Immune cells unexpectedly help the heart keep its beat

first_img The immune cells known as macrophages have a long to-do list in our bodies. They defend us from bacteria, coax wounds to heal, and perform other vital tasks. A surprising new study suggests they are also essential for the heart to beat normally. That could make macrophages prime targets for treating conditions like arrhythmias, in which the heart beats erratically.“It’s a very careful study all the way through,” says cardiac electrophysiologist Douglas Zipes of the Indiana University School of Medicine in Indianapolis, who wasn’t connected to the research. Macrophages “are clearly playing an important role” in the heart, he says.A heartbeat begins when the organ’s pacemaker, a structure called the sinoatrial node, fires off an electrical impulse that disperses across the atria, the blood-collecting chambers. But to reach the ventricles, the heart’s pumping chambers, the signal needs a little help. Another structure, the atrioventricular (AV) node, relays the impulse to the ventricles, allowing them to contract. Macrophages, researchers assumed, weren’t involved. Click to view the privacy policy. Required fields are indicated by an asterisk (*) By Mitch LeslieApr. 20, 2017 , 12:00 PM Immune cells unexpectedly help the heart keep its beat This macrophage is busy capturing bacteria, but its relatives may be helping your heart beat. Sign up for our daily newsletter Get more great content like this delivered right to you! Countrycenter_img Email Country * Afghanistan Aland Islands Albania Algeria Andorra Angola Anguilla Antarctica Antigua and Barbuda Argentina Armenia Aruba Australia Austria Azerbaijan Bahamas Bahrain Bangladesh Barbados Belarus Belgium Belize Benin Bermuda Bhutan Bolivia, Plurinational State of Bonaire, Sint Eustatius and Saba Bosnia and Herzegovina Botswana Bouvet Island Brazil British Indian Ocean Territory Brunei Darussalam Bulgaria Burkina Faso Burundi Cambodia Cameroon Canada Cape Verde Cayman Islands Central African Republic Chad Chile China Christmas Island Cocos (Keeling) Islands Colombia Comoros Congo Congo, the Democratic Republic of the Cook Islands Costa Rica Cote d’Ivoire Croatia Cuba Curaçao Cyprus Czech Republic Denmark Djibouti Dominica Dominican Republic Ecuador Egypt El Salvador Equatorial Guinea Eritrea Estonia Ethiopia Falkland Islands (Malvinas) Faroe Islands Fiji Finland France French Guiana French Polynesia French Southern Territories Gabon Gambia Georgia Germany Ghana Gibraltar Greece Greenland Grenada Guadeloupe Guatemala Guernsey Guinea Guinea-Bissau Guyana Haiti Heard Island and McDonald Islands Holy See (Vatican City State) Honduras Hungary Iceland India Indonesia Iran, Islamic Republic of Iraq Ireland Isle of Man Israel Italy Jamaica Japan Jersey Jordan Kazakhstan Kenya Kiribati Korea, Democratic People’s Republic of Korea, Republic of Kuwait Kyrgyzstan Lao People’s Democratic Republic Latvia Lebanon Lesotho Liberia Libyan Arab Jamahiriya Liechtenstein Lithuania Luxembourg Macao Macedonia, the former Yugoslav Republic of Madagascar Malawi Malaysia Maldives Mali Malta Martinique Mauritania Mauritius Mayotte Mexico Moldova, Republic of Monaco Mongolia Montenegro Montserrat Morocco Mozambique Myanmar Namibia Nauru Nepal Netherlands New Caledonia New Zealand Nicaragua Niger Nigeria Niue Norfolk Island Norway Oman Pakistan Palestine Panama Papua New Guinea Paraguay Peru Philippines Pitcairn Poland Portugal Qatar Reunion Romania Russian Federation Rwanda Saint Barthélemy Saint Helena, Ascension and Tristan da Cunha Saint Kitts and Nevis Saint Lucia Saint Martin (French part) Saint Pierre and Miquelon Saint Vincent and the Grenadines Samoa San Marino Sao Tome and Principe Saudi Arabia Senegal Serbia Seychelles Sierra Leone Singapore Sint Maarten (Dutch part) Slovakia Slovenia Solomon Islands Somalia South Africa South Georgia and the South Sandwich Islands South Sudan Spain Sri Lanka Sudan Suriname Svalbard and Jan Mayen Swaziland Sweden Switzerland Syrian Arab Republic Taiwan Tajikistan Tanzania, United Republic of Thailand Timor-Leste Togo Tokelau Tonga Trinidad and Tobago Tunisia Turkey Turkmenistan Turks and Caicos Islands Tuvalu Uganda Ukraine United Arab Emirates United Kingdom United States Uruguay Uzbekistan Vanuatu Venezuela, Bolivarian Republic of Vietnam Virgin Islands, British Wallis and Futuna Western Sahara Yemen Zambia Zimbabwe The Science Picture Company/Alamy Stock Photo However, the cells are prevalent in the heart, and cell biologist Matthias Nahrendorf of Harvard Medical School in Boston and colleagues wanted to determine what they did. When the team began examining mice that lacked macrophages, a technician noticed that the electrical rhythms of the animals’ hearts were abnormal. Nahrendorf says the rodents appeared to have an AV block, in which the AV node doesn’t forward the atrial signal to the ventricles.The researchers then discovered that macrophages were common in AV nodes from mouse and human hearts. The AV node also contains many heart muscle cells that can transmit the “contract now” signal. When the researchers grew macrophages and heart muscle cells together in culture dishes, they found that the two types of cells were connected physically and synchronized electrically. Because of these interactions, macrophages made it easier for their muscular neighbors to fire, the group reports today in Cell.But do macrophages have the same impact in a beating heart? To find out, the scientists studied the isolated hearts of genetically modified mice whose macrophages respond to light. The cells’ electrical charge becomes more positive, which in turn promotes firing by neighboring heart muscle cells. After spurring the animals’ hearts to beat faster with an electrical stimulus, the team used a fiber optic cable to illuminate the organ, thus prodding the macrophages. Because the heart is contracting rapidly in these experiments, it occasionally misses a beat when the AV node fails to relay the contraction signal to the ventricles. Fewer of these missed beats occurred in hearts whose macrophages had been stimulated by light, the researchers discovered. “If the light is switched on, the AV node functions better,” Nahrendorf says.The researchers found that the AV node was also faulty in two other groups of genetically modified mice, which are missing the protein that links macrophages to heart muscle cells or that kill off their macrophages. “The net effect is that macrophages make conduction more reliable” Nahrendorf says.For scientists studying AV node biology, “this paper really puts macrophages on the map,” says molecular and developmental biologist Vincent Christoffels of the University of Amsterdam in the Netherlands. But he notes that previous studies have implicated other non–heart muscle cells, such as fibroblasts, in the AV node’s operation. “So now we need to know how big [macrophages’] contribution is compared to other cells.”Why the AV node needs assistance from macrophages, when the heart muscle cells it harbors conduct electrical impulses perfectly well, remains unclear, Nahrendorf says. Whatever the reason, the results raise the question of whether macrophages deserve some of the blame for conditions in which the heart rhythm is abnormal. If they do, researchers might be able to restore the heartbeat to normal by, for example, dosing macrophages with drugs that alter their behavior. Some of these drugs are already in clinical trials.last_img