How do the sentinels of the immune system translate the microbial data they receive into immunological instructions?

In the blood, mucous membranes and lymphoid organs, dendritic cells provide a dual role as sentinels, but also as conductors of the immune orchestra. Hiding in the entryways used by pathogens, dendritic cells locate infectious agents, ingest them and release biochemical signals to alert the first line of defensive cells in the body and draw them to the site of the infection. Once the intruder has been digested, dendritic cells also expose on their their surface fragments of the pathogen: the antigens. They then migrate via the lymphatics to secondary lymphoid organs (spleen, lymph nodes, mucosa-associated lymphoid tissue of the digestive tract and lungs) where they present these antigens to T and B lymphocytes. Once "armed" against the pathogen as a consequence of this process, these highly precise defenders in turn migrate to the site of infection to ensure its eradication.

In this sequence of events, the team of Philippe Pierre is particularly interested in the key stage of maturation of dendritic cells. This is the moment when the cells detect the microbes, change their biological functions and begin their migration to secondary lymphoid organs.


Generation of a dendritic cell. Copyright, P Pierre, CIML

"Dendritic cells work like an interface," recalls Philippe Pierre. "they sense microbial products, classify them according to their type and then convert this information into instructions. They use these instructions not only for themselves - because the encounter with microbial products is not a trivial event and generates natural stress in the cells - but also for immune cells with which they cooperate.
Paradoxically, while we have known for a long time the key role played by these cells in the activation of B and T lymphocytes, and we identified the sensors that allow them to detect danger signals emitted by pathogens, operation of this interface remains partly a mystery. We are trying to solve this puzzle, to understand how the dendritic cells convert "microbial data" into "immunological instructions”.

To deliver these immunological instructions, antigen (in practice a peptide a few amino acids in length) is not presented in isolation to T lymphocytes by dendritic cells but is nestled in a pocket formed by a self molecule (such self proteins are known as the Major Histocompatibility Complex and are called HLA in humans). Found in identical form in all cells of the individual, the MHC proteins determine antigen recognition by lymphocytes and therefore their activation within secondary lymphoid organs. Logically, the team is interested in finding out whether and by what means the dendritic cell redirects the transport of MHC molecules depending on the nature of microbial products that it has detected.

"Under the microscope all dendritic cells change dramatically
in response to microbial products"

"By looking under the microscope at the behavior of MHC molecules in dendritic cells we found that the addition of microbial products induces dramatic changes in all cells in our culture," says Evelina Gatti, co-leader on this team subject. "Initially on the inside of the cells, MHC molecules were suddenly exposed on the outside of the cell. We tried to understand how the dendritic cell organized trafficking of MHC molecules. "

The case is even more complicated because there are actually two main families of MHC molecules, each with its own circuit of presentation.

Molecules called MHC1 load peptides derived from the degradation of proteins produced by the pathogen inside the dendritic cell it has infected (or by which it has been ingested). As soon as they are synthesized, microbial proteins are unfolded, labeled by enzymes then sectioned into antigenic peptides. Once transferred to the manufacturing plant of MHC1 proteins, each antigen is loaded into its pocket and then the couple migrates to the surface of the cell to be presented to lymphocytes.

Conversely, MHC2 molecules take charge of peptides derived from "debris." Microbes are engulfed by the dendritic cell and then transferred into small vesicles loaded with enzymes and partially degraded. The MHC2 molecules penetrate these vesicles to take charge of antigenic peptides before beginning their journey to the surface.

While the labeling system of microbial protein was known for many years, the team discovered that enzymes - the ubiquitin ligase family of molecules called MARCH - regulated the routing of MHC molecules in cells. This allows the molecules to have a "ticket" for restricted access into specialized compartments of the cell. Here the MHC encounter antigens from pathogens or peptides from the self to orient the response of the immune system, either to mount an attack or to signal that they are harmless.

Change in the distribution of MHC class II antigens within a dendritic cell in the presence (top) or absence (bottom) of MARCH 1. Copyright P Pierre , CIML.

In parallel, the team has lifted the veil on how determinants of pathogen interfere with translation, that is, the key moment when the RNA called messenger RNA (mRNA) derived from the transcription of active genes of the dendritic cell is translated into proteins.

"By controlling translation, we control the flow of antigens
presented at the surface of dendritic cells"

"Under the effect of different microbial products the synthesis of certain RNA is increased or decreased" summarizes Philippe Pierre. "In addition to mRNA, other RNAs called micro RNA, such as miR-146 or mi-155, accumulate inside the cell and stick to certain mRNAs thereby preventing their translation into proteins. This is the case for some molecules responsible for the production of certain cytokines such as interleukin-1. In response to the attack by pathogens, inflammatory proteins are released into the environment by dendritic cells to stimulate immune cells, starting by themselves! By selectively blocking the synthesis of interleukin-1, these micro RNA thereby preventing a runaway immune response leading to inflammatory diseases."

Using an original marking technique the team then measured in vivo and in real time, the impact of microbial products on the manufacture of proteins and the route of newly synthesized proteins to the surface of the dendritic cell.

"In dendritic cell recognition of microbial products, the use of stress pathways and regulation of translation are closely connected. The speed and precision of the response depends on this complex mechanism," point out Philippe Pierre and Evelina Gatti." As a result of these interactions, some molecules are presented on the surface of the cell, some not, others take a different path before being captured by autophagy (a process of self-cannibalism that allows the cell to eliminate excess or damaged proteins)."

The team is now trying to better understand how each of these channels functions and hopes soon to be able to manipulate them specifically: to slow them down to reduce the hyperactivity of the immune system that causes autoimmune disease or, conversely, to accelerate them to improve the effectiveness of vaccines against infectious diseases.