ABO: building B cells from blood stem cells

B cells are a crucial component of the immune system that protects our body against pathogens. Researchers from the group of reNEW Principal Investigator Frank Staal have succeeded in growing these cells from blood stem cells in the laboratory using a new method. The study marks an important step forward for research into B-cell deficiencies.

Blood stem cells in the bone marrow develop into red blood cells and a variety of cells that make up our immune system. Each type of immune cell follows its own developmental pathway, which is typically difficult to replicate in the laboratory. Staal’s research group primarily focuses on the development of cells that belong to the adaptive immune system: T cells and B cells. Previously, they employed a system — Artificial Thymic Organoids (ATOs) — that allows them to model T-cell development from blood stem cells. In a new study, published in Cell Reports Medicine on June 18^th, the group now presents a method that also captures complete B-cell development.

Better and simpler
“We adapted the ATO-protocol in such a way that blood stem cells develop into B cells rather than T cells. We call this new system ABO – Assay for efficient B-cell Output,” says Merijn Braams, PhD candidate in Staal’s group and first author of the study. “The protocol is not only efficient, but also surprisingly simple. We mix the blood stem cells with mouse stromal cells and create small aggregates. These are then placed in an air-liquid interface.”

Compared to other model systems, where B cell development often stalls at the pre-B cell stage, immune cells in the ABO efficiently develop towards immature B cells. This better reflects B cell formation in the human body as they are highly comparable to B cells that leave the bone marrow and possess fully functional B-cell receptors. Moreover, they are capable of ‘class switching’ and even form plasma cells, meaning they develop into cells that produce large quantities of different antibody types such as IgM, IgG and IgA.

When B cell development goes wrong
The ABO system therefore enables researchers to fully mature blood stem cells into healthy B cells in the laboratory and study their development in great detail. But that is not its only application. It can also be used in a disease context to study the underlying mechanisms of inherited B-cell deficiencies and to develop new therapies for these disorders.

“There are hundreds of known inborn errors of immunity, of which a subset specifically affect B cell development. For most of these conditions currently no curative treatments exist beyond transplantation with suitable donor bone marrow. At the same time, for many patients the causes of their disease and the underlying mechanisms remain poorly understood,” says Braams.

He continues: “Thanks to ABOs, we can now investigate complete B cell development in both known and unknown disorders in the laboratory, as well as develop and test new treatments such as gene therapy. As a proof of concept, we employed ABOs to replicate the disrupted B-cell development seen in the severe immune disorder RAG1-SCID, as well as the effects of a lentiviral gene therapy we currently use to treat patients. We next aim to expand our research to X-Linked Agammaglobulinemia (XLA), which is the most common B cell deficiency in boys for which no curative treatment exists.”

Unlocking ABO’s full potential with iPSCs
The research team is currently working on methods that allow them to exploit the ABO system to its full potential. In this context, Braams learned how to grow blood stem cells from human induced pluripotent stem cells (iHSCs) in the groups of reNEW Principal Investigators Elizabeth Ng and Andrew Elefanty at the Murdoch Children’s Research Institute in Melbourne. These iHSCs may eventually provide an attractive alternative to the mobilized or cord blood-derived hematopoietic stem cells currently in use.

Last author Dr. Sander de Kivit looks enthusiastically toward the possibilities this could provide: “We can easily introduce (potential) disease-associated genetic modifications into iHSCs to study the effects of these mutations on B-cell development, even without patient material.  Through our collaborators in Melbourne, we have access to unique expertise in iHSC biology and we are now able to make all major blood cell lineages in the laboratory. By combining this with our experience in gene therapy development, we are uniquely positioned to accelerate insights into severe, but often rare, immune disorders, and to translate this knowledge into treatments that can genuinely transform the lives of these patients.”

Tool for diagnostics
Finally, the ABO system opens opportunities for improved diagnostics of B cell deficiencies. Staal concludes: “The ABO system is highly useful for improving the diagnosis of B-cell immunodeficiencies because it can be performed using only small numbers of blood stem cells. These quantities can also be obtained from peripheral blood rather than bone marrow. This approach provides precise insights into subtle blocks in B-cell development without the need for painful and highly invasive bone marrow aspirations. It can therefore be particularly valuable for clinical immunologists and their patients, offering a less invasive method to investigate defects in B-cell maturation while maintaining diagnostic accuracy.”