医学免疫学课件：免疫细胞的分化发育

1、The Development of Immune CellsReferencesJaneways Immunobiology8th EditionBy Kenneth MurphyFundamental Immunology7th EditionBy William E. Paulwww.The origin of immune cellsFundamental Immunology, 7th editionThe cells of the immune system derive from precursors in the bone marrowQuestion: Are erythro

2、cytes and platelets immune cells? Development of B lymphocytesThe main phases of a B lymphocytes life historyThe early stages of B-cell development are dependent on bone marrow stromal cellsStromal cells form specific adhesive contacts with the developing lymphocytes by interactions between cell-adh

3、esion molecules and their ligands. Stromal cells soluble and membrane-bound cytokines and chemokines that control lymphocyte differentiation and proliferationFLT3 is required for differentiation to the next stage, the common lymphoid progenitor;IL-7 produced by stromal cells is required for the deve

4、lopment of B-lineage cells;The interaction of VLA-4 and VCAM-1 promotes the binding of Kit and SCF;B-cell Antigen ReceptorB cellantigen-binding siteThe germline organization of the immunoglobulin heavy- and light-chain loci in the human genomeNote: In approximately 50% of individuals, the entire clu

5、ster of V gene segments has undergone an increase by duplication. The heavy-chain locus contains a series of C regions arrayed one after the other, each of which corresponds to a different isotype. V-region genes are generated by the somatic recombination of separate gene segments“12-23”原则重组信号序列(rec

6、ombination signal sequence, RSS):七核苷酸12/23核苷酸的随机序列九核苷酸V(D)J RecombinationThe immunoglobulin isotypes are encoded by a cluster of immunoglobulin heavy-chain C-region genes.Immediately 3 to the C gene lies the C gene, which encodes the C region of the IgD heavy chain. Different classes of immunoglobul

7、ins are distinguished by the structure of their heavy-chain constant regionsMature nave B cells express both IgM and IgD at their surfaceIgM is the first Ig to be produced during B cell development.IgD is coexpressed with IgM on the surface of almost all mature B cells. But the unique function of Ig

8、D is still unclear. (may be involved in affinity maturation)B cells expressing IgM and IgD have not undergone class switching.Coexpression of IgD and IgM is regulated by RNA processing.The developmental stages of a B-lineage cell are marked by the rearrangement and expression of the immunoglobulin g

9、enesExpression of surface proteins, receptors, and transcription factors in B-cell developmentB cell markersB-cell development begins by rearrangement of the heavy-chain locusA productively rearranged immunoglobulin gene is immediately expressed as a protein by the developing B cell.The pre-B-cell r

10、eceptor tests for successful production of a complete heavy chain and signals for the transition from the pro-B cell to pre-B cell stage.Successful light-chain gene rearrangement results in the production of a light chain that binds the chain to form a complete lgM molecule, which is expressed toget

11、her with lg and Ig at the cell surface.Pre-B cell receptor signaling inhibits further heavy-chain locus rearrangement and enforces allelic exclusionAllelic exclusion in individual B cellsPre-B cells rearrange the light-chain locus and express cell surface immunoglobulinLight-chain loci lack D segmen

12、ts and rearrangement occurs by V to J joining.If a particular VJ rearrangement fails to produce a functional light chain, it can be rescued by further rearrangement. Light-chain rearrangement also exhibits allelic exclusion. Rearrangements at the light-chain locus generally take place at only one al

13、lele at a time.Light chains also display isotypic exclusion; that is, the expression of only one type of light chain- or -by an individual B cell. In mice and humans, the light-chain locus tends to rearrange before the locus.Expression of proteins involved in gene rearrangement and the production of

14、 pre-B-cell and B cell receptors The steps in immunoglobulin gene rearrangement at which developing B cells can be lostLight chains also display isotypic exclusion: the expression of only one type of light chain- or -by an individual B cell.Immature B cells are tested for autoreactivity before they

