The body’s immune system includes white blood cells called phagocytes that function like the video game character Pac-Man, circulating through the body to detect and ingest intruders like bacteria and fungi.

Once the invading microbe is engulfed, a process called phagocytosis, the phagocyte kills it. The microbe is killed by fusing with an intracellular organelle called a phagosome, which contains proteins, that destroy the invader.

David Underhill, PhD, chair of the Department of Biomedical Sciences and the Janis and William Wetsman Family Chair in Inflammatory Bowel Disease, notes that once the microbe is inside, a phagosome generates signals that guide local inflammatory responses, and many of these signals are microbe specific.

“With this knowledge, we developed a novel technique, termed PhagoPL, to identify all of the proteins involved in attacking different kinds of microbes,” said Underhill, senior and corresponding author of the study. “We then used this technique to study three types of phagosomes: one containing yeast, and two containing different types of bacteria.”

Underhill said his team of investigators also found that many proteins found in phagosomes are common to the different types of phagosomes.

“While we found that many phagosome proteins are common with different pathogens, we found that each microbe also recruits unique proteins that are specific to that microbe, suggesting that these proteins help drive inflammatory responses that are targeted for specific microbes,” Underhill said.

Even more unexpected, the Cedars-Sinai investigators found that a cell surface protein called PD-L1 is recruited specifically to yeast-containing phagosomes and turns out to be a fungal-binding receptor. PD-L1 has been previously known to regulate activation of T cells, which help the immune system fight off infections, and researchers say PD-L1 is an exciting protein being targeted by cancer immunotherapy strategies.

“We would never have guessed that PD-L1 would have such a specific role in sensing fungi,” said Kai Li, PhD, a project scientist and lead author of the study. “In fact, we focused our study on PD-L1 because it stood out to us as a protein that we felt really shouldn’t be found associated with these microbes.”

Underhill said the discovery of a novel function for PD-L1 is proof-in-principle of the power of PhagoPL, the new methodology created with broad applicability for the discovery of phagosome proteins and mechanisms of immunity.

Jeffrey A. Golden, MD, executive vice dean of Research and Education and director of the Burns and Allen Research Institute at Cedars-Sinai, said this study is likely to impact future research.

“These findings have provided new insight into how phagocytes function and has the potential to fundamentally change our current view of PD-L1. This information can potentially be leveraged to develop novel, or improve current, anti-tumor immunotherapies,” said Golden, who was not involved in the study.

Because of the versatility of the PhagoPL technique, Underhill and team are eager to apply their findings to study many more phagosomes containing different fungi, bacteria, or other nonmicrobial materials.

“We aim to uncover more mechanisms related to how we defend ourselves against infection and uncover how autoimmunity and anti-tumor immunity are regulated,” Underhill said. “We are also keen to collaborate with and provide technical support to those interested in using the PhagoPL technique to address scientific questions.”

Other Cedars-Sinai investigators involved in the study include Avradip Chatterjee, Chen Qian, Katherine Lagree, Yang Wang, Courtney A. Becker, Michael R. Freeman, Ramachandran Murali, and Wei Yang.

This work was supported by National Institutes of Health grant R01AI071116 (DMU).

Read more on the Cedars-Sinai Blog: Meet David Underhill, PhD