Germline-Encoded Affinity for Cognate Antigen Enables Vaccine Amplification of a Human Broadly Neutralizing Response against Influenza Virus.

Antibody paratopes are formed by hypervariable complementarity-determining regions (CDRH3s) and variable gene-encoded CDRs. The latter show biased usage in human broadly neutralizing antibodies (bnAbs) against both HIV and influenza virus, suggesting the existence of gene-endowed targeting solutions that may be amenable to pathway amplification. To test this, we generated transgenic mice with human CDRH3 diversity but simultaneously constrained to individual user-defined human immunoglobulin variable heavy-chain (VH) genes, including IGHV1-69, which shows biased usage in human bnAbs targeting the hemagglutinin stalk of group 1 influenza A viruses. Sequential immunization with a stalk-only hemagglutinin nanoparticle elicited group 1 bnAbs, but only in IGHV1-69 mice. This VH-endowed response required minimal affinity maturation, was elicited alongside pre-existing influenza immunity, and when IGHV1-69 B cells were diluted to match the frequency measured in humans. These results indicate that the human repertoire could, in principle, support germline-encoded bnAb elicitation using a single recombinant hemagglutinin immunogen.

An Integrated Signaling Threshold Initiates IgG Response toward Virus-like Immunogens.

Class-switched neutralizing Ab (nAb) production is rapidly induced upon many viral infections. However, due to the presence of multiple components in virions, the precise biochemical and biophysical signals from viral infections that initiate nAb responses remain inadequately defined. Using a reductionist system of synthetic virus-like structures, in this study, we show that a foreign protein on a virion-sized liposome can serve as a stand-alone danger signal to initiate class-switched nAb responses without T cell help or TLR but requires CD19. Introduction of internal nucleic acids (iNAs) obviates the need for CD19, lowers the epitope density (ED) required to elicit the Ab response, and transforms these structures into highly potent immunogens that rival conventional virus-like particles in their ability to elicit strong Ag-specific IgG. As early as day 5 after immunization, structures harboring iNAs and decorated with just a few molecules of surface Ag at doses as low as 100 ng induced all IgG subclasses of Ab in mice and reproduced the IgG2a/2c restriction that is long observed in live viral infections. These findings reveal a shared mechanism for the nAb response in mice. High ED is capable but not necessary for driving Ab secretion. Instead, even a few molecules of surface Ag, when combined with nucleic acids within these structures, can trigger strong IgG production. As a result, the signaling threshold for induction of IgG in individual B cells is set by dual signals originating from both ED on the surface and the presence of iNAs within viral particulate immunogens.

Self-Antigens Displayed on Liposomal Nanoparticles above a Threshold of Epitope Density Elicit Class-Switched Autoreactive Antibodies Independent of T Cell Help.

Epitope density has a profound impact on B cell responses to particulate Ags, the molecular mechanisms of which remain to be explored. To dissect the role of epitope density in this process, we have synthesized a series of liposomal particles, similar to the size of viruses, that display a model self-antigen peptide at defined surface densities. Immunization of C57BL/6J mice using these particles elicited both IgM and class-switched IgG1, IgG2b, and IgG3 autoreactive Abs that depended on the epitope density. In C57BL/6 gene knockout mice lacking either functional TCRs or MHC class II molecules on B cells, the liposomal particles also elicited IgM, IgG1, IgG2b, and IgG3 responses that were comparable in magnitudes to wild-type mice, suggesting that this B cell response was independent of cognate T cell help. Notably, the titer of the IgG in wild-type animals could be increased by more than 200-fold upon replacement of liposomes with bacteriophage Qß virus-like particles that displayed the same self-antigen peptide at comparable epitope densities. This enhancement was lost almost completely in gene knockout mice lacking either TCRs or MHC class II molecules on B cells. In conclusion, epitope density above a threshold on particulate Ags can serve as a stand-alone signal to trigger secretion of autoreactive and class-switched IgG in vivo in the absence of cognate T cell help or any adjuvants. The extraordinary immunogenicity of Qß viral-like particles relies, in large part, on their ability to effectively recruit T cell help after B cell activation.

Identification and Immunological Characterization of the Ligand Domain of Plasmodium vivax Reticulocyte Binding Protein 1a.

