How do bacteria prevent phagocytosis?

How immune cells kill bacteria with acid

Macrophages

The first line of defense of the immune system against invading pathogens are macrophages.

These phagocytes enclose their prey with a membrane and take them inside, where they are killed by increasing acidification within the so-called phagosomes - a process that is not yet fully understood. As part of the major project to systematically research all proteins that transport chemical substances through cell membranes, CeMM scientists have characterized the transporter SLC4A7; a membrane protein that is crucial for phagosome acidification. The results provide valuable insights into a variety of diseases.

Fluorescence micrograph of human macrophages (THP-1 cell line) incubated with blue and red colored particles.
Source: CeMM / Ruth Eichner

Among the diverse range of immune cells that patrol the body, macrophages are among the first to fight off invading foreign bodies. With their flexible and versatile surface, they wrap themselves over any intruder that could be dangerous for the organism and seal it in an intracellular membrane bubble, the so-called phagosome. Then the pH in the bladder must be gradually and effectively lowered until it resembles a strong acid in order to kill the trapped microbes. Many details of this crucial process, called phagocytosis in technical jargon, have not yet been fully understood. The team of Giulio Superti-Furga, Scientific Director of the CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, was able to show in its latest study that a membrane protein from the family of "Solute Carriers" (SLCs) is essential for macrophages to during phagocytosis actually killing their prey with acid. Her work has been published in the journal Cell Host & Microbe.

When SLC4A7 is switched off, positively charged protons accumulate in the cell plasma, which prevents acid formation in the phagosomes

Giulio Supert-Furga

SLCs are the largest group of transport proteins in a cell. They are responsible for moving chemical substances through cell membranes. Since phagocytosis requires the exchange of ions and other substances, the scientists from Superti-Furga's team hypothesized that SLCs play a central role in this process in macrophages. To test this hypothesis, they developed an experimental approach in which they individually switched off all 391 human SLC genes in a macrophage cell line using the genome editing technique CRISPR / Cas9. These cells - each with a single defective SLC gene - were then observed during phagocytosis to find out whether the genetic defects had an effect. Among all SLCs, SLC4A7, a sodium bicarbonate transporter, was the only one that was found to be essential for the acidification of phagocytosed microbes. Cells in which this gene was switched off were unable to acidify their phagosomes and thus also unable to kill ingested bacteria.

With SLC4A7, the researchers had identified their main suspect and now, in collaboration with the research group of Nicolas Demaurex from the University of Geneva, investigated in more detail how this protein influences the acidification of the membrane bladders. "SLC4A7 is located on the surface of the macrophages and is responsible for the import of bicarbonate, the salt of carbon dioxide, from the environment into the interior of the cell," explains Giulio Supert-Furga, the last author of the study. "This import is essential for the stabilization of the cellular pH value during phagocytosis. When SLC4A7 is switched off, positively charged protons accumulate in the cytoplasm, which prevents acid formation in the phagosomes."

The results of this study provide new, fundamental insights into the molecular mechanisms and biology of macrophages, one of the most important types of immune cells. Since phagocytosis plays an important role in a whole range of diseases, from infectious diseases to cancer, the results could also be useful for medical purposes, for example in the development of new therapies or active substances. The study, funded by the European Research Council, is another contribution in the effort to understand the role of the different cellular transporters.

Source: MEDICA / Medical University of Vienna