Respiratory Syncytial Virus (RSV) infects millions of people each year causing disease in the lungs and respiratory tract. For individuals with developing or compromised immune systems (such as infants or older adults), RSV infection can result in hospitalization or death. Proteins on the surface of the viral membrane can be targeted by the adaptive immune system. Furthermore, their function is critical to viral infection making them an attractive potential candidate for vaccine development. Of particular interest is the thermostability of an antigen at low temperatures in liquid formulation because it has been observed that the antigen exhibits reduced in vitro potency of a critical epitope more quickly when stored at 5 °C than 25 °C.
To understand this phenomenon, the physicochemical properties were assessed. The Gibbs free energy of folding (ΔG°fold) was determined with differential scanning calorimetry and the analysis revealed that the ΔG°fold at 5 °C and 25 °C is -1.2 and -3.5 kcal/mol, respectively. This result provides a biophysical explanation for the apparent cold denaturation, because the antigen gains approximately 2 kcal/mol in ΔG°fold when stored at 25 °C versus 5 °C. In order to understand the structural aspects of the instability at low temperature, an investigation was initiated to determine how the secondary structure, tertiary structure , and hydrodynamic radius of the antigen were affected. The results of the investigation revealed that the secondary structure was maintained, the tertiary structure was not preserved, and the hydrodynamic radius decreased. This study provides further detail into the physicochemical characteristics of the antigen as a vaccine candidate and deepens our understanding of its design limits.