Antibody-mediated immunity against viral respiratory infections

Summary

Respiratory syncytial virus (RSV) and influenza A virus (IAV) are two leading causes of lower respiratory tract infections in infants. Despite many years of research, a safe and effective vaccine against RSV is still lacking. The only protective drug against RSV is the monoclonal IgG antibody palivizumab, which is costly and only partly effective. For IAV, seasonal vaccines are available, however they need to be modified annually to match circulating IAV strains and their efficacy is highly variable. Therefore, more robust, effective and lower cost interventions are needed against RSV and IAV. This thesis focuses on antibody-mediated immunity against RSV and IAV and investigates the potential of recombinant human IgA antibodies as alternative to IgG antibodies for antiviral therapy.

Chapter 1 gives a general introduction into the field of antibody research and monoclonal antibody (mAb) therapy. In humans, five different antibody isotypes have been described: IgM, IgD, IgG, IgE and IgA. These antibodies (aka, immunoglobulins) can bind to their target using their variable (Fab) domain, which determines the antigen specificity. With their constant (Fc) domain, which determines the isotype, the antibodies can bind to Fc receptors on the surface of immune cells and initiate a wide range of cellular activities, e.g. phagocytosis and killing of the target cells. IgG antibodies have been extensively studied and only antibodies of this isotype have thus far been approved for therapy in humans, e.g. for prevention or treatment of infectious disease, inflammatory disease, autoimmune disease and cancer. However, IgA antibodies are the first line of defense against mucosal pathogens and may be a potential alternative to IgG antibodies for the therapy of viral respiratory infections. This chapter describes the structure and function of human IgA antibodies and provides general knowledge on RSV and IAV.

Chapter 2 focusses in more detail on Fc receptors, their expression, distribution and function. In humans, five mayor leucocyte Fc receptor classes are known: FcR for IgG, FcR for IgA, FcR for IgE, FcR for IgM and FcR for IgD. These receptors are expressed on different immune cells at different levels. Consequently, different cellular responses are induced by antibodies depending on the immune cells they engage. This chapter zooms into the genetic level and describes different Fc receptor genes and polymorphisms in these genes that affect receptor affinity and, thereby, can affect mAb therapy.

Chapter 3 extends our review of Fc receptors in the previous chapter by focusing on Fc receptor activation and the potential impact of this on mAb therapy. Three Fc receptors, FcRI, FcRIIa and FcRI, have been implicated previously in a process referred to as inside-out activation. This process is initiated by cytokine stimulation and involves the rapid increase in Fc receptor ligand binding capacity without changing receptor expression levels. This chapter thoroughly reviews available literature on Fc receptor modulation by different cytokines and addresses how this might affect mAb therapy.

Chapter 4 describes the specificity of IgG antibodies from cow’s milk for human respiratory pathogens and FcRs. The first milk of cows, the colostrum, contains high levels of IgG antibodies to protect the newborn calf against infections. Cow’s milk infant formulas are the alternative to breast milk for human infants, however, they lack intact antibodies because of heat treatment. Besides, it’s unknown whether colostrum IgG can protect against human respiratory pathogens. Therefore, we investigated whether intact IgG antibodies from colostrum would be a useful ingredient for infant formulas; we studied their ability to bind human respiratory pathogens and their ability to induce human FcR-dependent cellular functions. Our data show that colostrum IgG binds to human RSV and (components of) other human respiratory pathogens and induces cellular activities with human myeloid immune cells by binding to FcRII.

Chapter 5 describes a new mechanism to explain how healthy term infants are protected against RSV infection during neonatal age. RSV infections are very frequent in infants younger than six months, but very uncommon in the first month after birth. This has been attributed to the presence of maternal RSV-neutralizing antibodies in the serum of infants, however, most RSV infections occur at a time when serum RSV-neutralizing antibodies are still well above protective levels. Human amniotic fluid contains maternal antibodies and is in direct contact with the respiratory tract of the fetus, however, it’s unknown whether these antibodies can protect against RSV infection. Therefore, we investigated the ability of human amniotic fluid antibodies to protect against RSV infection and determined how long these antibodies may persist in the lungs. Our data strongly suggest that specific amniotic fluid antibodies contribute to protection of the neonate against life-threatening RSV infection during the first weeks of life.

The next two chapters focus on IgA antibodies as an alternative to IgG antibodies against viral respiratory infections. IgA is the most abundant antibody isotype produced in humans and the most prevalent antibody at mucosal surfaces. Human IgA comprises two subclasses, IgA1 and IgA2, which are present in mainly three forms, monomeric, dimeric and secretory IgA. In its secretory form, IgA acts as the first-line of defense against mucosal pathogens. In contrast to IgG, the prophylactic and therapeutic potential of recombinant human IgA antibodies is largely unexplored. The following two chapters compare for the first time the antiviral potential of different human IgA forms that have identical antigen specificity.
In Chapter 6, we investigated the potential of human IgA antibodies to prevent RSV infection in vitro and in vivo. We show that intranasal IgA prophylaxis significantly reduces the RSV load in the lungs after infection and that different forms of human IgA are effective against RSV infection in mice.

In Chapter 7, we investigated the therapeutic potential of human IgA antibodies against IAV infection in mice. We show that intranasal IgA therapy reduces the bodyweight loss after infection and that the effectiveness of IgA anti-IAV is enhanced in human FcRI transgenic mice compared to wildtype mice (which lack the FcRI for IgA).

In the final chapter, Chapter 8, the main findings described in this thesis are discussed in a broader perspective. New insights into the development of RSV and IAV interventions are discussed as well as the challenges and opportunities for IgA mAb therapy in humans. 

Conclusions
In this thesis, we investigated antibody-mediated protection against viral respiratory infections. We demonstrated that cow’s milk and human amniotic fluid contain naturally acquired virus-specific antibodies that provide protection against infection with human RSV, and presumably also IAV. The presence of protective antibodies in these mucosal secretions suggests that local delivery of neutralizing antibodies in mucosal compartments, e.g. intranasal administration of antibodies to the respiratory tract, is an effective way to prevent mucosal infections. Furthermore, we demonstrate in mouse models of virus infection that recombinant human IgA antibodies of the monomeric, dimeric and secretory forms are effective candidates for mAb therapy of IAV and prophylaxis against RSV infection, particularly when administered intranasally. Therefore, intranasal IgA mAb therapy should be considered for prophylaxis against RSV and for treatment of IAV infection in humans.