PD-1/PD-L1-CXCR3 Coexpression and CXCR3 on Intermediate Monocytes Predict Fibrosis, Leukemic Transformation, and Survival in Egyptian Philadelphia-Negative Myeloproliferative Neoplasms.

[BACKGROUND] Despite established prognostic scores, predicting disease progression in myeloproliferative neoplasms (MPNs) remains challenging, and the role of specific immune cell subsets-particularly the PD-1/PD-L1 axis-in mediating fibrosis and transformation is poorly characterized. This study aimed to investigate the prognostic utility of novel immune-inflammatory biomarkers, including PD-1/PD-L1 coexpression with C-X-C chemokine receptor type 3 (CXCR3), CXCR3 expression on monocyte subsets, and systemic indices (systemic immune-inflammation index [SII], systemic inflammation response index [SIRI]), for predicting fibrosis grade, leukemic transformation, and survival outcomes in patients with Philadelphia-negative MPNs.

[METHODS] In a prospective case-control study, including 90 Egyptian patients with Philadelphia-negative MPNs and 90 matched controls, we quantified PD-1/PD-L1 coexpression with CXCR3 and CXCR3 expression on monocyte subsets by flow cytometry. Validation employed targeted next-generation sequencing (NGS) in the full cohort, complemented by focused mechanistic validation using bulk RNA sequencing (n = 20 primary myelofibrosis [PMF], n = 10 controls), single-cell RNA sequencing (n = 10 PMF, n = 5 controls), and targeted serum cytokine profiling (IL-6 and TGF-β; n = 90 MPN, n = 90 controls). Diagnostic/prognostic performance used receiver operating characteristic (ROC) curves, decision curve analysis, logistic/Cox regression, random forest/gradient boosting, and external validation in an independent Egyptian cohort (n = 30). Patient-reported outcomes were assessed with the Myeloproliferative Neoplasm Symptom Assessment Form (MPN-SAF).

[RESULTS] PD-1/CXCR3 and PD-L1/CXCR3 were elevated in patients ( < .001). PD-L1/CXCR3 demonstrated high discrimination between advanced (grade 4) and grade 3 fibrosis (area under the curve [AUC] 0.975; cut-off >40%; sensitivity 92.9%; specificity 90%). In an exploratory analysis, reduced CXCR3 on intermediate monocytes was associated with leukemic transformation (AUC 0.876; cut-off ⩽ 12%) with high negative predictive value (98.4%). Higher PD-L1/CXCR3 correlated with worse fatigue ( = 0.58,  < .001) and pruritus ( = 0.45,  = .002) on MPN-SAF and was an independent predictor of poorer survival. A biomarker-enhanced prognostic score (incorporating high PD-L1/CXCR3, low CXCR3/intermediate, age, and Dynamic International Prognostic Scoring System) outperformed DIPSS/DIPSS-Plus/MIPSS70 (AUC 0.85; bootstrap AUC 0.84) and showed promising initial validation in an external cohort (AUC 0.82). Single-cell RNA sequencing revealed a CXCR3-low intermediate monocyte population enriched for profibrotic/inflammatory signatures; targeted cytokine analysis indicated higher IL-6/TGF-β in checkpoint-high states.

[CONCLUSION] PD-1CXCR3/PD-L1CXCR3 coexpression and relatively low CXCR3 expression on intermediate monocytes (within the patient cohort) are promising biomarkers for fibrosis, leukemic transformation, and survival in Egyptian MPNs. Their integration with clinical scores improved risk stratification and showed promising initial performance in an external cohort. These findings support incorporating immune checkpoint-chemokine profiling into precision medicine frameworks for MPNs.