Effects of Leishmania tropica infection on macrophage polarization

Macrophages are the primary host cells for Leishmania, and their polarization into either M1 (pro-inflammatory) or M2 (anti-inflammatory) phenotypes determines the course of the infection. This study aimed to investigate the effects of Leishmania tropica, the main agent of cutaneous leishmaniasis (CL) in Türkiye, on the polarization of primary human macrophages. Primary monocytes were isolated from human peripheral blood and differentiated into macrophages using GM-CSF. The cells were infected with L. tropica promastigotes, analyzed after 48 h. Polarization was evaluated through morphological changes, flow cytometry for surface markers (CD80, CD163, CD206, CD209), and qPCR for gene expression of M1 (NOS2, IDO1, IRF4) and M2 (ARG1, CCL22, CCL1) markers. An infection rate of 56.5% was achieved, with infected macrophages shifting from a spindle-like morphology to a rounded and granular appearance. Flow cytometry revealed a significant decrease in the expression of M1 marker CD80 and M2 markers CD206 and CD209 (p < 0.01), while CD163 expression showed a non-significant increase. qPCR analyses demonstrated a significant down regulation of M1-related genes and a significant upregulation of the M2-related gene ARG1 (p < 0.05). The findings suggest that L. tropica infection induces a shift in macrophage polarization from an M1-like phenotype toward an M2-like phenotype, with partial resemblance to an M2c-like profile, though the concurrent decrease in CD206 and CD209 underscores the complexity of the phenotype and indicates that the activated state does not conform neatly to any single M2 subtype. This shift likely serves as an immune escape strategy, promoting parasite persistence and influencing the chronicity of CL. Further studies incorporating additional M2c subtype markers, multiple time points, and functional assays are needed.