1. Adaptive Immunity I

2. Innate and Adaptive Immunity
Humoral
Cellular

3. Characteristics of the IR
Innate
Adaptive
Broad specificity
Recognizes molecular patterns
Rapid response
No memory
Narrow specificity
Recognizes specific antigens
Delayed response (why?)
Memory

4. Functions of the (innate) immune system
Barriers:
Recognition:
Remove and destroy:
Distinguish self and non-self:
Memory?
Physical, chemical, microbial
PAMPs, other receptors
Phagocytes
Phagocytes, NK cells
Ask for examples

5. Functions of the adaptive IR
Barriers:
Recognition:
Remove and destroy: Distinguish self and non-self:
Memory?

6. The adaptive immune response
A brief overview

7. Zing!! Zing!! T B M Th Tc Zing!! Zing!! Zing!! P M Zing!! Y Y Y Y Y Y
Treg
Th17

9. Humor = fluid (latin)

10. Topics for today Antigen receptors Antibody structure and function
Antigen presentation and MHC restriction
Antibody receptor rearrangement
MHC diversity

11. Antigens and antigen receptors
Antigen: A macromolecule (usually protein) that is specifically recognized by an antibody.
“Generates an antibody”
Antibody: A molecule that is produced in response to and is specific for a particular antigen.
Antigen = Anything that induces an adaptive immune response.

12. Antigen receptors come in two flavors
Antibodies
T cell receptors
Made by B cells
Cell-associated and soluble
Bivalent
Recognize antigen directly
Expressed by T cells
Cell associated
Monovalent
Recognize antigen in conjunction with MHC
Both:
Recognize specific antigens
Each B or T cell expresses a single antigen specificity
One cell, one antibody or T cell receptor, one antigen

13. Antibody structure: “lock and key”
Bivalent: Two identical antigen binding sites

14. Epitopes

16. Antigen-binding
Figure 3-1 part 3 of 3

17. Figure 3-1 part 1 of 3

18. Anatomy of an antibody Fab : Fc: Antigen binding Variable region
Confers antigen specificity
Different for every antibody
Fc:
Constant regions
Confers Fc receptor binding on immune cells
Crystalized

19. Classification of antibodies:
Idiotype: antigen binding specificity:
Variable region
B cells only produce a single idiotype
A single idiotype may recognize closely related antigens
Defined by Fab
Isotype: heavy chain structure (antibody “class”)
Defined by their Fc domain
Named for popular letters of the alphabet:
Ig(immunoglobulin)G, IgA, IgM
Different isotypes have different functions

20. Idiotype vs Isotype Antibody class Antigen specificity
IgG, IgM, IgA, IgE, etc
Conferred by constant region
Antibodies of different isotypes may have the same idotype (antigen specificity)
Antigen specificity
Conferred by variable region
Independent of isotype

21. Isotype structure Monomeric IgM IgG IgE IgA Pentameric IgM

22. Isotype structure Valency: Fc fragment domains: Monomers = bivalent
Multimers = multivalent
Fc fragment domains:
Two or three domains
Determine receptor binding:
Multimers
Immune cell activation
Fc receptors present on various types of immune cells
IgG
Pentameric IgM

23. Isotype function Fc fragment determines receptor binding IgM: IgG:
Surface receptor: cell associated antibody
Primary response: initial antigen recognition
IgG:
Most abundant isotype
Antigens in serum
Bacteria, viruses, toxins
IgE: Allergens, parasitic antigens
IgA: Mucosal immunity

24. Antibody function: Antigen removal
Agglutination
Complement fixation
Opsonization
Neutralization
Activation of immune cells:
B cell activation
Antibody-dependent cellular cytotoxicity Y P

25. Agglutination Pathogen/toxin inactivation Enhanced phagocytosis IgG
IgM
Pathogen/toxin inactivation
Enhanced phagocytosis

26. Complement fixation (activation)

27. Opsonization and phagocytosis

29. Neutralization
Toxin Neutralization C ^
Virus neutralization

31. The T cell receptor Surface receptor Structure:
a and b chains
Variable and constant regions
Antigen-specific
Recognizes antigen presented by the major histocompatibility complex (MHC)

32. T cell receptor: Antibody: Soluble or membrane bound
Heavy and light chains
Variable and constant regions
Bivalent
T cell receptor:
Membrane bound
a and b chains
Variable and constant regions
Monovalent

33. Antigen recognition
B cell receptors (antibody) recognize antigen directly
T cell receptors recognize antigen only in conjunction with MHC molecules on the surface of antigen presenting cells

34. Antibody-antigen interaction
T cell receptor antigen interaction

35. MHC restriction T cells recognize antigen bound to MHCTh
T cells recognize antigen bound to MHC
MHC molecules come in 2 flavors:
MHCI: Present on all nucleated cells
MHCII: Present on antigen presenting cells
T cells are MHC restricted
Helper T cells recognize antigen bound to MHCII
Cytotoxic T cells recognize antigen bound to MHCI Tc

36. Helper T cells are MHC II restricted
Cytotoxic T cells are MHC I restricted
Cytokines

37. Antigen presentation

38. MHC structure: MHC I Two chains: a, b2 micro-globulin
Peptide-binding cleft
Three globular domains
a1 and a2 peptide binding
a3 constant

39. MHC structure:MHCII a and b chains Peptide-binding cleft
Peptide binding and constant domains

40. How the peptide gets into the binding cleft: Antigen processing
MHCI
Intracellular antigens
Cytoplasmic processing
MHCII
Extracellular antigens
Endosome processing

41. Antigen Processing
Thought question: Why is processing different for MHCI vs MHCII?

42. Antigen receptors: Specificity and diversity
1015+ different antigen specificities
Virtually every antigen ever invented
But not self antigens
Where does the diversity come from?

