Targeting lysosomal proteases for a host-directed therapy for tuberculosis

Tuberculosis is a disease caused by the bacteria Mycobacterium tuberculosis (Mtb) which latently infects one-quarter of the human population and is the leading cause of death by an infectious agent. Contributing to the challenge of this old disease is the lack of (1) an effective vaccine, (2) reliable biomarkers for latent infection, (3) the limited number of effective antimicrobial drugs, (4) a prolonged therapeutical regime, and (5) the evolution of multi-drug resistant strains. This increasingly foments the need for novel therapies that target the bacterial niche or improve the host response, alone and in combination with the current conventional therapy. We have been probing the intracellular niche of Mtb, the macrophage, for how these bacteria survive and replicate inside them while impairing their bactericidal response. In that search, we found that Mtb induces a decrease in the expression of a group of proteolytic enzymes, the cathepsins, that participate in key cellular processes regulating homeostasis, cell death, inflammation, antigen presentation, and microbial killing. Regarding Mtb infection, this downregulation results in improved bacterial survival and replication inside macrophages as well as poor lymphocyte priming by the infected cells. To address this problem, we have been exploring the different pathways by which cathepsin activity is regulated in our cells. So far, we found three levels of cathepsins regulation that can be manipulated to our advantage: One is by targeting miRNAs to restore cathepsins gene expression; another by targeting cystatins, the natural inhibitors of cathepsins, to restore their activity; and finally, by using saquinavir, a repurposed inhibitor of the HIV protease that unexpectedly improves the activity of some human cathepsins. Together, these strategies were shown to improve the intracellular killing of Mtb by macrophages, as well as enhance the ability of these cells to prime CD4+ T-lymphocytes and induce their proliferation and IFNγ secretion. Our approach suggests a potential host-targeted strategy that can be developed as a complementary therapy to current antibiotics. Human monocytes were isolated from buffy-coats of healthy human donors provided by the National Blood Institute (Instituto Português do Sangue e da Transplantação, IP, Lisbon, Portugal).