Cellular Mediators of BCG Vaccine-Induced Protection

Abstract

Tuberculosis (TB) has surpassed human immunodeficiency virus as the world’s deadliest infectious disease. The intradermal TB vaccine, bacille Calmette-Guérin (BCG), prevents disseminated childhood TB yet fails to protect against the most prevalent form, pulmonary TB. The urgent need to develop a more effective TB vaccine has resulted in new TB vaccine candidates entering clinical trials without a solid understanding of the protective memory immune response against TB. All TB vaccines in current clinical trials target conventional T cells, primarily memory CD4 T cells. The contribution of innate immune cells to vaccine-induced protection to TB has been neglected in these vaccine approaches, as they have only recently been discovered to show features of immunological memory, known as trained immunity.

The aim of this thesis was to determine whether the long held dogma, that memory T cells mediate BCG vaccine-induced protection, held true. To address this question a novel murine memory T cell depletion model was established, in which the protective role of BCG-induced memory CD4 and CD8 T cells against a mycobacterial infection was evaluated. Effector T cells are essential for resistance against mycobacterial infection, however, previous studies using T cell depletion have failed to differentiate between effector and memory T cells. For this reason, the depletion model in this thesis evaluated the protective role of BCG-induced memory CD4 and CD8 T cells in the context of an effector T cell response. Subcutaneously BCG-vaccinated mice were treated with anti-mycobacterial drugs to clear remaining live bacilli. Then, mice were depleted of CD4, CD8 or CD4 and CD8 T cells. Following repopulation of the depleted naïve T cell pool, mice were challenged intranasally with BCG and the lung bacterial burden of memory T cell-depleted mice was compared to non-depleted control mice. Strikingly, the absence of memory CD4 and/or CD8 T cells did not abrogate protection induced by BCG. Thus, resistance against infection induced by BCG, appeared to be mediated largely by cells other than conventional memory T cells.

BCG vaccination is known to provide protection against unrelated pathogens. This broad-specific resistance could be based on the induction of trained immunity in innate cells. A newly discovered innate subset, innate lymphoid cells (ILCs), has been shown to become activated shortly after pathogen entry and provide early protection. A role for ILCs in mycobacterial infection has not been investigated yet. Since ILCs respond to microbial encounter, it was hypothesised that BCG vaccination would activate ILC responses in lungs of mice. Comparing intranasal, subcutaneous and intradermal vaccination routes for their ability to induce accumulation and effector function of ILCs in lungs, it was found that mucosal delivery was superior to parenteral vaccination.

Intranasal BCG vaccination increased ILCs numbers in lung tissue and enhanced their effector function. ILC1s, ILC3s and NK cells accumulated in lungs four weeks after intranasal challenge. In addition, these cells were major producers of interferon-gamma (IFN-γ), an essential cytokine in mycobacterial defense. More importantly, IFN-γ production by ILCs could be involved in mediating resistance against mycobacterial infection. The discovery that the BCG vaccination activates ILCs has paved the way for future research to elucidate the protective potential of ILCs against mycobacterial infection.

Findings of this thesis present a shift of the current paradigm that memory CD4 T cells are the main mediator of BCG vaccine-induced protection. A potential inability of BCG-induced memory T cells to protect against TB could indicate that BCG booster vaccine candidates that aim to enhance BCG-induced memory T cell responses might be ineffective in providing superior protection in comparison to BCG alone. It will be imperative to identify the protective mediator(s) of BCG-vaccine induced protection. This work also provides the first evidence that ILCs are activated in lungs in response to mucosal BCG vaccination and produce IFN-γ, one of the key cytokines in mycobacterial defense.