Sepsis is the leading cause of death in Brazilian intensive care units (ICUs). Among patients progressing to severe disease, 40% die and survivors often suffer from cardiovascular and neurological complications, as well as a significant decline in immunity that lasts for years after hospital discharge.
"Studies suggest that sepsis survivors are seven times more likely to develop serious infections than individuals who have never had the disease. They are also susceptible to opportunistic pathogens which do not usually cause disease in healthy people. We are starting to understand why this happens. “
José Carlos Farias Alves Filho, Professor, University of Sao Paulo
Filho was the principal investigator for a study on the subject which is due to be published on September
Sepsis, also known as generalized infection or septicemia, is actually systemic inflammation often triggered by a localized infection that gets out of control. The body’s immune response to the pathogen, which can be a bacterium, a fungus or even a virus such as SARS-CoV-2, produces excessive amounts of inflammatory substances that damage the body. More severe forms of sepsis can cause damage to vital organs, low blood pressure, and eventually a form of acute circulatory failure known as septic shock.
To understand how this leads to immunosuppression, Alves Filho and his team performed experiments on mice and immune cells isolated from the blood of septic patients. The survey was conducted by Daniele Carvalho Bernardo Nascimento, with a post-doctoral scholarship from the São Paulo Research Foundation – FAPESP. The researchers are affiliated with the Inflammatory Diseases Research Center, a center for research, innovation and dissemination hosted by the Ribeirão Preto Medical School (FMRP-USP) of the University of Sao Paulo.
“Put simply, our results show that sepsis induces the proliferation of a subpopulation of B lymphocytes that express large amounts of CD39, an enzyme that breaks down ATP[[[[adenosine triphosphate, the main energy source for cellular processes]to release adenosine into the bloodstream, “said Alves Filho.” Increasing the level of adenosine in the blood reduces the activity of macrophages, the defense cells that engulf bacteria, fungi and other potential threats to the body. “
Previous research had already found that adenosine levels rise in the early stages of sepsis and that this has an immunosuppressive effect. Adenosine binds to a receptor called A2aR on the surface of macrophages, inducing the production of interleukin-10 (IL-10), a molecule with an anti-inflammatory action.
The novelty in this study is its demonstration that adenosine plays a key role in the development of post-sepsis immunosuppression and that a specific cell type is the source of the additional adenosine found in the blood of septic animals and humans.
“All cells in the immune system express CD39 to varying degrees, but we found that a subclass of B lymphocytes called plasmablasts produces large amounts of this enzyme,” said Alves Filho. “We have seen plasmablasts proliferate strongly after sepsis and undergo metabolic reprogramming. They started consuming more glucose and thus producing more ATP. At the same time, they expressed large amounts of the enzyme[[[[CD39]responsible for the hydrolysis of ATP and the release of adenosine. It is as if the soldiers could produce their own ammunition. “
To induce severe sepsis in mice, the scientists used a pattern known as caecal ligation and puncture (CLP), which involves perforation of the cecum (the beginning of the large intestine) to allow the release of fecal material and bacteria into the cavity. peritoneal. The procedure simulates what happens in a patient with suppurative appendicitis. The sepsis survivors were monitored for 90 days and were highly susceptible to opportunistic pathogens throughout the period. None of the animals exposed to the bacterium Legionella pneumophila survived. Only 20% survived exposure to the fungus Aspergillus fumigatus.
To confirm the importance of adenosine in post-sepsis immunosuppression, they conducted experiments with drugs that inhibit CD39 or the adenosine A2A receptor. Survivors rose to 60% of those “treated” in this way before exposure to opportunistic pathogens.
When the same experiment was performed on animals genetically modified so as not to express CD39 or A2aR, the survival rate was 70%.
Analysis of blood samples from septic patients admitted to the general and university hospital of FMRP-USP (Hospital de Clínicas) showed that the worse their condition, the higher the level of adenosine in the blood and the higher the level of plasmablasts expressing CD39.
Several other tests were conducted on mice to clarify the immunosuppression mechanism step by step. In one experiment, healthy animals were given an infusion of plasmablasts isolated from septic animals, thereby becoming susceptible to opportunistic pathogens.
A similar experiment was performed using human cells. Macrophages isolated from healthy subjects were incubated with plasmablasts from septic patients and with an opportunistic bacterium. Due to the action of adenosine, the defense cells were unable to kill the microorganism.
The mechanisms by which sepsis causes the proliferation of CD39-producing B lymphocytes are still poorly understood. The researchers also plan to find out whether adenosine levels in the blood of patients who survive severe sepsis remain high for a long time, as they do in mice.
In a previous study, reported in an article published in Nature communications In 2017, the group led by Alves Filho showed that sepsis triggers the proliferation of another type of immunosuppressive cell known as regulatory T cells or Tregs.
“We had previously shown that sepsis triggers an increase in Tregs related to IL-10 levels. In this study, we demonstrate that adenosine produced by plasmablasts is a mediator leading to the production of IL-10 by macrophages in survivors of So these two studies are linked: Plasmablasts induce the production of IL-10 by macrophages, which in turn can promote an increase in Tregs and make the patient even more immunosuppressed, “said Nascimento.
The identification of this new mechanism should facilitate the search for interventions capable of combating immunosuppression. “We believe that if we can inhibit the growth of plasmablasts, we can reduce immunosuppression and thus increase the life expectancy of survivors of sepsis,” said Nascimento.
São Paulo Research Foundation
Birth, DC, et al. (2021) Sepsis expands a CD39+ plasmablast population that promotes immunosuppression via adenosine-mediated inhibition of macrophage antimicrobial activity. Immunity. doi.org/10.1016/j.immuni.2021.08.005.