Jeffrey Perkel has been a scientific writer and editor since 2000. He holds a PhD in Cell and Molecular Biology from the University of Pennsylvania, and did postdoctoral work at the University of Pennsylvania and at Harvard Medical School.
The immune system is full of what you might call cellular “chatty Cathys.” B cells talk to macrophages, macrophages talk to T cells, and T cells? They talk to everybody.
The language of this cellular communication is cytokines, and dozens upon dozens have been discovered. In fact, cytokines—or more broadly, cytokines, chemokines and growth factors—orchestrate cell-to-cell chatter throughout the body. R&D Systems categorizes hundreds of proteins with the search term “cytokine”; eBioscience lists 111. And that’s just the human proteins.
Growth factors are small protein molecules secreted by one cell to induce some physiological effect—such as proliferation, growth or differentiation—in the recipient cell. They include immune system regulators like interleukins and interferons, growth factors like fibroblast growth factor (FGF) and vascular endothelial growth factor (VEGF) plus chemokines like CCL2 and CXCL8. (Chemokines induce cellular migration, or chemotaxis.)
Researchers who study growth factor biology have a number of molecular tools at their disposal to measure the abundance and activity of these molecules, including antibodies, ELISAs, quantitative RT-PCR, and of course, functional bioassays.
Sometimes, though, researchers need a purified source of the cytokine or chemokine proteins themselves—say, to stimulate cells in culture or as a positive control in an ELISA. Life-science tool providers do market recombinant growth factors for such occasions, and you can search their catalogs online. Here are a few variables to consider before clicking “Checkout.”
Protein source
Some proteins must be post-translationally modified for activity. Mammalian cells can attach those modifications, but bacterial cells often cannot. So be sure to check the product’s data sheet for the source of your protein. R&D Systems’ human TGF-beta 1, for instance, is prepared in Chinese hamster ovary cells, but its human CCL2 is prepared in E. coli. InvivoGen’s recombinant human IL-4 is prepared in yeast.
Formulation
Recombinant growth factors generally come in one of two formulations: with bovine serum albumin (BSA) or without it (“carrier-free”). Which one you need depends on your application. According to eBioscience literature, BSA serves as a stabilizer, and the BSA-containing formulation is “Ideal for use in bioassays and as standards in ELISA applications.” Carrier-free formulations are “for use in in vivo applications or where BSA would interfere with the experiment.”
Bioactivity and applications
You should never be in doubt as to the quality of the product you’re buying. Reputable companies subject their protein products to a variety of quality-control checks. You’ll often see data regarding purity, for instance, or endotoxin concentration, with possibly an SDS-PAGE thrown in for good measure.
You should also see info on bioassays, which provide a measure of the protein’s functional activity. Recombinant growth factors are typically used as ELISA standards, in culture-based bioassays and in vivo. R&D Systems tests its CCL2 chemokine preps by their ability to “chemoattract BaF3 mouse pro-B cells transfected with human CCR2A” as well as “2-day cultured human monocytes.” eBiosciences quantifies the bioactivity of its TGF-beta 1 by the “inhibition of IL-4 induced proliferation in HT-2 cells.”
Bioassay metrics are often given in terms of units/ml or ED50 (the dose at which you will achieve 50% of maximal activity), expressed as a concentration in the ug/ml to pg/ml range. But ED50 is assay specific, as eBioscience notes, and in any event, companies cannot test every application.
Check for publications
Unfortunately, if your assay isn’t listed, there’s a good chance the company cannot offer much guidance. R&D Systems’ FAQ list says: “If a specific application is not listed on the datasheet, it indicates that R&D Systems has no further in-house data.” The company suggests you check its publication listings “to see if other researchers have published using your application, sample type, and/or species.”
That’s not a bad idea in any event. If nothing else, a quick trip through the literature can often identify useful controls and possible pitfalls. As the saying goes, “Six months in the lab can save an afternoon in the library.”
The image at the top of the page is from R&D Systems.