15、leave the bone marrow (central tolerance)Replacement of light chains by receptor editing can rescue some self-reactive B ells by changing their antigen specificityNotes:It is not clear whether receptor editing occurs at the heavy-chain locus.The fact that central tolerance is not perfect and some se

16、lf-reactive B cells are allowed to mature reflects the balance that the immune system strikes between purging all self-reactivity and maintaining the ability to respond to pathogens. If the elimination of self-reactive cells were too efficient, the receptor repertoire might become too limited and th

17、us unable to recognize a wide variety of pathogens. Some autoimmune disease is the price of this balance: ignorant self-reactive lymphocytes can be activated and cause disease under certain circumstances. Survival and maturation of B lymphocytes in peripheral lymphoid tissuesSpleenDifferent lymphocy

18、te subsets are found in particular locations in peripheral lymphoid tissuesThe homing of lymphocytes to specific regions of peripheral lymphoid tissues is mediated by chemokinesImmature B cells arriving in the spleen turn over rapidly and require cytokines and positive signals through the B-cell rec

19、eptor for maturation and survivalWhen B cells emerge from bone marrow into the periphery, they are still functionally immature. Immature B cells express high levels of sIgM but little sIgD, whereas mature B cells express low levels of sIgM and high levels of sIgD. Most immature B cells leaving the b

20、one marrow will not survive to become fully mature B cells.The follicle provides signals necessary for B-cell survival: BAFF (B cell activating factor)Secondary diversificatoin of the antibody repertoireThe RAG-mediated V(D)J recombination is responsible for the initial antibody repertoire of B cell

21、s developing in the bone marrow. This takes place without interaction of B cells with antigen.Although this primary repertoire is large, further diversification can occur that enhances both the ability of immunoglobulins to recognize and bind to foreign antigens and the effector capacities of the ex

22、pressed antibodies. This secondary phase of diversification occurs in activated B cells and is largely driven by antigen. Diversification is achieved through three mechanisms: somatic hypermutation, class switching or class switch recombination, and gene conversion-which alter the sequence of the se

23、creted immunoglobulin in distinct ways.The primary antibody repertoire is diversified by three processes that modify the rearranged immunoglobulin geneSomatic hypermutation introduces point mutations into the V regions of both chains, which alters the affinity of the antibody for antigen. Class swit

24、ch recombination involves the C region only: it replaces the original C heavy-chain C region with an alternative C region, thereby increasing the functional diversity of the immunoglobulin repertoire. Gene conversion diversifies the primary antibody repertoire in some animals, replacing blocks of se

25、quence in the V regions with sequences derived from the V regions of pseudogenes. These processes are initiated by an enzyme called activation-induced cytidine deaminase (AID), which is expressed specifically in activated B cells.These processes do not occur in T-cell receptor genes.Somatic hypermut

26、ation further diversifies the rearranged immunoglobulin V regions and improves antigen bindingClass switchB-1 cells and marginal zone B cells are distinct B-cell subtypes with unique antigen receptor specificityConventional B cells (B2 cell): reside in B-cell folliclesB1 cells are a unique subset co

27、mprising about 5% of all B cells in mice and humans, and are the major population in rabbits. They are called B-1 because they are the first to appear during fetal development. B-1 cells are found primarily in the peritoneal and pleural cavity fluid. It is not clear whether B-1 cells arise as a dist

28、inct lineage from a unique precursor cell or differentiate to the B-1 phenotype from a precursor cell that could also give rise to B-2 cellsThe weight of evidence favors the idea that commitment to the B-1 or B-2 subset is due to a selection step, rather than their being distinct lineages like the :

29、 and : T cells.Marginal zone B cells, so called because they reside in the marginal sinus of the white pulp in the spleen, are another unique subset of B cells.Marginal zone B cells have restricted antigen specificities, biased toward self antigens and common bacterial antigens. B-1 cells may play a

30、 role in defending the body cavities and marginal zone B cells defend against bacteria that penetrate the bloodstream.B-1 cells and marginal zone B cells are distinct B-cell subtypes with unique antigen receptor specificityA summary of the development of human conventional B-lineage cells (Antigen i

