Small set of T cell metaclones points to shared TB immune responses

Tuberculosis remains a disease where the behaviour of the immune system is central to whether people control infection or develop illness. T cells are a key part of that immune response, but previous measurements that treated the T cell response as a broad, polyclonal signal have not revealed clear links to outcomes in people. In new work led by Mahdad Noursadeghi, researchers took a more detailed, time-resolved look at the actual T cell clones that respond in the human body. They performed the first temporal evaluation of the human in vivo clonal repertoire of Mtb-reactive T cell responses at the site of a standardised antigenic challenge, the tuberculin skin test. By sampling the reaction at the skin test site and sequencing T cell receptors over time, the team could watch which T cells arrived and which expanded. This focused approach allowed them to separate the early, mixed influx of T cells from the later, specific expansion of Mtb-reactive clones, providing a clearer window onto what the immune system is doing during a defined exposure to Mtb antigens.

To follow the clonal dynamics, the researchers used T cell receptor (TCR) sequencing directly from the tuberculin skin test site. They observed an initial recruitment of non-Mtb reactive T cells to the challenge site, followed by an enrichment of Mtb-reactive clones that arose through oligoclonal T cell proliferation. To make sense of the many distinct TCR sequences and find those that recognize the same bacterial fragments, they developed a modular computational pipeline called Metaclonotypist. Metaclonotypist sensitively clusters distinct TCRs that share epitope specificity, enabling the team to compile a catalogue of public Mtb-reactive HLA-restricted T cell metaclones. The study found that, although most in vivo Mtb-reactive T cells are private to individuals, a surprisingly small number of metaclones—just 10—were sufficient to identify Mtb-T cell reactivity across the study population (N≥128), revealing population-level immunodominance of specific TCR-peptide interactions.

The findings change how we might look for meaningful immune signals in tuberculosis. Rather than treating the T cell response as an indistinct bulk, the work shows that following individual T cell clones over time at a defined challenge site can reveal which clones are truly Mtb-reactive and which are bystanders. The Metaclonotypist pipeline makes it possible to detect groups of different TCRs that nevertheless target the same Mtb epitopes, producing a set of public, HLA-restricted metaclones that recur across individuals. Because only a small number of metaclones were needed to flag Mtb-T cell reactivity in a large sample, these results suggest clear immunodominant TCR-peptide pairs that are generalisable across people. The study points toward new ways to stratify patients, to develop diagnostics that read out specific T cell reactivity, and to design vaccines that aim at the most commonly recognized Mtb targets identified by these metaclones.