class II-restricted T cells (TH) recognize exogenousantigen.

Manipulation of the location of a protein can determinewhether it elicits an MHC class I- or class II-restrictedresponse. For example: influenza virus hemagglutinin (HA), a glycoprotein

associated with the membrane of an infected host cell, normally elicits only a weak CTL response, butinfluenza virus HA can be generated in the cytoplasmby deleting that part of its sequence that encodes the Nterminal signal peptide (required for translation acrossthe membrane of the endoplasmic reticulum [ER]) and,when this is done, there is a strong CTL response to HA;

the introduction of ovalbumin into the cytoplasm of atarget cell (using an osmotic shock technique) generatesCTLs recognizing ovalbumin, whereas the addition ofexogenous ovalbumin generates an exclusively TH cellresponse.

Proteasomes are cytoplasmic organelles thatdegrade cytoplasmic proteinsAlthough the assembly of MHC class I molecules occursin the ER of the cell, peptides destined to be presented by MHC class I molecules are generated from cytosolicproteins. The initial step in this process involves anorganelle called the proteasome a complex of 1417different subunits, which form a barrel-like structure (Fig. 7.7).

Proteasomes provide the major proteolytic activity ofthe cytosol. They have a range of different endopeptidaseactivities and they degrade denatured or ubiquitinatedproteins to peptides of about 515 amino acids (ubiquitinis a protein that tags other proteins for degradation).

Two genes, LMP2 and LMP7, located in the class IIregion of the MHC (Fig. 7.8), encode proteasomecomponents that subtly modify the range of peptidesproduced by proteasomes. The expression of these genesis induced by interferon- (IFN), and the proteinproducts: displace constitutive subunits of the proteasome. LMP2

and LMP7 along with a third inducible proteasomecomponent encoded on a different chromosome;

influence processing of peptides by creating peptidefragments suitable for binding MHC class I moleculesfrom pathogens such as viruses, in addition to selfproteins.

Additional subunits associate with the ends of core (20S)proteasomes and may influence antigen processing. Theseinclude interferon-inducible PA28 molecules as well as acomplex of proteins that result in a larger 26S particle.

Proteasomes may not be the only proteases involved in producing peptides for presentation by MHC class Imolecules. There is evidence for the involvement ofenzymes, such as the giant tripeptidyl aminopeptidase II(TPPII) complex.

7. ANTIGEN PRESENTATION

150

Generation of immunoproteasomes by replacement ofactive subunits

Fig. 7.7 The 20S proteasome, shown in cartoon form, iscomposed of four stacked disks, two identical outer disks of subunits, and two similar inner disks comprised of subunits.Each disk has seven different subunits. Peptides enter into thebody of the proteasome for cleavage into peptides. Only threeof the subunits are active. In normal proteasomes, these arecalled MB1, delta, and Z (1). Interferon- treatment of cellsresults in replacement of these three subunits by the twoMHC-encoded proteins, LMP2 and LMP7, as well as a thirdinducible protein, MECL1 (2). These subunits are shownadjacent to each other here, whereas they are actually inseparate parts of the ring and some would be hidden at theback of the structure shown.

MB1

ZMB1

LMP7

MECL1LMP7

MECL1

LMP2

LMP2Z

!

!

"

"

!

!

"

"Delta

Delta

1 2

MHC genes involved in antigen processing and presentationFig. 7.8 Genes encoding the two subunitsof the peptide transporter (TAP) and twocomponents (LMP) of the multisubunitproteasome (see Fig. 7.7) are located inthe murine and human class II regions. TheTapasin gene is located just centromeric(left) of the MHC.

K2 K Pb Oa Eb2ObH-2 Ab Aa Eb Ea

Ob

DOB

B2

Tapasin

Tapasin

B2 A2 B3 B1A2 B1 A1 DOA DOB A1 B1 B2 B3 AHLA

DP DM DQ DR

LMP7TAP1LMP2 TAP2

LMP7TAP1LMP2 TAP2