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"Extended priming" immunization regimens that prolong exposure of the immune system to vaccines during the primary immune response have shown promise in enhancing humoral immune responses to a variety of subunit vaccines in preclinical models. We previously showed that escalating-dosing immunization (EDI), where a vaccine is dosed every other day in an increasing pattern over 2 weeks dramatically amplifies humoral immune responses. But such a dosing regimen is impractical for prophylactic vaccines. We hypothesized that simpler dosing regimens might replicate key elements of the immune response triggered by EDI. Here we explored "reduced ED" immunization regimens, assessing the impact of varying the number of injections, dose levels, and dosing intervals during EDI. Using a stabilized HIV Env trimer as a model antigen combined with a potent saponin adjuvant, we found that a two-shot extended-prime regimen consisting of immunization with 20% of a given vaccine dose followed by a second shot with the remaining 80% of the dose 7 days later resulted in increased total GC B cells, 5-10-fold increased frequencies of antigen-specific GC B cells, and 10-fold increases in serum antibody titers compared to single bolus immunization. Computational modeling of the GC response suggested that this enhanced response is mediated by antigen delivered in the second dose being captured more efficiently as immune complexes in follicles, predictions we verified experimentally. Our computational and experimental results also highlight how properly designed reduced ED protocols enhance activation and antigen loading of dendritic cells and activation of T helper cells to amplify humoral responses. These results suggest that a two-shot priming approach can be used to substantially enhance responses to subunit vaccines., Competing Interests: Competing interests S.H.B, D.J.I, A.K.C, and L.Y are inventors on a patent filing related to the extended-priming regimens described in this manuscript. For completeness, it is also noted that A.K.C. is a consultant (titled “Academic Partner”) for Flagship Pioneering, consultant and Strategic Oversight Board Member of its affiliated company, Apriori Bio, and is a consultant and Scientific Advisory Board Member of another affiliated company, Metaphore Bio.

Global containment of COVID-19 still requires accessible and affordable vaccines for low- and middle-income countries (LMICs). 1 Recently approved vaccines provide needed interventions, albeit at prices that may limit their global access. 2 Subunit vaccines based on recombinant proteins are suited for large-volume microbial manufacturing to yield billions of doses annually, minimizing their manufacturing costs. 3 These types of vaccines are well-established, proven interventions with multiple safe and efficacious commercial examples. 4-6 Many vaccine candidates of this type for SARS-CoV-2 rely on sequences containing the receptor-binding domain (RBD), which mediates viral entry to cells via ACE2. 7,8 Here we report an engineered sequence variant of RBD that exhibits high-yield manufacturability, high-affinity binding to ACE2, and enhanced immunogenicity after a single dose in mice compared to the Wuhan-Hu-1 variant used in current vaccines. Antibodies raised against the engineered protein exhibited heterotypic binding to the RBD from two recently reported SARS-CoV-2 variants of concern (501Y.V1/V2). Presentation of the engineered RBD on a designed virus-like particle (VLP) also reduced weight loss in hamsters upon viral challenge., Competing Interests: Competing interests L.E.C., K.R.L., and J.C.L. have filed patents related to the InSCyT system and methods. N.C.D., S.R.A., and J.C.L. have filed a patent related to the RBD-L452K-F490W sequence. K.R.L., L.E.C., and M.K.T. are current employees at Sunflower Therapeutics PBC. J.C.L. has interests in Sunflower Therapeutics PBC, Pfizer, Honeycomb Biotechnologies, OneCyte Biotechnologies, QuantumCyte, Amgen, and Repligen. J.C.L’s interests are reviewed and managed under MIT’s policies for potential conflicts of interest. J.M.S. is an employee of the Bill & Melinda Gates Medical Research Institute. H.K. is an employee of the Bill & Melinda Gates Foundation.