb3 is not just a binding point for bacterial lectins but was already known to be a tumour marker that is found in high concentrations on the surface of many cancer cells’, says Römer.

The researchers observed that two of the lectins under study bind specifically to Gb3, which demonstrates their potential for applications even beyond infection research. For one thing, these binding perspectives open up the possibility of preventing bacterial infections in a targeted manner, for example by blocking the lectin-Gb3 interaction. For another, Gb3 can also be used in cancer research as a target for identifying tumour cells specifically. ‘This dual function as a pathogenic factor and as a potential target for therapeutic approaches shows how versatile lectins can be as tools in biomedicine’, says Römer.

Lectins as tools for optimizing symbiotic partnerships in plants

The plant researcher Thomas Ott has also already conducted research on lectins. Römer’s findings motivated him and his team to investigate whether it is possible to observe similar changes caused by lectins in the membrane in root nodule symbiosis. ‘The similarity of the invaginations was fascinating, even if the lectins and mechanisms involved are ultimately different’, says Ott. In root nodule symbiosis, soil bacteria, so-called rhizobia, enter into a close partnership with plants by colonizing roots, causing the plants to form tiny nodules. There they convert nitrogen gas from the air into a form the plant can use – a process that would not be possible for the plants themselves and that provides them with an individual supply of nitrogen. This natural ‘fertilizer factory’ also improves the fertility of the soil, as the fixed nitrogen is available to other plants after crop rotation. In the long term, Ott also aims to develop strategies for optimizing this valuable process and perhaps even transferring it to other crop plants. That could save resources for agriculture and reduce the ecologically problematic use of synthetic fertilizers.

Ott’s team is working with the natural lectin LDP1 from the model plant Medicago truncatula. The researchers succeeded in demonstrating that LDP1 accumulates in the so-called infection chamber – the region of the root in which the symbiosis between plant and rhizobia originates. In collaboration with Winfried Römer, the team also investigated with the help of an experimental model how LDP1 acts outside of its natural plant context. ‘We were able to use Winfried Römer’s system without having to spend months or years to develop our own’, says Ott. ‘That was a great advantage.’

From research to practice: Applications with lectins

The researchers found indications that the lectin under study interacts with specific sugar molecules on bacterial cell walls under certain conditions – and thus possibly promotes membrane invaginations. This mechanism could play an important role for the uptake of rhizobia into plant cells. Unlike in the case of pathogens, where this effect is undesirable, it could be promoted in a targeted manner in plants to support symbiotic interactions.

The researched genetic constructs aim to regulate gene expression in plants and promote symbiotic processes. If the researchers manage to reconstruct the mechanisms under study in living cells, they could open up a new approach for the targeted uptake of symbiotic bacteria into plant cells – and possibly for the development of more sustainable agricultural applications.

Expertise in immune therapy in combination with lectin and membrane research

The key here is to carefully balance the signalling pathways created by CAR in the T cells: On the one hand, they must be strong enough to ensure an effective activation and the killing of the tumour cell, but on the other hand, they should not be so strong that they cause so-called T cell exhaustion – a dysfunctional state in which T cells lose their ability to multiply, kill, and send out inflammation-promoting signals. The exhaustion of T cells weakens the entire immune response to cancer and reduces the long-term effectiveness of a therapy.

Innovative immune therapies via lectin CAR T cells

One of Susana Minguet’s areas of expertise is so-called CAR T cell therapy. This modern method of immune therapy involves modifying the patient’s own T cells to identify cancer cells with the help of so-called chimeric antigen receptors (CAR) and attack them. Up to now, this technology has usually been based on the CAR T cells recognizing particular proteins on tumours. In the collaboration between Susana Minguet and Winfried Römer, the researchers hit on the idea of extending this strategy and integrating lectins into CARs, enabling T cells to identify tumour cells on the basis of altered sugar structures – so-called glycan patterns.

‘This opens up completely new possibilities for fighting tumours that have been regarded up to now as invulnerable to existing immune therapy strategies’, says Minguet. ‘That’s an exciting scientific breakthrough – and the interdisciplinary exchange played a key role in it.’ Now the team is focusing on optimizing how the CAR T cells react after identifying their target. ‘Identification alone is not enough to eliminate tumour cells effectively: CAR T cells must be properly activated.’