Background: Erythrocyte invasion by malaria parasites is essential for blood-stage development. Consequently, parasite proteins critically involved in erythrocyte invasion, such as the Plasmodium vivax reticulocyte binding proteins (RBPs) that mediate preferential invasion of reticulocytes, are considered potential vaccine targets. Thus, targeting the RBPs could prevent blood-stage infection and disease. The RBPs are large, and little is known about their functional domains and whether individuals naturally exposed to P. vivax acquire binding-inhibitory antibodies to these critical binding regions. This study aims to functionally and immunologically characterize Plasmodium vivax RBP1a. Methods: Recombinant proteins of overlapping fragments of RBP1a were used to determine binding specificity to erythrocytes and immunogenicity in laboratory animals. The naturally acquired antibody response to these proteins was evaluated using serum samples from individuals in regions of endemicity. Results: The N-terminal extracellular region, RBP1157-650 (RBP1:F8), was determined to bind both reticulocytes and normocytes, with a preference for immature reticulocytes. Antibodies elicited against rRBP1:F8 blocked binding between RBP1:F8 and erythrocytes. Naturally acquired anti-RBP1 binding-inhibitory antibodies were detected in serum specimens from P. vivax-exposed individuals from Papua New Guinea and Brazil. Conclusion: Recombinant RBP1:F8 binds human erythrocytes, elicits artificially induced functional blocking antibodies, and is a target of naturally acquired binding-inhibitory antibodies.

Vaccine strategies for lowering LDL by immunization against proprotein convertase subtilisin/kexin type 9.

PURPOSE OF REVIEW: mAbs targeting proprotein convertase subtilisin/kexin type 9 (PCSK9) have the potential to become groundbreaking therapies for the treatment of hypercholesterolemia. However, one major drawback of mAb-based therapy for a chronic condition like dyslipidemia is its relatively high cost. This review summarizes two recent studies describing novel vaccine approaches for lowering LDL-cholesterol by active immunization against PCSK9. RECENT FINDINGS: PCSK9 is a plasma protein secreted by the liver that controls cholesterol homeostasis by enhancing endosomal and lysosomal degradation of the LDL receptor. Two PCSK9 inhibitory mAbs (evolocumab and alirocumab) have recently been approved by the Food and Drug Administration and a third mAb (bococizumab) is in late stage clinical trials. Treatment with PCSK9 mAbs, in combination with statins, reduces LDL-cholesterol levels by as much as 40-60%. As an alternative to mAbs, there have been two recent studies describing the development of vaccines that target PCSK9. These studies have shown that PCSK9 vaccines can effectively induce high-titer antibody responses that reduce proatherogenic lipoproteins in animal models. SUMMARY: A PCSK9 vaccine-based approach could serve as a more widely applicable and a more cost-effective approach than mAb therapy for controlling hypercholesteremia and associated cardiovascular disease.

Optimized Formulation of a Thermostable Spray-Dried Virus-Like Particle Vaccine against Human Papillomavirus.

Existing vaccines against human papillomavirus (HPV) require continuous cold-chain storage. Previously, we developed a bacteriophage virus-like particle (VLP)-based vaccine for HPV infection, which elicits broadly neutralizing antibodies against diverse HPV types. Here, we formulated these VLPs into a thermostable dry powder using a multicomponent excipient system and by optimizing the spray-drying parameters using a half-factorial design approach. Dry-powder VLPs were stable after spray drying and after long-term storage at elevated temperatures. Immunization of mice with a single dose of reconstituted dry-powder VLPs that were stored at 37 °C for more than a year elicited high anti-L2 IgG antibody titers. Spray-dried thermostable, broadly protective L2 bacteriophage VLPs vaccine could be accessible to remote regions of the world (where ∼84% of cervical cancer patients reside) by eliminating the cold-chain requirement during transportation and storage.

A vaccine against CCR5 protects a subset of macaques upon intravaginal challenge with simian immunodeficiency virus SIVmac251.

