No abstract

Despite the prevalence and clinical importance of influenza, its long-term effect on lung immunity is unclear. Here we describe that following viral clearance and clinical recovery, at one month post-influenza, mice are better protected from Streptococcus pneumoniae infection due to a population of monocyte-derived alveolar macrophages (AMs) which produce increased IL-6. Influenza-induced monocyte-derived AMs have a surface phenotype similar to resident AMs but display a unique functional, transcriptional and epigenetic profile which is distinct from resident AMs. In contrast, influenza-experienced resident AMs remain largely similar to naive AMs. Thus, influenza changes the composition of the AM population to provide prolonged antibacterial protection. Monocyte-derived AMs persist over time but lose their protective profile. Our results help to understand how transient respiratory infections, a common occurrence in human life, can constantly alter lung immunity by contributing monocyte-derived, recruited cells to the AM population.

SummaryThe integrity of eukaryotic genomes requires rapid and regulated chromatin replication. How this is accomplished is still poorly understood. Using purified yeast replication proteins and fully chromatinized templates, we have reconstituted this process in vitro. We show that chromatin enforces DNA replication origin specificity by preventing non-specific MCM helicase loading. Helicase activation occurs efficiently in the context of chromatin, but subsequent replisome progression requires the histone chaperone FACT (facilitates chromatin transcription). The FACT-associated Nhp6 protein, the nucleosome remodelers INO80 or ISW1A, and the lysine acetyltransferases Gcn5 and Esa1 each contribute separately to maximum DNA synthesis rates. Chromatin promotes the regular priming of lagging-strand DNA synthesis by facilitating DNA polymerase α function at replication forks. Finally, nucleosomes disrupted during replication are efficiently re-assembled into regular arrays on nascent DNA. Our work defines the minimum requirements for chromatin replication in vitro and shows how multiple chromatin factors might modulate replication fork rates in vivo.

The cohesin complex is at the heart of many chromosomal activities, including sister chromatid cohesion and transcriptional regulation1-3. Cohesin loading onto chromosomes depends on the Scc2/Scc4 cohesin loader complex4-6, but the chromatin features that form cohesin loading sites remain poorly understood. Here, we show that the RSC chromatin remodeling complex recruits budding yeast Scc2/Scc4 to broad nucleosome-free regions, that the cohesin loader itself helps to maintain. Consequently, inactivation of the cohesin loader or RSC complex have similar effects on nucleosome positioning, gene expression and sister chromatid cohesion. These results reveal an intimate link between local chromatin structure and higher order chromosome architecture. Our findings pertain to the similarities between two severe human disorders, Cornelia de Lange syndrome, caused by mutations in the human cohesin loader, and Coffin-Siris syndrome, resulting from mutations in human RSC complex components7-9. Both could arise from gene misregulation due to related changes in the nucleosome landscape.