The teams of Minguet and Römer aim to continue researching the possibilities of lectins in the future. This includes identifying new lectins that bind specifically to abnormal sugar structures on cancer cells and further optimizing these lectins via protein engineering. In addition, they’re working on developing lectin CAR T cells further and additionally using lectin NK cells (natural killer cells) to broaden the spectrum of immune cells equipped with lectins. Römer: ‘The close link between Susana’s experience in immune therapy and our expertise in lectin and membrane research is crucial to making such innovative approaches a reality.’

Selected publications

• A lipid zipper triggers bacterial invasion. Eierhoff et al., 2014, Proceedings of the National Academy of Sciences of the United States of America. DOI: 10.1073/pnas.1402637111.
• Septin barriers protect mammalian host cells against Pseudomonas aeruginosa in-vasion. Aigal et al., 2022, Cell Reports Medicine. DOI: 10.1016/j.celrep.2022.111510.
• The Gb3-enriched CD59/flotillin plasma membrane domain regulates host cell inva-sion by Pseudomonas aeruginosa. Brandel et al., 2021, Cellular and Molecular Life Sciences. DOI: 10.1007/s00018-021-03766-1
• Novel lectin-based chimeric antigen receptors target Gb3-positive tumour cells. Meléndez et al., 2022, Cell Molecular Life Science, DOI: 10.1007/s00018-022-04524-7. PMID: 36097202
• A bispecific, crosslinking lectibody activates cytotoxic T cells and induces cancer cell death. Rosato et al., 2022, Journal of Translational Medicine. DOI: 10.1186/s12967-022-03794-w.
• Pseudomonas aeruginosa LecB suppresses immune responses by inhibiting tran-sendothelial migration. Sponsel et al., 2023, EMBO Reports. DOI: 10.15252/embr.202255971.
• Neutralizing the Impact of the Virulence Factor LecA from Pseudomonas aerugino-sa on Human Cells with New Glycomimetic Inhibitors. Zahorska et al., 2023, Ange-wandte Chemie International Edition in English. DOI: 10.1002/anie.202215535.
• Harnessing CD3 diversity to optimize CAR T cells. Velasco Cárdenas et al., 2023, Nature Immunology. DOI: 10.1038/s41590-023-01658-z
• Phospho-mimetic CD3ε variants prevent TCR and CAR signaling. Wössner et al., 2024, Frontiers in Immunology. DOI: 10.3389/fimmu.2024.1392933
• Dual targeting of cancer metabolome and stress antigens affects transcriptomic heterogeneity and efficacy of engineered T cells. Hernández-López et al., 2023, Na-ture Immunology. DOI: 10.1038/s41590-023-01665-0
• From TCR fundamental research to innovative chimeric antigen receptor design. Minguet et al., 2024 Nature Reviews Immunology, DOI: 10.1038/s41577-024-01093-7
• Cellular morphodynamics and signalling around the transcellular passage cleft during rhizobial infections of legume roots. Zhang and Ott et al., 2024, Current Opinion in Cell Biology. DOI: 10.1016/j.ceb.2024.102436.
• Competence for transcellular infection in the root cortex involves a post-replicative, cell-cycle exit decision in Medicago truncatula. Batzenschlager et al., 2023, eLife (reviewed preprint). DOI: 10.7554/eLife.88588.1.
• RPG acts as a central determinant for infectosome formation and cellular polari-zation during intracellular rhizobial infections. Lace et al., 2023, eLife. DOI: 10.7554/eLife.80741.
• Stabilization of membrane topologies by proteinaceous remorin scaffolds. Su et al., 2023, Nature Communications. DOI: 10.1038/s41467-023-35976-5.
• Transcellular progression of infection threads in Medicago truncatula roots is as-sociated with locally confined cell wall modifications. Su et al., 2023, Current Biol-ogy. DOI: 10.1016/j.cub.2022.12.051.

CIBSS profile of Prof. Dr. Winfried Römer

CIBSS profile of Prof. Dr. Thomas Ott

CIBSS profile of Prof. Dr. Susana Minguet

CIBSS profile of Dr. Ana Valeria Melendez

CIBSS profile of Dr. Casandra Hernández-Reyes

Original press release University of Freiburg