As an alternative to targeting human immunodeficiency virus (HIV), we have developed vaccines targeting CCR5, a self-protein critically involved in HIV replication and pathogenesis. By displaying peptides derived from CCR5 at high density on the surface of virus-like particles, we can efficiently induce high-titer IgG antibodies against this self-molecule. Here, we investigated whether prophylactic immunization of rhesus macaques with a particle-based vaccine targeting two regions of macaque CCR5 could prevent or suppress vaginal infection with highly virulent SIVmac251. Twelve macaques were vaccinated with a bacteriophage Qß-based vaccine targeting macaque CCR5 (Qß.CCR5). Six control animals were immunized with the Qß platform alone. All animals immunized with Qß.CCR5 developed high-titer anti-CCR5 antibody responses. Macaques were vaginally challenged with a high dose of SIVmac251. The mean peak viral RNA levels in the vaccinated groups were 30-fold lower than in the control group (10(6.8) versus 10(8.3) copies/ml plasma). Three of the 12 vaccinated macaques dramatically suppressed simian immunodeficiency virus (SIV) replication: peak viral loads were low (10(3) to 10(4) RNA copies/ml), and SIV RNA became undetectable from 6 weeks onward. No viral RNA or DNA could be detected in colon and lymph node biopsy specimens collected 13 months after challenge. In vivo depletion of CD8(+) cells failed to induce a viral rebound. However, once anti-CCR5 antibody responses had waned, the 3 animals became infected after intravaginal and/or intravenous rechallenge. In conclusion, vaccination against CCR5 was associated with dramatic suppression of virus replication in a subset (25%) of macaques. These data support further research of vaccination against CCR5 to combat HIV infection.

The use of hybrid virus-like particles to enhance the immunogenicity of a broadly protective HPV vaccine.

Virus-like particles (VLPs) can serve as a highly immunogenic vaccine platform for the multivalent display of epitopes from pathogens. We have used bacteriophage VLPs to develop vaccines that target a highly conserved epitope from the human papillomavirus (HPV) minor capsid protein, L2.VLPs displaying an L2-peptide from HPV16 elicit antibodies that broadly neutralize infection by HPV types associated with the development of cervical cancer. To broaden the cross-neutralization further, we have developed a strategy to display two different peptides on a single, hybrid VLP in a multivalent, highly immunogenic fashion. In general, hybrid VLPs elicited high-titer antibody responses against both targets, although in one case we observed an immunodominant response against only one of the displayed epitopes. Immunization with hybrid particles elicited antibodies that were able to neutralize heterologous HPV types at higher titers than those elicited by particles displaying one epitope alone, indicating that the hybrid VLP approach may be an effective technique to target epitopes that undergo antigenic variation.

A malaria vaccine candidate based on an epitope of the Plasmodium falciparum RH5 protein.

BACKGROUND: The Plasmodium falciparum protein RH5 is an adhesin molecule essential for parasite invasion of erythrocytes. Recent studies show that anti-PfRH5 sera have potent invasion-inhibiting activities, supporting the idea that the PfRH5 antigen could form the basis of a vaccine. Therefore, epitopes recognized by neutralizing anti-PfRH5 antibodies could themselves be effective vaccine immunogens if presented in a sufficiently immunogenic fashion. However, the exact regions within PfRH5 that are targets of this invasion-inhibitory activity have yet to be identified. METHODS: A battery of anti-RH5 monoclonal antibodies (mAbs) were produced and screened for their potency by inhibition of invasion assays in vitro. Using an anti-RH5 mAb that completely inhibited invasion as the selecting mAb, affinity-selection using random sequence peptide libraries displayed on virus-like particles of bacteriophage MS2 (MS2 VLPs) was performed. VLPs were sequenced to identify the specific peptide epitopes they encoded and used to raise specific antisera that was in turn tested for inhibition of invasion. RESULTS: Three anti-RH5 monoclonals (0.1 mg/mL) were able to inhibit invasion in vitro by >95%. Affinity-selection with one of these mAbs yielded a VLP which yielded a peptide whose sequence is identical to a portion of PfRH5 itself. The VLP displaying the peptide binds strongly to the antibody, and in immunized animals elicits an anti-PfRH5 antibody response. The resulting antisera against the specific VLP inhibit parasite invasion of erythrocytes more than 90% in vitro. CONCLUSIONS: Here, data is presented from an anti-PfRH5 mAb that completely inhibits erythrocyte invasion by parasites in vitro, one of the few anti-malarial monoclonal antibodies reported to date that completely inhibits invasion with such potency, adding to other studies that highlight the potential of PfRH5 as a vaccine antigen. The specific neutralization sensitive epitope within RH5 has been identified, and antibodies against this epitope also elicit high anti-invasion activity, suggesting this epitope could form the basis of an effective vaccine against malaria.

Virus-like particle (VLP)-based vaccine targeting tau phosphorylated at Ser396/Ser404 (PHF1) site outperforms phosphorylated S199/S202 (AT8) site in reducing tau pathology and restoring cognitive deficits in the rTg4510 mouse model of tauopathy.

