A general platform for targeting MHC-II antigens via a single loop.

Class-II major histocompatibility complexes (MHC-IIs) are central to the communications between CD4+ T cells and antigen presenting cells (APCs), but intrinsic structural features associated with MHC-II make it difficult to develop a general targeting system with high affinity and antigen specificity. Here, we introduce a protein platform, Targeted Recognition of Antigen-MHC Complex Reporter for MHC-II (TRACeR-II), to enable the rapid development of peptide-specific MHC-II binders. TRACeR-II has a small helical bundle scaffold and uses an unconventional mechanism to recognize antigens via a single loop. This unique antigen-recognition mechanism renders this platform highly versatile and amenable to direct structural modeling of the interactions with the antigen. We demonstrate that TRACeR-II binders can be rapidly evolved across multiple alleles, while computational protein design can produce specific binding sequences for a SARS-CoV-2 peptide of unknown complex structure. TRACeR-II sheds light on a simple and straightforward approach to address the MHC peptide targeting challenge, without relying on combinatorial selection on complementarity determining region (CDR) loops. It presents a promising basis for further exploration in immune response modulation as well as a broad range of theragnostic applications.

Molecular determinants of cross-reactivity and potency by VH3-33 antibodies against the Plasmodium falciparum circumsporozoite protein.

IGHV3-33-encoded antibodies are prevalent in the human humoral response against the Plasmodium falciparum circumsporozoite protein (PfCSP). Among VH3-33 antibodies, cross-reactivity between PfCSP major repeat (NANP), minor (NVDP), and junctional (NPDP) motifs is associated with high affinity and potent parasite inhibition. However, the molecular basis of antibody cross-reactivity and the relationship with efficacy remain unresolved. Here, we perform an extensive structure-function characterization of 12 VH3-33 anti-PfCSP monoclonal antibodies (mAbs) with varying degrees of cross-reactivity induced by immunization of mice expressing a human immunoglobulin gene repertoire. We identify residues in the antibody paratope that mediate cross-reactive binding and delineate four distinct epitope conformations induced by antibody binding, with one consistently associated with high protective efficacy and another that confers comparably potent inhibition of parasite liver invasion. Our data show a link between molecular features of cross-reactive VH3-33 mAb binding to PfCSP and mAb potency, relevant for the development of antibody-based interventions against malaria.

Modular adjuvant-free pan-HLA-DR-immunotargeting subunit vaccine against SARS-CoV-2 elicits broad sarbecovirus-neutralizing antibody responses.

Subunit vaccines typically require co-administration with an adjuvant to elicit protective immunity, adding development hurdles that can impede rapid pandemic responses. To circumvent the need for adjuvant in a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) subunit vaccine, we engineer a thermostable immunotargeting vaccine (ITV) that leverages the pan-HLA-DR monoclonal antibody 44H10 to deliver the viral spike protein receptor-binding domain (RBD) to antigen-presenting cells. X-ray crystallography shows that 44H10 binds to a conserved epitope on HLA-DR, providing the basis for its broad HLA-DR reactivity. Adjuvant-free ITV immunization in rabbits and ferrets induces robust anti-RBD antibody responses that neutralize SARS-CoV-2 variants of concern and protect recipients from SARS-CoV-2 challenge. We demonstrate that the modular nature of the ITV scaffold with respect to helper T cell epitopes and diverse RBD antigens facilitates broad sarbecovirus neutralization. Our findings support anti-HLA-DR immunotargeting as an effective means to induce strong antibody responses to subunit antigens without requiring an adjuvant.

B cell targeting by molecular adjuvants for enhanced immunogenicity.

INTRODUCTION: Adjuvants are critical components of vaccines to improve the quality and durability of immune responses. Molecular adjuvants are a specific subclass of adjuvants where ligands of known immune-modulatory receptors are directly fused to an antigen. Co-stimulation of the B cell receptor (BCR) and immune-modulatory receptors through this strategy can augment downstream signaling to improve antibody titers and/or potency, and survival in challenge models. AREAS COVERED: C3d has been the most extensively studied molecular adjuvant and shown to improve immune responses to a number of antigens. Similarly, tumor necrosis superfamily ligands, such as BAFF and APRIL, as well as CD40, CD180, and immune complex ligands can also improve humoral immunity as molecular adjuvants. EXPERT OPINION: However, no single strategy has emerged that improves immune outcomes in all contexts. Thus, systematic exploration of molecular adjuvants that target B cell receptors will be required to realize their full potential as next-generation vaccine technologies.

Vaccination against Clostridium difficile by Use of an Attenuated Salmonella enterica Serovar Typhimurium Vector (YS1646) Protects Mice from Lethal Challenge.

Clostridium difficile disease is mediated primarily by toxins A and B (TcdA and TcdB, respectively). The receptor binding domains (RBD) of TcdA and TcdB are immunogenic, and anti-RBD antibodies are protective. Since these toxins act locally, an optimal C. difficile vaccine would generate both systemic and mucosal responses. We have repurposed an attenuated Salmonella enterica serovar Typhimurium strain (YS1646) to produce such a vaccine. Plasmid-based candidates expressing either the TcdA or TcdB RBD were screened. Different vaccine routes and schedules were tested to achieve detectable serum and mucosal antibody titers in C57BL/6J mice. When given in a multimodality schedule over 1 week (intramuscularly and orally [p.o.] on day 0 and p.o. on days 2 and 4), several candidates provided 100% protection against lethal challenge. Substantial protection (82%) was achieved with combined p.o. TcdA and TcdB vaccination alone (days 0, 2, and 4). These data demonstrate the potential of the YS1646-based vaccines for C. difficile and strongly support their further development.