Restoration of calcium balance restores antimicrobial activity, providing a potential target for new therapies
In people with cystic fibrosis (CF), a defect that makes their lungs susceptible to infection also appears to weaken the most common immune cell’s ability to defend the lungs, researchers at Nationwide Children’s Hospital have found.
The defect in the protein called cystic fibrosis transmembrane conductance regulator (CFTR), which functions as a chloride channel, leads to an imbalance of ions in cells. In lung cells, the imbalance starts a chain of events that causes mucus to become sticky and hard to cough up, enabling bacteria to get a foothold and thrive.
In immune cells called neutrophils, the imbalance results in calcium overloading that correlates with increased activation and inflammatory properties but also with silencing of some of the neutrophils’ antimicrobial activities, that is, their ability to kill bacteria, the researchers show. Instead, the immune cells become hyperactive and contribute to lung cell inflammation, damage and cell death.
The research community has long supported that the lung cell microenvironment is mainly responsible for facilitated inflammation and infection in cystic fibrosis patients. The team’s study in The Journal of Immunology shows neutrophils’ inability to clear the infection is also contributing to infection persistence, the researchers say.
Respiratory infections are the main cause of death from cystic fibrosis. The researchers chose to study the opportunistic bacteria Burkholderia cenocepacia, which is known to cause heightened lung inflammation and is associated with high morbidity and mortality.
They showed that healthy neutrophils taken from human blood efficiently killed the bacteria but those cells taken from CF patients were significantly less effective.
Testing on a mouse model of CF as well as human CF neutrophils showed the neutrophils have a defect in producing reactive oxygen species (ROS). ROS is a key ingredient to two components neutrophils use to kill bacteria: phagocytosis and inducing neutrophil extracellular traps, which the cell uses to catch bacteria.
Digging into CF neutrophils’ ROS production, the researchers found that an excessive number of calcium ions build up inside CF neutrophils, caused by dysregulation of calcium homeostasis. This imbalance correlated with diminished immune responses and neutrophil extracellular trap production.
In lab tests using B. cenocepacia, the researchers found that by restoring the calcium ion balance in neutrophils, the cells regained their antimicrobial capabilities.
“If we can restore calcium balance in patients’ neutrophils, we can potentially reestablish a patient’s ability to kill bacteria,” says Santigo Partida-Sanchez, PhD, a principal investigator at the Center for Microbial Pathogenesis and leader of the research team. He and his colleagues found two routes to do this, which may prove useful therapy targets, they suggest.
New ways to improve current cystic fibrosis treatments are needed, says Dr. Partida-Sanchez, who is also an associate professor of pediatrics at The Ohio State University College of Medicine. Cystic fibrosis patients typically rely on antibiotics to treat lung infections, some of which can become persistent, enabling bacteria to develop resistance to even multiple drugs.
Dr. Partida-Sanchez and his team found that treatment with drugs that inhibit calcium ion channels or boost activity of CFTR channels restored calcium ion balance and CF neutrophils’ bacteria-killing capabilities.
The researchers are now investigating which specific calcium channels they may want to target. They are also looking at whether the CFTR defect affects other blood cells, such as macrophages, that play important roles in the disease.
Citation: Robledo-Avila FH, Ruiz-Rosado JD, Brockman KL, Kopp BT, Amer AO, McCoy K, Bakaletz LO, Partida-Sanchez S. Dysregulated calcium homeostasis in cystic fibrosis neutrophils leads to deficient antimicrobial responses. The Journal of Immunology 2018 Aug. 17 [epub ahead of print].