P1.152: Periostin marks fetal mesenchymal progenitors that give rise to hematopoietic niche stroma in the postnatal bone marrow

Background: Neonates with critical illness often develop low blood cell counts, which carries significant morbidity and mortality. The etiology of inadequate blood cell production during illness is poorly understood. Cells in the hematopoietic niche, including hematopoietic stem and progenitor cells (HSPC) and mesenchymal-derived stromal cells, regulate blood cell output. We hypothesize that abnormal bone marrow (BM) niche responses in sick neonates results in hematopoietic dysfunction. Few studies have examined factors that underlie hematopoietic niche development during the perinatal period, and little is known about the identities of early mesenchymal progenitor populations that develop into mature niche cells. We designed this study to define the cellular dynamics that contribute to hematopoietic niche emergence in the postnatal BM.

Methods: To define mesenchymal progenitor cell types and transcriptional programs across ontogeny, we created an integrated single cell RNA-sequencing (scRNA-seq) dataset from mouse BM spanning late gestation through adulthood. Periostin-Cre (PostnCre) Rosa26-tdTomato mice coupled with scRNA-seq and high-resolution microscopy was used for lineage tracing of mesenchymal progenitors, and PostnCre Cxcl12fl/fl and PostnCre Rosa26iDTR mice were used for the functional assessment of HSPC supporting capacity of stromal cells derived from Postn-expressing progenitors.

Results: We found that BM mesenchymal cell populations are developmentally regulated. Mesenchymal progenitors predominate in the fetal-perinatal period while hematopoietic niche stromal cells do not emerge until after birth. These differences are mediated by developmental stage-specific transcriptional programs. Transcription factor activities and gene expression changes associated with niche stroma maturation are enriched in postnatal mesenchymal progenitors, including Ebf1/3 and Foxc1 known to impact the function of critical cells in the hematopoietic niche. These gene signatures progressively increase in the BM after birth, suggesting continued development of the marrow postnatally. Mesenchymal progenitor metabolism was also developmentally regulated. Genes linked to lipid metabolism were upregulated after birth, representing a key metabolic function that emerges postnatally. We identify Postn as a marker of fetal mesenchymal progenitors within the BM. In vivo lineage tracing and single cell trajectory analyses confirm that Postn-expressing progenitors become hematopoietic niche stroma.

Conclusion: Our study demonstrates that there is marked heterogeneity within the BM with regards to mesenchymal lineage populations and their transcriptomes. In contrast to fetal-perinatal progenitors, those in the postnatal marrow appear primed for lineage commitment, which explains the absence of mature hematopoietic niche stroma in prenatal BM. Importantly, we identify Postn as a novel and previously unappreciated marker of fetal BM mesenchymal progenitors. Critical illness and preterm birth induce well-studied maladaptive responses in the lung, brain, and gastrointestinal tract, but almost no studies have examined how blood production is affected. Together, our work provides a mechanistic foundation of hematopoietic niche development that will be used to investigate inadequate blood cell production in critically ill and preterm neonates.