Simple diagram that shows the development of different blood cells from
hematopoietic stem cell to mature cells. From Wikipedia.
All blood cells arise from the common hematopoietic stem cell and are classified into two lineages: lymphoid cells (B-, T- and Natural Killer [NK] cells) which play an important role in adaptive and innate immune response, and myeloid cells which include red blood cells, cells involved in clotting and non-lymphoid immune cells. In the classical model of hematopoiesis, an early decision is made committing cells to one of these two fates then following a linear progression to one of ten distinct blood cell types. However, evidence from mice indicates that blood development isn’t quite so neat.
In a paper published in Nature Immunology a Toronto group led by Dr. John Dick has shown that the classical model may not be adequate to describe human blood development. Adult bone marrow and neonatal cord blood cells were separated according to surface markers and assessed for their ability to produce different cell types. While an early myeloid commitment was consistent with the current model – these progenitors were not able to become T, B or NK cells – lymphoid precursors were able to form macrophages and dendritic cells, cells traditionally part of the myeloid lineage.
This represents a challenge to the “textbook” model, and is an important step for better understanding human blood development. Furthermore, understanding these processes gives us better knowledge of how they can go wrong and how to treat the resulting diseases. For example, acute myeloid leukemia (AML) is a cancer of the blood. As the name implies, it is the result of uncontrolled growth of cells from the myeloid lineage caused by dysregulation of the corresponding stem cell. Effective treatment requires the identification and targeting of the appropriate cells. If, as this research suggests, these cells sometimes arise from the lymphoid compartment it may require different strategies for treatment. Creating an accurate road map of development is essential for understanding normal biology as well as abnormal. In this case it also identifies a different source for traditionally myeloid immune cells which could also prove useful as a source of material for tailored immunotherapy.