Immunology and Microbiology
First Thesis Advisor
Liisa K Selin, M.D., Ph.D.
Immunity, Cellular, CD8-Positive T-Lymphocytes, Herpesvirus 4, Human, Influenza A virus
Heterologous immunity is a mechanism by which immunological memory within an individual, developed in response to a previous infection, plays a role in the immune response to a subsequent unrelated infection. In murine studies, heterologous immunity facilitated by cross-reactive CD8 T-cell responses can mediate either beneficial (protective immunity) or detrimental effects (e.g. enhanced lung and adipose immunopathology and enhanced viral titers) (Selin et al., 1998; Chen et al., 2001; Welsh and Selin, 2002; Nie et al., 2010; Welsh et al., 2010). Protective heterologous immunity results in enhanced clearance of virus during a subsequent infection with an unrelated pathogen. Such is the case when mice are immunized with lymphocytic choriomeningitis virus (LCMV) and subsequently challenged with Pichinde virus (PV) or vaccinia virus (VACV) (Selin et al., 1998). However, heterologous immunity may also mediate enhanced immunopathology as mice immunized with influenza A virus (IAV) and challenged with LCMV show increased viral titers and enhanced lung immunopathology (Chen et al., 2003).
The role heterologous immunity plays during infection is not limited to the murine system. In fact, there have now been several reports of enhanced immunopathology due to heterologous immunity during human infections, involving viruses such as IAV, Epstein-Barr Virus (EBV), hepatitis C virus (HCV), and dengue virus (DENV) (Mathew et al., 1998; Wedemeyer et al., 2001; Acierno et al., 2003; Nilges et al., 2003; Clute et al., 2005; Urbani et al., 2005). Interestingly, in all reported cases in humans, heterologous immunity mediated enhanced immunopathology.
Upon infection with EBV the clinical presentation can range from asymptomatic to severe, occasionally fatal, acute infectious mononucleosis (AIM) (Crawford et al., 2006b; Luzuriaga and Sullivan, 2010) which is marked by a massive CD8 lymphocytosis. This lympho-proliferative effect in AIM was shown to be partially mediated by reactivation of cross-reactive IAV-M1 58-66 (IAV-GIL) specific CD8 memory T-cells in HLA-A2 patients reacting to the EBV-BMLF1 280 (EBV-GLC) epitope (Clute et al., 2005).
Interestingly, EBV infects ~90% of individuals globally by the third decade of life, establishing a life-long infection (Henle et al., 1969). However, it is unknown why 5-10% of adults remain EBV-sero-negative (EBV-SN), despite the fact that the virus infects the vast majority of the population and is actively shed at high titers even during chronic infection (Hadinoto et al., 2009). Here, we show that EBV-SN HLA-A2+ adults possess cross-reactive IAV-GIL/EBV-GLC memory CD8 T-cells that show highly unique properties. These IAV-GIL cross-reactive memory CD8 T-cells preferentially expand and produce cytokines to EBV antigens at high functional avidity. Additionally, they are capable of lysing EBV-infected targets and show the potential to enter the mucosal epithelial tissue, where infection is thought to initiate, by CD103 expression. This protective capacity of these cross-reactive memory CD8 T-cells may be explained by a unique T-cell receptor (TCR) repertoire that differs by both organization and CDR3 usage from that in EBV-seropositive (EBV-SP) donors.
The composition of the CD8 T-cell repertoire is a dynamic process that begins during the stochastic positive selection of the T-cell pool during development in the thymus. Thus, upon egress to the periphery a naïve T-cell pool, or repertoire, is formed that is variable even between genetically identical individuals. This T-cell repertoire is not static, as each new infection leaves its mark on the repertoire once again by stochastically selecting and expanding best-fit effectors and memory populations to battle each new infection while at the same time deleting older memory CD8 T-cells to make room for the new memory cells (Selin et al., 1999). These events induce an altered repertoire that is unique to each individual at each infection. It is this dynamic and variable organization of the T-cell repertoire that leads to private specificity even between genetically identical individuals upon infection with the same pathogens and thus a different fate (Kim et al., 2005; Cornberg et al., 2006a; Nie et al., 2010). It is this private specificity of the TCR repertoire that helps explain why individuals with the same epitope specific cross-reactive response, but composed of different cross-reactive T-cell clones, can either develop AIM or never become infected with EBV.
Our results suggest that heterologous immunity may protect EBV-SN adults against the establishment of productive EBV infection, and potentially be the first demonstration of protective T-cell heterologous immunity between unrelated pathogens in humans. Our results also suggest that CD8 T-cell immunity can be sterilizing and that an individual’s TCR repertoire ultimately determines their fate during infection.
To conclusively show that heterologous immunity is actively protecting EBV-SN adults from the establishment of a productive EBV infection, one would have to deliberately expose an individual to the virus. Clearly, this is not an acceptable risk, and it could endanger the health of an individual. A humanized mouse model could allow one to address this question.
However, before we can even attempt to address the question of heterologous immunity mediating protection from EBV infection in humanized mice, we must first determine whether these mice can be infected with, and build an immune response to the two viruses we are studying, EBV and IAV. We show here that these mice can indeed be infected with and also mount an immune response to EBV. Additionally, these mice can also be infected with IAV. However, at this time the immune responses that are made to these viruses in our established humanized mouse model are not substantial enough to fully mimic a human immune response capable of testing our hypothesis of heterologous immunity mediating protection from EBV infection.
Although the immune response in these mice to EBV and IAV infection is not suitable for the testing of our model the data are promising, as the humanized mouse model is constantly improving. Hopefully, with constant improvements being made there will be a model that will duplicate a human immune system in its entirety.
This thesis will be divided into 5 major chapters. The first chapter will provide an introduction to both general T-cell biology and also to the role of heterologous immunity in viral infection. The second chapter will provide the details of the experimental procedures that were performed to test our hypothesis. The third chapter will describe the main scientific investigation of the role of heterologous immunity in providing natural resistance to infection in human subjects. This chapter will also consist of the data that will be compiled into a manuscript for publication in a peer-reviewed journal. The fourth chapter will consist of work performed pertaining to the establishment of a humanized mouse model of EBV and IAV infection. The establishment of this model is important for us to be able to show causation for protection from EBV infection mediated by heterologous immunity.
Watkin, LB. The Role of Heterologous Immunity in Mediating Natural Resistance to Infection in Human Subjects: A Dissertation. (2012). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 586. DOI: 10.13028/kbrp-ny05. https://escholarship.umassmed.edu/gsbs_diss/586
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