A cell's ability to multiply and divide is necessary for life, resulting in the evolution of complex creatures from a single cell.

It also allows worn-out cells to be replaced by a small number of "stem" cells, which subsequently multiply and specialise. Yet, in cancer, cell growth is no longer under control and becomes disorderly.

Researchers at the GIGA Institute at the University of Liege have shown that certain blood immune cells, called monocytes, also possess this capacity to multiply in order to take the place of tissue macrophages, which are crucial for our body's healthy operation. This study is published in Nature Immunology.

The formation of complex multicellular organisms, which human beings belong to, requires the generation of billions of cells from a limited number of progenitor cells that have first proliferated and then acquire particular morphologies and functions while assembling into tissues and organs.

Our current knowledge indicates that most of the cells that constitute a living organism arise from so-called "stem" cells, which have been divided by a process called mitosis in order to give rise to a greater number of cells. These cells then stop proliferating to specialise, differentiate and form muscles, the brain, bones, immune cells, etc.

When proliferation is no longer properly regulated, this can lead to the development of various diseases, among which cancers represent the most striking example.

In a study published in Nature Immunology, Professor Thomas Marichal (Professor at ULiege, Welbio investigator at the WEL Research Institute) and his team from the GIGA Institute at ULiege discovered that this ability to proliferate is not merely restricted to stem cells, but is also an as-yet-unknown function of blood immune cells, the monocytes.

Indeed, blood monocytes, previously considered differentiated cells, are capable of proliferating and generating a pool of monocytes in the tissues in order to give rise to macrophages, which are important immune cells that protect us against microbes and support the proper functioning of our organs.

"This is a major fundamental discovery, which changes our conception of the involvement of cell proliferation in the constitution and maintenance of our immune system," explains Thomas Marichal, director of the study.

"Our finding also suggests that the information that can be drawn from an enumeration of blood monocytes, classically carried out during a blood test, would reflect only little of what is happening at the level of the tissues, during 'infection or inflammation, for example, since monocytes can proliferate when they enter tissues."

He also adds: "Fortunately, this proliferation is extremely well controlled and does not lead to a tumoral process. It has only one goal: to allow, as effectively as possible, the replacement of immune cells that populate our tissues: the macrophages."