Allelic polymorphisms in the FcgammaRIIC gene can influence its function on normal human natural killer cells

Enhanced binding affinity for FcgammaRIIIa of fucose-negative antibody is sufficient to induce maximal antibody-dependent cellular cytotoxicity

Antibody-dependent cellular cytotoxicity (ADCC) is considered to be an important therapeutic function for clinical efficacy of monoclonal antibodies. Recent studies have revealed two methods to increase binding affinity for FcgammaRIIIa and enhance ADCC more efficiently for antibodies: (i) fucose removal from antibody N-linked complex oligosaccharides and (ii) amino acid mutations in the antibody Fc region. In this study, we compare the biological activities of the methods of generating high ADCC antibodies. We used a fucose-negative antibody and two antibodies with sets of mutations, demonstrated previously to optimally enhance ADCC using the chimeric anti-CD20 antibody, rituximab, as the model. Both amino acid mutant antibodies showed a significantly higher affinity for recombinant FcgammaRIIIa than fucose-negative antibody when compared using biosensor analysis. The removal of fucose from the antibodies bearing amino acid mutations exhibited a further enhancement of binding to recombinant FcgammaRIIIa and significantly increased binding to natural killer (NK) cells. Despite the differences manifested in binding for the FcgammaR, ADCCs were indistinguishable between methods and even when the methods were combined. These results indicate that the affinity of binding to FcgammaRIIIa does not predict ADCC beyond a certain threshold and that each method alone is sufficient to induce maximal ADCC of the antibody.

A Novel Antibody-Dependent Cellular Cytotoxicity Mechanism Involved in Defense against Malaria Requires Costimulation of Monocytes FcγRII and FcγRIII

Clinical experiments have shown that the Ab-dependent cell-mediated inhibition of Plasmodium falciparum is a major mechanism controlling malaria parasitemia and thereby symptoms. In this study, we demonstrate that a single merozoite per monocyte (MN) is sufficient to trigger optimal antiparasitic activity. Using particulate Ag as pseudomerozoites, we show that only Ags, and no other parasite-derived factor, are required to trigger MN activation and that a single Ag is as potent as the complex combination of Ags constituting the merozoite surface. Moreover, we found that soluble Ags binding at least two Abs are as effective as the parasite at stimulating MN and that nonmalarial Ags are as efficient provided they are targeted by cytophilic Abs. Indeed, only cytophilic IgGs are potent and, in agreement with immunoepidemiological findings, IgG3 is superior to IgG1. Very low Ab concentrations (>700 pM), i.e., in the range of molecules having a hormonal effect, are effective, in contrast to Abs having a direct, neutralizing effect. Finally, Ab-dependent cell-mediated inhibition proved to require the synergistic activation of both FcgammaRIIa and FcgammaRIIIa which both distinguish it from other Ab-dependent cellular cytotoxicity and implies that all MN are not equally effective. These findings have both fundamental and practical implications, particularly for vaccine discovery.

FcgammaRII and multi-system autoimmune disease

The FcR are a crucial link in the immune response between humoral and cellular immunity and cell-based effector systems, mediating a wide variety of physiological and biochemical responses. The FcR for IgG (FcgammaR) and in particular the most widely expressed of these, FcgammaRII, are important in regulating adaptive immunity. Disruption of their function is a key factor in the development of autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), which are characterized by chronic, multi-organ inflammation. Studies of the FcgammaRII include structure/function relationships, investigation of the associations between FcR polymorphisms and human disease and animal studies using knockout or transgenic mouse models. These investigations showed that the various forms of FcgammaRII interact with immune complexes to either initiate or inhibit inflammation. In conjunction with environmental antigens and genotype, the FcgammaRII activating and inhibitory receptors determine the nature and magnitude of response to antigens. In this review, the structure and function of the FcgammaRIIs and their role in immune complex-mediated auto-immunity are discussed.

Fc gamma RIIa, not Fc gamma RIIb, is constitutively and functionally expressed on skin-derived human mast cells

The expression of FcgammaR by human skin-derived mast cells of the MC(TC) type was determined in the current study. Expression of mRNA was analyzed with microarray gene chips and RT-PCR; protein by Western blotting and flow cytometry; function by release of beta-hexosaminidase, PGD(2), leukotriene C(4) (LTC(4)), IL-5, IL-6, IL-13, GM-CSF, and TNF-alpha. FcgammaRIIa was consistently detected along with FcepsilonRI at the mRNA and protein levels; FcgammaRIIc was sometimes detected only by RT-PCR; but FcgammaRIIb, FcgammaRI, and FcgammaRIII mRNA and protein were not detected. FcgammaRIIa-specific mAb caused skin MC(TC) cells to degranulate and secrete PGD(2), LTC(4), GM-CSF, IL-5, IL-6, IL-13, and TNF-alpha in a dose-dependent fashion. FcepsilonRI-specific mAb caused similar amounts of each mediator to be released with the exception of LTC(4), which was not released by this agonist. Simultaneous but independent cross-linking of FcepsilonRI and FcgammaRIIa did not substantially alter mediator release above or below levels observed with each agent alone. Skin MC(TC) cells sensitized with dust-mite-specific IgE and IgG, when coaggregated by Der p2, exhibited enhanced degranulation compared with sensitization with either IgE or IgG alone. These results extend the known capabilities of human skin mast cells to respond to IgG as well as IgE-mediated signals.

