Cellular Biophysics SS Prof. Manfred Radmacher

Transcription

1 SS Manfred Radmacher Immune system has several layers 2

2 The immune system attacks pathogens macrophage attacking bacteria From: 3 The immune system is the largest organ 4

3 The memory of the adaptive immune system is in the B cells Figure A classic experiment showing that lymphocytes are required for adaptive immune responses to foreign antigens. An important requirement of all such cell-transfer experiments is that cells are transferred between animals of the same inbred strain. Members of an inbred strain are genetically identical. If lymphocytes are transferred to a genetically different animal that has been irradiated, they react against the foreign antigens of the host and can kill the animal. In the experiment shown, the injection of lymphocytes restores both antibody and cell-mediated adaptive immune responses, indicating that lymphocytes are required for both types of responses. 5 B and T lymphocytes can be resting or active Resting B lymphocyte Activated B lymphocyte producing antibodies Cytotoxic T lymphocyte 6

4 The innate immune systems activates the adaptive immune system Figure One way in which the innate immune system helps activate the adaptive immune system. Specialized phagocytic cells of the innate immune system, including macrophages (not shown) and dendritic cells ingest invading microbes or their products at the site of infection. The dendritic cells then mature and migrate in lymphatic vessels to a nearby lymph node, where they serve as antigen-presenting cells. The antigen-presenting cells activate T cells to respond to the microbial antigens that are displayed on the presenting cells' surface. The antigen-presenting cells also have special proteins on their surface (called costimulatory molecules) that help activate the T cells. Some of the activated T cells then migrate to the site of infection where they either help activate macrophages or kill infected cells, thereby helping to eliminate the microbes. As we discuss later, the costimulatory molecules appear on dendritic cells only after these cells mature in response to invading microbes. 7 The adaptive immune has a memory Figure Primary and secondary antibody responses. The secondary response induced by a second exposure to antigen A is faster and greater than the primary response and is specific for A, indicating that the adaptive immune system has specifically remembered encountering antigen A before. The same type of immunological memory is observed in T-cell-mediated responses. 8

5 Clonal selection theory: each B cell produces only one type of antibody Figure The clonal selection theory. An antigen activates only those lymphocyte clones (represented here by single cells) that are already committed to respond to it. A cell committed to respond to a particular antigen displays cell-surface receptors that specifically recognize the antigen, and all cells within a clone display the same receptor. The immune system is thought to consist of millions of different lymphocyte clones. A particular antigen may activate hundreds of different clones. Although only B cells are shown here, T cells operate in a similar way. 9 If a clone is deleted no antibodies of this clone can be generated anymore 10

6 Antibodies recognize highly specific their antigen Specific binding of antibody and antigen AFM images of antibodies From: Monika Fritz

7 How many different antibodies are needed? an antibody recognizes a part of the surface of an antigen: the epitope let's assume: epitopes are 2 by 3 amino acids in size this gives: 20 6 different epitopes: ~60 million they can not be encoded each by a single gene 13 The genes for antibodies are spliced together from several options Figure The V-J joining process involved in making a human! light chain. In the germ-line DNA (where the antibody genes are not being expressed and are therefor not rearranged), the cluster of five J gene segments is separated from the C-region exon by a short intron and from the 40 V gene segments by thousands of nucleotide pairs. During the development of a B cell, the randomly chosen V gene segment (V3 in this case) is moved to lie precisely next to one of the J gene segments (J3 in this case). The extra J gene segments (J4 and J5) and the intron sequence are transcribed (along with the joined V3 and J3 gene segments and the C-region exon) and then removed by RNA splicing to generate mrna molecules in which the V3, J3, and C sequences are contiguous. These mrnas are then translated into! light chains. A J gene segment encodes th C-terminal 15 or so amino acids of the V region, and the V-J segment junction coincides with the third hypervariable region of the light chain, which is the most variable part o the V region. 14

8 How many combinations? light chains: 300 V segments * 4 J segments: 1200 combintaions heavy chains: 500 V segments * 4 J segments * 12 D segments: combintaions total number: 3 * Where is biophysics in the immune system? specific molecular interactions in antigen antibody recognition chemotaxis of macrophages system theory in development of immune system 16

9 Immune System Theory Ein Antigen Ag X (mit einer Determinanten) stimuliert die Synthese eines Antikörpers Ab1 (den sog. Idiotyp), der seinerseits antigene Wirkung zeigt und einen zweiten Klon zur Produktion eines Antikörpers Ab2 (den sog. Anti- Idiotyp) anregt, der zu Ab1 komplementär ist. Ab2 stimuliert dann Ab3 usw. From: Sackmann Biophysik Skript 17 Immune System Theory Abb : Schematische Darstellung des verzweigten Immunnetzwerkes. Das Antigen AgX mit mehreren Determinanten stimuliert mehrere Klone zur Produktion von Antikörpern Ab1 (idiotyp), die ihrerseits die Synthese von anti-idiotypen Ab2 auslösen usw. Im Immunsystem sind sehr wahrscheinlich die B-Zellen an der ersten Stufe (Ab1) beteiligt, während die Antwort ab Ab2 durch T-Zellen vermittelt wird. From: Sackmann Biophysik Skript 18

10 Immune System Theory Derivation 19 Immune System Theory Abb : Verhalten des Immunsystems bei ständiger Zugabe einer niedrigen Konzentration [X] des Antigens AgX. Situation der Niedrig-Dosen-Toleranz. From: Sackmann Biophysik Skript 20

11 Immune System Theory Das Antigen AgX wird in höherer Konzentration injiziert. Die Unterdrückung der Ab1 durch Ab2 wird (durch Bindung von AgX an Ab1) soweit gehemmt, daß die Unterdrücker Ab2 durch Ab3 selbst elminiert werden. Auch spätere Zugaben von AgX werden durch Ab1 (B-Zellen) leicht eliminiert. From: Sackmann Biophysik Skript 21 Immune System Theory Das Immunsystem toleriert eine hohe Konzentration des Antigens ([X] ist um einen Faktor 103 höher als bei der Niedrig-Zonen-Toleranz). Der Grund liegt in der Elimination der B-Zellen Ab1 durch die Population 2. Die Konzentration [Ab2] steigt an, da die Helfer-Zellen 3 durch Ab4 eliminiert werden. Die 5. Stufe der Hierarchie wird nicht mehr aktiviert. From: Sackmann Biophysik Skript 22

12 Final remarks on the immune system tolerance against self antigens is trained in early childhood tolerance against self antigens is trained in early childhood there are auto-immune deseases there are hints that allergies may be related to development of immune system: hygiene hypothesies worm treatment 23 Applications of Antibodies: blood group test 24

13 Applications of Antibodies: blood group test Experiment 25 Elisa 26