Characterization of PPE19 as a novel mediator of <i>Mycobacterium tuberculosis</i> -macrophage interactions

<i>Mycobacterium tuberculosis</i> (Mtb) is a highly adapted human pathogen capable of manipulating host immunity. This study demonstrates that PPE19, a member of the PE/PPE protein family, facilitates Mtb adhesion to, and invasion of murine macrophages. PPE19-coated microspheres showed enhanced uptake by macrophages compared to control beads, while Mtb overexpressing <i>ppe19</i> (Rv1361c) was phagocytosed at a significantly greater rate than WT Mtb. <i>ppe19</i> is identified as pH responsive and displays reduced expression following macrophage entry. CRISPR interference-mediated knockdown of two highly related PPE proteins, <i>ppe18</i> (Rv1196) and <i>ppe60</i> (Rv3478), revealed an additive reduction in Mtb's ability to invade host macrophages, indicating a potential functional relationship. Furthermore, the absence of an <i>in vivo</i> phenotype following murine infection with a <i>ppe19</i> knockout strain suggests functional redundancy within this PPE protein family. Finally, PE13 has been identified here as a binding partner for PPE19, characterizing another relationship presumed important for successful PPE secretion. These findings reveal PPE19 as a secreted effector protein used by Mtb to modulate important early interactions with the innate immune system, enhancing entry into host macrophages.IMPORTANCETuberculosis remains a leading infectious disease killer worldwide, with approximately one-quarter of the global population infected with <i>Mycobacterium tuberculosis</i> (Mtb). Understanding how this pathogen initially establishes infection is crucial for developing more effective vaccines and treatments. This study identifies PPE19, a previously uncharacterized bacterial protein, as a key factor that helps Mtb invade and colonize human immune cells called macrophages during the earliest stages of infection. The research shows that PPE19 acts like a molecular "key" that facilitates bacterial entry into host cells but is then downregulated once the bacteria are safely inside. Importantly, PPE19 belongs to a family of similar proteins that can compensate for each other, explaining why targeting individual members may not be sufficient for treatment. These findings provide new insights into tuberculosis pathogenesis and suggest that early infection factors like PPE19 could serve as targets for next-generation vaccines designed to prevent initial infection rather than just disease progression.