Induction of Antiviral Immune Response through Recognition of the Repeating Subunit Pattern of Viral Capsids Is Toll-Like Receptor 2 Dependent
Authors: Shepardson Kelly M. , Schwarz Benjamin , Larson Kyle , Morton Rachelle V. , Avera John , McCoy Kimberly , Caffrey Alayna , Harmsen Ann , Douglas Trevor , Rynda-Apple Agnieszka 
Published: November 2017

Although viruses and viral capsids induce rapid immune responses, little is known about viral pathogen-associated molecular patterns (PAMPs) that are exhibited on their surface. Here, we demonstrate that the repeating protein subunit pattern common to most virus capsids is a molecular pattern that induces a Toll-like-receptor-2 (TLR2)-dependent antiviral immune response. This early antiviral immune response regulates the clearance of subsequent bacterial superinfections, which are a primary cause of morbidities associated with influenza virus infections. Utilizing this altered susceptibility to subsequent bacterial challenge as an outcome, we determined that multiple unrelated, empty, and replication-deficient capsids initiated early TLR2-dependent immune responses, similar to intact influenza virus or murine pneumovirus. These TLR2-mediated responses driven by the capsid were not dependent upon the capsid’s shape, size, origin, or amino acid sequence. However, they were dependent upon the multisubunit arrangement of the capsid proteins, because unlike intact capsids, individual capsid subunits did not enhance bacterial clearance. Further, we demonstrated that even a linear microfilament protein built from repeating protein subunits (F-actin), but not its monomer (G-actin), induced similar kinetics of subsequent bacterial clearance as did virus capsid. However, although capsids and F-actin induced similar bacterial clearance, in macrophages they required distinct TLR2 heterodimers for this response (TLR2/6 or TLR2/1, respectively) and different phagocyte populations were involved in the execution of these responses
in vivo
. Our results demonstrate that TLR2 responds to invading viral particles that are composed of repeating protein subunits, indicating that this common architecture of virus capsids is a previously unrecognized molecular pattern.

IMPORTANCE
Rapid and precise pathogen identification is critical for the initiation of pathogen-specific immune responses and pathogen clearance. Bacteria and fungi express common molecular patterns on their exteriors that are recognized by cell surface-expressed host pattern recognition receptors (PRRs) prior to infection. In contrast, viral molecular patterns are primarily nucleic acids, which are recognized after virus internalization. We found that an initial antiviral immune response is induced by the repeating subunit pattern of virus exteriors (capsids), and thus, induction of this response is independent of viral infection. This early response to viral capsids required the cell surface-expressed PRR TLR2 and allowed for improved clearance of subsequent bacterial infection that commonly complicates respiratory viral infections. Since the repeating protein subunit pattern is conserved across viral capsids, this suggests that it is not easy for a virus to change without altering fitness. Targeting this vulnerability could lead to development of a universal antiviral vaccine.