Does the BCG vaccine protect infants from severe forms of childhood tuberculosis by trained innate and humoral immune mechanisms?

Tuberculosis (TB) is a significant global health burden, with 1.8 million TB-related deaths
(0.2 million in children) in 2015, as reported by the WHO. Recent research suggested that
infant immune responses may be suboptimal, predisposing them to TB and its
complications. However, the current anti-TB vaccine, Mycobacterium bovis BCG, protects
them from TB complications and may exert non-specific effects, reducing overall infant
mortality from other, unrelated infections.
The mechanisms mediating these properties are yet unknown; however, one candidate may
be a recently discovered monocyte immune memory (or training), regulated through
epigenetic and metabolic modifications. This phenomenon was shown to improve monocyte
activation and the inflammatory cytokine production in BCG vaccinated adult monocytes for
up to 3 months post-vaccination in response to stimuli other than BCG. This mechanism
may contribute to infant protection from TB, possibly improving monocyte function and so
enhancing T-cell (a major effector of anti-TB immunity) activation or proinflammatory
responses.
Anti-BCG antibodies may also contribute to protecting infants from severe TB forms. Recent
research associated anti-mycobacterial antibodies with reduced infant TB risk, and the
earlier studies suggested that such antibodies could improve BCG phagocytosis, control of
its growth and even T-cell responses. As the innate immune cells express receptors for the
crystallisable antibody domains, they may possibly act synergistically protecting infants from
TB complications.
As neither BCG-trained innate immunity, nor the anti-mycobacterial antibody responses
have been extensively studied in infants, this study aims to fill the gap in the current
understanding on the role of these mechanisms in infant TB and BCG immunobiology,
potentially contributing to novel TB vaccine design strategies. To do so, the study will
combine a mixture of immunology, cell and molecular biology, metabolic and quantitative
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approaches characterising the multi-layered effects of BCG humoral immunity and training
in monocytes.
First, infant monocyte surface activation marker and cytokine profiles will be determined by
flow cytometry/Luminex/intracellular staining to associate monocyte surface and functional
phenotypes with their training status in infants prior to and post the BCG vaccination. The
epigenetic histone modification and DNA methylation analyses will then be applied to
determine which gene regulation changes correlate with monocyte phenotypes induced by
BCG training upon vaccination. In addition, monocyte metabolic phenotypes prior to and
post the infant BCG vaccination will be determined. Computational analysis via R/Matlab
will be used to test the relationship between the surface, functional, epigenetic and
metabolic markers detected and BCG-induced monocyte training.
This study will also test the anti-BCG antibody role in infant protection from TB. Pre- and
post-BCG vaccination infant blood sample anti-mycobacterial antibody titres will be
measured by ELISA/Luminex. To test their functional contribution to anti-TB immunity,
cellular infant phagocyte and sera assays from the pre- or post-vaccination samples will be
used to characterise their activation marker and cytokine profiles. In addition, their ability to
activate anti-mycobacterial T-cells will be measured by flow cytometry and ELISA/Luminex
assays. To study their clinical implications in anti-TB immunity, the pre- and post-BCG
vaccination sera antimicrobial effect on mycobacterial growth will be tested. Computational
R/Matlab analysis will be used to determine the correlation between serological markers
and other immune compartment component activation.