Conversion of vaccines from low to high immunogenicity by antibodies with epitope complementarity

Intrinsic immunogenicity is a major determinant of type-specific responses in SARS-CoV-2 infections

Few type-specific antibodies that recognize drifted epitopes are made during post-vaccination exposures to SARS-CoV-2 variants1-12, perhaps due to suppression by previous immunity. We compared type-specific B cell responses in unvaccinated and vaccinated individuals with Delta and Omicron BA.1 SARS-CoV-2 variant infections. For both Delta, which is antigenically similar to the vaccine strain, and the more distant BA.1 variant, neutralizing antibodies were greater in post-vaccination variant infections than in primary variant infections. Delta type-specific memory B cells were reduced in post-vaccination Delta infections relative to primary variant infections. Yet some drifted epitopes in the Delta variant elicited minimal responses even in primary infections. For BA.1 infections, type-specific antibodies and memory B cells were mostly undetectable, irrespective of previous immunity. Thus, poor intrinsic antigenicity of drifted epitopes in Delta and BA.1 infections superseded the impact of previous immunity. Enhancing the immunogenicity of vaccine antigens may promote type-specific responses.

Highlights of 2024: Advances in Germinal Centers

In this article for the Highlights of 2024 series, we review the latest advances in the biology of the germinal center response. These discoveries provide key insights into germinal center function and dysregulation, uncovering new opportunities for the development of more effective vaccines.

Syndecans and glycosaminoglycans influence B-cell development and activation

Syndecans (SDCs) are glycosaminoglycan-containing cell surface proteins with diverse functions in the immune system with SDC1 (CD138) and SDC4 expressed in B-lineage cells. Here, we show that stem cells lacking either molecule generate fewer B-cell progenitors but give rise to mature B cells in vivo. Deletion of the plasma cell "marker" CD138 has no effect on homeostatic or antigen-induced plasma cell formation. Naive B cells express high SDC4 and encounter with cognate antigen results in transient CD138 upregulation and SDC4 loss, both further modulated by IL-4, IL-21, and CD40 ligation. SDC4 is downregulated on germinal center B cells and absent on most memory B cells. Glycosaminoglycans such as those attached to SDCs, and heparin, a commonly used therapeutic, regulate survival and activation of naive B cells by limiting responsiveness to cognate antigen. Conversely, ablation of SDC4 results in increased baseline and antigen-induced B-cell activation. Collectively, our data reveal B-cell activation- and subset-dependent SDC expression and show that SDC4 and GAGs can limit antigen-induced activation to promote B-cell survival and expansion.

Mimicking immune complexes for efficient antibody responses

Efficient antibody responses are crucial for combating infectious diseases and vaccination remains a cornerstone of this effort. This study introduces a novel approach for enhancing immune responses in wild-type mice by utilizing pre-formed immune complexes, using the receptor-binding domain (RBD) of SARS-CoV-2 as a model antigen to illustrate the broader potential of the concept. Specifically, we found that pre-treating the antigen with bis-maleimide, a chemical linker that facilitates protein cross-linking, significantly enhances antibody production. Moreover, in vitro cross-linking of antigen to unrelated IgG using bis-maleimide generated immune complexes that markedly enhanced antigen-specific antibody responses, likely by mimicking natural memory-like mechanisms, suggesting that bis-maleimide pre-treated antigens may similarly engage IgG in vivo. In contrast, antigen crosslinking with IgA or IgM did not yield comparable effects, highlighting the unique capacity of IgG to boost immunogenicity. By leveraging the principles of immune memory, this study demonstrates the potential of pre-formed immune complexes to significantly enhance vaccine efficacy using an antigen-independent strategy broadly applicable to diverse pathogens.

Diverse priming outcomes under conditions of very rare precursor B cells

Rare naive B cells have special pathogen-recognition features that enable outsized contributions to protective immunity but infrequently participate in immune responses. We investigatee how germline-targeting vaccine delivery and adjuvant selection affect priming of exceptionally rare BG18-like HIV broadly neutralizing antibody-precursor B cells (<1-in-50 million) in non-human primates. Only escalating dose (ED) priming immunization using the saponin adjuvant SMNP elicited detectable BG18-like cells in germinal centers (GCs) compared with other conditions. All groups had strong GC responses, but only ED+SMNP and bolus+SMNP induced BG18-like memory B cells in >50% of animals. One group had vaccine-specific GC responses equivalent to ED+SMNP but scarce BG18-like B cells. Following homologous boosting, BG18-like memory B cells were present in a bolus priming group but with lower somatic hypermutation and affinities than ED+SMNP. This outcome inversely associated with post-prime antibody titers, suggesting antibody feedback significantly influences rare precursor B cell responses. Thus, antigen and inflammatory stimuli extensively impact priming and affinity maturation of rare B cells.

