6 - Macrophages and Cardiac Physiology Flashcards
How many times will a heart beat in a lifetime?
During 80-yr. lifespan, heart will beat ca. 3 billion times
Common causes of heart failure
- Myocardial infarction, leading to ischaemic injury
- Myocarditis, e.g.viral infection
- Endocarditis
- Arrhythmia
Layers of the heart wall
The wall of the heart separates into the following layers: epicardium, myocardium, and endocardium
What surrounds the heart?
- A double-layer, fluid-filled sac known as the pericardium surrounds the heart
- The two layers of the pericardium are called the outer fibrous/parietal pericardium and the inner serous/visceral pericardium
What is the epicardium?
The epicardium constitutes the visceral pericardium, underlying fibro-elastic connective tissue, and adipose tissue
What runs below the epicardium?
Coronary arteries and veins, lymphatic vessels, and nerves run below the epicardium
What is the endocardium composed of?
- The endocardium is composed of the endothelium and the subendothelial connective tissue layer.
- The subendocardium is found between the endocardium and myocardium and contains the impulse-conducting system
What did mice studies show that the heart contains?
Studies from mice showed that healthy, adult heart contains all major leukocyte classes, i.e. mononuclear phagocytes, neutrophils, B cells and T cells (12x greater frequency than skeletal muscle)
What is found to be interspersed throughout the heart?
Resident macrophages interspersed throughout heart and lodged between cardiomyocytes. Mφ shown to be numerous, heterogeneous and ontogenically diverse
What did surface marker profiling show?
Surface marker profiling confirmed 4 subsets, differing in expression of
MHC class II molecules, CC- chemokine receptor 2 (CCR2) and
lymphocyte antigen 6C (Ly6C)
What subsets were found through surface marker profiling?
(1) CCR2 neg. macrophages: comprise 2 numerically dominant MHC
class II subsets
(a) MHC II high and (b) MHC class II low subsets
(2) CCR2 pos. macrophages (minor population)
(3) Ly6C-pos. Lowest frequency subset
When do some macrophages arise?
Some cardiac macrophages arise before birth, e.g. from foetal liver
precursors, while others arise from monocytes post-birth
What have detailed studies shown about the subsets?
Detailed studies showed the CCR2– and Ly6C+ subsets populate
the heart during embryogenesis (from embryonic progenitors) prior to definitive haematopoiesis.
Which subsets require monocytes and which don’t?
Post-birth, CCR2- and Ly6C+ subsets self- renew without input from monocytes
In contrast, the minor CCR2+ subset comes from circulating monocytes
What happens when cells lose/gain expression of surface markers?
As cells lose/gain expression of surface markers, the subsets mix, making ontogenic lines harder to distinguish
What is believed about cardiac macrophages?
Generally believed that circulating monocytes make only a small contribution to cardiac macrophages in healthy heart tissue in mice and humans
Immune cell heart locations
- cardiac valve
- conduction system (atrioventricular node)
- coronary artery
- pericardium
What immune cells are present in the cardiac valve?
- macrophage
- dendritic cells
What immune cells are found in the conduction system (atrioventricular node)?
Macrophages
What immune cells are present in the coronary artery?
- mast cells
- CCR2- macrophage
- CCR2+ macrophage
- cardiomyocyte
- monocyte
- neutrophil
- B cell
- T cell
- CD103+ DC
- CD103- DC
What are steady-state macrophages?
Steady-state heart macrophages proliferate locally and self-renew independently of circulating monocytes
What can macrophages act as?
Mφ act as sentinels and in normal defence
What is proposed about cardiac macrophages?
- Proposed that cardiac macrophages aid maintenance of cell/matrix turnover, removal of local cell debris, adaptations to altered and increased tissue strain
- This may include physiological remodelling of cardiac tissue, e.g. during endurance training or pregnancy
What kind of communication is possible in cardiac macrophages?
Neural communication possible – e.g. regulating communication of sympathetic and vagal nerves with cardiomyocytes
What do cardiac macrophages form with cardiomyocytes?
