From therapeutic antibodies to immune complex vaccines | npj Vaccines

Antigen–antibody ICs can either cause immune pathological effects or potentiate beneficial immune effects, depending on various factors, including the subclasses of the antibody, the ratio between the antigen and the antibody forming the IC, the biological characteristics of the IC components, the sites where the ICs were formed, the cells involved and how the ICs were introduced into hosts etc. Table 1 shows comparisons between ICs causing pathological outcomes versus ICs inducing immune regulatory effects.

Table 1 Comparison of ICs exerting immune regulatory versus immune pathological functions

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With the evolving progress of using therapeutic antibodies or immunoglobulins for treatment, in recent years, the immunopathological effects of ICs alongside with their therapeutic efficacies have been studied in depth. The conventional concept of IC-mediated immunological pathogenesis has been that, when ICs were not cleared by phagocytosis system, they remained in blood circulation and deposited on small vessel walls of various organs. These deposited ICs could exert damaging effects by binding to complement receptors on innate immune effector cells and result in inflammation and tissue injury. However, through studies on soluble ICs and their effects, it was observed that the fate of ICs in blood circulation is either to initiate immunopathological outcomes, or to react with receptors on immune cells initiating immunological regulations. Decreased binding of ICs to Fc receptors could affect biological outcomes. In a study to analyze elements involved in ICs binding to Fc receptors, the size of IC, IgG subclasses, glycosylation of IgG, all were found of relevance.45 Mechanistic study of the pathological injuries in arthritis patients and IC-induced nephritis revealed that binding of ICs to FcγRI (CD64) contributed to the severity of arthritis and hypersensitivity responses.46 In lupus nephritis, intra-capillary IC deposits selectively accumulated a proinflammatory population of 6-sulfo LacNAc+ (slan) monocytes (slanMo), which locally expressed TNF-α.47 The recruitment of slanMo from the microcirculation was via interaction with Fc γ receptor IIIA (CD16) and the slanMo then induced the production of neutrophil-attracting chemokine CXCL2, as well as TNF-α.

In microbial infections, more pathogenic mechanisms have been described. When ICs formed between non-neutralizing IgG and microorganisms that can replicate in macrophages, increased intracellular infections can occur and this was named intrinsic antibody-dependent enhancement (ADE) of infections.48 This ADE of infection modulates the severity of diseases such as dengue hemorrhagic fever and leishmaniasis. Intrinsic ADE is distinct from extrinsic ADE, because intrinsic ADE leads to an increased number of infected cells.48 The mechanism manifests as suppression of host innate immunity through idiosyncratic Fcγ, increased production of IL-10, a bias of Th1 responses towards Th2 responses and increased numbers of infected cells.

Recently, another new immune inhibitory mechanism of ICs was reported in a mouse model of persistent lymphocytic choriomeningitis virus (LCMV) infection.49 The increased amounts of IC in the circulation during persistent infections, competed with FcγR binding and suppressed multiple aspects of FcγRs-dependent responses in vivo. The FcγR-mediated processes that were suppressed in vivo included activation of innate cells such as NK cells. By using transgenic mice expressing human CD20 and chronically infected with LCMV, virus antibody IC in circulation was shown to hamper the depletion of B cells by an anti-CD20 antibody (rituximab), a drug for treatment of B-cell lymphoma. In addition, FcR-dependent activation of dendritic cells by agonistic ant-CD40 antibody was decreased by the persistence of IC in these mice.50 Though these findings are not directly associated with IC pathogenicity, the data suggest that ICs could limit the effectiveness of therapeutic antibodies in humans.

Consistent with the role of ICs as a double-edged sword, ICs have shown immune regulatory functions that potentiate or restore favorable immune responses. Long before the discovery of Fc receptors, Terres et al. observed that when antibody was combined with its antigen at an appropriate ratio, IC could enhance antibody response in animals.51 Later, the potentiating effects of IC were shown with structural protein and antibody to paramyxovirus Simian virus 5,52 with hepatitis B surface antigen (HBsAg) and its antibodies53 and with antibody to HIV in an in vitro study with peptides.54

Following the discovery of Fcγ receptors, mechanistic studies on how ICs potentiate immune responses progressed with focus on IC–cell interactions. Hamano et al. showed that the efficient priming of Th cell responses by APCs in vivo was IC dependent.55 In cancer antitumor vaccine studies, IC-loaded dendritic cells (DCs) were found superior to soluble ICs.56 Circulating antibodies were shown to enhance systemic cross priming by delivery of antigens to DCs57 and ICs not only induced DC maturation in vitro, but also enabled DCs to prime peptide-specific CD8+ CTLs in vivo.58 These dual roles in enhancement of Ag uptake and activation of DCs and in priming of CD8+ CTL responses to exogenous antigens, resulted in a “license to kill” function. In experimental studies, formation of complexes of cellular antigen with antibody resulted in activation of dendritic cells, facilitation of cross-presentation of antigens to tumor-specific CD8+ T cells and inhibition of tumor growth Fc receptor-targeted antigen uptake was shown to initiate cross-presentation pathways as an immune regulatory mechanism for effective tumor immunity. In consequence, Fc receptor targeting was considered in tumor vaccine development.59 ICs potentiation effect on B cells was shown to be mediated by ICs retention in follicular dendritic cells (FDCs), and reappearance on the cell surface, thereby becoming available to B cells.60,61