Tauopathies, including Alzheimer's disease (AD) and Frontotemporal Dementia (FTD), are histopathologically defined by the aggregation of hyperphosphorylated pathological tau (pTau) as neurofibrillary tangles in the brain. Site-specific phosphorylation of tau occurs early in the disease process and correlates with progressive cognitive decline, thus serving as targetable pathological epitopes for immunotherapeutic development. Previously, we developed a vaccine (Qß-pT181) displaying phosphorylated Thr181 tau peptides on the surface of a Qß bacteriophage virus-like particle (VLP) that induced robust antibody responses, cleared pathological tau, and rescued memory deficits in a transgenic mouse model of tauopathy. Here we report the characterization and comparison of two additional Qß VLP-based vaccines targeting the dual phosphorylation sites Ser199/Ser202 (Qß-AT8) and Ser396/Ser404 (Qß-PHF1). Both Qß-AT8 and Qß-PHF1 vaccines elicited high-titer antibody responses against their pTau epitopes. However, only Qß-PHF1 rescued cognitive deficits, reduced soluble and insoluble pathological tau, and reactive microgliosis in a 4-month rTg4510 model of FTD. Both sera from Qß-AT8 and Qß-PHF1 vaccinated mice were specifically reactive to tau pathology in human AD post-mortem brain sections. These studies further support the use of VLP-based immunotherapies to target pTau in AD and related tauopathies and provide potential insight into the clinical efficacy of various pTau epitopes in the development of immunotherapeutics.

Virus-like particle (VLP)-based vaccine targeting tau phosphorylated at Ser396/Ser404 (PHF1) site outperforms phosphorylated S199/S202 (AT8) site in reducing tau pathology and restoring cognitive deficits in the rTg4510 mouse model of tauopathy.

Tauopathies, including Alzheimer's disease (AD) and Frontotemporal Dementia (FTD), are histopathologically defined by the aggregation of hyperphosphorylated pathological tau (pTau) as neurofibrillary tangles in the brain. Site-specific phosphorylation of tau occurs early in the disease process and correlates with progressive cognitive decline, thus serving as targetable pathological epitopes for immunotherapeutic development. Previously, we developed a vaccine (Qß-pT181) displaying phosphorylated Thr181 tau peptides on the surface of a Qß bacteriophage virus-like particle (VLP) that induced robust antibody responses, cleared pathological tau, and rescued memory deficits in a transgenic mouse model of tauopathy. Here we report the characterization and comparison of two additional Qß VLP-based vaccines targeting the dual phosphorylation sites Ser199/Ser202 (Qß-AT8) and Ser396/Ser404 (Qß-PHF1). Both Qß-AT8 and Qß-PHF1 vaccines elicited high-titer antibody responses against their pTau epitopes. However, only Qß-PHF1 rescued cognitive deficits, reduced soluble and insoluble pathological tau, and reactive microgliosis in a 4-month rTg4510 model of FTD. Both sera from Qß-AT8 and Qß-PHF1 vaccinated mice were specifically reactive to tau pathology in human AD post-mortem brain sections. These studies further support the use of VLP-based immunotherapies to target pTau in AD and related tauopathies and provide potential insight into the clinical efficacy of various pTau epitopes in the development of immunotherapeutics.

Minimal determinants for lifelong antiviral antibody responses in BALB/c mice from a single exposure to virus-like immunogens at low doses.

However, due to the complex compositions of natural virions, the molecular determinants of Ab durability from viral infection or inactivated viral vaccines have been incompletely understood. Here we used a reductionist system of liposome-based virus-like structures to examine the durability of Abs in primary immune responses in mice. This system allowed us to independently vary fundamental viral attributes and to do so without additional adjuvants to model natural viruses. We show that a single injection of antigens (Ags) orderly displayed on a virion-sized liposome is sufficient to induce a long-lived neutralizing Ab (nAb) response. Introduction of internal nucleic acids dramatically modulates the magnitude of long-term Ab responses without alteration of the long-term kinetic trends. These Abs are characterized by exceptionally slow off-rates of ~0.0005 s-1, which emerged as early as day 5 after injection and these off-rates are comparable to that of affinity-matured monoclonal Abs. A single injection of these structures at doses as low as 100 ng led to lifelong nAb production in BALB/c mice. Thus, a minimal virus-like immunogen can give rise to potent and long-lasting antiviral Abs in a primary response in mice without live infection. This has important implications for understanding both live viral infection and for optimized vaccine design.

