Klas, J. M. | Search bioRxiv

bioRxiv2026

A DNA-scaffolded Retron Ring Mediates Antiphage Immunity

Retrons are a diverse family of bacterial defence systems that couple reverse transcriptase (RT) activity to antiphage immunity by producing multicopy single-stranded DNA (msDNA). Yet, the molecular mechanisms by which most systems detect infection and execute immunity remain unknown. Here, we define the structure, immune sensing and effector mechanisms of retron type IX, composed of an RT, a non-coding RNA, a tandem KH-HTH subunit and a HEPN RNase effector. Retron IX confers robust protection against diverse bacteriophages. Cryo-EM and biochemical analyses reveal a supramolecular ring assembly composed of 8-10 repeating modules, spanning up to 230 [A] in diameter. Within this assembly, the msDNA scaffolds the outer rim inhibiting the HEPN ribonuclease in a catalytically poised state, with the active site occluded within the inner layer. We identify a phage-encoded PD-(D/E)XK nuclease as the immune trigger that cleaves the exposed msDNA stem-loop, leading to ring disassembly and HEPN effector release. Upon activation, the HEPN ribonucleases cleave tRNAs indiscriminately, inducing dormancy and blocking phage propagation. Our findings unveil how ring-shaped higher-order assembly controls enzymatic activities while allowing signal-specific activation and establish the ncRNA as an evolvable scaffold underlying the remarkable structural and defensive diversity of bacterial retron immune complexes.

microbiology↗

bioRxiv2024

CD4-mediated immunity shapes neutrophil-driven tuberculous pathology

Pulmonary Mycobacterium tuberculosis (Mtb) infection results in highly heterogeneous lesions ranging from granulomas with central necrosis to those primarily comprised of alveolitis. While alveolitis has been associated with prior immunity in human post-mortem studies, the drivers of these distinct pathologic outcomes are poorly understood. Here, we show that these divergent lesion structures can be modeled in C3HeB/FeJ mice and are regulated by prior immunity. Using quantitative imaging, scRNAseq, and flow cytometry, we demonstrate that Mtb infection in the absence of prior immunity elicits dysregulated neutrophil recruitment and necrotic granulomas. In contrast, prior immunity induces rapid recruitment and activation of T cells, local macrophage activation, and diminished late neutrophil responses. Depletion studies at distinct infection stages demonstrated that neutrophils are required for early necrosis initiation and necrosis propagation at chronic stages, whereas early CD4 T cell responses prevent neutrophil feedforward circuits and necrosis. Together, these studies reveal fundamental determinants of tuberculosis lesion structure and pathogenesis, which have important implications for new strategies to prevent or treat tuberculosis.

immunology↗