A study led by scientists at Osaka University Graduate School of Medicine (OUGSM) has identified the previously unknown molecular mechanism underlying bone marrow regeneration after chemotherapy, which damages hematopoietic stem and progenitor cells (HSPCs).

"Several studies have reportedly elucidated the molecular mechanisms underlying bone marrow regeneration after chemotherapy-induced damage," noted study leader Masaru Ishii, professor and chairman of the Department of Immunology and Cell Biology at OUGSM.

"However, ours is the first to show that bone marrow-resident group 2 innate lymphoid cells (ILC2s) support recovery of HSPCs from chemotherapy-induced stress by secreting granulocyte-macrophage colony-stimulating factor (GM-CSF)," said Ishii.

Resident in bone marrow, HSPCs constantly proliferate into new blood cells, giving rise to oxygen-carrying red blood cells, most immune system white blood cells and the platelets necessary for coagulation and bleeding cessation.

Because they are constantly proliferating, HSPCs are particularly sensitive to injury induced by chemotherapy, but can also regenerate following such injury.

"The bone marrow is a very active organ because it has to constantly produce new blood cells, so once bone marrow loses its function during chemotherapy, conditions including anemia, neutropenia and bleeding can occur," said Ishii.

"Sequential chemotherapy and total body irradiation can both result in prolonged bone marrow suppression, which can lead to high risk of severe infections due to reduced numbers of white blood cells in peripheral blood," he added.

Therefore, "in this study we wanted to understand how bone marrow hematopoietic stem cells regenerate upon chemotherapy-induced injury to recover their full function."

To this end, the researchers focused on ILC2s, a specific subset of blood cells produced from HSPCs that are found in several tissue types.

ILC2s play key roles in immunity and tissue repair, with those in bone marrow being thought to have a distinct location-specific role, but to date the nature of their function has remained unclear.

In their new study reported in the March 5, 2021, edition of Journal of Experimental Medicine, the researchers treated mice with 5-fluorouracil (5-FU), a chemotherapy toxic to HSPCs, then transplanted fresh HSPCs into the mice to recapitulate stem cell transplantation in leukemia patients.

This demonstrated that the injured HSPC microenvironment in 5-FU-treated mice promoted proliferation of transplanted HSPCs.

Analysis at the molecular level then showed that ILC2s in the bone marrow of treated mice produced GM-CSF to promote HSPC regeneration.

"RT-PCR [reverse transcriptase polymerase chain reaction] analysis of various types of bone marrow cells before and after 5-FU treatment showed that ILC2s expressed extremely high levels of the Csf2 gene encoding GM-CSF,-- explained Ishii.

Moreover, "flow cytometric analysis to detect intracellular GM-CSF further revealed that the ILC2 population of 5-FU-treated mice expressed GM-CSF," he told BioWorld Science.

"We further demonstrated that ILC2s support HSPC recovery by producing GM-CSF, with HSPCs expressing GM-CSF receptors, although bone marrow ILC2s have not been shown to originate from HSPCs," said Ishii.

To determine why ILC2s produce GM-CSF after bone marrow injury, the researchers then investigated whether other cells or molecules promote ILC2s GM-CSF production.

They discovered that progenitors of antibody-producing B cells in bone marrow produce the cytokine IL-33 after injury, which in turn activated ILC2s, demonstrating how multiple molecular factors are required to recover damaged bone marrow.

"Our body has developed a remarkable hematopoietic system to stabilize the number of cells in bone marrow, whereby ILC2 hematopoietic cells sense tissue damage and support recovery," said Ishii.

"Although we do not know why B cell progenitors specifically produce IL-33 then stimulate ILC2s, they may transmit a 'dying message' to ILC2s, because B cell progenitors are vulnerable to chemotherapy-induced damage," he speculated.

Importantly, the researchers then demonstrated that transferring isolated ILC2s to 5-FU-treated mice accelerated hematopoietic recovery, while reducing ILC2s had the opposite effect, suggesting that ILC2s may sense bone marrow damage.

"These findings show that the bone marrow regenerates after chemotherapy," commented study first author Takao Sudo, an assistant professor in the Department of Immunology and Cell Biology at OUGSM.

"Our results may contribute to the development of a novel therapeutic approach for chemotherapy-induced myelosuppression," said Sudo.

On this note, "while we have discovered a mechanism of HSPC recovery in murine models, we hope that our findings will also be applicable in humans," concluded Ishii.

"Our study findings suggest that the adoptive transfer of cultured ILC2s might be a useful treatment to induce HSPC recovery after chemotherapy and in future we intend to analyze human samples, in order to develop a novel adjunct therapy aimed at early hematopoietic recovery."