Transcript Slide 1

Bioinformática
Inmunológica
Grupo 5
The Mammalian Immune System
• A complex and adaptive learning system
• Evolved to defend an individual against foreign
invaders
• Operates at multiple levels: from molecule to
cell, organ, organism and community
ANTIGENO
• Sustancia genética y estructuralmente extraña
para el organismo receptor
• Molécula que genera una respuesta inmune:
Inmunógeno (contraparte Tolerógeno)
• Molécula que reacciona con receptores
específicos de células T o B (anticuerpos
libres)
INMUNÒGENO
• Molécula que desencadena una respuesta
inmune con producción de anticuerpos.
• Ej: Microorganismos enteros, aislados,
productos metabólicos; otras sustancias
Características
• Alto PM
• Químicamente compleja
PROTEINAS
Moléculas complejas y mucho
más inmunogénicos que los
polisacáridos
Formado por cientos de Aa
Tienen muchos epítopes de
diferente especificidad.
Proteínas conjugadas:
glicoproteinas, lipoproteínas,
nucleoproteínas
RECEPTOR DE CELULA B - RECEPTOR DE CELULAS T
What Are Epitopes
• Antigenic determinants or Epitopes are the portions of the
antigen molecules which are responsible for specificity of the
antigens in antigen-antibody (Ag-Ab) reactions and that
combine with the antigen binding site of Ab, to which they are
complementary.
Properties of Epitopes
• They occur on the surface of the protein
and are more flexible than the rest of the
protein.
• They have high degree of exposure to the
solvent.
• The amino acids making the epitope are
usually charged and hydrophilic.
Epitopes
• In protein antigens epitopes can be defined in terms of:
– Amino acid composition
– Protein location
– Length (5-15 amino acids)
• Immunodominant epitopes:
– Epitopes bound by a greater proportion
of antibodies than others in a normal
in vivo immune response.
– Also known as Major Antigenic Sites.
• Epitopes can be divided into 2 classes:
– Discontinuous epitopes
– Continuous (linear) epitopes
Epitopes could be contiguous (when Ab binds to a
contiguous sequence of amino acids)
non-contiguous (when Ab binds to
non-contiguous residues, brought
together by folding).
Sequential epitopes are contiguous
epitopes.
Conformational epitopes are noncontiguous antigenic determinants.
Discontinuous Epitopes
• Constitutive residues are non-sequential in the
primary sequence.
• Highly conformational dependant.
• Account for approx. 90% of epitopes
on a given antigenic (globular) protein.
Linear (continuous) Epitopes
•
Constitutive residues are sequential in the primary sequence of the protein.
•
Fewer conformational constraints on Ab recognition.
•
Often contain residues that are not
implicated in antibody interaction.
Epitopes
Sequential
Conformational
Ab-binding sites
Types of Epitopes
• Conformational / Discontinuous epitopes:
• recognized by B cells
• non-linear discrete amino acid sequences, come
together due to folding.
• Sequential / Continuous epitopes:
• recognized by T cells & B cells
• linear peptide fragments
Types of Peptide Epitope
Conformational
Antibody
or “B cell”
Epitope
Linear
B cell
Epitope
NonConformational
T cell
Epitope
Class I MHCs
Class II MHCs
all cells
Professional Antigen
Presenting cells
Foreign and self proteins
Foreign proteins
8-10 amino acids
8-20 amino acids
T cells and B cells use Distinct Antigen Receptors
to Recognize Fundamentally Different Forms of Antigen
B cells can recognize linear or conformational epitopes on cell surfaces, of proteins,
carbohydrates or of lipids. The B cell antigen receptor is a form of membrane Ig.
T cells recognize linear peptide fragments bound to MHC class I or class II molecules
Sperm whale myoglobin (1vxg) contains five sequential
epitopes (red, green, magenta, blue, orange) and two
conformational epitopes (yellow, pink).
B cells and T cells recognize different epitopes of the
same protein antigen
T cell epitope
B cell epitope
Denatured antigen
Native or denatured (rare) antigen
Linear peptide 8-30 ac
Sequential or conformational
Internal (often)
Accessible, hydrophilic, mobile, usually on
the surface or could be exposed as a result
of physicochemical change
Binding to T cell receptor:
Kd 10-5 – 10-7 M (low affinity)
Slow on-rate, slow off-rate (once
bound, peptide may stay associated
for hours to many days)
Binding to antibody:
Kd 10-7 – 10-11 M (high affinity)
Rapid on-rate, variable off-rate
Why is the knowledge of antibody epitopes is so important?
