How Mycobacterium tuberculosis Disarms Macrophages: Ahri Han Investigates Actin and Migration

At the 2025 Theobald Smith Society Spring Symposium, MD/PhD student Ahri Han from Samantha Bell’s Lab at Rutgers New Jersey Medical School presented compelling early-stage work investigating how Mycobacterium tuberculosis (Mtb) may suppress the ability of macrophages to move—disrupting a key component of host immune defense. Her talk explored the intersection of bacterial effectors, actin regulation, and macrophage migration using a combination of imaging, molecular modeling, and protein interaction studies.

Mtb remains a top global health threat, killing over a million people each year. Although macrophages are usually well-equipped to destroy pathogens, Mtb has evolved an arsenal of more than 100 secreted effector proteins that help it persist within host cells. Prior research suggests that intracellular pathogens can manipulate host actin networks to impair cell migration, but the specifics of how Mtb impacts macrophage motility remain largely unexplored.

To address this, Han developed a scratch assay adapted for murine macrophages (iBMDMs) to measure how quickly they repopulate a cleared area in a cell monolayer. Initial experiments showed that activated macrophages migrate faster than untreated cells, confirming the assay's sensitivity and utility for studying motility changes.

The project then turned to a specific Mtb effector protein: PPE15. Using protein structure prediction tools (AlphaFold3 and ChimeraX), Priyanka et al. identified a region in PPE15 with structural similarity to the SH3 domain found in many eukaryotic proteins. SH3 domains typically bind proline-rich sequences in actin-regulating proteins like N-WASP, a key player in actin polymerization.

Their structural predictions suggested that PPE15 might interact with N-WASP through this SH3-like domain. Han followed up with a co-immunoprecipitation assay in HEK293T cells and confirmed that PPE15 does, in fact, bind to N-WASP—supporting the idea that Mtb uses this effector to interfere directly with actin regulation in host cells.

Looking ahead, Han plans to test whether PPE15 expression in macrophages or its deletion in Mtb alters cell migration. She also aims to create mutants lacking the SH3-like domain to determine its role in binding and motility disruption. This work not only sheds light on Mtb's stealth strategies but could ultimately identify host-directed therapy targets to support macrophage function during infection.

See Ahri’s talk at: https://youtu.be/Byy4Q7lOUkM