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B cells modulate the expression of MHC-II on cardiac CCR2 macrophages

https://doi.org/10.1016/j.yjmcc.2021.05.003Get rights and content

Highlights

  • Intravascular myocardial associated B cells are found in proximity to cardiac macrophages.

  • B cell depletion leads to a reduction in the number of CCR2MHC-IIhigh resident macrophages in the heart.

  • B cell modulate MHC-II expression on cardiac macrophages through a paracrine effect.

Abstract

The uninjured murine heart contains a heterogeneous population of macrophages with disparate ontogenies and functions. These macrophages are often associated with blood vessels and can be subclassified based on the expression of CC chemokine receptor 2 (CCR2) and major histocompatibility complex class II (MHC-II). The biological cues that modulate these macrophage pool subpopulations have not been completely identified. It has been recently shown that a sub-population of circulating naïve B cells adheres to the myocardial microvasculature. We hypothesized that B cells might modulate the phenotype of myocardial macrophages. To test this hypothesis, we analyzed both the relative location of B cells and macrophages in myocardial histological section and the prevalence of myocardial macrophage subsets in hearts from B cell–deficient mice (μMT) and mice depleted of B cells through administration of an anti-CD20 antibody. We found that B cells pause in the microvasculature in proximity of macrophages and modulate the number of myocardial CCR2MHC-IIhigh cells. Through in vitro studies we found that this is likely the result of a paracrine effect of B cells on the expression of MHC-II in CCR2 cells. These results reveal an unexpected relationship between B cells and resident macrophages and, highlighting a direct intramyocardial effect of circulating B cells, challenge the currently held belief that naïve recirculating B lymphocytes merely shuttle between lymphoid stations.

Introduction

Current models of B cell biology posit that naïve B cells merely recirculate between primary and secondary lymphoid stations until meeting their cognate antigen [1,2]. However, recent studies have highlighted that: 1) the naïve myocardium contains a sizeable population of naïve circulating B cells that pause on the myocardial microvascular endothelium; 2) B cell–deficient and B cell–depleted mice have alterations in myocardial mass, myocardial contractility and number of myocardial T cells [3,4]. These studies pointed to an unexpected biological relationship between B cells and the heart but did not investigate whether B cells exhibit any local effects in the myocardium [3]. The demonstration of a local B cell effect within the myocardium would broaden our understanding of the relationship between circulating immune cells and the myocardium and challenge current paradigms of B cell biology.

The murine myocardium contains four distinct subsets of CD64+ macrophages defined by different expression levels of CC chemokine receptor 2 (CCR2) and major histocompatibility complex class II (MHC-II) [5]. The vast majority of macrophages in the naïve heart are CCR2 resident macrophages (MHC-IIhigh or MHC-IIlow) of embryonic origin. CCR2+ cells are bone marrow–derived monocytic cells that acquire higher levels of MHC-II expression as they fully differentiate into tissue macrophages [[5], [6], [7]]. These macrophage subsets have several important roles in the context of myocardial physiology and pathology [6,[8], [9], [10], [11]]. Recently, a population of intra parenchymal myocardial macrophages has been identified in close association with the vasculature [8]. The factors that modulate the myocardial macrophage pool composition in the naïve heart have not been completely understood.

Here, we used histology, flow cytometric analysis of myocardial macrophages from B cell–deficient mice and B cell–depleted mice, and primary cardiac macrophage cultures to investigate the hypothesis that B cells modulate the phenotype of myocardial macrophages through a paracrine effect.

Section snippets

Mice

To investigate the effects of B cell deficiency, we used 4–5 week-old male and female wild-type (WT) (background C57BL/6 J; Stock No. 000664) and μMT mice (B cell deficient) (background C57BL/6 J; B6.129S2-Ighmtm1Cgn/J; Stock No. 002288) age and sex matched. These strains were purchased from The Jackson Laboratory and bred in house. The CD19 reporter model was generated as previously reported [3]. Mice were bred and maintained at the Washington University School of Medicine Animal Care Facility

Results and discussion

We began investigating the relationship between B cells and myocardial macrophages by immunostaining cryosections of naïve murine hearts and analyzing the relative positions of B cells and CD68+ macrophages. We found that myocardial-associated B cells were almost exclusively intravascular and myocardial macrophages were frequently perivascular (Fig. 1A and Supplementary movies S1 and S2). Intravascular myocardial associated B cells and perivascular myocardial macrophages were typically in close

Contribution to the field statement

The naïve myocardium hosts a heterogeneous pool of resident macrophages with disparate ontogeny and function. The biological cues that regulate the composition of the myocardial macrophage pool in the naïve heart remain mostly unexplored. Studying both B cell-deficient and B cell-depleted animals, we found that B cells modulate the composition of the myocardial macrophage pool in the naïve murine heart. We performed histological analysis and in vitro studies that support the notion that

Declaration of Competing Interest

LA is cofounder of i-Cordis LLC, which is focused on the development of B cell–modulating therapies for the treatment of heart failure, and coinventor on patent WO2019028062, “Pirfenidone derivatives for modulation of B lymphocyte activity and organ protection,” owned by Washington University in St Louis. CRR and FP have no conflict of interest to disclose.

Acknowledgments

This study was supported by NIH grant 1K08HL145108-01A1 and institutional funds from the Washington University in St. Louis Division of Cardiology and the Johns Hopkins University Division of Cardiology. The authors acknowledge Dr. Doug Mann for critical discussions of this work. The authors acknowledge Dr. Erica Lantelme, Dr. Hao Zhang, the Washington University Flow Cytometry & Fluorescence Activated Cell Sorting Core and the Bloomberg School of Public Health Cell Sorting Facility for

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