The transmission of most respiratory pathogens, including SARS-CoV-2, occurs via virus-containing respiratory droplets, and thus, factors that affect virus viability in droplet residues on surfaces are of critical medical and public health importance. Relative humidity (RH) is known to play a role in virus survival, with a U-shaped relationship between RH and virus viability. The mechanisms affecting virus viability in droplet residues, however, are unclear. This study examines the structure and evaporation dynamics of virus-containing saliva droplets on fomites and their impact on virus viability using four model viruses: vesicular stomatitis virus, herpes simplex virus 1, Newcastle disease virus, and coronavirus HCoV-OC43. The results support the hypothesis that the direct contact of antiviral proteins and virions within the "coffee ring" region of the droplet residue gives rise to the observed U-shaped relationship between virus viability and RH. Viruses survive much better at low and high RH, and their viability is substantially reduced at intermediate RH. A phenomenological theory explaining this phenomenon and a quantitative model analyzing and correlating the experimentally measured virus survivability are developed on the basis of the observations. The mechanisms by which RH affects virus viability are explored. At intermediate RH, antiviral proteins have optimal influence on virions because of their largest contact time and overlap area, which leads to the lowest level of virus activity.

Complement receptor type 2 (CR2) is an important membrane molecule expressed on B cells and follicular dendritic cells. Human CR2 has been shown to play a critical role in bridging the innate complement-mediated immune response with adaptive immunity by binding complement component 3d (C3d). However, the chicken CR2 (chCR2) gene has not been identified or characterized. In this study, unannotated genes that contain short consensus repeat (SCR) domains were analyzed based on RNA sequencing data for chicken bursa lymphocytes, and a gene with >80% homology to CR2 from other bird species was obtained. The gene consisted of 370 aa and was much smaller than the human CR2 gene because 10–11 SCRs were missing. The gene was then demonstrated as a chCR2 that exhibited high binding activity to chicken C3d. Further studies revealed that chCR2 interacts with chicken C3d through a binding site in its SCR1–4 region. An anti-chCR2 mAb that recognizes the epitope 258CKEISCVFPEVQ269 was prepared. Based on the anti-chCR2 mAb, the flow cytometry and confocal laser scanning microscopy experiments confirmed that chCR2 was expressed on the surface of bursal B lymphocytes and DT40 cells. Immunohistochemistry and quantitative PCR analyses further indicated that chCR2 is predominantly expressed in the spleen, bursa, and thymus, as well as in PBLs. Additionally, the expression of chCR2 varied according to the infectious bursal disease virus infection status. Collectively, this study identified and characterized chCR2 as a distinct immunological marker in chicken B cells.