Immuno-potentiating pathway of HBsAg-HBIG immunogenic complex visualized

The role of hepatitis B surface antibodies in HBV infection, disease and clearance

The clinical sequelae associated with chronic HBV infection is generally regarded as a consequence of an inadequate and inappropriate immune response to active viral replication, predominantly at the T-cell level. However, recent studies on hepatitis B surface antigen (HBsAg)-specific B cells and hepatitis B surface antibody (anti-HB) responses have identified their previously unrecognized role in the pathogenesis of chronic hepatitis B (CHB). These studies have also uncovered novel therapeutic approaches to more effectively target HBsAg loss and seroconversion, an important end point and regarded as a functional cure. Anti-HBs IgG has also been shown to have multiple direct acting antiviral roles with the Fab component directly blocking viral entry, and release while the Fc component has been linked to antibody dependent cellular cytotoxicity. Likewise, the HBsAg-specific B-cell dysfunctionality can be reversed providing new therapeutic opportunities to achieve functional cure in CHB.

Restoring, releasing or replacing adaptive immunity in chronic hepatitis B

Multiple new therapeutic approaches are currently being developed to achieve sustained, off-treatment suppression of HBV, a persistent hepatotropic infection that kills ~2,000 people a day. A fundamental therapeutic goal is the restoration of robust HBV-specific adaptive immune responses that are able to maintain prolonged immunosurveillance of residual infection. Here, we provide insight into key components of successful T cell and B cell responses to HBV, discussing the importance of different specificities and effector functions, local intrahepatic immunity and pathogenic potential. We focus on the parallels and interactions between T cell and B cell responses, highlighting emerging areas for future investigation. We review the potential for different immunotherapies in development to restore or release endogenous adaptive immunity by direct or indirect approaches, including limitations and risks. Finally, we consider an alternative HBV treatment strategy of replacing failed endogenous immunity with infusions of highly targeted T cells or antibodies.

From therapeutic antibodies to immune complex vaccines

In recent years, therapeutic monoclonal antibodies have made impressive progress, providing great benefit by successfully treating malignant and chronic inflammatory diseases. Monoclonal antibodies with broadly neutralizing effects against specific antigens, or that target specific immune regulators, manifest therapeutic effects via their Fab fragment specificities. Subsequently therapeutic efficacy is mediated mostly by interactions of the Fc fragments of the antibodies with their receptors (FcR) displayed on cells of the immune system. These interactions can trigger a series of immunoregulatory responses, involving both innate and adaptive immune systems and including cross-presentation of antigens, activation of CD₈⁺ T cells and CD₄⁺ T cells, phagocytosis, complement-mediated antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). The nature of the triggered effector functions of the antibodies is markedly affected by the glycosylation patterns of the Fc fragments. These can cause differences in the conformation of the heavy chains of antibodies, with resultant changes in antibody binding affinity and activation of the complement system. Studies of the Fc glycosylation profiles together with the associated Fc effector functions and FcR/CR interactions promoted interest and progress in engineering therapeutic antibodies. Furthermore, because antigen–antibody immune complexes (ICs) have shown similar actions, in addition to certain novel immunoregulatory mechanisms that also reshape immune responses, the properties of ICs are being explored in new approaches for prevention and therapy of diseases. In this review, both basic studies and experimental/clinical applications of ICs leading to the development of preventive and therapeutic vaccines are presented.

Modulation of Antibody Responses to the V1V2 and V3 Regions of HIV-1 Envelope by Immune Complex Vaccines

Prophylactic HIV vaccines must elicit antibodies (Abs) against the virus envelope glycoproteins (Env) to effectively prevent HIV infection. We investigated a vaccine platform that utilizes immune complexes made of Env proteins gp120 and monoclonal Abs (mAbs) against different gp120 epitopes. We previously observed alterations in V3 antigenicity upon formation of certain gp120/mAb complexes and demonstrated the ability of these complexes to modulate the elicitation of V3 Ab responses. However, the effects on the V1V2 domain, an important target for Abs that correlate with vaccine-induced protection against HIV, have not been studied, nor have immune complex vaccines made with non-B subtype Env. This study compared subtypes B (JRFL) and CRF_01.AE (A244) Env gp120 proteins in complex with selected gp120-specific mAbs. Allosteric and antigenic changes were detected on these immune complexes, indicating that gp120/mAb interaction induces alterations on the Env surface that may modify the Env immunogenic properties. To evaluate this idea, mice were immunized with gp120/mAb complexes or their uncomplexed gp120 counterparts. The overall serum IgG titers elicited against gp120 were comparable, but a marked skewing toward V1V2 or V3 was evident and dependent on the gp120 strain and the specificity of the mAb used to form the complexes. Compared with uncomplexed gp120_JRFL, gp120_JRFL complexed with CD4bs or V1V2 mAbs, but not with C2 or V3 mAbs, elicited V3 Abs of greater titers and breadth, and Abs more capable of neutralizing tier 1 virus. Epitope mapping revealed a shift to a more conserved site in the V3 crown. However, the complexes did not enhance V1V2 Ab response, and the elicited V1V2 Abs were not cross-reactive. This profile contrasts with Ab responses to gp120_A244/mAb complexes. Notably, gp120_A244/mAb complexes induced higher levels of V1V2 Abs with some cross-reactivity, while also stimulating weak or strain-specific V3 Abs. Sera from gp120_A244/mAb complex-immunized animals displayed no measurable virus neutralization but did mediate Ab-dependent cellular phagocytosis, albeit at levels similar to that induced by gp120_A244 alone. These data indicate the potential utility of immune complexes as vaccines to shape Ab responses toward or away from Env sites of interest.

