Immune cells in single file

How do these networks form and how are they maintained? For the long-lived macrophages (phagocytes), the answer is already known: They settle in so-called niches. These are environments of connective tissue cells that supply the macrophages with nutrients and keep them alive.

A team led by Professors Georg Gasteiger, Dominic Grün and Wolfgang Kastenmüller from the Institute of Systems Immunology at Julius-Maximilians-Universität Würzburg (JMU) / Max Planck Research Group has now turned its attention to a related type of immune cells, the so-called dendritic cells.

These immune cells are essential for the control of immune responses because they are at the first line of defense of the immune system: They recognize foreign structures, take them in and process them into a kind of mugshot. They then present the photo to other immune cells and trigger a specific immune response, for example against pathogens or cancer cells.

Dendritic cells migrate through the tissue

The special thing about dendritic cells: They only live for about a week and during this time they continuously migrate through the body's tissues. “In this respect, it was clear that the classic niche concept would not work here" , says Wolfgang Kastenmüller.

The JMU team found a completely new concept for this, according to which three-dimensional cell networks can organize themselves: Dendritic cells orient themselves to the blood vessels and migrate one after the other along their outer wall – similar to children walking in single file. The blood vessels thus determine the three-dimensional arrangement of the cells.

Cytokines keep the cells together

"We wanted to understand how this process is regulated and how the cells manage to close gaps in their network," explains Dr. Milas Ugur, a scientist in Professor Kastenmüller's group. Closing such gaps is important because otherwise the immune defense no longer functions optimally.

As the JMU team reports in the journal Immunity, it is due to a locally acting cytokine, the FLT3 ligand, that the dendritic cells always stay close together during their developmental migration.

The cytokines are continuously and evenly produced locally and consumed by the dendritic cells. If there are gaps in the group, more cytokines are available for the isolated dendritic cells. This surplus speeds them up in their development and movement and helps them to reconnect with the group. When the cells have moved up, they have a little less cytokines available again due to competition from their neighbours. Accordingly, they slow down their developmental speed.

Of prognostic value for tumour diseases

These findings are for example important  for cancer therapy: dendritic cells have a high prognostic value for tumour diseases: The higher their abundance in the tumour, the better the prognosis for the patient. This is especially true after immunotherapy.

"Increasing our basic insights on dendritic cell biology will help us to restore the networks of these cells in tumours and thereby tailor optimal therapies in the future" explains Kastenmüller.

How the researchers are moving forward

The JMU researchers’ data so far is based on the analysis of lymph nodes from animal models. The team next wants to test whether the same principles of network organisation of dendritic cells apply to all tissues and also in humans.

The work described was done in cooperation with researchers from the Würzburg Helmholtz Institute for RNA-based Infection Research (HIRI) and with scientists from France and Japan.