Introduction

Bacillus Calmette–Guérin (BCG)-vaccination affords variable protection against tuberculosis (TB), which is unexplained. The project hypothesises that there is inter-individual variation in both the naïve T cell (NTC) response to a standard stimulation and in the ‘trained’ immune response of BCG-vaccine-primed T cells on secondary exposure to antigen and that relationships exist between the two. The aim of the study is to identify factors by which T cell vaccine design may be improved.

Methods

Blood was collected from 107 adults immediately prior to BCG-vaccination. NTCs were enriched from pre-BCG frozen/thawed peripheral blood mononuclear cells (PBMC), subjected to anti-CD3/CD28 stimulation and then characterised by fluorescence-activated cell-sorting (FACS) on markers of activation, differentiation and proliferation. Participants were then re-bled eight weeks later and their frozen/thawed PBMC incubated with purified protein derivative (PPD) in three separate assays before FACS to measure activation induced markers, Th1 cytokines and proliferation. A DNA sequencing pipeline and computational analysis were employed to examine 67 participants’ peripheral blood TCR repertoires in response to BCG.

Results

There was inter-individual variation amongst all pre-vaccination NTC stimulation assay parameters, some of which were strongly positively correlated. In the post-BCG-vaccination assays, all parameters were significantly upregulated in response to incubation with PPD and demonstrated wide ranges of inter-individual variation. In both sets of assays, results were found to be reproducible and inter-individual variation surpassed technical noise. Post-BCG expanded TCRs generated clusters of homologous sequences, were shared between multiple participants and shared homology with annotated TB-specific TCRs.

Discussion

There is inter-individual variation in multiple parameters to standard NTC stimulation, possibly reflecting intrinsic variance in the T cell intracellular signalling capacity. BCG has variable T cell immunogenicity, which may bear relation to pre-vaccination parameters. BCG-vaccination induces a polyclonal population of T cells with the ability to recognise mycobacterial antigens. Improved understanding of the T cell component to variable BCG vaccine efficacy may aid next generation TB vaccine development.