WEBVTT 1 00:00:00.140 --> 00:00:03.020 All vaccines are based on the same principle: 2 00:00:03.020 --> 00:00:08.480 stimulate our immune system to prepare it for a pathogen before it infects us. 3 00:00:09.600 --> 00:00:15.200 Injecting an inactive portion of the virus, or the complete inactivated virus, 4 00:00:15.200 --> 00:00:19.160 stimulates sentinel cells and then B lymphocytes 5 00:00:19.160 --> 00:00:23.300 to produce pathogen-specific antibodies in large numbers. 6 00:00:23.800 --> 00:00:27.180 The body is thus prepared in case of infection. 7 00:00:27.760 --> 00:00:29.900 Let's see how it works. 8 00:00:30.500 --> 00:00:35.600 In the case of the SARS-CoV-2 virus, responsible for COVID-19, 9 00:00:35.600 --> 00:00:38.460 we know that the "S" protein of its envelope 10 00:00:38.460 --> 00:00:42.100 is able to trigger an immune response inside our bodies. 11 00:00:42.980 --> 00:00:47.380 Where many vaccines inject this S proteins in our body, 12 00:00:47.380 --> 00:00:52.000 m-RNA vaccines rely on a genetic approach to vaccination. 13 00:00:52.260 --> 00:00:55.440 m-RNA, developed in the laboratory, 14 00:00:55.440 --> 00:01:01.320 contains the genetic instruction to teach our cells how to make this S protein. 15 00:01:04.400 --> 00:01:08.970 Messenger RNA strands are encapsulated in tiny vesicles: 16 00:01:08.970 --> 00:01:11.180 lipid nanoparticles. 17 00:01:12.100 --> 00:01:17.720 Their function is just to transport their cargo of messenger RNA to their target: 18 00:01:17.720 --> 00:01:19.720 muscle cells. 19 00:01:22.720 --> 00:01:27.400 The lipid membrane of nanoparticles is similar to that of our cells. 20 00:01:27.400 --> 00:01:33.340 They will therefore fuse according to a natural mechanism called endocytosis 21 00:01:33.340 --> 00:01:37.880 and release their precious content into the cytoplasm of the cell. 22 00:01:39.740 --> 00:01:42.960 The genetic information of messenger RNAs 23 00:01:42.960 --> 00:01:48.140 is decoded by small units of protein synthesis: ribosomes. 24 00:01:48.820 --> 00:01:51.920 Encoding one amino acid after another, 25 00:01:51.920 --> 00:01:56.800 the S protein fragment lengthens as the ribosome advances 26 00:01:57.620 --> 00:02:01.160 until the protein acquires optimal conformation. 27 00:02:03.620 --> 00:02:05.840 The proteins thus synthesized are 28 00:02:05.840 --> 00:02:10.500 released to be taken care of by specialized cells of our immune system: 29 00:02:10.500 --> 00:02:13.140 the follicular dendritic cells. 30 00:02:20.440 --> 00:02:25.340 These cells play the role of sentinel patrolling the tissues of the body. 31 00:02:25.340 --> 00:02:28.500 They are able to recognize foreign elements. 32 00:02:34.300 --> 00:02:38.660 Follicular dendritic cells digest these foreign elements. 33 00:02:39.060 --> 00:02:45.920 They possess all the enzymatic equipment to cut the S protein into small antigenic fragments. 34 00:02:46.420 --> 00:02:50.140 These fragments are expressed on the membrane surface 35 00:02:50.140 --> 00:02:52.140 of the follicular dendritic cells. 36 00:02:53.960 --> 00:03:00.180 The next step will involve other cells part of the immune system: B lymphocytes. 37 00:03:01.280 --> 00:03:05.260 They are able to recognize the antigenic fragments 38 00:03:05.260 --> 00:03:07.720 expressed by follicular dendritic cells. 39 00:03:09.000 --> 00:03:12.780 This recognition is based on the principle of a key in a lock. 40 00:03:12.780 --> 00:03:15.380 When the complementarity is perfect, 41 00:03:15.380 --> 00:03:18.100 the B lymphocyte is activated. 42 00:03:19.640 --> 00:03:25.000 It then undergoes multiple cell divisions forming a clone. 43 00:03:26.180 --> 00:03:30.720 Some of the cells will differentiate into memory lymphocytes 44 00:03:31.560 --> 00:03:36.660 while others become antibody-producing cells called plasma cells. 45 00:03:37.320 --> 00:03:42.200 The plasma cells secrete a large number of antibodies into the bloodstream. 46 00:03:42.200 --> 00:03:48.200 If, later, the adequately vaccinated subject is in contact with the virus, 47 00:03:48.200 --> 00:03:52.980 it will be immediately recognized and neutralized by antibodies. 48 00:03:53.760 --> 00:03:57.600 Vaccination will have played its role in preparing the body 49 00:03:57.600 --> 00:04:00.260 against a particular infectious agent.