Tiny nanobody spots TB protein for molecular imaging

Tuberculosis remains a disease that forms complex immune structures called granulomas, where Mycobacterium tuberculosis (Mtb) and pieces of its secreted proteins can accumulate on the surfaces of bacteria and host cells. These surface-exposed antigens are attractive because they could serve as biomarkers that antibodies recognize and light up, enabling targeted molecular imaging of TB. In work led by Pramod Gupta, researchers turned to nanobodies — very small antibody fragments derived from camels — because they move quickly into target tissue and clear rapidly from non-target organs, traits that make them especially useful for imaging. The team immunized a camel with secretory proteins of Mtb and built a phage display library displaying nanobodies. From that library they isolated a single nanobody, named C8 Nb, that binds the PstS-1 protein. The PstS-1 protein has roles in phosphate uptake for the bacterium and in adhesion to host cells, making it a meaningful target for detecting bacterial presence within granulomas. This study set out to produce and test C8 Nb and to evaluate whether it recognizes PstS-1 where it appears in infected tissue and cells.

The investigators characterized C8 Nb both in the laboratory and in living mice to test its immunoreactivity against the PstS-1 protein. Using the nanobody isolated from the phage display library, the team showed that C8 Nb binds PstS-1 protein presented on the surface of Mtb bacilli and also recognizes PstS-1 when it is adhered to macrophages. To demonstrate behavior in an animal model, they injected BCG cells intramuscularly in mice and observed localization of the nanobody around those cells, indicating that C8 Nb homes to bacterial structures in tissue. These results were gathered using standard immunoreactivity assays and in vivo localization studies described in the study. By showing binding to PstS-1 in vitro and a pattern of localization around BCG cells in mice, the work demonstrates that C8 Nb can detect the PstS-1 protein in contexts relevant to TB infection and granuloma biology.

The findings point toward a practical route for developing molecular imaging tools tailored to tuberculosis. Because nanobodies like C8 Nb reach targets rapidly and are cleared from non-target organs, an imaging agent based on this nanobody might provide clearer, faster pictures of where M. tuberculosis or its antigens concentrate within granulomas. Targeting PstS-1 protein specifically could allow visualization of bacterial presence on bacilli or on host cell surfaces, which may help researchers study how granulomas form and change, and could ultimately support the design of diagnostics that reveal active infection sites. While additional work will be needed to translate this nanobody into a clinical imaging agent, the demonstration that C8 Nb binds PstS-1 on Mtb and localizes around BCG in mice establishes a proof of concept for further development of targeted TB molecular imaging tools.