Form gap junctions with cardiomyocytes
Participation of macrophages in normal electrical conduction
- sinus node and atrioventricular node are important in conduction
- ventricular QRS complex involves an atrial wave (P), QRS, and the ventricular depolarisation (T)
- macrophages in contact with cardiomyocytes
- connexion 43 (CX43) - containing gap junctions
- +ve charge travels through the gap junction - direction of flux depends on the phase of the cycle
Macrophages in diastole
- cations move from the cytoplasm of the macrophage into the cardiomyocyte cytoplasm via the CX43-containing gap junction
Macrophages in cardiomyocyte repolarisation?
- Cardiomyocyte becomes more + than Mφ Cations migrate support rapid repolarization of the cardiomyocyte.
- cations move into the macrophage
What happens when macrophages are absent in electrical conduction?
- Mφ absent - cardiomyocyte depolarization is not passed from atria to ventricles.
- Atrial P waves and QRS complexes are uncoupled. QRS occurrence now rare.
Immune cells in coronary heart disease timeline
- MINUTES: increased expression of adhesion molecules by endothelial cells
- 1 DAY: inflammatory cell recruitment & phagocytosis of dead cardiomyocytes
- 3-7 DAYS: neoangiogenesis, ongoing monocyte recruitment, attenuation of inflammation, & collagen production
- WEEKS: immune cell recruitment wanes & formation of scar tissue
Myocarditis
Inflammation of the heart, caused by infectious agents
Immune mechanisms in viral myocarditis
a. cardiomyocyte infection
b. cell death
c. DAMPs
d. phagocyte recruitment
e. migration of DCs to draining lymph node
f. activation of effector lymphocytes
g. lymphocyte recruitment to the heart
h. monocyte and neutrophil recruitment
Cardiomyocytes
- large, highly specialized cells, containing many organelles, including mitochondria and sarcomeres
- long-lived cells, minimal turnover in the heart
What do cardiomyocytes have to endure?
Cells have to endure high levels of metabolic and physical demand from the heart beats
Cardiomyocytes and mitochondria
Recently shown that cardiomyocytes eject mitochondria and other material in subcellular vesicles, which are then phagocytosed by cardiac Mφ
Cardiomyocytes and homeostais
Uptake aids homeostasis by preventing extracellular accumulation of
waste material, inflammasome activation, and autophagic block
Changes observed in mitochondrial numbers, morphology and function in Nicolas Avila study
- Mitochondria showed increased area, but reduced cristae density, inferring compromised fitness in the absence of cMacs.
- ATP production by purified cardiac mitochondria was significantly reduced in cMac-depleted mice
- Significant increase in cardiomyocyte mitochondria in cMφ depleted mice
Phagocytic activity of cMφ in Nicolas Avila study
- Bone marrow transplantation used to generate chimeric mice models in which circulating leukocytes, endothelial cells or cardiomyocytes expressed red fluorescent protein
- cMacs ingested abundant fluorescent material from circulating cells, cardiomyocytes and some material from endothelial cells
- Demonstrated that cMacs actively and specifically take up material from surrounding cardiac cellsin the steady state
- Cardiomyocytes produce mitochondria-containing exophors, which are taken up by cMφ
Mitochondrial uptake my macrophages in Nicolas Avila study
Heart infected with adeno-associated-virus (AAV9-mt-Keima) bearing mt- Keima (specific mitochondrial protein).
Role of subsets in lymphatic modelling
Surgical model of heart failure in mice showed role of the subsets influenced lymphatic remodelling and that experimental treatment with relevant antigenic molecules could manipulate this behaviour
Does the heart have an immune system?
The heart has a distinct immune system, including resident
macrophages comprising distinct subtypes, the majority of which arise
from embryonic and foetal progenitors
What do macrophages occupy?
Macrophages occupy particular niches in the heart and under steady state conditions, perform their sentinel and defence roles
How do macrophages facilitate conduction?
Through association with cardiomyocytes via gap junctions, macrophages facilitate normal electrical conduction
What do cardiac-resident macrophages do?
Cardiac-resident macrophages remove damaged/defunct mitochondria which are ejected in exophors from the cardiomyocytes
