Deciphering the functional roles of PE18 and PPE26 proteins in modulating pathogenesis and immune response.

Introduction: Tuberculosis (TB), caused by (Mtb), remains a leading cause of mortality worldwide. A crucial factor in virulence is the ESX-5 secretion system, which transports PE/PPE proteins such as PE18 and PPE26. These proteins modulate host-pathogen interactions, immune responses, and intracellular survival mechanisms. Despite their importance, the roles and molecular interactions of PE18 and PPE26 in pathogenesis require further investigation.

Methods: We explored the roles of PE18 and PPE26 using recombinant () as a model organism. Protein-protein interactions were analyzed biochemically to identify partners within the ESX-5 secretion system, including EspG5 and other PE/PPE proteins. Subcellular localization of these proteins was assessed via cell fractionation studies. Functional assays, including cytokine production and antigen presentation studies, were performed using TLR2/Myd88 knockout and wild-type macrophages. experiments were conducted to assess effector T-cell activation and intracellular survival. Mechanistic insights into endosome-phagosome maturation and actin cytoskeleton dynamics were obtained through fluorescence microscopy.

Results: Our biochemical analyses confirmed interactions between PE18/PPE26, PE18/PPE27, PE19/PPE25, and EspG5/PPE, highlighting their involvement in ESX-5-mediated secretion. Cell fractionation studies revealed that PE/PPE proteins predominantly localize to the cell wall, with PE18 also secreted extracellularly. In vitro and experiments demonstrated that PE18 and PPE26 activate cytokine production and antigen presentation via TLR2/Myd88-dependent signaling pathways, inducing robust effector memory T-cell responses. Recombinant expressing PE18, PPE26, or their combination exhibited enhanced intracellular survival by disrupting endosome-phagosome maturation, likely through interference with actin cytoskeletal organization.

Discussion: Our findings elucidate the pivotal roles of PE18 and PPE26 in pathogenesis, emphasizing their contributions to immune modulation and intracellular persistence. The observed disruption of actin dynamics and endosome-phagosome maturation underscores a novel mechanism by which evades host defenses. The ability of PE18 and PPE26 to induce effector T-cell responses highlights their potential as targets for host-directed therapies or vaccine development against TB. Further studies focusing on their structure-function relationships and interactions with host proteins could accelerate the development of innovative therapeutic strategies.