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The TAM receptor tyrosine kinases Axl and Mer drive the phagocytic differentiation of tissue-resident macrophages

ABSTRACT: Many apoptotic thymocytes are generated during the course of T cell selection in the thymus, yet the machinery through which these dead cells are recognized and phagocytically cleared is incompletely understood. We find that the TAM receptor tyrosine kinases Axl and Mer, which are co-expressed by a specialized set of phagocytic thymic macrophages, are essential components of this machinery. Axl-/-Mertk-/- double mutant mice exhibited a marked accumulation of apoptotic cells during the time that autoreactive and nonreactive thymocytes die. Unexpectedly, the double mutants also displayed a profound deficit in the total number of phagocytic macrophages in the thymus, and concomitantly exhibited diminished expression of key non-TAM engulfment systems in the macrophages that remain. These previously unrecognized deficits were not confined to the thymus, and were also evident in the bone marrow and spleen. They had pleiotropic metabolic consequences for the double mutants, which included severe dysregulation of hemoglobin turnover, iron metabolism, and erythropoiesis.

ORGANISM(S): Mus musculus

PROVIDER: GSE192363 | GEO | 2022/07/12

REPOSITORIES: GEO

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Project description:Cells undergoing apoptosis are known to modulate their tissue microenvironments. By acting on phagocytes, notably macrophages, apoptotic cells inhibit immunological and inflammatory responses and promote trophic signaling pathways. Paradoxically because of their potential to cause death of tumor cells and thereby militate against malignant disease progression, both apoptosis and tumor-associated macrophages (TAM) are often associated with poor prognosis in cancer. In order to better understand the influence of tumor cell apoptosis and in particular its effect on TAM, we investigated global gene expression signatures of undisturbed TAM engaged in engulfment of apoptotic tumor cells. We studied a xenograft model of an aggressive ‘starry-sky’ non-Hodgkin’s lymphoma, Burkitt’s lymphoma (BL), in which apoptotic tumor cells are common and frequently observed in association with the starry-sky TAM (SS-TAM, so called because they appear histologically as ‘stars’ in a ‘sky’ of tumor cells) that accumulate in these tumors. We used a BL cell line (BL2) whose cells phenotypically resemble the tumor biopsy cells from which the line was derived including the capacity to undergo apoptosis constitutively. BL xenografts in SCID mice closely recapitulated the starry-sky histological picture of the human lymphoma. Due to the high sensitivity of macrophages to their environments, we adopted laser-capture microdissection of individual SS-TAM in BL xenografts in order to obtain unbiased in situ transcriptional profiles of these cells, which we compared specifically with those of similarly-captured macrophages, the tingible-body macrophages from normal germinal centers (GCM). The rationale for this comparison was based upon BL being a germinal center malignancy and tingible-body macrophages being regarded as normal equivalents of SS-TAM. Gene expression profiles of SS-TAM from BL2 xenograft tumors were compared to splenic GCM profiles. Three mice from each group were analysed. RNA was isolated from 1000 captured macrophages from each mouse and global gene expression signatures were obtained using Affymetrix Mouse Gene 1.0 GeneChip arrays.