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Summary Generation of potent antibodies by a mutation-selection process called affinity maturation is a key component of effective immune responses. Antibodies that protect against highly mutable pathogens must neutralize diverse strains. Developing effective immunization strategies to drive their evolution requires understanding how affinity maturation happens in an enviroment where variants of the same antigen are present. We present an in silico model of affinity maturation driven by antigen variants which reveals that induction of cross-reactive antibodies often occurs with low probability because conflicting selection forces, imposed by different antigen variants, can frustrate affinity maturation. We describe how variables such as temporal pattern of antigen administration influence the outcome of this frustrated evolutionary process. Our calculations predict, and experiments in mice with variant gp120 constructs of the HIV envelope protein confirm, that sequential immunization with antigen variants is preferred over a cocktail for induction of cross-reactive antibodies focused on the shared CD4 binding site epitope.

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Nature provides many examples of soft gel-like materials where performance is determined by tuning time-dependent mechanical properties with dynamic bonding interactions between biomacromolecules.[1] Accordingly, synthetic gels that contain dynamic bonding interactions have undergone intense investigation in fundamental and applied material science. The balance of solid-like and fluid-like behavior in these gel networks results from the binding equilibrium of reversible crosslinks between polymer chains. [2][3][4][5] Contemporary research on gels has focused on self-assembled systems [6][7][8][9][10][11][12][13][14][15] usually exploiting hydrogen bonding interactions. However, gels crosslinked with reversible covalent bonding chemistries [16][17][18] would provide an energetically favorable, [19] specific and controlled mechanism for engineering functional dynamic networks. [20][21][22][23][24] We have synthesized hydrogel networks that form in the physiological pH range by the reversible covalent interaction of polymer-bound phenylboronic acid and salicylhydroxamic acid (Fig. 1). These gels demonstrate a spectrum of pH-dependent viscoelastic behavior that can be controlled by the chemical composition of the polymer backbone. Moreover, the reversible crosslinks allow these networks to restructure dynamically and self-heal after mechanical disruption. Phenylboronate-salicylhydroxamate gels provide a new class of self-assembled materials enabling precise control over network viscoelasticity and pH responsiveness. Water-soluble polymers containing 10 mol % phenylboronic acid (PBA) or 10 mol % salicylhydroxamic acid (SHA) were synthesized by free-radical polymerization of functionalized monomers with 2-hydroxypropylmethacrylamide (HPMA) or acrylic acid (AA; Fig. 1b). When aqueous solutions of PBA and SHA containing polymers are mixed at physiological pH, the PBA and SHA moieties associate to form coordinate covalent bonds [25][26][27] (PBA-SHA; Fig. 1a). c Figure 1. Self-healing, viscoelastic hydrogel networks are formed using reversible covalent crosslinking chemistry. a) Covalent bonds that form between polymer-bound phenylboronic acid (PBA) and salicylhydroxamic acid (SHA) have pH-dependent binding equilibria that are shifted to the uncrosslinked state under acidic conditions. b) Linear water-soluble polymers containing either PBA or SHA moieties can be easily synthesized with different polymer backbones (e.g., 2-hydroxypropylmethacrylamide (HPMA) or acrylic acid (AA)) of controlled molar feed ratios. Two degrees of substitution for each polymer were made with x = 90 mol % or 95 mol % (see Supporting Information Table S1 for details). c) When PBA-and SHA-containing polymer solutions are mixed under physiological conditions, a reversible semisolid gel forms due to the dynamic restructuring of the crosslinked gel network. The specific pH range at which gels behave reversibly can be controlled with choice of polymer backbone (in b); HPMA-based PBA-SHA crosslinked gels are reversible at mildly acidic pH (pH 4-5) whi...

New subunit vaccine formulations with increased potency are of interest to improve immune responses against poorly immunogenic antigens, avoid vaccine shortages in pandemic situations, and to promote dose-sparing of potent adjuvant molecules that can cause unacceptable side effects in prophylactic vaccination. Here we report strong class-switched, high avidity humoral immune responses elicited by a vaccine system based on poly(lactide-co-glycolide) micro- or nano-particles enveloped by PEGylated phospholipid bilayers, with protein antigens covalently anchored to the lipid surface and lipophilic adjuvants inserted in the bilayer coating. Strikingly, these particles elicited high endpoint antigen-specific IgG titers (>106) sustained for over 100 days after two immunizations with as little as 2.5 ng of antigen. At such low doses, the conventional adjuvant alum or the molecular adjuvants monophosphoryl lipid A (MPLA) or α-galactosylceramide (αGC) failed to elicit responses. Co-delivery of antigen with MPLA or αGC incorporated into the particle bilayers in a pathogen-mimetic fashion further enhanced antibody titers by ~12-fold. MPLA provided the highest sustained IgG titers at these ultra-low antigen doses, while αGC promoted a rapid rise in serum IgG after one immunization, which may be valuable in emergencies such as disease pandemics. The dose of αGC required to boost the antibody response was also spared by particulate delivery. Lipid-enveloped biodegradable micro- and nano-particles thus provide a potent dose-sparing platform for vaccine delivery.