Hematopoietic and nonhematopoietic cells promote Type I interferon- and TLR7-dependent monocytosis during low-dose LCMV infection

Review of Mammarenavirus Biology and Replication

The family Arenaviridae is divided into three genera: Mammarenavirus, Reptarenavirus, and Hartmanivirus. The Mammarenaviruses contain viruses responsible for causing human hemorrhagic fever diseases including New World viruses Junin, Machupo, Guanarito, Sabia, and Chapare virus and Old World viruses Lassa, and Lujo virus. These two groups of arenaviruses share the same genome organization composed of two ambisense RNA segments. These segments contain four open reading frames that encode for four proteins: the nucleoprotein, glycoprotein precursor, L protein, and Z. Despite their genome similarities, these groups exhibit marked differences in their replication life cycles. This includes differences in attachment, entry, and immune evasion. By understanding the intricacy of replication in each of these viral species we can work to develop counter measures against human diseases. This includes the development of vaccines and antivirals for these emerging viral threats. Currently only the vaccine against Junin virus, Candid#1, is in use as well as Ribavirin for treatment of Lassa Fever. In addition, small molecule inhibitors can be developed to target various aspects of the virus life cycle. In these ways an understanding of the arenavirus replication cycle can be used to alleviate the mortality and morbidity of these infections worldwide.

MicroRNA-22 controls interferon alpha production and erythroid maturation in response to infectious stress in mice

MicroRNA-22 (miR-22) is a highly conserved microRNA that can regulate cell proliferation, oncogenesis, and cell maturation, especially during stress. In hematopoietic stem cells (HSCs), miR-22 has been reported to be involved in the regulation of key self-renewal factors, including Tet2. Recent work demonstrates that miR-22 also participates in regulation of the interferon (IFN) response, and expression profiling studies suggest that it is variably expressed at different stages in erythroid differentiation. We thus hypothesized that miR-22 regulates maturation of erythroid progenitors during stress hematopoiesis through its interaction with IFN. We compared the blood and bone marrow of wild-type (WT) and miR-22-deficient mice at baseline and upon infectious challenge with systemic lymphochoriomeningitis (LCMV) virus. miR-22-deficient mice maintained platelet counts better than WT mice during infection, but they showed significantly reduced red blood cells and hemoglobin. Analysis of bone marrow progenitors demonstrated better overall survival and improved HSC homeostasis in infected miR-22-null mice compared with WT, which was attributable to a blunted IFN response to LCMV challenge in the miR-22-null mice. We found that miR-22 was expressed exclusively in stage II erythroid precursors and downregulated upon infection in WT mice. Our results indicate that miR-22 promotes the IFN response to viral infection and that it functions at baseline as a brake to slow erythroid differentiation and maintain adequate erythroid potential. Impaired regulation of erythrogenesis in the absence of miR-22 can lead to anemia during infection.

BCAP inhibits proliferation and differentiation of myeloid progenitors in the steady state and during demand situations

B-cell adaptor for phosphatidylinositol 3-kinase (BCAP) is a signaling adaptor expressed in mature hematopoietic cells, including monocytes and neutrophils. Here we investigated the role of BCAP in the homeostasis and development of these myeloid lineages. BCAP^-/- mice had more bone marrow (BM) monocytes than wild-type (WT) mice, and in mixed WT:BCAP^-/- BM chimeras, monocytes and neutrophils skewed toward BCAP^-/- origin, showing a competitive advantage for BCAP^-/- myeloid cells. BCAP was expressed in BM hematopoietic progenitors, including lineage^-Sca-1⁺c-kit⁺ (LSK), common myeloid progenitor, and granulocyte/macrophage progenitor (GMP) cells. At the steady state, BCAP^-/- GMP cells expressed more IRF8 and less C/EBPα than did WT GMP cells, which correlated with an increase in monocyte progenitors and a decrease in granulocyte progenitors among GMP cells. Strikingly, BCAP^-/- progenitors proliferated and produced more myeloid cells of both neutrophil and monocyte/macrophage lineages than did WT progenitors in myeloid colony-forming unit assays, supporting a cell-intrinsic role of BCAP in inhibiting myeloid proliferation and differentiation. Consistent with these findings, during cyclophosphamide-induced myeloablation or specific monocyte depletion, BCAP^-/- mice replenished circulating monocytes and neutrophils earlier than WT mice. During myeloid replenishment after cyclophosphamide-induced myeloablation, BCAP^-/- mice had increased LSK proliferation and increased numbers of LSK and GMP cells compared with WT mice. Furthermore, BCAP^-/- mice accumulated more monocytes and neutrophils in the spleen than did WT mice during Listeria monocytogenes infection. Together, these data identify BCAP as a novel inhibitor of myelopoiesis in the steady state and of emergency myelopoiesis during demand conditions.