Isolating single human hematopoietic stem cells

by David Kent

For those who followed the Stem Cell Foundation campaign last year, you may have come across the article I wrote about what inspired me to enter the field of stem cell biology. The power of a single blood stem cell to recreate all of the elements of the blood system for the lifetime of a mouse (and beyond) and the ability to isolate them prospectively at near purity in mammals is a luxury primarily limited to the mouse blood stem cell system. With the breast (John Stingl, former SCN trainee under Connie Eaves) and prostate (Owen Witte) fields catching up quickly, scientists are very excited to pursue high-level studies of pure populations in multiple tissues of the mouse.

This week, even more excitement was generated with news from one of the pioneering stem cell labs in Toronto: John Dick and his team of researchers at the Ontario Cancer Institute have achieved this feat in human blood stem cells. In a splendid display of teamwork indicative of the collaborative atmosphere in the Dick lab, the dynamic duo of Faiyaz Notta and Sergei Doutlatov (PhD students and SCN trainees) led the work that demonstrated for the first time, the capacity of single blood cells isolated from a human cord blood sample to make all of the cell types of the blood system for a sufficiently long time post-transplantation to be deemed a bona fide blood stem cell. This is the same pair of students who also recently published the Nature Immunology paper that mapped the various progenitor compartments in the human blood system by phenotype and function – a strategy now widely used to help study the panoply of diseases arising in the blood system.

Prospective isolation of highly purified blood stem cells paves the way for studies in human that have been happening in the mouse for the last decade. Gene/protein expression and stem cell expansion studies will no longer be confused by large numbers of non-stem cells and questions around heterogeneity of stem cells and intrinsic potential of each cell can now be addressed.

Importantly, this study highlights the rarity of these functional blood stem cells (less than 1/100,000), exemplifying the large caveat of making conclusions based on non-purified fractions. Many researchers still use single marker (e.g.: CD34) or two markers (e.g.: CD34+38-) to isolate “blood stem cells” – far more numerous, but sadly not very representative of the actual blood stem cell. The problem with such careless use of terms means that in many cases, studies are looking at a population of cells that are more than 98% impure—in other words, actual blood stem cells represent at most 2% of the sample in question, which obviously could raise doubts about the conclusions drawn. The Dick group’s study, on the other hand, builds on a steady stream of additional markers in the literature and improves isolates to 1 in 5, making studies of these fractions much more relevant to stem cell biology.

If we want to understand the biology that drives disease, the importance of being able to isolate the cell that is capable of causing the disease is critical. While many experiments are not doable with such a rare fraction, those that fail to look at purified cells should be taken with a grain of salt unless they exclude the non-stem cells in some sort of functional assay. This is something that is of particular importance for those working in clinical trials and screening for novel regulators and/or inhibitors of blood stem cell diseases (e.g.: many different leukemias).

The Dick group’s study is an impressive accomplishment and opens many doors for blood stem cell scientists as well as researchers trying to accomplish the same feat in other human tissues.

Additional reports of the July 8 publication can be found on Cell News and in the Globe and Mail.