31、ndependent stage)A summary of the development of human conventional B-lineage cells(Antigen dependent stage and terminal differentiation)The development of T lymphocytes in the thymusB-cell Receptor and T-cell ReceptorB cellantigen-binding siteantigen-binding siteT cell TCR shows similar structure a

32、s TCRThe germline organization of the human TCR and lociThe TCR gene segments are arranged in a similar pattern to BCR gene segments and are rearranged by the same enzymeT-cell receptor - and -chain gene rearrangement and expressionT cells undergo development in the thymus and migrate to the periphe

33、ral lymphoid organs, where they are activated by foreign antigensThymocytes at different developmental stages are found in distinct parts of the thymusThymocytes at different developmental stages are found in distinct parts of the thymusThe cellular organization of the human thymusThe thymus is crit

34、ical for the maturation of bone marrow derived cells into T cellsSurgical removal of the thymus (thymectomy) at birth resulted in immunodeficient mice.The scid mutation mice have a defect in the maturation of B and T lymphocytes due to the impairment of antigen-receptor rearrangement.The nude mutati

35、on (Foxn1 ) have a defect in the development of the cortical epithelium of the thymus.T cells do not develop in either strain of mouse.However, scid bone marrow cannot develop T cells, even in a wild-type recipient.Notes:In mice, the thymus continues to develop for 3-4 weeks after birth, whereas in

36、humans it is fully developed at birth. The rate of T-cell production by the thymus is the greatest before puberty. After puberty, the thymus begins to shrink and the production of new T cells in adults is lower, although it does continue throughout life. In both mice and humans, removal of the thymu

37、s after puberty is not accompanied by any notable loss of T-cell function or numbers. Thus, it seems that once the T-cell repertoire is established, immunity can be sustained without the production of significant numbers of new T cells. The pool of peripheral T cells is instead maintained by long-li

38、ved T cells and also by some division of mature T cells.T cell precursors proliferate extensively in the thymus, but most die thereThe thymus of a young adult mouse contains about 12X 108 thymocytes.5 X 107 new cells per dayOnly 12 X 106 cells grow to mature T cells (24%)98% of thymocytes die by apo

39、ptosis, and cleaned by macrophages by phagocytosis.This apparently profligate waste of thymocytes is a crucial part of T-cell development because it reflects the intensive screening that each thymocyte undergoes for the ability to recognize self-peptide: self MHC complexes and for self-tolerance.Two

40、 distinct lineages of thymocytes are produced in the thymus.In the fully developed thymus, immature double-negative T cells constitute about 60% of the thymocytes. about 20% of the double-negative cells, comprises cells that have rearranged and are expressing the genes encoding the : T-cell receptor

41、; another 20% of all double negative thymocytes, includes cells bearing : T-cell receptors of very limited diversity. These cells also express the NK1.1 receptor commonly found on NK cells; they are therefore known as invariant NKT cells (iNKT cells)Successive stages in the development of thymocytes

42、 are marked by changes in cell-surface moleculesThe correlation of stages of : T-cell development in the mouse thymus with the program of gene rearrangement and the expression of cell-surface proteinsT cells with : or : receptors arise from a common progenitorThe , , and loci begin to undergo rearra

43、ngement almost simultaneously in developing Thymocytes.The decision of a precursor to commit to the : or the : lineage is thought to depend on which type of receptor-a functional : receptor or the pre-T-cell receptor (: pT)- is expressed first at the DN stage of thymocyte development.This difference

44、 in fate between the two lineages is thought to result from differences in the quality of signaling from the two types of receptors.In most thymocytes a -chain gene rearranges successfully before productive rearrangements of both the and genes have occurred.The -chain locus rearranges first, in CD4-

45、CD8- double-negative thymocytes expressing CD25 and low levels of CD44. Successful synthesis of a rearranged chain allows the production of a pre-T-cell receptor that triggers cell proliferation and blocks further -chain gene rearrangement.The rearrangement of the - and -chain loci closely parallels

