Beta-1 Integrin Controls Homing and Expansion of Erythroid Cells in Stress Erythropoiesis and ß-Thalassemia

After blood loss, the production of red cells must be increased by stress erythropoiesis. This phenomenon is associated with increased proliferation and reduced differentiation of the erythroblasts, leading to a net increase in the number of progenitor erythroid cells and red cells (erythron). In normal conditions, after expansion of the pool of erythroblasts, these cells eventually differentiate to erythrocytes and the anemia resolves. However, in diseases such as β-thalassemia, production of healthy mature erythrocytes is impaired, resulting in anemia. Over time, the expansion, rather than the differentiation, of the erythron further exacerbates the ineffective erythropoiesis (IE), reducing the ability of the erythroid progenitors to generate erythrocytes.

Interupting the interaction between macrophages and erythroblasts (macrophage-erythroblast interaction, MEI) in thalassemia models is efficacious in reducing IE and alleviating the disease phenotype. Targeting MEI, using a number of approaches, caused a significant improvement in blood parameters in β-thalassemia intermedia (BTI) mouse models (Hbb^th3/+) and a rapid and dramatic improvement in splenomegaly, an outcome that is relevant for clinical practice. Importantly, MEI is not critical for hematopoiesis under non-stress conditions, and ablation of this interaction in normal mice showed minimal effects on blood parameters. As our initial observations indicate that MEI is essential to support stress erythropoiesis, we investigated adhesion molecules that might activate downstream pathways in erythroblasts that regulate cell proliferation. We also speculate that these molecules are also responsible for the homing of erythroid progenitor cells to extramedullary organs, such as the spleen and liver.

Our studies in erythroblasts indicate that integrin beta 1 (Itgb1) and also intracellular molecules such as Fak1, Talin1 and Sharpin might play a role in stress erythropoiesis. There is increased interaction between Itgb1 and Fak1 in erythroblasts co-cultured with macrophages as demonstrated by immunocytochemistry and in vitro proximity ligation assays. In addition, targeting either Itgb1 and Fak1 prevents expansion of erythroid cells when cultured in the presence of macrophages. Strikingly, using Itgb1 together with Ter119 as selection parameters in flow cytometry, a distinct subset of erythroblasts, not discernable using CD44 or CD71, was observable, which we found to be part of the mixed orthochromatic erythroblast/reticulocyte population as determined with CD44 expression. More specifically, when measuring the content of DNA, we were able to demonstrate that enucleation of erythroblasts was accompanied by a marked loss of Itgb1 expression, indicating that there may be an important role for Itgb1 in erythroblast enucleation, and differentiation in general. Lack of Itgb1 in thalassemic mice prevents erythroid cells from homing to and expanding in the spleen, the major source of chronic stress erythopoiesis in this disorder. In particular, such a role of Itgb1 is supported by our analysis of thalassemic mice in which this molecule was partially depleted by induction of the Cre recombinase. These animals were generated by crossing th3/+ mice with animals in which Itgb1 was floxed and carrying an inducible Cre-recombinase (Mx1-CRE). We utilized the BM of these animals (Hbb^th3/+, Itgb^1fl/fl, Mx1-CRE) to generate thalassemic animals that expressed the floxed Itgb1 only in hematopietic cells. After serial administration of polyI:C the animals were analyzed for their erythropoiesis in the bone marrow and spleen. Interestingly, all the animals analyzed show chimeric populations of Itgb1 positive and negative erythroid cells in the bone marrow. This indicated that not all the HSCs were successfully depleted of the Itgb1 gene. However, when we investigated Itgb1 in the spleen, we observed only erythroid cells positive for the expression of this adhesion molecule. This last observation strongly suggests that depletion of Itgb1 prevents homing and expansion of erythroid cells in the spleen and drugs that by inhibit Itgb1 could reduce erythroid spleen colonization, splenomegaly and limit erythropoiesis.

We are now in the process of identifying compounds that target MEI. Such molecules might be utilized for development of new treatments for thalassemia or additional disorders of aberrant erythropoiesis.