Blood M2a monocyte polarization and increased formyl peptide receptor 1 expression are associated with progression from latent tuberculosis infection to active pulmonary tuberculosis disease

Objectives: This study aims to explore the role of M2a polarization and formyl peptide receptor (FPR) regulation in the reactivation of Mycobacterium tuberculosis (Mtb) infection. Methods: M1/M2a monocyte percentage and FPR1/2/3 protein expression of blood immune cells were measured in 38 patients with sputum culture (+) active pulmonary TB disease, 18 subjects with latent TB infection (LTBI), and 28 noninfected healthy subjects (NIHS) using flow cytometry method. Results: M1 percentage was decreased in active TB versus either NIHS or LTBI group, while M2a percentage and M2a/M1 percentage ratio were increased. FPR1 expression on M1/M2a, FPR2 expression on M1, and FPR3 expression of M1 were all decreased in active TB versus LTBI group, while FPR1 over FPR2 expression ratio on NK T cell was increased in active TB versus either NIHS or LTBI group. In 11 patients with active TB disease, M1 percentage became normal again after anti-TB treatment. In vitro Mtb-specific antigen stimulation of monocytic THP-1 cells resulted in M2a polarization in association with increased FPR2 expression on M2a. Conclusions: Increased M2a and decreased M1 phenotypes of blood monocyte may serve as a marker for active TB disease, while decreased FPR1 on blood monocyte may indicate LTBI status.