In the work, the researchers show that blood stem cells have all the components needed to detect an invasion and to mount an inflammatory response. They show, as others have previously, that these cells have on their surface a type of receptor called a toll-like receptor.
The researchers then identify an entire internal response pathway that can translate activation of those receptors by infection-related molecules, or danger signals, into the production of cytokines, signaling molecules that can crank up immune-cell production. Interestingly, they show for the first time that the transcription factor NF-?B, known to be the central organizer of the immune response to infection, is part of that response pathway.
To examine what happens to a blood stem cell once it is activated by a danger signal, the Baltimore lab teamed up with chemists from the lab of James Heath, the Elizabeth W. Gilloon Professor and professor of chemistry at Caltech.
They devised a microfluidic chip — printed in flexible silicon on a glass slide, complete with input and output ports, control valves, and thousands of tiny wells — that would enable single-cell analysis. At the bottom of each well, they attached DNA molecules in strips and introduced a flow of antibodies — pathogen-targeting proteins of the immune system — that had complementary DNA.
Caltech researchers separate blood stem cells from other bone marrow cells and load them
onto a newly developed microfluidic chip. Fluorescent signals indicate the presence of
secreted proteins with one “barcode†representing each cell.
Image credit:Â Caltech/James Heath/Chao Ma/Young Shik Shin
They then added the stem cells along with infection-related molecules and incubated the whole sample. Since the antibodies were selected based on their ability to bind to certain cytokines, they specifically captured any of those cytokines released by the cells after activation. When the researchers added a secondary antibody and a dye, the cytokines lit up.
The team found that blood stem cells produce a surprising number and variety of cytokines very rapidly. In fact, the stem cells are even more potent generators of cytokines than other previously known cytokine producers of the immune system. Once the cytokines are released, it appears that they are able to bind to their own cytokine receptors or those on other nearby blood stem cells. This stimulates the bound cells to differentiate into the immune cells needed at the site of infection.
“This does now change the view of the potential of bone marrow cells to be involved in inflammatory reactions.†said Baltimore.
Heath notes that the collaboration benefited greatly from Caltech’s support of interdisciplinary work.Â
“It is a unique and fertile environment, one that encourages scientists from different disciplines to harness their disparate areas of expertise to solve tough problems like this one.†he concluded.
References
-Â http://www.caltech.edu
– immy L. Zhao, Chao Ma, Ryan M. O’Connell, Arnav Mehta, Race DiLoreto, James R. Heath, David Baltimore. (2013). Conversion of Danger Signals into Cytokine Signals by Hematopoietic Stem and Progenitor Cells for Regulation of Stress-Induced Hematopoiesis. Cell Stem Cell, DOI: 10.1016/j.stem.2014.01.007