Megakaryocyte emperipolesis in myeloproliferative neoplasms: Are neutrophils friends or foes?

Abstract

Megakaryocyte emperipolesis is a biological process in which a cell penetrates and exists as a viable intact cell within another. It is a recognized morphological feature of myeloproliferative neoplasms (MPNs), in which neutrophils can be seen within megakaryocytes in bone marrow smears and sections. We aimed to determine whether neutrophil contents, specifically protein and RNA, are deposited within megakaryocytes due to emperipolesis. Evaluation of hematoxylin and eosin-stained bone marrow showed that 84% of MPN patients (n = 163) had megakaryocyte emperipolesis, most notably in enlarged megakaryocytes and those with pyknotic/condensed nuclei. Morphological assessment and immunohistochemical staining for CD15-neutrophil membrane antigen confirmed that majority of intramegakaryocytic cells were neutrophils, and that emperipolesis was more frequent in myelofibrosis patients and patients with pathologic reticulin. Furthermore, megakaryocytes in MPNs were observed to have intracellular positivity for neutrophil azurophilic granule protein myeloperoxidase (MPO) (n = 21 MPN patients) and specific granule lactoferrin (n = 56). Platelets were also used as a surrogate to establish if neutrophil contents had been transferred into megakaryocytes intracellularly of MPN patients, using mass spectrometry to assess protein and transcriptomic next-generation sequencing to assess messenger RNA (mRNA). A total of 109 neutrophil mRNA transcripts and 20 neutrophil granule proteins were upregulated in platelets of MPN patients compared with control subjects, including cathepsin-G and lactoferrin, with 5.1- and 4.6-fold increase in mRNA and 1.8- and 1.4-fold protein increases, respectively. This suggests that the transfer of neutrophil material occurs during emperipolesis in disease state, which could be a consequence of neutrophil degranulation or apoptosis.