The finding by researchers from the U.S. Department of Energy's Lawrence Berkeley National Laboratory attain significance as it has long been thought that a link between the two systems may yield new weapons in the fight against tuberculosis and other deadly infectious diseases.
Research groups led by Carolyn Bertozzi, director of Berkeley Lab's Molecular Foundry nanoscience center, and Jay Keasling, director of Berkeley Lab's Physical Biosciences Division, obtained profiles for 546 different types of proteins in the membrane of a phagosome, an organelle of macrophages-a type of white blood cell- that essentially "eats" and destroys invading organisms in a process called phagocytosis.
This represents the most comprehensive proteomic analysis of a phagosomal membrane to date, according to an article in the Proceedings of the National Academy of Sciences.
Phagocytosis is the process by which a macrophage type white blood cell engulfs a bacterium in a membrane-bound shell called a phagosome, which fuses with a lysosome that carries digestive enzymes that destroy the bacterium.
"We were able to identify many new proteins that were not previously known to reside in the phagosome," said Wenqing Shui, a member of both the Bertozzi and Keasling research groups, and a proteomics specialist who was the lead author on a paper reporting the study's results.
"One of the new proteins exclusively found in our study, LC3-II, is considered a marker of autophagy, the process that enables cells to clean up their own cytoplasm. Not only was LC3-II present in the phagosome, its level was increased upon the induction of autophagy in macrophages, and reduced when autophagy was suppressed. This indicates cross-talking between autophagy and phagocytosis that may play an important role in the response of the immune system," Shui added.
When bacteria or other foreign particles invade the body, the first line of defence are the macrophages that engulf and contain the invaders within the membrane-bound shells of their phagosomes.
The invaders can be killed with digestive enzymes from another cell organelle called a lysosome, which fuses with the phagosome.
Just like other types of cells, macrophages also use lysosomal enzymes for internal housekeeping, but no direct biochemical evidence of a link between phagocytosis and autophagy has been found to date.
Shui and her colleagues worked with latex bead-containing phagosomes isolated in cell lines from mice to perform a detailed analysis of the protein contents of the phagosomal membrane.
The Bertozzi-Keasling groups revealed that their research was membrane-specific and included hydrophobic protein species, unlike earlier proteomic studies that profiled the entire organelle and focused on abundant water-soluble protein species.
"We were able to demonstrate the endogenous level of LC3-II in macrophage phagosomes through the combination of sensitive proteomic techniques and biochemical assays. This is an excellent show-case of how a non-biased high-throughput proteomic study can shed new light on the diverse functions and pathways an organelle may engage in," said Shui.
The researchers say that the discovery that the presence of the LC3-II protein, a critical component of the autophagy machinery, in phagosomes is modulated by autophagic activity points to autophagy playing a heretofore unknown role in immune response, particularly against intracellular pathogens such as mycobacterium tuberculosis.
Shui said: "After mycobacterium tuberculosis are phagocytosed into the macrophage cell, they are able to subvert various host defense mechanisms, including the killing of bacilli in the phagosome, and survive well inside the cell. Given that induced autophagy activity appears to enhance mycobacterial killing in the phagosome, from a study by the University of New Mexico's Vojo Deretic and his research group, we speculate that mycobacteria may produce specific factors to counteract the bactericidal effect of autophagy activation."
The researchers are presently studying whether certain mycobacterial products can modulate macrophage autophagy activity. They are also looking for proteins that could specifically mediate the autophagy as well as the phagosome maturation process.
Shui said: "We might be able to open new avenues for pharmacologic intervention of tuberculosis as well as other infectious diseases."