The Development and Mechanisms of Protective Humoral Immunity to HIV
A successful HIV-1 vaccine is likely to be comprised of a polyclonal response with multiple antiviral functions that generate both broadly neutralizing antibodies (bNAbs) that neutralize multiple strains of HIV and/ or functional non-neutralizing antibodies that participate in numerous antiviral mechanisms. It is suggested that poly-/autoreactivity is a feature of bNabs but the interplay of autoimmunity and bNab development is unknown. Furthermore the non-neutralizing antibody specificities and functions are unknown for generating a potentially protective anti-HIV response. Understanding the development of rare neutralizing antibodies and the specificity and effector functions of non-neutralizing antibodies is critical for generating a preventative HIV vaccine. This thesis focuses on 1) the development of broadly neutralizing antibodies; specifically those that are present in both the face of autoimmunity when both self-antigen and HIV-1 is a determinant in disease outcome and 2) the mechanisms of non-neutralizing antibodies in mediating protection against HIV-1.
In the first part of this thesis, I chart the evolution of an HIV-1 antibody lineage that has reactivity to self antigen. One HIV-1 broadly neutralizing antibody targeting the membrane proximal external region, 2F5, shares sequence identity with the human self-antigen kynureninase, (KYNU). We previously reported that an HIV-1 chronically infected patient had both evidence of antibodies targeting self-proteins and circulating 2F5-like antibodies in serum. Whether development of 2F5-like antibodies to a self-antigen can lead to a broadly neutralizing antibody response or whether reactivity to self-antigen is a requirement for the neutralization breadth of 2F5-like antibodies has been a question in the HIV vaccine community. Evolution of 2F5 mAb-like antibody neutralization breadth and autoreactivity has not been previously demonstrated due to the scarcity of this specificity of antibody found circulating in HIV-1 infected individuals. In this study, I found that an unmutated common ancestor derived from a isolated mAb from this patient did not bind KYNU but more mature antibodies in the constructed antibody lineage developed reactivity to KYNU and other self-antigens due to an accumulation of antibody VH mutations. At the same time, the accumulation of mutations in the m66 lineage VH resulted in decreased 2F5 mAb binding affinity and virus neutralization potency, consistent with host mechanisms that limit antibody autoreactivity by placing constraints on affinity maturation. This work highlights the complex maturation pathways involving autoantigen reactivity in the development of broadly neutralizing antibodies.
The second part of the study centers on non-neutralizing antibody functions in defense against HIV-1. Elucidation of antibody effector functions responsible for protective immunity against HIV-1 acquisition is a major goal for the HIV-1 vaccine field. Immunoglobulin A (IgA) is an important part of the host defense against pathogens; however, little is known about the role of vaccine-elicited IgA and their capacity to mediate antiviral functions. To identify the antiviral functions of HIV-1-specific IgA elicited by vaccination, we cloned HIV-1 envelope specific IgA monoclonal antibodies by memory B cell cultures from peripheral blood mononuclear cells from a vaccinee from the only partially efficacious HIV vaccine regimen to date, RV144. We produced two IgA clonal cell lines (HG129 and HG130) producing native, non-recombinant IgA monoclonal antibodies (mAbs). HG129 is crossreactive and binds the variable loop (V3) of the HIV envelope; HG130 is a conformational gp120 antibody that interacts with the CD4 binding site. HG129 and HG130 mAbs mediated phagocytosis of HIV infectious virions and HIV antigen-coated beads respectively, and HG129 blocked HIV-1 Env glycoprotein binding to galactosylceramide, an alternative HIV-1 receptor. These findings elucidate potential antiviral functions of vaccine-elicited HIV-1 envelope specific IgA that may act to block HIV-1 acquisition at the portal of entry by preventing HIV-1 binding to galactosylceramide and mediating antibody Fc receptor mediated virion phagocytosis. Furthermore, these findings highlight the complex and diverse interactions of vaccine-elicited IgA with pathogens that depend on IgA fine specificity and form (e.g. multimeric, monomeric) in the systemic circulation and mucosal compartments.
The results of this work demonstrate that whether an antibody is broadly neutralizing or non-broadly neutralizing, it is the fine specificity to the HIV-1 envelope that is crucial for its functional activity. The functional activity is dependent on antibody specificity and isotype due to engagement of effector cells at mucosal sites. The production of potentially protective antibody envelope specificities, antibody isotypes, and functions should be considered in the immunogen design for the development of safe and effective vaccine approaches.
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