A core viral protein binds host nucleosomes to sequester immune danger signals

Viral proteins mimic host protein structure and function to redirect cellular processes and subvert innate defenses(1). Small basic proteins compact and regulate both viral and cellular DNA genomes. Nucleosomes are the repeating units of cellular chromatin and play an important role in innate immune responses(2). Viral encoded core basic proteins compact viral genomes but their impact on host chromatin structure and function remains unexplored. Adenoviruses encode a highly basic protein called protein VII that resembles cellular histones(3). Although protein VII binds viral DNA and is incorporated with viral genomes into virus particles(4,5), it is unknown whether protein VII impacts cellular chromatin. Our observation that protein VII alters cellular chromatin led us to hypothesize that this impacts antiviral responses during adenovirus infection. We found that protein VII forms complexes with nucleosomes and limits DNA accessibility. We identified post-translational modifications on protein VII that are responsible for chromatin localization. Furthermore, proteomic analysis demonstrated that protein VII is sufficient to alter protein composition of host chromatin. We found that protein VII is necessary and sufficient for retention in chromatin of members of the high-mobility group protein B family (HMGB1, HMGB2, and HMGB3). HMGB1 is actively released in response to inflammatory stimuli and functions as a danger signal to activate immune responses(6,7). We showed that protein VII can directly bind HMGB1 in vitro and further demonstrated that protein VII expression in mouse lungs is sufficient to decrease inflammation-induced HMGB1 content and neutrophil recruitment in the bronchoalveolar lavage fluid. Together our in vitro and in vivo results show that protein VII sequesters HMGB1 and can prevent its release. This study uncovers a viral strategy in which nucleosome binding is exploited to control extracellular immune signaling.