Person:

He, Shun

Iron is an essential component of the erythrocyte protein hemoglobin and is crucial to oxygen transport in vertebrates. In the steady state, erythrocyte production is in equilibrium with erythrocyte removal1. In various pathophysiological conditions, however, erythrocyte life span is severely compromised, which threatens the organism with anemia and iron toxicity2,3. Here we identify an on-demand mechanism that clears erythrocytes and recycles iron. We show that Ly-6Chigh monocytes ingest stressed and senescent erythrocytes, accumulate in the liver via coordinated chemotactic cues, and differentiate to ferroportin 1 (FPN1)-expressing macrophages that can deliver iron to hepatocytes. Monocyte-derived FPN1+ Tim-4neg macrophages are transient, reside alongside embryonically-derived Tim-4high Kupffer cells, and depend on Csf1 and Nrf2. The spleen likewise recruits iron-loaded Ly-6Chigh monocytes, but these do not differentiate into iron-recycling macrophages due to the suppressive action of Csf2. Inhibiting monocyte recruitment to the liver leads to kidney and liver damage. These observations identify the liver as the primary organ supporting rapid erythrocyte removal and iron recycling and uncover a mechanism by which the body adapts to fluctuations in erythrocyte integrity.

Background—Healing after myocardial infarction (MI) involves the biphasic accumulation of inflammatory Ly-6Chigh and reparative Ly-6Clow monocytes/macrophages. Excessive inflammation disrupts the balance between the 2 phases, impairs infarct healing, and contributes to left ventricle remodeling and heart failure. Lipoprotein-associated phospholipase A2 (Lp-PLA2), a member of the phospholipase A2 family of enzymes, produced predominantly by leukocytes, participates in host defenses and disease. Elevated Lp-PLA2 levels associate with increased risk of cardiovascular events across diverse patient populations, but the mechanisms by which the enzyme elicits its effects remain unclear. This study tested the role of Lp-PLA2 in healing after MI.

Methods and Results—In response to MI, Lp-PLA2 levels markedly increased in the circulation. To test the functional importance of Lp-PLA2, we generated chimeric mice whose bone marrow–derived leukocytes were Lp-PLA2–deficient (bmLp-PLA2−/−). Compared with wild-type controls, bmLp-PLA2−/− mice subjected to MI had lower serum levels of inflammatory cytokines tumor necrosis factor-α, interleukin (IL)-1β, and IL-6, and decreased number of circulating inflammatory myeloid cells. Accordingly, bmLp-PLA2−/− mice developed smaller and less inflamed infarcts with reduced numbers of infiltrating neutrophils and inflammatory Ly-6Chigh monocytes. During the later, reparative phase, infarcts of bmLp-PLA2−/− mice contained Ly-6Clow macrophages with a skewed M2-prone gene expression signature, increased collagen deposition, fewer inflammatory cells, and improved indices of angiogenesis. Consequently, the hearts of bmLp-PLA2−/− mice healed more efficiently, as determined by improved left ventricle remodeling and ejection fraction.

Conclusions—Lp-PLA2 augments the inflammatory response after MI and antagonizes healing by disrupting the balance between inflammation and repair, providing a rationale for focused study of ventricular function and heart failure after targeting this enzyme acutely in MI.

Background— Healing after myocardial infarction (MI) involves the biphasic accumulation of inflammatory Ly-6Chigh and reparative Ly-6Clow monocytes/macrophages. Excessive inflammation disrupts the balance between the 2 phases, impairs infarct healing, and contributes to left ventricle remodeling and heart failure. Lipoprotein-associated phospholipase A2 (Lp-PLA2), a member of the phospholipase A2 family of enzymes, produced predominantly by leukocytes, participates in host defenses and disease. Elevated Lp-PLA2 levels associate with increased risk of cardiovascular events across diverse patient populations, but the mechanisms by which the enzyme elicits its effects remain unclear. This study tested the role of Lp-PLA2 in healing after MI. Methods and Results— In response to MI, Lp-PLA2 levels markedly increased in the circulation. To test the functional importance of Lp-PLA2, we generated chimeric mice whose bone marrow–derived leukocytes were Lp-PLA2–deficient (bmLp-PLA2−/−). Compared with wild-type controls, bmLp-PLA2−/− mice subjected to MI had lower serum levels of inflammatory cytokines tumor necrosis factor-α, interleukin (IL)-1β, and IL-6, and decreased number of circulating inflammatory myeloid cells. Accordingly, bmLp-PLA2−/− mice developed smaller and less inflamed infarcts with reduced numbers of infiltrating neutrophils and inflammatory Ly-6Chigh monocytes. During the later, reparative phase, infarcts of bmLp-PLA2−/− mice contained Ly-6Clow macrophages with a skewed M2-prone gene expression signature, increased collagen deposition, fewer inflammatory cells, and improved indices of angiogenesis. Consequently, the hearts of bmLp-PLA2−/− mice healed more efficiently, as determined by improved left ventricle remodeling and ejection fraction. Conclusions— Lp-PLA2 augments the inflammatory response after MI and antagonizes healing by disrupting the balance between inflammation and repair, providing a rationale for focused study of ventricular function and heart failure after targeting this enzyme acutely in MI.

Myocardial infarction (MI) elicits massive inflammatory leukocyte recruitment to the heart. Here, we hypothesized that excessive leukocyte invasion leads to heart failure and death during acute myocardial ischemia. We found that shortly and transiently after onset of ischemia, human and mouse cardiac fibroblasts produce granulocyte/macrophage colony-stimulating factor (GM-CSF) that acts locally and distally to generate and recruit inflammatory and proteolytic cells. In the heart, fibroblast-derived GM-CSF alerts its neighboring myeloid cells to attract neutrophils and monocytes. The growth factor also reaches the bone marrow, where it stimulates a distinct myeloid-biased progenitor subset. Consequently, hearts of mice deficient in either GM-CSF or its receptor recruit fewer leukocytes and function relatively well, whereas mice producing GM-CSF can succumb from left ventricular rupture, a complication mitigated by anti–GM-CSF therapy. These results identify GM-CSF as both a key contributor to the pathogenesis of MI and a potential therapeutic target, bolstering the idea that GM-CSF is a major orchestrator of the leukocyte supply chain during inflammation.

Communication within the glial cell ecosystem is essential to neuronal and brain health1–3. The influence of glial cells on β-amyloid (Aβ) and neurofibrillary tau accumulation and clearance in Alzheimer’s disease (AD) is poorly understood, despite growing awareness that these are therapeutically important interactions4,5. Here we show, in humans and mice, that astrocyte-sourced interleukin-3 (IL-3) reprograms microglia to ameliorate AD pathology. Upon recognition of Aβ deposits, microglia augment IL-3Rɑ, IL-3’s specific receptor, rendering them responsive to IL-3. Astrocytes constitutively produce IL-3, which elicits transcriptional, morphological, and functional reprograming of microglia endowing them with an acute immune response program, enhanced motility, and the capacity to cluster and clear Aβ and tau aggregates. These changes restrict AD pathology and cognitive decline. This study identifies IL-3 as a critical mediator of astrocyte-microglia crosstalk and a node for therapeutic intervention in AD.