Blood stem cell transplants may not fully reconstitute blood cells in the immune system, according to a study using humanized mice.

If the study led by Stanford University scientists is confirmed in human transplant patients, it means they could be more vulnerable to common infections such as the flu, the study found.

At issue is a subset of B cells called B-1a. Like all other immune cells, these are currently believed to be restored by blood stem cell and bone marrow transplants, which transfer blood-forming or hematopoietic stem cells found in bone marrow.

The study says the progenitors of B-1a cells do not appear to be found in HSCs. Moreover, it says, these cells may remain stably located in peripheral tissues, and not normally circulate in the blood at all.


The study, published in the Dec. 24 issue of Stem Cell Reports, says the finding adds to evidence that hematopoetic stem cells from bone marrow does not contain the full spectrum of cells required to make immune cells. Eliver Eid Bou Ghosn was the study’s first author; the senior author was Leonore A. Herzenberg.

A 2012 study published in PNAS concluded that B-1a cells are not produced from HSCs taken from bone marrow. The new study performed a similar experiment using fetal human liver, where HSCs are believed to exist before entering the bone marrow. The most important are known as long-term or LT HSCs, which persist for life, providing a perpetually fresh source of new blood cells.

Two experts who reviewed the study gave it a mixed assessment, stressing that that humanized mouse model is subject to error in replicating the human immune system.

Many cells, one source


Blood formation is a complex process, involving many cell types that develop from more primitive stem cell states to more developed ones, and finally to fully adult cells. Their relationship to each other is still being debated.

However, the transplant procedure’s usefulness is not in doubt. It has been successfully used for decades to restore the immune systems of patients with blood cancers, whose therapy destroyed their original immune systems.

The procedure has been changed over the years. It once consisted of transplanting whole bone marrow, before the specific stem cells involved in blood formation were identified.

In recent years, it has become more common to filter blood from a donor to remove circulating hematopoietic stem cells. The purified stem cells are then infused into the recipient’s bloodstream, from where they migrate to the bone marrow and begin making blood cells.


It is this newer process that the researchers say fails to regenerate the B-1a cells. If the cells aren’t normally present in circulating blood, their lack would be a logical consequence of transplant procedures using circulating blood.

In the new study, researchers tested female mice given a human immune system. These mice were lethally irradiated to destroy their immune system, then given injections of fetal HSCs taken from the fetal human liver. As with HSCs from adult bone marrow, the fetal liver HSCs failed to produce B1a-like cells, although they did regenerate many other immune cells.

“These findings provide further evidence that B-1a emerge as a separate B-cell lineage that develops independently of LT-HSCs,” the study stated. “Consistent with these findings, we show that fetal liver LT-HSCs also fail to reconstitute a key component of the B-1a repertoire (i.e., VH11), which produce anti-phospholipid antibodies known to promote housekeeping activities and prevent autoimmunity.”

Since previous work has shown that grafts of whole fetal liver tissue can regenerate the B-1a cells, the new study concluded that these cells come from progenitor cells in the fetal liver that are not hematopoietic stem cells.


These cells may reside in peripheral tissues, not actually circulating in the blood, the authors suggested. If that is true, then one important component of what are considered blood cells mainly act in specific locales.

“Furthermore, from a medical standpoint, we call into question current human regenerative therapies in which HSC transplantation is used to restore immunity in immune-compromised individuals, and consider the probable shortcomings of an immune system that lacks the B-1a cell subset,” the study stated.

Critical responses

The study makes several unwarranted assumptions, said Daniel Salomon, medical program director of the Scripps Center for Organ and Cell Transplantation at The Scripps Research Institute in La Jolla.


Concluding that hematopoietic stem cell transplant patients are immune-deficient in B-1a cells is “a very big leap and not justified at this time for many reasons,” Salomon said by email.

“Mice are not humans. Human B-1a cells are actually poorly characterized and may be very different than their mouse versions. The authors lethally irradiated their mice but this is never done for human patients. Thus, there is no reason to assume that B-1a cells located in peripheral immune compartments like the gut are killed by the drugs optimized for depletion of the bone marrow necessary for human transplants.”

“Finally, it is critical to note that most human bone marrow transplant patients after successful recovery are not falling left and right from the supposed immune deficiencies predicted by the authors,” Salomon stated.

Dr. Rafael Bejar, a blood cancer physician/researcher at UC San Diego, gave a more positive assessment, calling the study “fairly compelling.” But like Salomon, he cautioned that the mouse model could be an inaccurate reflection of human blood formation.


“In this study, the authors cleanly show that these cells they are calling LT-HSCs do not reconstitute B1a lymphocytes in the time frames they examine. And, they look for them a long time after transplantation,” Bejar said by email.

Playing the “devil’s advocate role,” Bejar said it could simply be possible that production of the B1a cells from HSC is very inefficient, but eventually generates long-lived progenitor cells.

“Their interpretation also doesn’t rule out the possibility that a more primitive cell than the LT-HSC can’t give rise to the LT-HSC and the B1a precursors,” Bejar said. “Maybe this more primitive cell is only present in fetal development. In any case, the consequences would be similar. That is, reconstitution with an LT-HSC might fail to generate all of the hematopoietic repertoire needed for a completely healthy immune system.”

Bejar called the mouse model the “weakest aspect” of the study.


“First, it is well known that reconstitution of human hematopoiesis in mice is not uniform,” Bejar said “It is often easier to engraft cells with a strong lymphoid bias, for example. Myeloid cells might engraft less well, even in ‘humanized’ mouse models and it can be donor specific. A failure to engraft a cell type may not necessarily be a consequence of the underlying biology. It may just be due to the differences between the mouse and human microenvironments.”

As with Salomon, Bejar said B1a cells in humans are not well defined, and it’s not even clear that they exist.

“There are significant differences in the intrinsic immune cells of mice and the range of antigens that they respond to.” Bejar said. “However, the article suggests several testable hypotheses to explore further.”

Following up


Looking for this effect in human transplant patients is one logical path to explore, Bejar said.

“The authors touched on this a little, but it could be stressed more,” he said. “They argue that these missing cells have gone unnoticed because they are largely not circulating populations. They are more tissue-bound like other cells of dubious lineage including Langerhans cells and microglia.”

Changes in bone marrow transplant methods could also be involved, he said.

“In the past, we used to do actual bone marrow transplants where bone marrow extracted from a donor was transfused into the recipient, largely unprocessed,” Bejar said. “Since then, we have switched to collecting hematopoietic stem cells from the peripheral blood of donors in the majority of cases. This is analogous to the experiments in the paper where unsorted bone marrow reconstitutes B1a cells but LT-HSCs don’t.”


“The speculation raised by the paper is that by switching to peripherally collected stem cells, we may have missed transplanting cells in the bone marrow that are needed for complete hematopoietic reconstitution. This might be testable,” Bejar said.

“If the human cells analogous to the B-1a cells can be better defined or the antibodies they produce better characterized, we can look for evidence of these donor-derived cells in patients that have received a stem cell transplant,” he said.

“It’s also possible that this finding is only of scientific interest, as it challenges a long-held dogma, but has little or no clinical consequence. Both methods of donor stem cell collection might allow for reconstitution of B1a-like cells in patients, for example.”