46、 the rearrangement of immunoglobulin heavy-chain and light-chain loci during B-cell development.The stages of gene rearrangement in : T cellsThe temporal expression pattern of some important proteins in early T cell developmentPositive and negative selection of T cellsUp to the stage at which an : r

47、eceptor is produced, T-cell development has been independent of antigen. From this point onward, developmental decisions in the : T-cell lineage depend on the interaction of the receptor with peptide:MHC ligands it encounters in the thymus.Double-positive cells have a life-span of only about 3-4 day

48、s unless they are rescued by positive selection. Positive selection: only thymocytes whose receptors interact with self-peptide: self-MHC complexes can survive.Double positive cells also undergo negative selection: T cells whose receptors recognize self-peptide:self-MHC complexes too strongly underg

49、o apoptosis, thus eliminating potentially self-reactive cells. Positive selection is revealed by bone marrow chimeric miceThe MHC molecules present in the environment in which T cells develop determine the MHC restriction of the mature T-cell receptor repertoire.Positive selection: only thymocytes w

50、hose receptors interact with self-peptide: self-MHC complexes can survive and matureIn mice transgenic for rearranged : T-cell receptor genes, the maturation of T cells depends on the MHC haplotype expressed in the thymus. If the transgenic mice express the same MHC haplotype in their thymic stromal

51、 cells as the mouse from which the rearranged TCR-chain and TCR-chain genes originally developed (both MHCa), then the T cells expressing the transgenic T-cell receptor will develop from the double-positive stage into mature T cellsIf the MHCa-restricted TCR transgenes are genetically crossed into a

52、 different MHC background (MHCb, yellow), then developing T cells expressing the transgenic receptor will progress to the double-positive stage but will fail to mature further. This failure is due to the absence of an interaction between the transgenic T-cell receptor with MHC molecules on the thymi

53、c cortex, and thus no signal for positive selection is delivered, leading to apoptotic death by neglect.The MHC molecules that induce positive selection determine co-receptor specificityPositive selection coordinates the expression of CD4 or CD8 with the specificity of the T-cell receptor and the po

54、tential effector functions of the T cell.The specificity of the T-cell receptor for self-peptide:self-MHC molecule complexes determines which co-receptor a mature T cell will express.If the transgenes encode a T-cell receptor specific for antigen presented by self-MHC class I molecules, mature T cel

55、ls that express the transgenic receptor are CD8 T cells. Similarly, in mice made transgenic for a receptor that recognizes antigen with self-MHC class II molecules, mature T cells that express the transgenic receptor are CD4 T cells.In both cases, normal numbers of immature, double-positive thymocyt

56、es are found.Stages in the positive selection of : T cells as identified by FACS analysisDN cells that have successfully rearranged a chain receive a signal through pre-TCR and undergo proliferation, and then induce expression of the CD8 and CD4 to DP cells (green). DP cells express T-cell receptor

57、on the cell surface and induce positive selection.The cell initially reduces CD8 and CD4 expression (orange), followed by a subsequent increased expression of CD4 to generate the CD4+CD8low population(red). If selection was provided by an MHC class II molecule, signaling in the CD4+CD8low T cell is

58、of a longer duration and commitment to CD4 occurs, with maintenance of CD4 and loss of CD8 expression (blue). If the selection was provided by an MHC class I molecule, signaling in the CD4+CD8low T cell will be of shorter duration, and this leads to commitment to the CD8 lineage, with reexpression o

59、f CD8 and loss of CD4 (purple).Thymic cortical epithelial cells mediate positive selection of developing thymocytesEquivalent studies of CD8 interaction with MHC class I molecules showed that coreceptor binding is also necessary for the positive selection of CD8 cells.Treg CellsThe majority of doubl

60、e-positive thymocytes that undergo positive selection develop into either CD4 or CD8 single-positive T cells. The thymus also generates a smaller population of single-positive CD4 T cells that represent a distinct lineage known as natural regulatory T cells (Treg cells). Like CD4 and CD8 cells, Treg