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 by Elise Wachspress

If you think the invention of the microscope was a pivotal moment in the development of biological knowledge, you might be pretty impressed with the flow cytometer.

This technology allows scientists to “see” an entire stream of individual cells, detecting the features of each as they rush single file through a tiny tube. A laser (or sometimes several) shines through or bounces off the cells as they pass by. Depending on the specific kinds of fluorescent indicators applied, flow cytometry can efficiently characterize 30 or more factors in hundreds of thousands of individual cells and even sort them as they surge through the tube.

For scientists studying the microbiome, the immune system, and their intersection, flow cytometry—we celebrate its 50th anniversary this year—was a breakthrough. The tool provides an efficient way to both distinguish the various bacteria in a sample and identify human immune cells and the particular antibodies they carry.

Last fall, a team led by Jeffrey Bunker (a student in the University of Chicago’s revered Medical Scientist Training Program) and his mentor, Albert Bendelac, MD, PhD (A.N. Pritzker Professor of Pathology) used flow cytometry to understand an important interaction between the gut and the bacteria that live there.

The gut is the source of large quantities of immunoglobulin A (IgA), the most abundant antibody protein in mammals. In our intestines, where we depend on a diverse community of bacteria to digest our food and make important by-products like vitamins, IgA sticks to the surfaces of the microbes, allowing them do their work while keeping them from settling in on the mucous membranes that line the gut. These membranes are the critical barrier that separates the energy furnace in our intestines from the rest of our bodies.

But how do IgA proteins—which defang bacteria with a kind of key-in-lock technique—recognize the many different kinds of bacterial locks and latch so specifically on to each? Thanks to flow cytometry, the team could identify the many types of bacteria involved, so they knew the complicated job IgA was up against.

What they found was that IgA cells in the gut were “polyreactive”: they could clasp very specifically onto many different types of bacteria. This Swiss-army-knife ability was not the result of intervention by other parts of the immune system or hyperactive mutations within the IgA cells themselves. Even changes in diet (and the presumed alterations in the balance of bacteria induced by these changes) did not significantly affect their ability as quick-change artists.

This research demonstrated that IgAs, part of our adaptive immune system, actually have the innate—born inability to recognize individual types of bacteria, even new ones, and do what they need to do to create the right latch. The takeaway is that the bacteria in our guts and our own immune systems have clearly been evolving together, probably for millennia, and that the immune systems of new humans have innate ability to recognize “old” bacteria—those that have been fellow travelers with the human race for generations.

This is just one more bit of evidence that none of us are truly individuals. Each human being is a colony of species living together in community—communities with very long histories and intense cultures—and we’re wise to go with the flow.

This past summer, Bunker and Bendelac went on to publish a major review article on IgA biology for the journal Immunity. There they presented a new framework integrating two distinct types of immunity that protect the gastrointestinal mucus membranes: the polyreactive IgA described above and much more “bespoke” key-and-lock responses to pathogens and vaccines provided by other kinds of immune cells.

Elise Wachspress is a senior communications strategist for the University of Chicago Medicine & Biological Sciences Development office