Microglia are resident macrophages in the central nervous system that are involved in immune responses driven by Toll-like receptors (TLRs). Microglia-mediated inflammation can lead to central nervous system disorders, and more than one TLR might be involved in these pathological processes. The cysteine peptidase cathepsin X has been recognized as a pathogenic factor for inflammation-induced neurodegeneration. Here, we hypothesized that simultaneous TLR3 and TLR4 activation induces synergized microglia responses and that these phenotype changes affect cathepsin X expression and activity. Murine microglia BV2 cells and primary murine microglia were exposed to the TLR3 ligand polyinosinic-polycytidylic acid (poly(I:C)) and the TLR4 ligand lipopolysaccharide (LPS), individually and simultaneously. TLR3 and TLR4 co-activation resulted in increased inflammatory responses compared to individual TLR activation, where poly(I:C) and LPS induced distinct patterns of proinflammatory factors together with different patterns of cathepsin X expression and activity. TLR co-activation decreased intracellular cathepsin X activity and increased cathepsin X localization at the plasma membrane with concomitant increased extracellular cathepsin X protein levels and activity. Inhibition of cathepsin X in BV2 cells by AMS36, cathepsin X inhibitor, significantly reduced the poly(I:C)- and LPS-induced production of proinflammatory cytokines as well as apoptosis. Additionally, inhibiting the TLR3 and TLR4 common signaling pathway, PI3K, with LY294002 reduced the inflammatory responses of the poly(I:C)- and LPS-activated microglia and recovered cathepsin X activity. We here provide evidence that microglial cathepsin X strengthens microglia activation and leads to subsequent inflammation-induced neurodegeneration. As such, cathepsin X represents a therapeutic target for treating neurodegenerative diseases related to excess inflammation.

Cathepsin X is a lysosomal peptidase that is involved in tumour progression and represents a potential target for therapeutic interventions. In addition, it regulates important functions of immune cells and is implicated in the modulation of tumour cell–immune cell crosstalk. Selective cathepsin X inhibitors have been proposed as prospective antitumour agents to prevent cancer progression; however, their impact on the antitumour immune response has been overlooked. Previous studies indicate that the migration and adhesion of T cells and dendritic cells are affected by diminished cathepsin X activity. Meanwhile, the influence of cathepsin X inhibition on natural killer (NK) cell function has not yet been explored. Here, we examined the localization patterns of cathepsin X and the role of its inhibitors on the cytotoxicity of cell line NK-92, which is used for adoptive cellular immunotherapy in cancer patients. NK-92 cells depend on lymphocyte function-associated antigen 1 (LFA-1) to form stable immunoconjugates with target cells, providing, in this way, optimal cytotoxicity. Since LFA-1 is a substrate for cathepsin X activity in other types of cells, we hypothesized that cathepsin X could disturb the formation of NK-92 immunoconjugates. Thus, we employed cathepsin X reversible and irreversible inhibitors and evaluated their effects on the NK-92 cell interactions with target cells and on the NK-92 cell cytotoxicity. We show that cathepsin X inhibition does not impair stable conjugate formation or the lytic activity of NK-92 cells. Similarly, the conjugate formation between Jurkat T cells and target cells was not affected by cathepsin X activity. Unlike in previous migration and adhesion studies on T cells, in NK-92 cells cathepsin X was not co-localized with LFA-1 at the plasma membrane but was, rather, redistributed to the cytotoxic granules and secreted during degranulation.

Background: Trophoblast cell surface antigen 2 (TROP2) is widely overexpressed in epithelial cancers and associated with poor prognosis, representing an attractive target for cancer treatment. Recent clinical studies reported favorable outcomes of TROP2-targeting antibody–drug conjugates. However, heterogeneity of human cancers and development of resistance encourage the exploration of alternative cytotoxic payloads. In this regard, bacterial immunotoxin is a viable treatment option owing to the unique and potent mechanism of action. Methods: We first constructed three immunotoxins by fusing deimmunized Pseudomonas Exotoxin A (PE24) to novel TROP2-targeting single-chain variable fragments identified through phage display, which were screened for high binding and cytotoxic activity against tumor cell lines with distinct TROP2 expression. The lead immunotoxin T3PE was engineered by introducing extra disulfide bonds and an albumin-binding domain (ABD) to generate disulfide-stabilized T3PE (dsT3PE and dsT3PE_ABD_CC). Then, both variants were characterized in stability, cell binding, internalization, pharmacokinetic profiles, and in vitro and in vivo antitumor activities. Results: T3PE bound strongly to TROP2 (EC₅₀ = 0.3824 μg/mL) and exerted potent cytotoxicity against NCI-N87 cells (EC₅₀ = 0.09074 μg/mL), but it displayed poor serum stability and was prone to aggregation. Introduction of disulfide bonds stabilized the immunotoxins while preserving the potent cytotoxic activity, and fusion of ABD extended the plasma half-life 6-fold in mice. Additionally, five doses of either dsT3PE or dsT3PE_ABD_CC at 3 mg/kg could significantly reduce tumor progression in the NCI-N87 mouse xenograft model. Conclusions: Together, these results demonstrated the therapeutic potential of these novel anti-TROP2 immunotoxins against TROP2-overexpressing tumors.