Giant viruses are ecologically important players in aquatic ecosystems that have challenged concepts of what constitutes a virus. Herein, we present the giant Bodo saltans virus (BsV), the first characterized representative of the most abundant group of giant viruses in ocean metagenomes, and the first isolate of a klosneuvirus, a subgroup of the Mimiviridae proposed from metagenomic data. BsV infects an ecologically important microzooplankton, the kinetoplastid Bodo saltans. Its 1.39 Mb genome encodes 1227 predicted ORFs, including a complex replication machinery. Yet, much of its translational apparatus has been lost, including all tRNAs. Essential genes are invaded by homing endonuclease-encoding self-splicing introns that may defend against competing viruses. Putative anti-host factors show extensive gene duplication via a genomic accordion indicating an ongoing evolutionary arms race and highlighting the rapid evolution and genomic plasticity that has led to genome gigantism and the enigma that is giant viruses.

Deeg et al. show a novel line of transgenic mice expressing restriction factor MxA exhibits robust resistance to influenza viruses of avian but not human origin. In vivo evasion of MxA is mediated by distinct amino acids in the nucleoprotein of human influenza viruses.

Members of the major candidate phylum Dependentiae (a.k.a. TM6) are widespread across diverse environments from showerheads to peat bogs; yet, with the exception of two isolates infecting amoebae, they are only known from metagenomic data. The limited knowledge of their biology indicates that they have a long evolutionary history of parasitism. Here, we present Chromulinavorax destructans (Strain SeV1) the first isolate of this phylum to infect a representative from a widespread and ecologically significant group of heterotrophic flagellates, the microzooplankter Spumella elongata (Strain CCAP 955/1). Chromulinavorax destructans has a reduced 1.2 Mb genome that is so specialized for infection that it shows no evidence of complete metabolic pathways, but encodes an extensive transporter system for importing nutrients and energy in the form of ATP from the host. Its replication causes extensive reorganization and expansion of the mitochondrion, effectively surrounding the pathogen, consistent with its dependency on the host for energy. Nearly half (44%) of the inferred proteins contain signal sequences for secretion, including many without recognizable similarity to proteins of known function, as well as 98 copies of proteins with an ankyrin-repeat domain; ankyrin-repeats are known effectors of host modulation, suggesting the presence of an extensive host-manipulation apparatus. These observations help to cement members of this phylum as widespread and diverse parasites infecting a broad range of eukaryotic microbes.

Although classical dendritic cells (DCs) arise from distinct progenitors in the bone marrow, the origin of inflammatory DCs and the distinction between monocyte-derived DCs and macrophages is less clear. In vitro culture of mouse bone marrow cells with GM-CSF is a well-established method to generate DCs, but GM-CSF has also been used to generate bone marrow–derived macrophages. In this article, we identify a distinct subpopulation of cells within the GM-CSF bone marrow–derived DC culture based on their ability to bind hyaluronan (HA), a major component of the extracellular matrix and ligand for CD44. HA identified a morphologically distinct subpopulation of cells within the immature DC population (CD11c+ MHC IImid/low) that were CCR5+/CCR7− and proliferated in response to GM-CSF, but, unlike immature DCs, did not develop into mature DCs expressing CCR7 and high levels of MHC II, even after stimulation with LPS. The majority of these cells produced TNF-α in response to LPS but were unable to activate naive T cells, whereas the majority of mature DCs produced IL-12 and activated naive T cells. This HA binding population shared many characteristics with alveolar macrophages and was retained in the alveolar space after lung instillation even after LPS stimulation, whereas the MHC IIhigh mature DCs were found in the draining lymph node. Thus, HA binding in combination with MHC II expression can be used to identify alveolar-like macrophages from GM-CSF–treated bone marrow cultures, which provides a useful in vitro model to study alveolar macrophages.