Experiments with two prototype anti-β-amyloid vaccines in mice showed that a booster dose given 15 days after priming persistently impairs immunological memory, measured as the ratio of the secondary to the primary anti-β-amyloid IgG peak after a recall dose nine months later; day-21 boosters had no effect in either system, and a day-7 booster had no significant effect in the phage system, while in the protein system the day-7 group appeared intermediate between the unboosted control and the day-15 group. The discoverers named this vulnerable window a 'consolidation phase' of immunological memory, by analogy with memory consolidation in the nervous system. The phenomenon is documented for a multimeric protein antigen and for a filamentous-bacteriophage-displayed B-cell epitope, but its cellular and molecular mechanism is unknown: it is not settled what cellular mechanism the day-15 booster disrupts — whether an impaired development of memory cells (one hypothesis is interference with the acquisition of self-renewal capacity by nascent memory B cells) or a dominant inhibitory mechanism — how vulnerability changes over time, whether the phase exists for other antigens, adjuvants, injection routes, and doses, or why only a minority of phage-immunized mice develop an enhanced recall response. The open problem is to determine this mechanism and establish the scope of the consolidation phase.

Background

Immunological memory is the ability of the adaptive immune system to mount an antibody (humoral) response of greater magnitude and faster kinetics upon re-encounter of the same antigen, and it is the desired outcome of vaccination. In the humoral arm, memory is carried by memory B cells — long-lived, antigen-experienced B lymphocytes that can re-enter cell division and differentiate upon antigen re-exposure and can self-renew (maintain their own pool) — and by long-lived plasma cells, which continuously secrete antibody (for this response, immunoglobulin G, IgG). Memory B cells and plasma cells are produced in germinal centers (GCs): transient structures in lymphoid organs where B cells proliferate, mutate their antibody genes, and are selected by antigen and T-cell help; the GC output changes over time, with memory B cells generated during the reaction and isotype-switched (class-switched, i.e. producing a different antibody class such as IgG rather than IgM) bone-marrow long-lived plasma cells appearing late. A B-cell epitope is the short antigen segment an antibody recognizes. β-amyloid (Aβ) is the peptide that aggregates in Alzheimer's disease, and several experimental vaccines target defined Aβ B-cell epitopes conjugated to carriers, so as to avoid the Aβ T-cell epitopes blamed for meningoencephalitis in an early clinical trial. The source studies used two such vaccines in Balb/c mice: the multimeric protein (1-11)E2, displaying many copies of Aβ residues 1–11, and fdAD(2-6), a filamentous bacteriophage (an Inovirus, a rod-shaped bacterial virus used as a highly repetitive antigen-display platform) engineered to display about 800 copies of the Aβ B-cell epitope AEFRH (residues 2–6) on its major capsid protein pVIII. Mice were primed with antigen in Complete Freund's Adjuvant (CFA, a strong oil-based immune stimulant) and boosted in Incomplete Freund's Adjuvant (IFA, the same emulsion without the mycobacteria); a recall dose was given nine months later, and memory was scored per animal as the ratio of the secondary to the primary anti-Aβ IgG peak, with a ratio above two operationalized as memory. The groups observed that a booster injected 15 days after priming significantly lowered this ratio, while a day-21 booster had no significant effect in either study and a day-7 booster had no significant effect in the phage study (in the protein study the day-7 group appeared intermediate between the control and the day-15 group); they named the vulnerable window a 'consolidation phase' of immunological memory, by analogy with the post-learning stabilization of memory traces in the nervous system. This is distinct from the known booster-induced hyporesponsiveness of polysaccharide vaccines (for example pneumococcal polysaccharide, which abrogates the germinal-center reaction and depletes antibody-secreting cells): in the consolidation-phase system the primary response and antibody titers are not reduced, indicating the booster specifically impairs the development of recall capacity rather than antibody production. The phenomenon was found with both a TH2-skewed protein antigen and a TH1-skewed phage antigen (TH1/TH2 denote the two canonical helper-T-cell response programs that accompany the humoral response), under the same CFA/IFA protocol, and with the same timing — but it has otherwise not been mechanistically characterized, and no cellular correlate has been identified. The phage system is a promising model in which to test interventions that could enhance the development of recall memory in protocols where only a minority of individuals currently develop it.

Problem Statement

Determine the cellular and molecular basis of the consolidation phase of immunological memory — the time window, about two weeks after priming, in which a booster dose disrupts the development of memory in the mouse IgG response to bacteriophage- or protein-displayed β-amyloid B-cell epitopes — and establish its scope. The objective is to explain quantitatively how developmental processes in the early primary response determine the magnitude of late recall responses. Specifically: (1) identify the cellular and molecular mechanism disrupted by a day-15 booster — whether an impaired development of memory cells or a dominant inhibitory mechanism caused the effect is unresolved; one proposed hypothesis is that at day 15 nascent memory B cells have not yet acquired self-renewal capacity, so that booster-driven activation and proliferation depletes the developing memory pool rather than expanding it; (2) characterize how this vulnerability changes over time, beyond the tested booster days, on which the day-15 booster had a significant effect while a day-21 booster did not (the day-7 result was endpoint- and system-dependent); (3) determine whether a consolidation phase occurs in immunizations with other antigens, adjuvants, injection routes, or doses; and (4) establish whether the interindividual variability in secondary-response strength — where only a minority of phage-vaccinated mice achieve an enhanced (secondary/primary IgG ratio above two) recall response — can be attributed to differences in early germinal-center dynamics. The required deliverables are items (1) and (3) together with the quantitative objective stated above; items (2) and (4) are optional extensions.

References

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