Immune regulatory effects of ICs have been shown to restore effective immune responses against infections. In a mouse model of persistent infection, IL-2/anti-IL-2 (IL-2 IC) was shown to increase the numbers of virus-specific CD8+ T cells and enhance cytotoxicity mediated by the perforin–granzyme pathway.62 Optimized adenovirus–antibody complexes were shown to stimulate strong cellular and humoral immune responses via a significantly extended duration of antigen availability and enhanced lymphocyte activation kinetics. Formation of IC with antibody and rabies virus G antigen on cell surface redirected the native intracellular pathway,63 suggesting that some new immunoregulating mechanisms might be generated by viral ICs in cells. Furthermore, study of the functions of dendritic cells carrying IC showed prolonged presentation of antigen. This effect was virus specific and was dependent on a switch of antibody isotypes.64

The use of new technologies has enabled new progress on immune regulatory functions associated with glycosylation profiles of IgG and detailed studies on IC-induced immunoregulatory pathways. The uptake of IC after ligation activated FcγR on DC, leading to 100 times more antigen presentation than uptake of free antigen. The activated FcγRs elicited signaling via the ITAM domain of the FcγR chain.65 Splenic DCs from NOTAM mice were used to identify the role of ITAM domain signaling in cross-presentation of soluble IC by DCs. Results showed that signaling by the ITAM domain of FcγR chain was critically required for IC presentation, but not for MHC class II antigen presentation.65 In a study to reveal the immunological mechanisms leading to the development of HIV broadly neutralizing antibodies, differences in IC biology in a group of spontaneous controllers of HIV (≤2000 copies/ml) were identified in comparison with normal progressors. Polyclonal ICs and monoclonal IC from neutralizers were more effective than those from progressors in inducing higher antibody titers, higher-avidity antibodies, and expanded DC–B-cell reactions after immunization of mice.66 The results implicated altered Fc profile/complement interactions exerted differentially shaping the maturation of the humoral immune response. It was speculated that the enhanced Fc functions could actively contribute to the evolution of a broader HIV-specific neutralization range.66

In addition to their immune regulatory functions, ICs can effectively inhibit inflammatory responses. ICs were shown to inhibit the adaptive immune responses in an NLRP3-dependent model during priming of immune responses in vivo,67 suppression of both inflammasome activation and the generation of IL-1 alpha and IL-1 beta from antigen-presenting cells were observed. Recently, IL-2/IL-2 antibody IC was found to regulate HSV-induced inflammation through induction of IL-2 receptors alpha, beta, and gamma in a mouse model.68 The anti-inflammatory function has been widely employed in therapeutics for various diseases. A favored approach has been to use ICs in combination with cytokines and their antibodies. IL-2 complex treatment expanded both the NK and CD8+ T memory cell pool, including preexisting memory-phenotype T cells. In a renal ischemia–reperfusion injury (IRI) mouse model, IL-2 IC reduced expression of inflammatory cytokines and attenuated the infiltration of neutrophils and macrophages in renal tissue.69 IL-2 IC treatment has also been studied in experimental renal cancer.69 In experimental atherosclerosis, IL-2 IC in combination with anti-CD3 antibody markedly reduced atherosclerosis lesions.70 This effect was accompanied by a striking increase in the Treg/Teff ratio in the T cells in lymphoid organs and atherosclerotic lesions. Naive mice treated with a short course of IL-2 complexes showed enhanced protection from newly encountered bacterial and viral infections.71 However, increased IL-2 complex treatment generated CD8+ T cells and NK cells with a reduced capacity to produce IFN-γ, potentially suggesting some form of exhaustion occurred. Figure 1 summarizes the various known immunological functions of ICs (Fig. 1).

Fig. 1

The immune regulatory functions of immune complex (IC) in therapy and vaccine. Summary of the major functions and mechanisms of ICs, showing the immune pathological effects and immune regulatory effects. The blue “Y” shape figure represents antibody, and the red round represents antigen. Immune pathological effects (a inflammation, tissue injury; b antibody-dependent enhancement; c suppression of FcγR-dependent antibody functions) and immune regulatory effects (d T-cell response enhancement; e antibody response enhancement; f inhibition of inflammatory responses)

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