P1409: MACROPHAGES PROVIDE ESSENTIAL SUPPORT FOR ERYTHROPOIESIS DURING CHRONIC PSYCHOLOGICAL STRESS

Background: Under steady-state conditions, a population of tissue-resident macrophages within erythroblastic islands is required for erythroid homeostasis. In addition to tissue resident macrophages, newly recruited monocyte-derived macrophages also contribute to erythropoietic niche during stress erythropoiesis. However, recent evidence suggests that the extent of macrophage contribution to stress erythropoiesis is likely to be both tissue- and stress-type dependent. Using a murine model of psychological stress, we have previously shown that chronic stress induces erythropoiesis both in the bone marrow and the spleen, but tissue-specific contribution of macrophages to erythropoiesis during chronic psychological stress remains unknown. Aims: The purpose of this study is to examine the effects of macrophage depletion on bone marrow and splenic erythropoiesis under chronic stress conditions. Methods: Adult male BALB/c mice were subjected to 2h daily restraint stress for 7 consecutive days. Clodronate liposomes were used to deplete resident macrophages from the bone marrow and spleen two days prior to first restraint procedure, as well as newly recruited macrophages every third day for the duration of the experiment. Mice were randomly assigned to following groups: (1) R - restraint group exposed to daily restraint stress; (2) CLOD + R group, received clodronate liposomes (i.p. 200μl /20g), and subjected to daily restraint; (3) CLOD group, treated with clodronate liposomes only; (4) CTRL + R received control liposomes (i.p. 200μl /20g) and subjected to daily restraint; and (5) control, untreated group. In the bone marrow and spleen number of erythroid progenitors burst forming units-erythroid (BFU-E) and colony-forming unit-erythroid (CFU-E) were determined using colony asays and CD71/Ter119 profiles of erythroid cells were analyzed by flow citometry. Results: Chronic stress induced a great increase in the number of BFU-E and CFU-E progenitors in the bone marrow and spleen. Macrophage depletion did not significantly alter the number of erythroid progenitors under basal conditions, but completely abolished the effect of repeated stress on BFU-E and CFU-E cells in both the bone marrow and spleen. Flow cytometric analysis of erythroid precursors revealed that macrophage depletion under steady-state conditions substantially reduced the percentage of CD71+/Ter119+ cells in the bone marrow, while the percentage of CD71+/Ter119+ cells in the spleen was not significantly altered. Chronic exposure to daily restraint resulted in considerably increased percentage of CD71+/Ter119+ cells in the bone marrow and spleen, and macrophage depletion fully abrogated the effect of stress on these cells. The treatment with clodronate liposomes under basal conditions increased the percentage of CD71-/Ter119+ cells in the spleen, while the percentage of these cells in the bone marrow remained almost unchanged. The percentage of CD71-/Ter119+ cells was significantly increased in the bone marrow and spleen of chronically stressed mice, and the depletion of macrophages has prevented the effect of stress on these cells. Summary/Conclusion: Under basal conditions, treatment with clodronate liposome induced tissue-specific changes in the percentage of CD71+/Ter119+ and CD71-/Ter119+ cells in the bone marrow and spleen. Macrophage depletion completely abolished the stimulatory effect of repeated stress on erythroid progenitors and precursors in both the bone marrow and spleen, pointing towards a critical role for macrophages in erythropoiesis under chronic stress conditions.