Cytokines, interleukins, growth factors, hormones, and other proteins secreted by activated immune cells are key intercellular messengers—they propagate biological information that controls a broad range of functions, coordinating homeostasis.
However, for as important as they are, scientists still know very little about the range of such secreted proteins and their functions. To date, investigations into protein secretion have relied heavily on antibody-based methods, which, beyond being marred by specificity concerns and limited experimental scope, can be quite costly.
Researchers at the Max Planck Institute of Biochemistry have come up with a workaround. They’ve developed an approach to detect and quantify proteins released from immune cells upon receptor ligation using high-resolutions mass spectrometry (MS). The team describes its approach in a paper published online today in Science.
“One of the benefits of our tech is that we are very sensitive at the moment, so we can look at very rare subsets [of secreted proteins],” Max Planck’s Felix Meissner, study co-author, tells GEN.
In their paper, Meissner and his colleagues describe and report on the results of their experimental induction of the Toll-like receptor 4 (TLR4) secretome via lipopolysaccharide (LPS) delivery in both wild-type and MyD88 knockout mouse macrophages, as measured using this new quantitative MS-based approach. Upon LPS-stimulated TLR4 induction, the researchers were surprised to find a whopping 775 proteins released by the macrophages, less than half of which had known extracellular functions.
“Usually, people look at what is known from genetics and predictive research, like cytokines, because that’s what they expect,” Meissner says. However, in his team’s experience, “the known secreted proteins are only a small proportion of what in fact comes out of a cell upon stimulation,” he adds.
That, of course, leaves plenty of unknowns. What are these mystery proteins? How do they function?
“At the moment we don’t know which are relevant,” says Meissner.
While the team is currently working with murine models, Meissner says this approach could also be applied to studies on human cells. Going forward, “we are interested in finding novel factors that are released in real infection settings,” he adds.
“We hope some of the proteins we find to be released in this and other settings might have modulatory functions and might be used as a key target during infections,” Meissner says, noting, of course, that such discovery work will require biological follow-up.