Recent evidence has revealed that oncogenic mutations may confer immune escape. A better understanding of how an oncogenic mutation affects immunosuppressive programmed death ligand 1 (PD-L1) expression may help in developing new therapeutic strategies. We show that oncogenic JAK2 (Janus kinase 2) activity caused STAT3 (signal transducer and activator of transcription 3) and STAT5 phosphorylation, which enhanced PD-L1 promoter activity and PD-L1 protein expression in JAK2-mutant cells, whereas blockade of JAK2 reduced PD-L1 expression in myeloid JAK2-mutant cells. PD-L1 expression was higher on primary cells isolated from patients with JAK2-myeloproliferative neoplasms (MPNs) compared to healthy individuals and declined upon JAK2 inhibition. JAK2 mutational burden, pSTAT3, and PD-L1 expression were highest in primary MPN patient-derived monocytes, megakaryocytes, and platelets. PD-1 (programmed death receptor 1) inhibition prolonged survival in human MPN xenograft and primary murine MPN models. This effect was dependent on T cells. Mechanistically, PD-L1 surface expression in JAK2-mutant cells affected metabolism and cell cycle progression of T cells. In summary, we report that in MPN, constitutive JAK2/STAT3/STAT5 activation, mainly in monocytes, megakaryocytes, and platelets, caused PD-L1-mediated immune escape by reducing T cell activation, metabolic activity, and cell cycle progression. The susceptibility of JAK2-mutant MPN to PD-1 targeting paves the way for immunomodulatory approaches relying on PD-1 inhibition.
Sterile alpha motif domain protein 9 (SAMD9) and its paralogue SAMD9-like (SAMD9L) are cytoplasmic proteins encoded by two juxtaposed single-exon genes on chromosome 7q21. They share a 60% amino acid sequence identity and likely originated from a duplication of a common ancestral gene 1 . Their function remains enigmatic; they have been linked to tumor suppression 2 , inflammation 3 , stress response 4 , development 4 , endosomal fusion 5,6 and protein translation 7,8 . Both proteins were also shown to function as restriction factors forming a cross-species barrier for poxvirus infection [9][10][11][12] . Structural analysis of these large proteins has been limited to homology modeling, which predicted identical domains in either protein (SAM, ALBA2, SIR2, P-loop/ NTPase and OB-fold) 13 . Moreover, these genes exhibit tight regulation during embryonic development and transition to ubiquitous expression levels in adult tissues 14,15 .Notably, Samd9l-haploinsufficient mice develop myeloid neoplasia mimicking human MDS with monosomy 7 5 . Several groups reported germline SAMD9 or SAMD9L mutations (SAMD9/9L mut ) underlying new human syndromes with a propensity for cytopenia, bone marrow failure (BMF) and MDS with non-random monosomy 7 or deletion of 7q 6,16-28 . SAMD9 mutations (SAMD9 mut ) were initially linked to a fatal, early-onset MIRAGE syndrome (myelodysplasia, infections, restriction of growth, adrenal hypoplasia, genital phenotypes and enteropathy) 6,29 . In contrast, SAMD9L mutations (SAMD9L mut ) were originally described in families with a progressive neurological phenotype, multi-lineage cytopenia and bone marrow hypoplasia (ataxia-pancytopenia syndrome) 16,17 . Recent reports broadened this spectrum and found missense SAMD9/9L mut in non-syndromic familial MDS [30][31][32][33] , truncating SAMD9L mut in children with autoinflammatory panniculitis resembling CANDLE
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