Cellular and molecular mechanisms for induction of broad anti-viral B cell responses through vaccination

Abstract

Viral infections contribute to significant morbidity and mortality worldwide. The immense diversity across and within viral families poses a substantial challenge for the development of prophylaxis and therapeutics. Vaccines are one of the most effective medical interventions to prevent infectious diseases. This thesis focused on characterizing anti-viral B cell responses elicited by vaccination and the mechanisms for inducing diversified responses that encompass broad reactivity. All studies were performed in the non-human primate (NHP) model to mimic the human immune system and increase the translational value of the results.

In paper I, we characterized how two common routes of parenteral immunization, intramuscular (IM) and subcutaneous (SC) injections, differentially affect the early innate immune responses and the development of adaptive immune responses. The main difference observed between the SC and IM routes was the specific lymph node (LN) clusters to which the vaccine was transported. SC immunization targeted the more superficial LNs in the SC fat while the IM route targeted LNs deeper in the tissue located near major veins. The induction of vaccine-specific adaptive immune responses did not differ.

In papers II and III, we performed a detailed analysis of the responses to a novel selfassembling protein nanoparticle that displayed multiple copies of the surface fusion (F) glycoprotein of human respiratory syncytial virus (HRSV). In mice, the multivalent display by the nanoparticle enhanced antibody responses compared to single copies of the HRSV-F protein. This occurred in a valency-dependent manner and relied on the assembly of the multivalent nanoparticle. Importantly, the improved responses were also observed in NHPs. In NHPs, the increased antigen display valency skewed antibody specificities, epitope-focused B cells, and led to an increase in the genetic diversity of responding B cell clonotypes. This resulted in the elicitation of pneumovirus cross-reactive antibodies. We could partly attribute this effect to increased avidity and/or B cell receptor cross-linking from repetitive arraying of antigen on the nanoparticle surface.

To follow up on this phenomenon and understand the development of antibody breadth, in paper IV we characterized a pneumovirus crossneutralizing antibody lineage elicited by nanoparticle immunization. Through molecular and structural analyses of antibody variants and evolutionary intermediates, we found that this antibody had acquired cross-reactivity through affinity maturation, with critical residues located in the second heavy chain complementarity determining region (HCDR2), and that similar antibody lineages with the potential to also acquire breadth may have been elicited in multiple other animals.

In conclusion, this thesis improves our understanding of the mechanisms by which vaccine formulation and delivery can modulate the quality and breadth of anti-viral B cell responses. This type of information is important for development and refinement of vaccines that are broadly protective, “universal”, within viral families.