Immune Cells Skew Toward Inflammation in Children With Newly Diagnosed ITP

In a comparison of 32 children with acute ITP against 21 age- and sex-matched healthy controls, the researchers found that the patients’ monocyte-macrophage cells were shifted markedly toward a pro-inflammatory M1-like state, while the anti-inflammatory M2-like compartment was reduced and internally reshuffled. At the same time, the expression of two key immune checkpoint molecules, PD-L1 (CD274) and PD-L2 (CD273), was altered on these cells in ways that tracked with the severity of the children’s blood abnormalities and, in part, with how they responded to standard steroid therapy.

ITP is an immune-mediated disorder in which the platelet count falls below 100 × 10⁹/L because the body’s own immune system turns against its platelets. In the classic picture, autoantibodies coat platelets, and macrophages in the spleen engulf and destroy the opsonized cells. But platelet destruction is only part of the story; impaired platelet production also contributes, and the disease involves broader dysregulation of cellular immunity. Macrophages are central players here. Depending on the signals they receive, they can polarize into classically activated M1-like cells, which drive inflammatory and autoimmune responses, or alternatively activated M2-like cells, which are generally anti-inflammatory. The M2 compartment itself is further divided into subtypes with distinct functions: M2a-like cells are associated with tissue repair and fibrosis, M2b-like cells with mixed immune regulation, and M2c-like cells with the strongest anti-inflammatory and tolerogenic activity, mediated largely through interleukin-10.

To map this landscape in pediatric ITP, the team drew blood at diagnosis, before any corticosteroid treatment had begun, and analyzed the cells by flow cytometry. Using a panel of surface markers, they defined total CD11b-positive monocyte-macrophage-like cells and then subdivided them: M1-like cells were identified as CD86⁺CD11b⁺, M2a-like cells as CD206⁺CD11b⁺CD86⁻CD163⁻, M2b-like cells as CD86⁺CD206⁺CD11b⁺, and M2c-like cells as CD86⁻CD163⁺CD206⁺CD11b⁺. On each subset they quantified PD-L1 and PD-L2, the two ligands that bind the PD-1 receptor on T cells. That interaction normally acts as a brake on the immune system, switching off autoreactive T cells and inducing peripheral tolerance, so reduced ligand expression on macrophages could theoretically loosen the restraint on platelet-directed autoimmunity.

The polarization shift the researchers measured was striking. M1-like cells made up a median of 14.4 percent of the relevant population in the ITP patients, with an interquartile range of 12.1 to 16.8 percent, compared with just 3.4 percent in controls. Total M2-like cells moved in the opposite direction, accounting for a median of 81.6 percent in patients versus 92.7 percent in controls, and both differences were highly statistically significant. Within the M2 compartment, the subtypes diverged in unexpected ways. M2a-like cells were nearly doubled in the patients, at a median of 58 percent versus 29 percent in controls, while M2b-like cells fell to 11 percent from 24 percent and M2c-like cells dropped to 7 percent from 38 percent. In other words, the residual M2 population in ITP was not simply smaller; it had been restructured toward a repair-and-remodeling profile and away from the immunoregulatory M2b and M2c phenotypes.

The checkpoint molecule results added a second layer of dysregulation. On M1-like cells, both PD-L2 and PD-L1 were significantly reduced in the patients: median PD-L2 expression was 9 in patients versus 25 in controls, and PD-L1 was 3.9 versus 6.7. On the M2 side, PD-L2 was significantly lower across all three subtypes, with the largest relative drop on M2a-like cells, where the median fell from 25 in controls to 6 in patients. PD-L1 on M2-like subtypes, by contrast, showed only a non-significant tendency toward elevation. The authors interpret this pattern as a selective defect in inhibitory signaling rather than a uniform shutdown, and they suggest that the paired PD-L1 and PD-L2 signals on M2b-like macrophages may constitute a dual tolerogenic checkpoint axis that is disturbed in pediatric ITP.

