The Role of Macrophages in the Immune Response to Peritoneal Infection with Dirofilaria immitis Larvae in the Mongolian Gerbil (Meriones unguiculatus)

The host cellular mechanisms determining whether Dirofilaria immitis (canine heartworm) larvae establish infection remain poorly understood. The Mongolian gerbil (Meriones unguiculatus), a naturally nonpermissive host, mounts a rapid macrophage-dominated peritoneal cellular response to intraperitoneal (IP) larval challenge. To test whether macrophages mediate larval clearance, peritoneal macrophages were selectively depleted with IP clodronate liposomes (100 µL/10 g body weight; days −4 and −1 relative to infection) before challenge with approximately 40 D. immitis third-stage larvae (L3). A rat anti-mouse anti-F4/80 antibody was validated as a cross-reactive macrophage marker in jird peritoneal exudate cells (PECs) by standard and imaging flow cytometry, and depletion efficacy was confirmed prior to infection. Significantly more L3 were recovered from macrophage-depleted jirds (mean = 17.7%; SEM = 6.0%) than from PBS liposome controls (mean = 3.8%; SEM = 1.9%) at 1-day post-infection (p < 0.03). Cytology and flow cytometry confirmed near-complete macrophage depletion, and a significant compensatory neutrophilia was observed in depleted animals (64.5% ± 5.7% vs. 19.6% ± 5.1% in controls; p < 0.0001). These findings establish peritoneal macrophages as the primary innate cellular mediators of D. immitis L3 clearance in a nonpermissive host, providing a validated in vivo framework for mechanistic dissection of filarial host specificity.