Study population
We collected bone marrow biopsies of patients with PMF done at Tulane Medical Center from 1990 to 2019. All these patients had a known JAK2 status and a well-preserved specimen for adequate PD-L1 staining. We only used the initial bone marrow biopsy obtained to diagnose patients with PMF. We collected 10 additional samples of patients that came to clinic with transient cytopenias from benign hematologic conditions. These patients had normal karyotype, FISH panel for myelodysplastic syndrome (MDS), and a negative 64-gene next generation sequencing (NGS) myeloid mutation panel. An institutional review board approval was obtained before collecting these bone marrow samples and all participants gave informed consent. The authors analysed the data to which all authors had access. All relevant participant data was deidentified and shared as appropriate in the text.