Natural infections expose the immune system to escalating antigen and inflammation over days to weeks, whereas nonlive vaccines are single bolus events. We explored whether the immune system responds optimally to antigen kinetics most similar to replicating infections, rather than a bolus dose. Using HIV antigens, we found that administering a given total dose of antigen and adjuvant over 1-2 wk through repeated injections or osmotic pumps enhanced humoral responses, with exponentially increasing (exp-inc) dosing profiles eliciting >10-fold increases in antibody production relative to bolus vaccination post prime. Computational modeling of the germinal center response suggested that antigen availability as higheraffinity antibodies evolve enhances antigen capture in lymph nodes. Consistent with these predictions, we found that exp-inc dosing led to prolonged antigen retention in lymph nodes and increased Tfh cell and germinal center B-cell numbers. Thus, regulating the antigen and adjuvant kinetics may enable increased vaccine potency.vaccination kinetics | antigen retention | humoral response | computational immunology | germinal center formation S ubunit vaccines based on recombinant protein antigens combined with adjuvants can safely elicit protective humoral immune responses in humans, and they have become a cornerstone of modern public health (1, 2). Recent advances in structure-based vaccine design (3, 4) and progress in the development of adjuvants that are safe and effective for prophylactic vaccines (5) have helped drive the field. However, several challenges remain: A number of protein vaccines, such as candidate vaccines against HIV and malaria, have tended to elicit short-lived immunity (6, 7). In HIV, broadly neutralizing antibodies (BNAbs) isolated from infected patients are generally characterized by high degrees of somatic hypermutation (SHM) (8), but methods to generate such highly mutated antibodies by vaccination remain unknown. SHM occurs in germinal centers (GCs) within lymphoid organs, and data from animal models demonstrate a critical role for follicular helper T cells in the induction of GCs and promotion of affinity maturation (9, 10). To date, methods to promote Tfh generation and long-lived germinal centers during vaccination remain unclear (11-15). Much attention has focused on the use of adjuvants to promote affinity maturation, but it remains unclear if adjuvants alone can provide the necessary immunological driving forces for promoting extensive affinity maturation (16).During acute infections, which often provoke robust germinal center responses and durable humoral immunity, microorganism replication typically occurs over the course of one to several weeks (17-19). During this time, recognition of molecular danger signals contained within the pathogen sustains stimulation of the innate immune system, and a continuous supply of antigen is provided to the adaptive immune system. In contrast to these patterns of antigen and inflammatory cues during infection, typical subunit vaccines...

Upon natural infection with pathogens or vaccination, antibodies are produced by a process called affinity maturation. As affinity maturation ensues, average affinity values between an antibody and ligand increase with time. Purified antibodies isolated from serum are invariably heterogeneous with respect to their affinity for the ligands they bind, whether macromolecular antigens or haptens (low molecular weight approximations of epitopes on antigens). However, less is known about how the extent of this heterogeneity evolves with time during affinity maturation. To shed light on this issue, we have taken advantage of previously published data from Eisen and Siskind (1964). Using the ratio of the strongest to the weakest binding subsets as a metric of heterogeneity (or affinity inequality), we analyzed antibodies isolated from individual serum samples. The ratios were initially as high as 50-fold, and decreased over a few weeks after a single injection of small antigen doses to around unity. This decrease in the effective heterogeneity of antibody affinities with time is consistent with Darwinian evolution in the strong selection limit. By contrast, neither the average affinity nor the heterogeneity evolves much with time for high doses of antigen, as competition between clones of the same affinity is minimal.