Analysis of immunological mechanisms exerted by HBsAg-HBIG therapeutic vaccine combined with Adefovir in chronic hepatitis B patients

An HBsAg-HBIG therapeutic vaccine (Yeast-derived Immune Complexes, YIC) for chronic hepatitis B (CHB) patients has undergone a series of clinical trials. The HBeAg sero-conversion rate of YIC varied from 21.9% to 14% depending on the immunization protocols from 6 to 12 injections. To analyze the immunological mechanisms exerted by 6 injections of YIC, 44 CHB patients were separately immunized with YIC, alum as adjuvant control or normal saline as blank control, with add on of antiviral drug Adefovir in all groups. Kinetic increase in Th1 and Th2 cells CD4⁺ T cell sub-populations with association in decrease in Treg cells and increase of Tc1 and Tc17 cells in CD8⁺ T cells were observed in YIC immunized group. No such changes were found in the other groups. By multifunctional analysis of cytokine profiles, significant increase of IL-2 levels was observed, both in CD4⁺ and CD8⁺ T cells in the YIC immunized group, accompanied by increase in IFN-gamma and decrease of inhibitory factors (IL-10, TGF-β and Foxp3) in CD4⁺ T cells. In the alum immunized group, slight increase of IL-10, TGF-β and Foxp3 in CD4⁺ T cells was found after the second injection, but decreased after more injections, suggesting that alum induced early inflammatory responses to a certain extent. Similar patterns of responses of IL-17A and TNF-α in CD8⁺T cells were shown between YIC and the saline group. Results indicate that add on of Adefovir, did not affect host specific immune responses.

Immunoregulatory functions of immune complexes in vaccine and therapy

Clinical and experimental preparations of IgG/soluble antigen complexes, as well as those formed following antibody therapy in vivo, are multifaceted immune regulators. These immune complexes (ICs) have been tested in humans and animal models, mostly in forms of experimental or clinical vaccination, for at least a century. With intensified research on Fcγ receptor-mediated immune modulation, as well as with immune complex-directed antigen processing, presentation, and inflammatory responses, there are renewed interests of using ICs in vaccines and immunotherapies. Currently, IC-based immune therapy has been broadly experimented in HBV and HIV viral infection control and antitumor treatments. However, mechanistic insights of IC-based treatments are relatively recent subjects of study; strong efforts are needed to establish links to connect laboratory findings with clinical practices. This review covers the history, mechanisms, and in vivo outcomes of this safe and effective therapeutic tool, with a clear aim to bridge laboratory findings with evolving clinical applications.

Converting monoclonal antibody-based immunotherapies from passive to active: bringing immune complexes into play

Monoclonal antibodies (mAbs), which currently constitute the main class of biotherapeutics, are now recognized as major medical tools that are increasingly being considered to fight severe viral infections. Indeed, the number of antiviral mAbs developed in recent years has grown exponentially. Although their direct effects on viral blunting have been studied in detail, their potential immunomodulatory actions have been overlooked until recently. The ability of antiviral mAbs to modulate antiviral immune responses in infected organisms has recently been revealed. More specifically, upon recognition of their cognate antigens, mAbs form immune complexes (ICs) that can be recognized by the Fc receptors expressed on different immune cells of infected individuals. This binding may be followed by the modulation of the host immune responses. Harnessing this immunomodulatory property may facilitate improvements in the therapeutic potential of antiviral mAbs. This review focuses on the role of ICs formed with different viral determinants and mAbs in the induction of antiviral immune responses in the context of both passive immunotherapies and vaccination strategies. Potential deleterious effects of ICs on the host immune response are also discussed.