Cell sorting protein SNX5 shapes immunity to tuberculosis

Tuberculosis remains a leading cause of death from an infectious disease, driven by the ability of Mycobacterium tuberculosis (Mtb) to live inside cells and evade immune attack. A central step in protective immunity is antigen presentation: infected cells place fragments of microbial proteins onto MHC-II molecules for recognition by helper T cells, which then orchestrate the inflammatory response needed to control infection. In work led by Michael U Shiloh, researchers investigated an unexpected player in this process, the endosomal sorting protein SNX5. Rather than changing how much MHC-II sits on the cell surface, the team found SNX5 influences the loading of peptides onto MHC-II. Their findings focus on pulmonary Mtb infection and reveal that SNX5 acts as a regulator of which bacterial fragments are displayed to T cells. By tying SNX5 to MHC-II peptide loading and to the downstream inflammatory outcome in the lung, this research highlights a previously unrecognized connection between intracellular trafficking machinery and the immune system’s ability to sense and respond to Mtb.

The key observation reported is that SNX5 affects antigen presentation without altering surface levels of MHC-II or the expression of costimulatory molecules, two features commonly used to judge a cell’s ability to activate T cells. In other words, cells with altered SNX5 still carried similar amounts of MHC-II on their surfaces and displayed normal costimulatory signals, yet the repertoire of peptides presented to helper T cells changed. This points specifically to a role for SNX5 in MHC-II peptide loading within the endosomal pathway where antigens are processed. The result was measurable differences in T cell responses and in the inflammatory landscape of the infected lung during pulmonary Mtb infection. Taken together, these results identify SNX5 as a previously unrecognized regulator of MHC-II peptide loading and place the endosomal sorting machinery squarely in the pathway that shapes immune recognition of intracellular pathogens like Mycobacterium tuberculosis.

These findings have several important implications. First, they expand the understanding of antigen presentation by showing that control over which peptides get loaded onto MHC-II can be decoupled from surface abundance of MHC-II and from classical costimulatory signals. Second, by implicating SNX5 and the endosomal sorting machinery, the work suggests that intracellular trafficking steps are active determinants of how the immune system ‘sees’ an intracellular pathogen. For tuberculosis research, this insight could redirect attention toward host pathways that influence antigen selection and T cell activation, complementing the usual focus on the pathogen itself. While the abstract does not detail therapeutic strategies, the identification of SNX5 as a regulator raises the possibility that modulating endosomal sorting or peptide loading could alter inflammatory outcomes and improve control of pulmonary Mtb infection. Further work will be needed to translate these mechanistic discoveries into interventions, but the study opens a new avenue for thinking about host-directed approaches to infectious disease.