Persistent BCG bacilli perpetuate CD4 T effector memory and optimal protection against tuberculosis

Abstract

Tuberculosis (TB) remains one of the most important infectious diseases of man and animals, and the only available vaccine (BCG) requires urgent replacement or improvement. To facilitate this, the protective mechanisms induced by BCG require further understanding. As a live attenuated vaccine, persistence of BCG bacilli in the host may be a crucial mechanism. We have investigated the long term persistence of BCG following vaccination and the influence on the induced immune response and protection, using an established murine model. We sought to establish whether previously identified BCG-specific CD4 TEM cells represent genuine long-lived memory cells of a relatively high frequency, or are a consequence of continual priming by chronically persistent BCG vaccine bacilli. By clearing persistent bacilli, we have compared immune responses (spleen and lung CD4: cytokine producing T effector/TEM; TCR-specific) and BCG-induced protection, in the presence and absence of these persisting vaccine bacilli. Viable BCG bacilli persisted for at least 16 months post-vaccination, associated with specific CD4 T effector/TEM and tetramer-specific responses. Clearing these bacilli abrogated all BCG-specific CD4 T cells whilst only reducing protection by 1log10. BCG may induce two additive mechanisms of immunity: (i) dependant on the presence of viable bacilli and TEM; and (ii) independent of these factors. These data have crucial implications on the rational generation of replacement TB vaccines, and the interpretation of BCG induced immunity in animal models.

Parenteral adenoviral boost enhances BCG induced protection, but not long term survival in a murine model of bovine TB

“…Analysis of total virus particles in batches of Ad-TB10.4 & Ad-empty revealed no differences, and therefore does not explain this differential effect. It is known that Ad vectors induce innate responses [34], and data demonstrate BCG to persist long after vaccination [35][36][37]. Combined with previous reports [38][39][40], we propose the Ad vector is adjuvanting persistent BCG, enhancing specific immune responses.…”

Vaccine-mediated immunity to experimental Mycobacterium tuberculosis is not impaired in the absence of Toll-like receptor 9

“…While both TCM and TEM provide protection, only TCMs are long-lived [42] and also provide better and long-term protection [43–45]. BCG vaccination induces predominantly TEMs, [46–48], which could partly explain its variable efficacy in protection. Consistent with this theory, BCG ΔureC::hly vaccine, a recombinant variant of BCG, induces an increased TCM response in the vaccinated mice and confers better protection against challenge Mtb infection in comparison to BCG which induces predominantly a TEM response [49].…”

Towards new TB vaccines

“…Despite this, the antigen-specific CD4+ T cell response of BCG-vaccinated human new-borns wanes over the first year of life, suggesting that the T EM population induced is unable to maintain persistent memory [165]. In response to continuous antigen exposure, T EM become terminally differentiated T effector (T eff ) cells, losing the ability to proliferate and migrate into the lung parenchyma, expressing the KLRG1 marker [166,167].…”

“…In addition to their cytolytic role, CD8+ T RM are also capable of activating bystander NK and B cells through IFN-ɣ, TNFα and IL-2 [173]. Maintenance of T RM may be reliant on the presence of live bacilli, as clearance of BCG in mice with chemotherapy abrogates the antigen-specific CD4+ T cell response [166]. Of all the T memory cell subtypes, the mucosal transfer of CD8+ T RM cells was associated with the most protection against M. tuberculosis challenge on a per-cell basis [141].…”