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Hox‐driven conditional immortalization of myeloid and lymphoid progenitors: Uses, advantages, and future potential

Those who study macrophage biology struggle with the decision whether to utilize primary macrophages derived directly from mice or opt for the convenience and genetic tractability of immortalized macrophage‐like cell lines in in vitro studies. Particularly when it comes to studying phagocytosis and phagosomal maturation—a signature cellular process of the macrophage—many commonly used cell lines are not representative of what occurs in primary macrophages. A system developed by Mark Kamps' group, that utilizes conditionally constitutive activity of Hox transcription factors (Hoxb8 and Hoxa9) to immortalize differentiation‐competent myeloid cell progenitors of mice, offers an alternative to the macrophage/macrophage‐like dichotomy. In this resource, we will review the use of Hoxb8 and Hoxa9 as hematopoietic regulators to conditionally immortalize murine hematopoietic progenitor cells which retain their ability to differentiate into many functional immune cell types including macrophages, neutrophils, basophils, osteoclasts, eosinophils, dendritic cells, as well as limited potential for the generation of lymphocytes. We further demonstrate that the use of macrophages derived from Hoxb8/Hoxa9 immortalized progenitors and their similarities to bone marrow‐derived macrophages. To supplement the existing data, mass spectrometry‐based proteomics, flow cytometry, cytology, and in vitro phagosomal assays were conducted on macrophages derived from Hoxb8 immortalized progenitors and compared to bone marrow‐derived macrophages and the macrophage‐like cell line J774. We additionally propose the use of a standardized nomenclature to describe cells derived from the Hoxb8/Hoxa9 system in anticipation of their expanded use in the study of leukocyte cell biology.