• Vaccine design (immunogenicity, i.e. ability of vaccine to elicit in the naïve
individual the production of pathogen neutralizing antibodies, is required):
 Purified antigen (subunit) vaccines:
• Inactivated toxins “toxoids”: tetanus toxoid, diphteria toxoid
• Vaccines composed of bacterial polysaccharide antigens: flu,
pneumococcus
 Synthetic antigen vaccines:
• hepatitus B (recombinant protein), herpes simplex virus
• Diagnostic design (antigenicity, i.e. ability of synthetic antigen to be recognized
by the original antibody, is required):
• Autoimmune diseases: lupus, rheumatoid arthritis
• Allergic reactions
• Basic knowledge of antigenicity.
Respuesta Inmune
The Immune Response
The humoral response involves
interaction of B cells with
antigen (Ag) and their
differentiation into antibodysecreting plasma cells. The
secreted antibody (Ab) binds
to the antigen and facilitates
its clearance from the body.
The cell-mediated responses
involve various subpopulations
of T cells that recognize
antigen presented on selfcells. Helper T cells respond to
antigen by producing
cytokines. Cytotoxic T cells
respond to antigen by
developing into cytotoxic T
lymphocytes (CTLs), which
mediate killing of altered selfcells (e.g., virus-infected cells).
Complejo Mayor
Histocompatibilidad I
MHC
• Molécula de reconocimiento de lo propio y extraño.
• Involucrada en la respuesta inmune adquirida.
• Importante en la presentación de antígenos.
• No es exclusiva de humanos.
The genetic organization of the major
histocompatibility complex (MHC) in human
Carracteristícas de las moleculas de las MHC
clase I y II
Característica
MHC I
MHC II
Cadenas peptídicas
α (44-47 kD)
β2-microglobulina(12kD)
α (32-34kD)
β (29-32kD)
Localización de residuos
polimorfos
Dominio α1 y α2
Dominio α1 y β1
Punto de unión al
receptor de linfocito T
Región α3 se une al CD8
Región β2
se une al CD4
Tamaño de la hendidura
de unión a peptidos
8-11 a.a
10-30 a.a
Genes que codf.
HLA A, HLA B, HLA C
HLA DR HLA DP, HLA
DQ
The structure of an
MHC class I molecule
determined by X-ray
crystallography
The MHC class I pathway
Antigen
Proteasome
Peptides
T-cell epitope
ER
MHC I
Antigen Presenting Cell
TCD8+
MHC class I molecules present antigen derived
from proteins in the cytosol
MHC class I molecules do not leave the endoplasmic reticulum
unless they bind peptides
Reconocimiento del linfocito T de un complejo péptido -MHC
TCR-Class I MHC peptide complex
Complejo Mayor de
Histocompatibilidad II
MHC II
• La generación de péptidos antigénicos y su asociación con las MHC
requiere acción concertada de moléculas accesorias como
chaperonas, transportadores de péptidos y proteasas encargadas
de degradar los Ags.
• Los péptidos se originan por vía endógena o exógena.
• Las MHCI, presentan péptidos de vía endógena, degradados por el
proteasoma y presentados al LTCD8+.
• Las MHCII, presentan péptidos de vía exógena fagocitadas por una
APC, son presentados al LTCD4+.
• En circunstancias especiales péptidos de la vía endógena son
presentados por MHCII y viceversa.
•
MHC
class I
MHC
class II
Peptide recognition by MHC molecules
Peptide binding to MHC class I
-8 to 10 amino acids long
-importance of N and C term
-two or more anchor residues
Peptide binding to MHC class II
-up to 20 amino acids long
-importance of backbone contacts
-two or more anchor residues
MHC molecules present antigen from 3 main sources
The immunoglobulin fold
Common Structures - Both the antibodies of
the humoral response and the molecules
involved in the cellular response (antibody,
TCR, most CD [cell surface molecules expressed
on various cell types in the immune system])
contain elements of common structure.
The domains in these molecules are built on a
common motif, called the immunoglobulin
fold, in which two anti-parallel sheets lie face
to face. This structure probably represents the
primitive structural element in the evolution of
the immune response. The immunoglobulin
fold is also found in a number of other
proteins.