Porcine antibodies reveal novel non-neutralizing universal epitopes on FMDV and their overlaps with neutralization sites

Foot-and-mouth disease virus (FMDV) is highly infectious and lacks cross-protection among serotypes, with antibodies playing a key role in antiviral immunity. To map conserved epitopes on the FMDV surface that exhibit cross-serotype reactivity, we constructed a pig-specific B-cell receptor (BCR) library through single B-cell sorting and high-throughput sequencing. This led to the identification of 16 broadly reactive, non-neutralizing monoclonal antibodies (mAbs), with 10 targeting VP2 (pOTB-1, pOTB-10, pOTB-13, pOTB-33, pOTB-37, pONY-14, pONY-17, pONY-23, pONY-30, pONY-60) and 6 targeting VP3 (pOTB-6, pOTB-11, pOTB-22, pOTB-23, pONY-3, pONY-59). Among these, a novel free linear epitope was identified at the C-terminus of VP2, recognized by pOTB-1, with the minimal recognition motif "KE." Key residues, T53 and W101, within the complementarity-determining region (CDR) of the pOTB-1 heavy chain, interact with the carboxyl group of the C-terminal glutamate through hydrogen bonding, contributing to the free-form nature of the epitope. Competitive enzyme-linked immunosorbent assays (cELISA) showed that most non-neutralizing antibodies (nNAbs) interfered with the binding of neutralizing antibodies B82 (site 2) and C4 (site 4), confirming the overlap between non-neutralizing and neutralizing epitopes. It has been confirmed that nNAbs mediate antiviral activity in vivo through various mechanisms, such as the formation of immune complexes. These findings reveal new epitopes on VP2 and VP3 and their spatial overlap with neutralizing sites, enhancing our understanding of FMDV immunogenicity and providing novel targets for vaccine and therapeutic development.

Divalent HIV-1 gp120 Immunogen Exhibits Selective Avidity for Broadly Neutralizing Antibody VRC01 Precursors

A major goal for the vaccine field is elicitation of broadly neutralizing antibodies (bnAbs) against pathogens that exhibit extensive antigenic diversity. In this study, we designed a rigid divalent immunogen for high avidity binding to the bnAb, VRC01, which targets the CD4 binding site (CD4bs) of HIV spike protein. This was accomplished by covalently linking two HIV-1 gp120 antigens to a complementary antibody and crosslinking the light chains. The divalent immunogen exhibits a higher affinity for VRC01-class antibodies compared to a non-Fab-Fab-crosslinked control, likely due to antigen pre-organization limiting the entropic penalty for divalent binding. Importantly, this immunogen exhibited divalent binding to VRC01 and monovalent binding to a non-CD4bs Ab, A32 - a characteristic we refer to as "selective avidity." This report supports future in vivo vaccination experiments to test the immune focusing properties of this immunogen, the results of which may suggest broad application of the selective avidity concept.

Highlights

We designed a rigid divalent immunogen containing two copies of gp120 antigenThe gp120s are positioned to bind divalently to both Fabs of a target B cell receptorThe immunogen binds monovalently to non-target B cell receptorsThis "selective avidity" effect may be used for immune focusing.

Mechanism for evolution of diverse autologous antibodies upon broadly neutralizing antibody therapy of people with HIV

Antiretroviral therapy (ART) inhibits Human Immunodeficiency Virus (HIV) replication to maintain undetectable viral loads in people living with HIV, but does not result in a cure. Due to the significant challenges of lifelong ART for many, there is strong interest in therapeutic strategies that result in cure. Recent clinical trials have shown that administration of broadly neutralizing antibodies (bnAbs) when there is some viremia can lead to ART-free viral control in some people; however, the underlying mechanisms are unclear. Our computational modeling shows that bnAbs administered in the presence of some viremia promote the evolution of autologous antibodies (aAbs) that target diverse epitopes of HIV spike proteins. This "net" of polyclonal aAbs could confer control since evasion of this response would require developing mutations in multiple epitopes. Our results provide a common mechanistic framework underlying recent clinical observations upon bnAb/ART therapy, and they should also motivate and inform new trials.

Antigen affinity and site of immunization dictate B cell recall responses

Protective antibodies against HIV-1 require unusually high levels of somatic mutations introduced in germinal centers (GCs). To achieve this, a sequential vaccination approach was proposed. Using HIV-1 antibody knockin mice with fate-mapping genes, we examined if antigen affinity affects the outcome of B cell recall responses. Compared to a high-affinity boost, a low-affinity boost resulted in decreased numbers of memory-derived B cells in secondary GCs but with higher average levels of somatic mutations, indicating an affinity threshold for memory B cells to enter GCs. Furthermore, upon boosting local lymph nodes (LNs), the composition of primary GCs was modified in an antigen-affinity-dependent manner to constitute less somatically mutated B cells. Our results demonstrate that antigen affinity and location of the boost affect the outcome of the B cell recall response. These results can help guide the design of vaccine immunogens aiming to selectively engage specific B cell clones for further diversification.

Diverse priming outcomes under conditions of very rare precursor B cells

Rare B cells can have special pathogen-recognition features giving them the potential to make outsized contributions to protective immunity. However, rare naive B cells infrequently participate in immune responses. We investigated how germline-targeting vaccine antigen delivery and adjuvant selection affect priming of exceptionally rare BG18-like HIV broadly neutralizing antibody-precursor B cells (~1 in 50 million) in non-human primates. Only escalating dose (ED) priming immunization using the saponin adjuvant SMNP elicited detectable BG18-like cells in germinal centers (GCs). All groups had strong GC responses, but only ED+SMNP and bolus+SMNP induced BG18-like memory B cells in >50% of animals. One group had vaccine-specific GC responses equivalent to ED+SMNP, but BG18-like memory B cells were rarely detected. Following homologous boosting, BG18-like memory B cells were more frequent in a bolus priming group, but had lower somatic hypermutation and affinities. This outcome was inversely associated with post-prime antibody titers, suggesting antibody feedback can significantly influence rare precursor B cell responses.