Role of antibody-dependent cell-mediated cytotoxicity in the efficacy of therapeutic anti-cancer monoclonal antibodies

In recent years, interest in anti-cancer therapeutic monoclonal antibodies (mAb) has been renewed. Several of these reagents have been approved for therapy in a variety of cancer patients and many others are in different stages of development. It is believed that multiple mechanisms are involved in the anti-cancer effects of these reagents. However, several in vitro and in vivo studies have demonstrated that antibody-dependent cell-mediated cytotoxicity (ADCC) is their predominant mode of action against cancer cells. The requirement for a direct interaction between mAb and receptors for the Fc region of the antibodies (FcR) has been demonstrated for anti-tumor effects of these antibodies. Consequently, FcR-bearing immune effector cells play an important role in mediating their effects. It is not surprising that cancer cells have developed different strategies to evade these antibodies. Several strategies are proposed to potentiate the mAb-mediated ADCC in cancer patients. They may enhance anti-cancer therapeutic effects of these regents.

Engineered antibody Fc variants with enhanced effector function

Antibody-dependent cell-mediated cytotoxicity, a key effector function for the clinical efficacy of monoclonal antibodies, is mediated primarily through a set of closely related Fcgamma receptors with both activating and inhibitory activities. By using computational design algorithms and high-throughput screening, we have engineered a series of Fc variants with optimized Fcgamma receptor affinity and specificity. The designed variants display >2 orders of magnitude enhancement of in vitro effector function, enable efficacy against cells expressing low levels of target antigen, and result in increased cytotoxicity in an in vivo preclinical model. Our engineered Fc regions offer a means for improving the next generation of therapeutic antibodies and have the potential to broaden the diversity of antigens that can be targeted for antibody-based tumor therapy.

Fc gamma R expression on NK cells influences disease severity in rheumatoid arthritis

Rheumatoid arthritis (RA) is associated with autoantibodies, the best known of which is rheumatoid factor (RF). RF/IgG complexes interact with FcgammaR on the surface of neutrophils, NK cells and monocyte/macrophages. We have analyzed the expression pattern and allelic polymorphisms of three FcgammaR genes (FcgammaRIIA, FcgammaRIIC and FcgammaRIIIA) in a large sample of RA patients and normal donors. We have found that the level of FcgammaR (CD16 and CD32) expression on NK cells is lower in RA patients than in normal individuals. Genotypic analysis demonstrated that the CD32 isoform expressed by the majority of RA patients was not the activating FcgammaRIIc1 isoform, commonly seen in normal individuals, but rather the inhibitory FcgammaRIIb isoform. The combination of the FcgammaRIIIA-176F allele with a lack of CD32 expression in NK cells appeared to be characteristic of RA subjects with aggressive disease. Since FcgammaRII and FcgammaRIIIA are predominantly expressed by NK cells, these data further suggest that FcgammaR-mediated activation of NK cells could be a disease-determining factor in RA patients.

Folate receptor-targeted immunotherapy: Induction of humoral and cellular immunity against hapten-decorated cancer cells

We previously exploited the frequent overexpression of folate receptors on cancer cells to decorate malignant cell surfaces selectively with folate-hapten conjugates. In antihapten-immunized hosts, this targeted localization of foreign haptens to tumor cells led to rapid accumulation of autologous antihapten IgG, which in turn yielded potent antitumor activity upon stimulation with cytokines (IL-2, IFN-alpha). In an effort to understand the effector mechanisms responsible for tumor regression, we have now investigated the involvement of both humoral and cellular immune components in the tumor destruction process. We report that the dependence of therapeutic efficacy on folate-hapten concentration is bimodal, suggesting that the conjugate must bridge between a cell surface FR and an antihapten IgG in order to mediate killing. Studies with cancer cells in vitro further demonstrate that folate-fluorescein-marked tumor cells are killed primarily by antibody-dependent cellular cytotoxicity and phagocytosis, with no contribution from complement-dependent mechanisms. Investigations of specific immune cell involvement also reveal that asialo-GM1(+)-natural killer cells, macrophages, CD4+ T cells and CD8+ T cells contribute significantly to recognition/removal of the cancer mass, and that elimination of these cell types markedly compromises the therapy. Because the initial antibody-dependent stage of tumor cell killing is shown to lead to a long-term antibody-independent cellular immunity that involves both CD4+ and CD8+ T cells, we propose that F(c) receptor-expressing immune cells not only initiate destruction of the IgG-marked tumor cells, but also participate in presentation of endogenous tumor antigens in a manner that leads to long-term cellular immunity.

FcgammaR polymorphisms: Implications for function, disease susceptibility and immunotherapy

Leukocyte Fcgamma receptors (FcgammaR) confer potent cellular effector functions to the specificity of IgG. FcgammaR-induced leukocyte functions, including antibody-dependent cellular cytotoxicity, phagocytosis, superoxide generation, degranulation, cytokine production and regulation of antibody production, are essential for host defense and immune regulation. The efficacy of IgG-induced FcgammaR function displays inter-individual heterogeneity due to genetic polymorphisms of three FcgammaR subclasses, FcgammaRIIa (CD32a), FcgammaRIIIa (CD16a), and FcgammaRIIIb (CD16b). FcgammaR polymorphisms have been associated with infectious and autoimmune disease, or with disease severity. FcgammaR polymorphisms may furthermore serve as markers for therapeutic efficacy and side-effects of treatment with monoclonal antibodies. In this review, FcgammaR function and the relevance of FcgammaR polymorphisms as prognostic markers for inflammatory disease and antibody-based immunotherapy are discussed.