An Approach for Antigen-Agnostic Identification of Virus-Like Particle-Displayed Epitopes that Engage Specific Antibody V Gene Regions.

Antibody complementarity determining regions (CDRs) participate in antigen recognition, but not all participate equally in antigen binding. Here we describe a technique for discovering strong, specific binding partners to defined motifs within the CDRs of chimeric, engineered antibodies using affinity selection and counter-selection of epitopes displayed on bacteriophage MS2-based virus-like particles (VLPs). As an example, we show how this technique can be used to identify families of VLPs that interact with antibodies displaying the CDRs encoded by the germline precursor of a broadly neutralizing monoclonal antibody against HIV-1.

Virus-like particle-based vaccines targeting the Anopheles mosquito salivary protein, TRIO.

Malaria is a highly lethal infectious disease caused by Plasmodium parasites. These parasites are transmitted to vertebrate hosts when mosquitoes of the Anopheles genus probe for a blood meal. Sporozoites, the infectious stage of Plasmodium , transit to the liver within hours of injection into the dermis. Vaccine efforts are hindered by the complexity of the parasite's lifecycle and the speed at which the infection is established in the liver. In an effort to enhance immunity against Plasmodium , we produced a virus-like particle (VLP)-based vaccine displaying an epitope of TRIO, an Anopheles salivary protein which has been shown to enhance mobility and dispersal of sporozoites in the dermis. Previous work demonstrated that passive immunization with TRIO offered protection from liver infection and acted synergistically with a Plasmodium targeted vaccine. Immunization of mice with TRIO VLPs resulted in high-titer and long-lasting antibody responses that did not significantly drop for over 18 months post-immunization. TRIO VLPs were similarly immunogenic when combined with an anti-malaria vaccine targeting the L9 epitope of the Plasmodium falciparum circumsporozoite protein.However, when used in a malaria challenge mouse model, TRIO VLPs only provided modest protection from infection and did not boost the protection provided by L9 VLPs.

Minimal Determinants for Lifelong Antiviral Antibody Responses in Mice from a Single Exposure to Virus-like Immunogens at Low Doses.

The durability of an antibody (Ab) response is highly important for antiviral vaccines. However, due to the complex compositions of natural virions, the molecular determinants of Ab durability from viral infection or inactivated viral vaccines have been incompletely understood. Here we used a reductionist system of liposome-based virus-like structures to examine the durability of Abs from primary immune responses in mice. This system allowed us to independently vary fundamental viral attributes and to do so without additional adjuvants to model natural viruses. We show that a single injection of protein antigens (Ags) orderly displayed on a virion-sized liposome is sufficient to induce a long-lived neutralizing Ab (nAb) response. The introduction of internal nucleic acids dramatically modulates the magnitude of Ab responses without an alteration of the long-term kinetic trends. These Abs are characterized by very slow off-rates of ~0.0005 s-1, which emerged as early as day 5 after injection and these off-rates are comparable to that of affinity-matured monoclonal Abs. A single injection of these structures at doses as low as 100 ng led to lifelong nAb production in mice. Thus, a minimal virus-like immunogen can give rise to potent and long-lasting antiviral Abs in a primary response in mice without live infection. This has important implications for understanding both live viral infection and for optimizing vaccine design.

An integrated signaling threshold initiates IgG response towards virus-like immunogens.

Class-switched neutralizing antibody (nAb) production is rapidly induced upon many viral infections. However, due to the presence of multiple components in typical virions, the precise biochemical and biophysical signals from viral infections that initiate nAb responses remain inadequately defined. Using a reductionist system of synthetic virus-like structures (SVLS) containing minimal, highly purified biochemical components commonly found in enveloped viruses, here we show that a foreign protein on a virion-sized liposome can serve as a stand-alone danger signal to initiate class-switched nAb responses in the absence of cognate T cell help or Toll-like receptor signaling but requires CD19, the antigen (Ag) coreceptor on B cells. Introduction of internal nucleic acids (iNAs) obviates the need for CD19, lowers the epitope density (ED) required to elicit the Ab response and transforms these structures into highly potent immunogens that rival conventional virus-like particles in their ability to elicit strong Ag-specific IgG. As early as day 5 after immunization, structures harbouring iNAs and decorated with just a few molecules of surface Ag at doses as low as 100 ng induced all IgG subclasses of Ab known in mice and reproduced the IgG2a/2c restriction that has been long observed in live viral infections. These findings reveal a shared mechanism for nAb response upon viral infection. High ED is capable but not necessary for driving Ab secretion in vivo . Instead, even a few molecules of surface Ag, when combined with nucleic acids within these structures, can trigger strong antiviral IgG production. As a result, the signaling threshold for the induction of neutralizing IgG is set by dual signals originating from both ED on the surface and the presence of iNAs within viral particulate immunogens. One-sentence summary: Reconstitution of minimal viral signals necessary to initiate antiviral IgG.

