The Role of M2 Macrophage in Primary Immune Thrombocytopenia

Background: Primary immune thrombocytopenia (ITP) is an acquired autoimmune hemorrhagic disorder, characterized by immune-mediated platelet destruction and impaired megakaryocyte maturation. Although impaired T cells have been implicated to participate in the pathogenesis of ITP, another immune cell signified as M2 macrophages has not been investigated properly in ITP patients. This study aimed to investigate the role of M2 macrophage subsets in primary immune thrombocytopenia (ITP).

Methods: Peripheral blood mononuclear cells (PBMC) from newly diagnosed ITP patients and healthy controls (HC) were isolated. M2-like macrophages (CD68⁺CD163⁺) and M2 macrophages (CX3CR1⁺CD163⁺) were measured by flow cytometry. The correlation between CD68⁺CD163⁺ cells and CX3CR1⁺CD163⁺ cells was also analyzed. The CX3CR1⁺cells were sorted by magnetic bead, CD68⁺CD163⁺ in PBMC of ITP patients and healthy controls were then isolated, and the proportion of m2-like macrophages before and after the sorting was analyzed. The PPAR gamma and arg-1 levels of mRNAs and proteins of CX3CR1⁺ M2 macrophages were examined by Real-time PCR and Western Blot, respectively.

Results: CX3CR1⁺CD163⁺M2 macrophages were positively correlated with CD68⁺ CD163⁺ M2-like macrophages in ITP patients (r = 0.54, p < 0.01). After magnetic bead separation, the proportion of CD68⁺CD163⁺ cells in CX3CR1⁺ cells was significantly increased (p = 0.02). Compared with HC, both the mRNA and protein levels of arg-1 of CX3CR1⁺ M2 macrophages were significantly increased in patients with ITP. The expression level of PPAR gamma protein was significantly increased in ITP than that of HC. However no statistical difference was detected at mRNA expression level, although it was numerically higher in ITP patients than in HC ( p = 0.19).

Conclusion: The peripheral CX3CR1⁺ M2 macrophage exercises similar phenotypes and functions of M2 macrophage. The remarkably increased expression of arg-1 at both transcription and protein levels and PPAR gamma at protein level of CX3CR1⁺M2 macrophages in ITP patients suggests potential immunomodulatory functions of these macrophage subsets during ITP pathogenesis. However, no significant change at mRNA level of PPAR gamma indicating that the increased PPAR gamma protein level might be caused by other mechanisms, such as after transcription abnormalities, which warrants further investigation.