A key antibody, IgG, links cells' capture and disposal of germs

Scientists have found a new task managed by the antibody that's the workhorse of the human immune system: Inside cells, Immunoglobulin G (IgG) helps bring together the phagosomes that corral invading pathogens and the potent lysosomes that eventually kill off the germs.

The research, by Axel Nohturfft at Harvard University and colleagues at Harvard, Massachusetts General Hospital, and the Massachusetts Institute of Technology, appears this week in the Proceedings of the National Academy of Sciences.

“The IgG class of antibodies is a critical part of the human immune system, guarding us against infection by an endless array of microorganisms,” says Nohturfft, associate professor of molecular and cellular biology in Harvard's Faculty of Arts and Sciences. “Our findings add yet another immunological task to the list of those handled by IgG.”

While just one of several broad classes of human antibodies, IgG is by far the most important — so much so that patients incapable of making their own antibodies to fight off infections are routinely treated with IgG alone. Broadly speaking, the immunological powerhouse manages the processes by which cells isolate and then kill invading microbes, viruses, and other antigens.

In a process called phagocytosis, intruding germs are first swallowed up by amoeba-like white blood cells and stored in membrane pouches called phagosomes. These compartments then fuse with lysosomes, toxic cellular reservoirs that kill and degrade the sequestered antigens by flooding the phagosomes with acid and destructive proteins.

IgG, Nohturfft and his colleagues report, plays a key role in this merger of phagosomes and lysosomes into the so-called phagolysosomes that finally do in most invading microbes. Specifically, the antibody prompts phagosomes and lysosomes to dock and bind to each other with actin filaments, the first step in the unification of the two vesicles.

Among the antibody's other known roles, Nohturfft's group has now shown that IgG serves to accelerate the creation of phagolysosomes. Under physiological conditions, the scientists found that latex beads coated with IgG formed phagolysosomes in just a third the time it took the cellular machinery to process uncoated beads, 15 minutes versus 45 minutes.

“This process is central to the human immune response,” Nohturfft says. “But some of the most destructive microbial pathogens, such as those responsible for tuberculosis and salmonellosis, are able to hijack cells and use them as a breeding ground precisely because they block the merger of phagosomes and lysosomes. It had long been known that coating these germs with IgG can restore their destruction and our recent results reveal a new branch of this IgG-led counterattack.”