43. The origins of antibody diversity
Germ line theory:
One gene in the germ line for every variable region
1015 antibody genes?
Nope
Somatic mutation theory:
Only a few variable region genes
Contact with antigen induces mutations resulting in (with luck) a matching sequence
Both are true
Neither is true

44. Combined theory of antibody diversity
Antibody genes contain many different coding sequences
Extensive recombination occurs during B cell maturation
Diversity is generated by:
Random combinations
Junctional diversity
Sloppy recombination
Pairing of heavy and light chains
Each B or Tcell clone expresses a unique antibody sequence
Relevant clones are selected

45. Antibody genes: Germline
Heavy and light chain loci
Each has multiple coding sequences:
Variable region: multiple sequences
Constant region: multiple sequences
Random recombination

46. Variable region genes
Each cell has two light chain loci (k and l) and one heavy chain locus.
Each light chain variable locus has:
V (variable) regions
J (joining) regions
Each heavy chain variable locus has:
V regions
J regions
D (diversity) regions
Multiple alternative sequences for each region
Random rearrangement during maturation results in one VL, one JL, one VH, one JH, and one DH

47. Antibody genes: Germ-line structure
Two light chain loci
V, D, and J regions
Up to 40 regions in each category
Randomly recombine H L

49. T cell receptor rearrangement

50. Factors that contribute to antigen receptor diversity
V, J, D gene variability: Large number of genes
V, J, D gene joining:
Selection
Order
Orientation
Junctional diversity:
Sloppy recombination
N region insertion
Heavy and light gene selection

52. Recombination activating genes (RAG)
RAG1 and RAG2
Enzyme complex
Catalyze recombination and joining
Essential for development of mature B and T cells

54. Somatic hypermutation
Enhanced diversity and specificity after antigen contact
Occurs in peripheral tissues in mature B cells only
Rapidly accumulating point mutations in V regions
Selection for best fit
Increases strength of binding

55. Somatic hypermutation

56. MHC diversity B and T cells are clonal:
Each cell expresses a single receptor type
Each cell is specific for one antigen type
Antigen-presenting cells have multiple specificities
Each cell may express several different MHC types
Each MHC molecule may associate with several different antigens

57. B Y T B Y T
APC B Y T

58. Genetics of the MHC Germ-line expression: no rearrangement
Multiple genes present in the germ line
Co-dominant expression
Multiple alleles present in the population

59. MHC locus Three regions: Each MHC region has several genes
MHCI and MHCII: Encode antigen-presentation molecules
MHCIII: Complement genes, some cytokine genes, others
Each MHC region has several genes
MHCI: A, B, C
MHCII: DP, DQ, DR
Each MHCII gene encodes a and b chains

61. MHC diversity MHC presents all possible epitopes
No somatic rearrangement
Each cell expresses multiple MHC molecules
Genetic polymorphism
High mutation rate
Degenerate peptide binding

62. Genetic polymorphism Multiple MHC genes Many alleles for each gene
Multiple combinations
Inherited independently
Expression is co-dominant
Population-wide diversity > individual diversity

63. Polymorphism of MHCII genes
Locus
Number of alleles
HLA-DPA 12
HLA-DPB 88
HLA-DQA 17
HLA-DQB 42
HLA-DRA 2
HLA-DRB1
269
HLA-DRB3 30
HLA-DRB4 7
HLA-DRB5

64. MHC Peptide binding
MHC-antigen binding differs from B and T cell antigen receptor binding:
“Degenerate” binding: Binding is not antigen-specific. A single MHC molecule can associate with many different peptides
Each antigen-presenting cell expresses several different MHC specificities
Different MHC on one cell can bind different peptides
Each cell can present many different peptides

65. Diversity: antigen receptor vs MHC
Each cell encodes multiple genes and multiple alleles
multiple MHC proteins per cell
Degenerate peptide binding
No gene rearrangement
Polymorphic: up to 600 alleles for each locus
Antigen receptor :
Each cell encodes a single polypeptide sequence
One receptor specificity per cell
V region selection
Junctional diversity
Somatic hypermutation

66. The role of diversity Antigen receptor diversity MHC diversity
Limited diversity of individuals
Extensive diversity of the population
Mediates population-wide disease resistance
Large gene pool: more resistant population
Limited gene pool: higher frequency of disease susceptibility and genetic disease
Recognition of all possible antigens by the individual
Protects the organism

67. Definitions: The language of MHC
Genetic region MHCI, II, III
Gene products of MHCI and II
HLA = Human Leukocyte Antigen
Human MHC molecules
Allele names: Popular numbers
DR5, DQ2, DP1, etc
MHC2 with a and b
Haplotype = An individual’s combination of alleles

68. MHCII association with autoimmune diseases
Haplotype
MHCII