HLA and Antigen Processing Flashcards
histocompatability
first discovered in skin graft transplantation
-genes involved in acceptance or rejection
HLA
human leukocyte antigens
-same as MHC (which is in mouse)
what do T cells recognize?
need to be presented as a peptide
-by HLA
HLA genes
tightly linked and highly polymorphic
Class III genes of HLA?
code for complement proteins and cytokines
what chromosome are HLA genes on?
chromosome 6 (short arm)
HLA haplotype
total set of all HLA alleles on both chromosomes
-have one from mom and dad
-expressed co-dominance - more diversity
important in transplantation (want a match)
what does the HLA haplotype allow?
encode protein antigens central for immune system to discriminate between self and non-self
Class I HLA
three separate regions (membrane bound glycoproteins)
HLA-A,
HLA-B,
HLA-C
present antigen to CD8+ T cells***
expressed on all nucleated cells
-not on RBCs
also inhibitory receptors for NK cells***
why don’t NK cells kill your RBCs?
???
structure of HLA Class I
heterodimer of two proteins
alpha chain encoded by HLA class locus
- alpha 1, 2, and 3
- 3 of the 4 globular domains
beta-2 microglobulin is not HLA encoded
-associates non-covalently with alpha 3 domain of alpha chain
peptide binding groove of class I?
area between alpha 1 and alpha 2
- binds peptides 8-10 amino acids
- bc it has closed ends
conformation of groove dictates what can bind
each allele has a different range of peptides it can bind
what site of Class I HLA has the greatest polymorphisms?
alpha 1 and 2 domains (peptide binding groove)
synthesis of Class I HLA alpha chain?
translated into the ER as glycoprotein
interacts with beta-2 microglobulin in the ER and associates with peptides from cytosolic proteins
Class I HLA/peptide complex transported to cell surface
how many different HLAs in an individual?
6 different HLAs
all have slightly different shape and present a different set of peptides
Class II HLA genes
encoded by the HLA-D region
-HLA-DP, DQ, DR
membrane bound glycoproteins
present to CD 4+ T cells
-primarily on macrophages, dendritic cells, and B cells
Class II HLA structure?
two proteins
-alpha and beta chain
both encoded in the HLA-D region
four globular domain structure
alpha and beta are not covalently linked
location of the binding groove in Class II HLA
alpha 1 and beta 2 domains
-binds 13-18 amino acids (has open ends)
beta 2 is highly conserved (where CD4 binds)
range of different Class II HLA?
6-8 possible
synthesis of HLA class II
alpha and beta are synthesized in the ER
-invariant chain binds to the alpha and beta chain binds to block the groove
in endocytic compartment - invariant chain degrade
-free peptides then bind in groove
**class II bind peptides that have entered cell via endocytosis (from outside of the cell)
then transported to the cell surfaces
alpha and beta chain interactions?
any alpha can associate with any beta chain
-adds to the diversity of peptide binding groove
**greater range of peptides that can bind to class II HLA
peptide binding to Class II HLA?
pockets in floor of groove that bind side chains of amino acids on antigen peptide
-rest of peptide bows upward
this is what is recognized by T cells
-rates of HLA and antigen association?
slow on rate and very slow off rate
-allows peptide-HLA complex to persist for a long time
how many peptides can bind HLA cleft?
only one at a time
how many peptides can single HLA cleft bind?
many (but only bind one at a time)
H-Y HLA
antigen encoded on Y chromosome
-associated with acute rejection of male grafts in female recipients
HLA restriction
ability of T cells to recognize antigen when associated with own HLA
-dual recognition critical to T cell funcion
leads to CD4 only binding Class II and CD8 only binding Class I
APC
antigen-processing cells
-convert proteins to peptides for presentation
three types of APCs?
macrophages, dendritic cells, and B cells
**other cells can express class II HLA and can act as APCs in some cases (thymic epithelial cells)
most efficient APC?
dendritic cells
activity of dendritic cell?
pinocytose antigen and process it for presentation
-draw in their dendrites and home to the T cell rich areas in nodes and spleen
activate naive CD4+ and CD8+ T cells
macrophage activity?
phagocytose or pinocytose antigen
not as effective as dendritic cells
**activate memory T cells (not naive as well)
B cell activity?
bind soluble antigen via surface Ig
-ingest by pinocytosis
bind antigen with high affinity thus effective when antigen low levels
**very effective at presenting to memory T cells
antigen capture
microbes enter body and are phagocytosed or pinocytosed by APCs
lose adhesive markers and upregulate CCR7 and increase HLA and B7 expresion
what do lymph node and spleen filter?
lymph node - antigens from periphery
spleen - antigens in blood
two procesing pathways of the APC?
intracellular - class I extracellular - class II
Class II HLA pathway
exogenous protein ingested and degraded
alpha and beta and invariant chain in ER
-transported through golgi
in late endosome - peptide loaded to the HLA class II
fate of the invariant chain?
degraded and leaves CLIP protein behind
- HLA-DM acts as peptide exchanger
- removes CLIP and adds peptide to groove
unbound HLA are not displayed
CLIP
fragment left over after the invariant chain is degraded in the Class II binding groove
Class I HLA Pathway
for cytosolic antigens
-proteasome - degrades proteins
protein transported to the ER (via TAP - uses ATP)
- ER forms Class I alpha chain
- beta-2 microglobulin associated with TAP via tapasin
in the ER - alpha chain associates with beta-2
TAP
transports cytosolic antigen peptides to the ER to form the class I HLA
tapasin
binds the beta-2 microglobulin to TAP while HLA class I is forming
outcome of dendritic cell antigen presentation?
naive T cell activation
outcome of macrophage antigen presentation?
effector (memory) T cell activation
-class II
-activation of macrophage
(cell-mediated)
outcome of B cell antigen presentation?
effector (memory) T cell activation
-class II
-antibody production of B cells
(humoral)
cross-presentation
dendritic cells can ingest virally infected cells and display antigen class I and class II
what is the major factor limiting transplantation?
HLA
large number of autoimmune disease due to what?
HLA mutations
ankylosing spondylitis
inflammation of spine
over 88% express HLA-B27 allele
-each allele has a limited number of peptides it can present
possible HLA-B27 doesn’t present critical peptide
rheumatic fever
can lead to antibodies agains cardiac tissue
HLA-DR4 allele involved
Sjogrens syndrome
associated with HLA-DR3
-defects in salvation and lacrimation
Insulin-dependent diabetes mellitus
associated with HLA-DQw8
psoriasis
associated with HLA-B3
every immune response is polyclonal
see different part of antigen every time you respond to it
ex/ hepatitis
-display different pieces of the virus
immune response is not one and done**
