IRF5 Regulates the Balance between Erythropoiesis and Myelopoiesis during Aging

During steady state, erythropoiesis takes place in the bone marrow within anatomic niches called erythroblastic islands (EBIs). The EBI consists of a central macrophage surrounded by differentiating erythroblasts. Recent data indicate that myeloid cells are also present in the island. While many studies have focused on the interactions between the macrophage and surrounding cells, much less is known about the transcription factors that regulate the formation and function of EBIs. So far, KLF1, a master regulator of erythropoiesis, has been the best studied in this context. Previous data from our lab found that mice lacking the transcription factor interferon regulatory factor 5 (Irf5^-/- mice) have decreased erythropoiesis during aging based on CD71/Ter119 expression by flow cytometry . We performed scRNAseq on purified EBIs and found that Irf5 is specifically expressed by the central macrophages within EBIs. Therefore, we hypothesized that Irf5 regulates the balance between erythropoiesis and myelopoiesis during aging.

To test this hypothesis, we isolated EBIs from the bone marrow of both wild-type and Irf5^-/- littermate mice at 3 months and 9 months. While no difference was found at 3 months of age, we found that at steady state in the 9-month old cohort, Irf5^-/- mice had a significantly decreased number of EBIs with an altered structure – decreased CD71 (erythroid marker) and increased CD11b and Ly6G (myeloid markers). Moreover, analysis of terminal erythroid differentiation based on CD44/Ter119 and CD44/FSC in the bone marrow revealed that the Irf5^-/- mice had a higher reticulocyte count (mean 38.61%) than wild-type mice (mean 26.28%, p value 0.032). Accordingly, the precursor populations were reduced in the bone marrow and the spleens of the Irf5^-/- mice characteristic of stress erythropoiesis. Using colony-forming assays, we observed that Irf5^-/- mice also had a higher number of early progenitor BFU-E when compared to wild-type mice (p value 0.0003 for 3 month-old and 0.038 for 9 month-old mice) as well as CFU -GEMM.

We then induced stress erythropoiesis through submandibular bleeding on 2 consecutive days and found that recovery from stress anemia was delayed in the Irf5^-/- mice accompanied with a decreased number of EBIs. 5 days after inducing stress anemia, the Irf5^-/- mice had a significantly lower number of bone marrow EBIs (mean 25.29% of the total cells) when compared to wild-type mice (mean 40.47% of the total cells, p value 0.03). Response to stress was also delayed, and the reticulocyte count was lower in the knockout mice compared to the wild-type 5 days post bleeding.

Altogether, these data identify Irf5 as a novel regulator of mammalian erythropoiesis under steady-state and stress conditions.