Abstract
Tuberculosis (TB) is the leading cause of mortality from a single infectious agent, Mycobacterium tuberculosis Relevant immune targets of the partially efficacious TB vaccine bacille Calmette-Guérin (BCG) remain poorly defined. Mucosal-associated invariant T (MAIT) cells are MHC-related protein 1 (MR1)-restricted T cells, which are reactive against M. tuberculosis, and underexplored as potential TB vaccine targets. We sought to determine whether BCG vaccination activated mycobacteria-specific MAIT cell responses in humans. We analyzed whole blood samples from M. tuberculosis-infected South African adults who were revaccinated with BCG after a six-month course of isoniazid preventative therapy. In vitro BCG stimulation potently induced IFN-γ expression by phenotypic (CD8+CD26+CD161+) MAIT cells, which constituted the majority (75%) of BCG-reactive IFN-γ-producing CD8+ T cells. BCG revaccination transiently expanded peripheral blood frequencies of BCG-reactive IFN-γ+ MAIT cells, which returned to baseline frequencies a year following vaccination. In another cohort of healthy adults who received BCG at birth, 53% of mycobacteria-reactive-activated CD8 T cells expressed CDR3α TCRs, previously reported as MAIT TCRs, expressing the canonical TRAV1-2-TRAJ33 MAIT TCRα rearrangement. CD26 and CD161 coexpression correlated with TRAV1-2+CD161+ phenotype more accurately in CD8+ than CD4-CD8- MAIT cells. Interestingly, BCG-induced IFN-γ expression by MAIT cells in vitro was mediated by the innate cytokines IL-12 and IL-18 more than MR1-induced TCR signaling, suggesting TCR-independent activation. Collectively, the data suggest that activation of blood MAIT cells by innate inflammatory cytokines is a major mechanism of responsiveness to vaccination with whole cell vaccines against TB or in vitro stimulation with mycobacteria (Clinical trial registration: NCT01119521).
Highlights
MHC-related protein 1 (MR1)–5-OP-RU tetramer staining was suboptimal on samples processed for WB-Intracellular cytokine staining (ICS) because leukocytes were fixed before cryopreservation
89.5%, interquartile range [IQR] 82.7–92.7) of MR1–5-OP-RU tetramer+ CD8+ T cells stained positive for CD26 and CD161 in adults and met the phenotypic definition (Fig. 1B)
As a proof of principle, we evaluated whether frequencies of bacille Calmette–Guerin (BCG)-reactive CD26+CD161+ CD8+ Mucosal-associated invariant T (MAIT) cells could be boosted in vivo by BCG revaccination of M. tuberculosis–sensitized adults (Fig. 2A)
Summary
Participants received a 6-mo course of isoniazid preventative treatment (West-Ward Pharmaceutical, Eatontown, NJ) prior to revaccination with BCG (Danish strain 1331 vaccination), delivered as an intradermal injection at the recommended dose for adults at 2–8 3 105 CFUs. Heparinized whole blood was stimulated within 2 h of collection for the whole blood intracellular cytokine staining (WB-ICS) assay described below. We retrieved cryopreserved blood cells from 5- or 9-wk-old infants who received routine BCG vaccination at birth or in whom BCG vaccination was delayed until 6 or 10 wk of age, respectively. For the delayed BCG group, pregnant mothers were contacted antenatally and asked to participate in the study through hospitals in Worcester, South Africa. Heparinized blood was collected for WB-ICS assays to investigate TCR, MR1, and cytokine dependence of BCG-mediated MAIT cell activation
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