Background/Purpose: Hemophagocytes (HPCs) are activated macrophages identified in situ by having engulfed other hematopoietic cells. HPCs are rarely seen in normal bone marrow, but are abundant in a variety of cytokine storm syndromes. HPCs are the pathologic hallmarks of two related diseases: Macrophage Activation Syndrome (MAS) and Hemophagocytic Lymphohistiocytosis (HLH). The function of the HPC is controversial. Some evidence suggests HPCs are inflammatory and pathogenic, while other evidence supports a housekeeping or anti-inflammatory role for these cells by pointing out their absence in some patients with overt MAS or HLH and their high expression of scavenger receptors like CD163. However, no study has yet attempted to directly phenotype these cells. Using two distinct approaches, we now show that HPCs display an M2, or anti-inflammatory phenotype.

Methods: Splenic HPCs induced in an animal model of MAS utilizing repeated Toll-like Receptor 9 (TLR9) stimulation in the context of IL-10 receptor blockade were isolated by single cell laser capture microdissection with the assistance of a board certified hematopathologist (MP). Gene expression levels of these cells were measured using microarray and compared to those of laser-captured resting splenic macrophages. Highly differentially expressed genes were verified using quantitative RT-PCR. Differential regulation of pre-determined gene sets was analyzed using a Gene Set Enrichment Analysis (GSEA). Additionally, bone marrow biopsies from patients with MAS or HLH noted to have excessive hemophagocytosis as part of their clinical evaluation were subjected to immunohistochemistry for markers of classical (M1) or alternative (M2) activation and scored by a blinded hematopathologist (MP).

Results: Of 6 treatment and 6 control samples, the RNA of 4 samples in each group were of sufficient quality for analysis. The gene sets meeting statistical significance for upregulation in HPCs were those related to the proteasome, M2 gene regulation, cytoskeleton regulation, and Nod-like receptor signaling. The gene set for M1 gene regulation was not found to be different between HPCs and resting macrophages. Human bone marrow biopsies stained for the mannose receptor and M2 marker CD206 showed HPCs with a membrane-bound staining pattern. HPCs with such staining for the M1 differentiation marker CD64 were very rarely identified.

Conclusion: For the first time, we have described the functional program of in situ HPCs as alternatively activated (M2) and potentially anti-inflammatory. This raises questions about elimination of HPCs as a therapeutic rationale, and supports future investigations into optimal treatment strategies for hemophagocytic diseases.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/transcriptome-and-surface-phenotype-analyses-suggest-an-alternatively-activated-m2-function-for-hemophagocytes/