Charging the batteries of immunity

White blood cells help defend the body against bacteria and other disease-causing organisms by swallowing them and digesting them in internal compartments called phagosomes. These compartments contain highly reactive oxygen atoms that send a bacteria's chemistry into overdrive and break it apart. To accumulate such superoxides, the phagosome has to be "charged" by the cell. Like the battery of a mobile phone, at some point it should become fully charged – but cells continue to load their phagosomes with energy. Researchers at the MDC and the Charité University Hospital have now made an important discovery about how they do so. The study, by the laboratories of Ralph Kettritz, Maik Gollasch, Friedrich Luft and their collaborators, appears in the February 12, 2009 issue of Blood. The work sheds light on a basic mechanism in white blood cells that is disrupted in some diseases.

Oxygen atoms are crucial to living systems because they lack electrons and acquire them by binding to other atoms. This process of linking and unlinking drives chemical reactions within cells. Sometimes oxygen atoms are combined in a superoxide – a group that still lacks electrons and is even more unstable. Superoxides would be dangerous to the white blood cell, so they are held inside the phagosome by a membrane; it acts as a barrier and also contains proteins which regulate the phagosome's charge.

An important group of proteins that promotes the generation of superoxides is called the NAPDH oxidase. It tranfers electrons from outside into the phagosome, and uses them to generate these highly reactive forms of oxygen. The electrons have a negative charge that would eventually overload the compartment and stop the reaction, but other membrane proteins step in to balance things out. Kettritz and most scientists have believed that this is managed by proton channels, which snatch protons and shuttle them in, counteracting the electrons being brought in at the same time.

But a paper from another lab, recently published in Nature, suggested a different scenario: a passageway called a BK channel might be balancing the phagosome's charge. BK channels permit the passage of positively-charged potassium ions through membranes. In other types of cells this channel plays an important role; for example, it allows neurons in the brain to communicate with each other.

Kettritz's work supported the proton channel hypothesis. It was important to be sure because the behavior of the NAPDH oxidase has been linked to disease. For example, mutations in the protein have been linked to infections as well as to atherosclerosis, commonly known as "hardening of the arteries." In this condition, white blood cells accumulate in the linings of blood vessels and damage them. Superoxides are part of the problem, and one way of treating atherosclerosis involves drugs that block the NAPDH oxidase's activity. Blocking the channels that regulate the phagosome's charge ought to have an impact on superoxides as well, because the oxidase only works if the charge is right.

This gave post-doctoral fellow Kirill Essin from the group in Berlin-Buch and his colleagues a way to find out which channel was really involved. The group took advantage of a mouse strain in which BK channels had been "knocked out" (removed through genetic engineering techniques). When the scientists looked at the animals' white blood cells, they found that NAPDH oxidases were just as active as before. In fact, the loss of the channels didn't seem to have any effect on NADPH oxidase or innate immunity at all; the mice weren't any more susceptible to bacterial infections than strains with the channels. Nor was any effect seen from drugs that blocked BK channels.

Kettritz says these findings rule out a role for BK channels in counterbalancing the electron transfer that is so important for the the oxidase activity. Do the channels have any important role at all in white blood cells? Superoxides are only one tool these cells use to ward off infections; they also release signaling molecules that sound a general immune system "alarm," calling up other cells and amplifying the body's response to a threat. Other groups had proposed that BK channels participate in this process, too. But Kettritz says the current study dismisses this idea.

Kettritz acknowledges that the work provides insight into a fundamental mechanism that has no immediate applications. But superoxides are important factors in immunity, chemical processes throughout the body, and many diseases. Through this and other projects, Kettritz and his colleagues are assembling a global picture of how white blood cells regulate their production and the effect that this has on processes in the immune system.

Highlight Reference:

Essin K, Gollasch M, Rolle S, Weissgerber P, Sausbier M, Bohn E, Autenrieth IB, Ruth P, Luft FC, Nauseef WM, Kettritz R. BK channels in innate immune functions of neutrophils and macrophages. Blood. 2009 Feb 5;113(6):1326-31.

Link to full article
Wikipedia entry on the NADPH oxidase