Antigen processing

of cytosolic pathogens that are degraded in the cytosol

presented to effector CD8 T-cells inducing cell death on the presenting cell

1. intravesicular pathogens degraded in endocytic vesicles, presented to CD4 T-cells inducing killing of intravesicular bacteria and parasites

2. extracellular pathogens and toxins degraded in endocytic vesicles, presented to CD4 T-cells activating B cells to secrete Ig to eliminate extracellular bacteria / toxin (differentiation of B-cell)

1. partly folded MHC class I α-chain binds to calnexin until b2-microglobulin binds

2. MHC class I α:ß2m complex is released from calnexin & binds a complex of chaperone proteins (calreticulin, ERp57) and TAP via tapasin

   3 chaperones to stabilize structure before loading, protects MHC molecule by forming a ring around it that is closed by tapasin

   peptide generation: defective ribosomal products (DRiPs) are peptides translated from introns upon improper splicing, translation of frameshifts (don’t start with met. codon) or correct translation but unfolding—>tagged for ubiquitinylation

3. protease stage: processing in proteasome—>protein cleaved into peptide fragments (from DRiPs & cytosolic proteins

   TAP stage: peptides have to cross ER membrane via TAP transporter (transporter associated with antigen presentation) —>has size restriction only allowing peptides to enter that are slightly longer than peptides bound in groove

   ERAAP cleaves the N-terminus so that the peptides can fit into the MHCI groove (peptide trimming)

4. surface export: peptide binds MHCI molecule and completes its folding, release from the TAP complex and exported to the cell membrane

   —>never find a MHCI molecule w/o a peptide at the cell surface ( not stable)

   
   

two cores: 20 S (4 rings with 7 subunits) has a catalytic domain for cleavage

2x 19S binds ub-proteins, located at extremities

constitutive proteasome; 20S composed of ß1, ß2, ß5

immunoproteasome with enhanced proteolytic activity: 3 additional subunits LMP-2,-7 & MECL-1, 19S core replace by PA28, stimulated by IFNγ signaling

PA28 binds 20S and leads to open conformation —>changes conformation

transporter associated with antigen processing

heterodimer of TAP1 & TAP2

has ATP-binding cassette (ABC)—>transport requires energy consumption

peptides 8-16 AA

TAP

TAPBP = tapasin

LMP = protein associated to the proteasom

endoplasmatic reticulum aminopeptidase associated with antigen processing

TAP transports up to 16 AA peptides—>too long for MHC class I

C-ter selected by TAP compatible to MHC I binding

—>N-terminus cut

cross-presentation by MHCI

presentation of exogenous antigens by MHCI by DC

internalized in phagolysosome of DC, protein cleavage & peptide processing

crossing of phagolysosome membrane and loading via TAP

—>important to not only present intracellular peptides

would miss otherwise many pathogens

invariant chain li forms a complex with MHC II molecule blocking the binding of peptides and misfolded proteins

li cleaved in an acidified endosome, leaving a short peptide fragment CLIP still bound to the MHCII molecule (hinders loading of a peptide)

—>chaperone li used for protection from peptides presented by MHCI

antigen taken up from the extracellular space into intracellular vesicles

in early endosome of neutral pH endosomal proteases inactive

acidification of vesicles actives proteases to degrade antigen into peptide fragments (MIIC compartment acidic, MHC class II compartment)

vesicle-containing peptides fude with vesicles containing MHC class II molecules

HLA-DM (non-classical MHCII) binds to the MHC class II molecule releasing CLIP and allowing other peptides to bind

HLA-DO represses HAL-DM—>for ongoing immune response upregulation of HLA-DM to evade negative regulation by HLA-DO via IFNγ signaling (high efficiency to present antigens needed)

HLA-DO & HLA-DM encoded in the MHC locus

MHC class II molecule bound with an antigen travels to the cell surface

never without a peptide at the cell surface

presentation of intracellular proteins after autophagy

autophagy = natural process of protein turnover with cytosolic proteins delivered to lysosomes (self-digestion)

antigens in auto-phagosome vesicle

as diverse as possible for both classes

intracellular, extracellular, non-coding sequences to represent each pathogen