A bacteriophage virus-like particle vaccine against oxycodone elicits high-titer and long-lasting antibodies that sequester drug in the blood.

Opioid use disorder (OUD) and opioid overdoses are public health emergencies. In 2021, 80,000 opioid overdose associated deaths were reported in the United States. Despite the availability of treatment strategies, including medications for opioid use disorder (MOUD) and naloxone, opioid overdoses continue to increase at an alarming rate. Opioid vaccines are a novel approach to combat the growing crisis with several candidates recently entering human clinical trials. In this study, we investigated Qß bacteriophage virus-like particles (VLPs) as a vaccine platform for immunogenic display of oxycodone. A derivative of oxycodone was conjugated to pre-formed Qß VLPs using a sulfhydryl-amine reactive heterobifunctional crosslinker with high loading of oxycodone. In mice, intramuscular immunization with Qß-oxycodone elicited high-titer, high-avidity and long-lasting antibody responses. Qß-oxycodone was also immunogenic after storage at ambient room temperature for over two weeks, demonstrating that the vaccine is highly thermostable. In mice, immunization with Qß-oxycodone elicited antibodies that sequester oxycodone in the serum, an important mechanism for preventing the adverse effects of opioid activity. Finally, Qß-oxycodone is immunogenic in nonhuman primates, eliciting serum oxycodone antibodies after intramuscular immunization of rhesus macaques. These data establish Qß-oxycodone as a promising opioid vaccine candidate.

A two-dose regimen of Qß virus-like particle-based vaccines elicit protective antibodies against heroin and fentanyl.

Opioid overdoses and the growing rate of opioid use disorder (OUD) are major public health concerns, particularly in the United States. Current treatment approaches for OUD have failed to slow the growth of the opioid crisis. Opioid vaccines have shown pre-clinical success in targeting multiple different opioid drugs. However, the need for many immunizations can limit their clinical implementation. In this study, we investigate the development of novel opioid vaccines by independently targeting fentanyl and the active metabolites of heroin using a bacteriophage virus-like particle (VLP) vaccine platform. We establish the successful conjugation of haptens to bacteriophage Qß VLPs and demonstrate immunogenicity of Qß-fentanyl, Qß-morphine, and Qß-6-acetylmorphine in animal models after one or two immunizations. We show that in independently or in combination, these vaccines elicit high-titer, high-avidity, and durable antibody responses. Moreover, we reveal their protective capacities against heroin or fentanyl challenge after two immunizations. Overall, these findings establish Qß-VLP conjugated vaccines for heroin and fentanyl as very promising opioid vaccine candidates.

The Immunogenicity of a VLP-based Malaria Vaccine Targeting CSP in Pregnant and Neonatal Mice.

Maternal antibodies are passively transferred to the fetus via the placenta during gestation and can play an important role in protecting the newborn from infection. For example, in malaria-endemic regions, maternal antibodies likely provide substantial protection against Plasmodium falciparum malaria in the first 6 months of life. However, circulating maternal antibodies can also interfere with vaccine efficacy. Here, we used a mouse maternal transfer model to evaluate whether maternal antibodies interfere with the responsiveness to a virus-like particle (VLP)-based vaccine targeting the CIS43 epitope of the malaria circumsporozoite protein (CSP). We found immunized dams passively transfer to pups high levels of anti-CSP IgG antibodies that steadily decline as the animals age. We also found that the neonatal offspring of immunized mice do not respond to de novo immunization with the CIS43-targeted VLP vaccine until maternal antibody titers decline below an inhibitory threshold. These findings may have important implications for delineating the delicate balance between protection conferred by maternal antibodies and the offspring's ability to respond to immunization.