Cellular sorting reveals how immune cells spot tuberculosis

Tuberculosis remains a major infectious disease, and understanding how the immune system senses infected cells is essential to improve prevention and treatment. One part of that sensing system uses a molecule called Major Histocompatibility Complex Class I-Related molecule (MR1) to display small chemical fragments made by microbes to a special class of immune cells called Mucosal Associated Invariant T (MAIT) cells. However, scientists did not yet know exactly how MR1 picks up and shows pieces of Mycobacterium tuberculosis (Mtb) inside infected cells. In work led by Elham Karamooz, researchers focused on the internal shipping routes inside cells — the endosomal system — to learn how MR1 acquires Mtb-derived ligands. They asked whether MR1 waits at the cell surface or travels through internal compartments called sorting endosomes to sample the inside of infected cells. By tracing MR1 and the traffic machinery that controls movement between endosomes, the team set out to reveal the cellular steps that connect infection with MR1-dependent recognition by MAIT cells.

The study tested the role of sorting endosome machinery and several trafficking proteins in MR1 antigen presentation during Mtb infection. The authors noted that sorting endosomes are controlled by proteins including Syntaxin 6, Syntaxin 12, Syntaxin 16 and VAMP4. Prior work had shown VAMP4 is important for MR1 presentation during Mtb infection; Karamooz and colleagues found that Syntaxin 12 (Stx12) and Syntaxin 16 (Stx16) are also required. Using siRNA-mediated knockdown to reduce Stx12 or Stx16, they observed less MR1 antigen presentation of Mtb, indicating these proteins support MR1’s ability to display bacterial ligands. The team used RFP-tagged constructs to visualize trafficking and found that Stx16 co-localized more with MR1 vesicles than Stx12 in MR1-GFP expressing airway epithelial cells. Blocking Stx12 and Stx16 increased MR1 surface stabilization and total MR1 expression, consistent with impaired endosomal trafficking slowing MR1 internalization.

Taken together, these findings support a model in which sorting endosomes selectively sample the inside of Mtb-infected cells and load MR1 with microbial ligands for presentation to MAIT cells. By identifying Stx12 and Stx16 as contributors alongside VAMP4 and Syntaxin 6, the work narrows down the specific trafficking steps that control whether MR1 reaches the compartments where it can capture Mtb-derived molecules. The observation that interfering with Stx12 or Stx16 increases MR1 on the cell surface suggests that normal endosomal movement pulls MR1 into internal compartments for loading, and when that movement is blocked MR1 accumulates outside. For researchers and clinicians, these details refine our picture of how immune surveillance operates at the cellular level and point to molecular checkpoints that determine whether infected cells are detected by MAIT cells.