Design of soluble and particulate immunogens derived from the stem of Influenza hemagglutinin

Abstract

Influenza is a highly contagious virus, belonging to the family Orthomyxoviridae that causes acute febrile respiratory illness which can be fatal in some cases. The highest risk groups for influenza viral infection includes elderly persons and children. Seasonal influenza leads to 3,00,000-5,00,000 deaths worldwide annually, thus posing a serious health threat to the people. Vaccination has been the most effective protective measure against influenza. However, influenza virus undergoes rapid evolution through ‘antigenic shift’ and ‘antigenic drift’ mechanisms to avoid the host immune pressure. Owing to the the continuous changes in the virus, the currently available trivalent and quadrivalent vaccines are mainly strain specific and need to be annually updated. Thus, there is a growing need to develop a ‘universal vaccine candidate’ that can confer broad protection. Influenza is an enveloped virion having eight negative sense RNAs in its core and two important suface glycoproteins- Hemagglutinin (HA) and Neuraminidase(NA). Most neutralizing antibodies are elicited against HA, thus making it an attractive vaccine candidate. HA0, the trimeric precursor of HA is cleaved to HA1 and HA2 subunits. Cleavage activates the fusion capacity of HA. HA comprises of a globular head domain and a coiled-coil stem domain. The HA head domain is much more immunodominant than the HA stem, having four antigenic sites at the receptor binding domain. However, the head domain is subjected to heightened immune pressure leading to escape variants. This eventually limits the potency of head directed neutralizing antibodies. Sequence analysis studies have revealed that the HA stem domain is much more conserved and can be targeted by several broadly neutralizing antibodies. Hence, several groups have tried to mimic stem based conserved epitopes through headless stem domain immunogen designs, that can elicit antibodies capable of protecting against infection by diverse influenza strains. Advancements in structural biology and nanotechnology have led to the development of new strategies to enhance the efficacy of stem domain based immunogens. One of them is to use self-assembling protein nanoparticles that can display several copies of the same immunogen simultaneously, thus enhancing B cell and T cell immune responses through “avidity effects”. In this thesis , we discuss the design and characterization of several stem domain immunogens, as well as different protein nanoparticle fusion immunogens