Differential characteristics of antigens during Mycobacterium tuberculosis infection: Implications for next-generation tuberculosis vaccines

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis (Mtb). To this day, TB remains a global health problem killing 1.4 million people annually. A number that is expected to increase over the coming years due to the impact of the current COVID-19 pandemic on TB case findings and timely diagnosis.

The only available vaccine against TB, the Bacillus Calmette–Guérin (BCG), has variable efficacy in adults and does not prevent pulmonary TB. Thus, the development of a more effective TB vaccine is desperately needed in order to combat the TB pandemic. However, vaccine development is hindered by the lack of complete knowledge of protective immunity against Mtb.

CD4 T cells are required for the control of Mtb infection and excessive antigen stimulation from the infection is known to drive the impairment of CD4 T cell functionality in a process termed terminal differentiation. Based on three original manuscripts, this thesis studies the conditions for antigen-specific T cell differentiation during murine Mtb infection and designs vaccination strategies to mitigate T cell differentiation for optimal immunity, including the best vaccination regimen for combining BCG and a subunit vaccine.

Together, these three studies address knowledge gaps in TB immunology, provide new means to identify optimal antigens and guide efficient TB vaccine design. Manuscript I explores to what extent immunodominant antigens confer protection in models of long-term chronic infection and whether CD4 T cell differentiation state influences long-term protection of vaccine antigens.

In conclusion, manuscript I shows that T cells recognizing immunodominant antigens identified by high IFN-γ responses do not necessarily display equivalent T cell functionality or result in comparable long-term protection. Therefore, this thesis suggests the functional differentiation score (FDS) as a more precise measure of CD4 T cell subsets accumulated differentiation state and could represent a new way of identifying optimal TB vaccine antigens.

In the search for other factors that shape T cell differentiation state, the study of manuscript II examined the impact of antigen availability for T cell differentiation and the link between in vivo antigen expression and protective capacity in a vaccination setting. Data of manuscript II suggests that T cell differentiation is a proxy for in vivo antigen expression.

The results of manuscripts I and II accentuate that immunodominant and high in vivo expressed antigens are superior vaccine targets. Vaccination with these compensate for excessive Mtb-driven T cell differentiation and result in the greatest long-term protection and lung-homing ability. Manuscript III introduces the next-generation TB subunit vaccine, H107, and explores the optimal design for combining BCG with a subunit vaccine that mitigates BCG-mediated T cell differentiation and captures BCG/subunit synergies.

The results of manuscript III showed that BCG co-administered with the adjuvanted Mtb-specific vaccine, H107/CAF01, did compensate for BCG-mediated T cell differentiation and conferred significant protection over BCG and H107 vaccination alone. This was in contrast to vaccines consisting of BCG-shared antigens (H65 and H4), which interfered with BCG immunity and replication.

Co-administration of BCG and a subunit vaccine revealed a synergistic reciprocally adjuvant effect, which improved the immune response to both vaccines. In summary, manuscript III reports on a highly protective TB vaccine that has potential as a standalone vaccine as well as in a BCG co-administration setting, which complements rather than boosts BCG-induced immunity.

H107 is currently being prepared for clinical testing and this vaccine has the potential for enhanced vaccine efficacy in infants or as an adjuvant to BCG revaccination in adolescents and adults.