These cellular patterns correlated with the children’s blood counts. Patients with ITP had significantly lower hemoglobin, platelet counts, and lymphocyte counts than controls, along with elevated mean platelet volume and platelet distribution width, markers that can reflect stressed or accelerated platelet production and turnover. Within the patient group, platelet count correlated positively with total M2-like cells, and the checkpoint-expressing M2b subsets stood out: CD274-positive M2b-like cells showed a strong positive correlation with platelet count and hemoglobin and inverse correlations with mean platelet volume and platelet distribution width, while CD273-positive M2b-like cells correlated positively with platelet count and negatively with the same platelet indices. M1-like cells, in contrast, correlated with adverse platelet distribution width but not significantly with mean platelet volume. The authors note that, to their knowledge, this is the first study to demonstrate direct correlations between macrophage checkpoint molecule expression and platelet indices in pediatric ITP.

The treatment data provided a clinical angle. All enrolled children received first-line oral prednisone at 2 to 4 mg/kg/day for 5 to 7 days, following American Society of Hematology guidance, and were later classified as responders or non-responders using international consensus criteria for pediatric ITP. Nineteen of the 32 patients showed no response, while 13 achieved a partial or complete response. The non-responders had higher M1 polarization, elevated M2a levels, and reduced M2b and M2c subsets compared with responders. Checkpoint expression also differed: PD-L2-positive M1 cells and PD-L2-positive M2a cells were significantly lower in non-responders, PD-L2 on M2b-like cells showed a non-significant trend in the same direction, and PD-L2 on M2c-like cells was essentially identical between the groups. The authors are careful to frame these as associations, suggesting but not proving that a shift toward M2b and M2c polarization accompanies better treatment response.

The findings fit with a broader body of work on macrophage biology in platelet disorders. Previous studies have reported an M1 predominance at ITP onset with recovery of the M1/M2 balance during remission, and animal and translational work has linked the PI3K/AKT signaling pathway to the promotion of the M2 phenotype and to platelet production, with disrupted signaling favoring M1 polarization, platelet apoptosis, and T-cell autoimmunity. Prior studies have also documented altered PD-1 and PD-L1 levels in ITP, including increased PD-1 on T cells and reduced PD-L1 on dendritic cells in active disease. The new study extends this picture to children and to the finer M2 subtypes, proposing that the rise of M2a-like cells represents a compensatory but ineffective response to inflammation, since M2a cells lack the strong interleukin-10-mediated immunosuppressive capacity of M2c cells.

Important caveats temper these conclusions. The study was single-center and small, which limits generalizability; the correlations were not adjusted for multiple comparisons; the cross-sectional design could not capture how macrophage polarization changes over the disease course or with treatment; the cells examined came from peripheral blood rather than the bone marrow, where much of platelet regulation occurs; and macrophage subsets were defined by surface marker phenotyping alone, without functional testing of what the cells actually do. The association with corticosteroid response, in particular, awaits confirmation in larger prospective cohorts and functional studies. Within those limits, the work points to macrophage polarization and the PD-L1/PD-L2 checkpoint system as candidate immunomodulatory targets in childhood ITP, and it offers a measurable cellular signature that may eventually help clinicians anticipate which children are likely to resist first-line steroid therapy.

Subject of Research: Monocyte-macrophage polarization and PD-L1/PD-L2 checkpoint expression in children with newly diagnosed immune thrombocytopenia

Article Title: Unveiling the role of circulating monocyte-macrophage polarization state and PD-L1/PD-L2 expression in newly diagnosed pediatric ITP

Article References: Zahran, A. M., El-Badawy, O., Youssef, M. A. M., Ghattas, M. S. Y., Mohamed, M. H., Morsy, S. G., Elsayh, K. I., Elashmawy, H., & Zahran, Z. A. M. (2026). Unveiling the role of circulating monocyte-macrophage polarization state and PD-L1/PD-L2 expression in newly diagnosed pediatric ITP. Blood Research, 61(1), Article 59. https://doi.org/10.1007/s44313-026-00163-0

Image Credits: AI Generated

DOI: 10.1007/s44313-026-00163-0

Keywords: immune thrombocytopenia, pediatric ITP, macrophage polarization, M1 macrophages, M2 macrophages, PD-L1, PD-L2, immune checkpoints, flow cytometry, corticosteroid response, platelet count, case-control study