- Email: [email protected]
Antigen processing and presentation
798
E N ! I 1990
molecules such as MEL-14. The in vivo function of these proteins is still insufficiently known, and A. Hamann's data indicate that lymphocyte homing ~s regulated by a complex interplay between different receptor-ligand interactions rather than by a few highly specific homing receptors. The precise function of Thy-I has been hard to define, although a role in cell-cell interactions has been suspected for a long time. Investigating the adhesive interactions between thymocytes and thymic epithelial cell lines, Hai Tan He presented evidence that Thy-1 functions as an adhesion molecule in this system via a heterophilic Ca + +-independent mechanism.
The binding of CD4 molecules to MHC class II molecules on other cells may be regarded as an adhesive interaction. However, as reported in the final talk of the session by Fabienne Mazerolles (Nogent), the CD4-MHC class II interaction does not seem to contribute to the adhesive strength of T-B cell conjugate formation. By contrast, ligand binding to CD4 appears to modulate T-cell adhesiveness, since conjugate formation between CD4+ and MHC class II+ cells (but not between cells lacking one or the other) results in reduced adhesiveness during a subsequent time period, an effect which can be mimicked by soluble CD4 ligands.
A N T I G E N PROCEE~_~ING A N D P R E S E N T A T I O N
A. Lanzavecchia Basel Institute for Immunology Grenzacherstrasse 487, 4005 Basel (Switzerland)
This session was mainly devoted to the analysis of the intracellular pathways of class 11and class II molecules and their assoc'iation with antigenic peptides. The mechanism for the preferential association1 of class II molecules with exogenous antigens has been reviewed by Peter Cresswell. Newly synthesized class II molecules are sorted to an endosomal compartment where they bind peptides derived from the proteo1.vtic degradation of exogenous antigens. Cresswell reported peptide-binding studies using different types of class II molecules purified from human EBV-B cells. Newly synthesized class II molecules which are associated with the invariant chain fail to bind peptides, while the class II molecules that spontaneously dissociate from the invariant chain bind peptides with an efficiency
close to 100 °70. Finally, these results demonstrate that one of the functions of the invariant chain is to protect the peptide-binding site of class II molecules until these molecules reach the endosomal compartment. The fate of membrane MHC molecules was studied by Colin Watts in human EBV-B cells. He showed that both class I and class II molecules are internalized and recycle through a small intracellular pool that can be increased by the addition of primaquine, a drug that slows receW_or recycling. He calculated tha' a single MHC molecule recycles 60 times during its lifespan. Jacques Davoust reported that mouse class II moJecules also recycle in a B-cell lymphoma, but not in fibroblasts. Although these reports were the first indication that MHC molecules are internalized
S U M M A R Y OF SESSION'S and recycle, it was not clear what could be the functional consequence of class I and class II recycling and whether this would necessarily lead to an exchange of immunogenic peptides. Vincenzo Cerundolo reviewed the class I pathway. Class I molecules bind peptides early in the endoplasmic reticulum and are then rapidly transported to the cell surface. Studies on class I have been greatly boosted by the availability of mutant cell lines of mouse (RMA-S) or human (0.174) origin with a defect in cell surface cxpression of class I molecules due to failure of peptide loading. These mutants have been used by several groups in a collaborative effort to understand the site and mechanism of peptide-class I association. Cerundolo reported that the defect of 0.174 cells can be mapped to the short arm of human chromosome 6 between DP and complement genes. It is not yet clear what this gene product .might be, but it is likely to be involved in peptide loading. Empty class I molecules are very unstable and can be stabilized by addition of peptides. It seems that this might occur on the cell surface and even in cell lysates. The availability of empty unstable class I molecules that can be stabilized by peptide
hinding
nff~rc
n n~ur
hlnrl~
. . . . . . . .
to measure the interaction between peptides and class i.
799
The identification of T-cell antigens by DNA transfection has enabled identification of the first tumour-specific antigens. Thierry Boon has reviewed his findings on the characterization of T u m - antigens which are generated by point mutations in normal cellular genes !eading to the production of an antigen~c peptide. Interestingly, not all tumourspecific antigens are produced by mutation, since at least one has been found to be a normal cellular constituent. Cees Melief has described the case of mouse tumours induced by adenovirus 5. In this case, the El viral product is required to maintain the transformed phenotype and can be recognized by specific T-ccll clones that eradicate the tumour in nude mi.ce. Relatively few reports dealt with structural aspects of T-cell recognition. Perarnau reported that allelic forms of mouse [3~microglobulin may influence antigen presentation to T cells. Romagnoli showed that Nickel, which is recognized by T c.ells from patients with atopic dermatitis, binds to class II molecules by interacting with histidine residues of peptides that are already bound in the groove. Finally, Terry Delovitch reported attempts to isolate and characterize naturally processed antigen fragments in v, v o a , nu a i t o w t : u t u u t t dendritic cells and thymic epithelial cells display different sets of antigen fragments.
PERIPH ERAL TOLERA NCE S. Cobbold
Cambridge University Department of Pathology, Level 3, Laboratories block, addenbrook's Hospital, Hill's Road, Cambridge CB2 2QQ
This session was probably the first devoted entirely to peripheral mechanisms of tolerance. The chairman
(H. Waldmann, Cambridge) used CD4 antibody-induced T-cell tolerance to human gamma globulin (HGG) to in-
E N ! I 1990
molecules such as MEL-14. The in vivo function of these proteins is still insufficiently known, and A. Hamann's data indicate that lymphocyte homing ~s regulated by a complex interplay between different receptor-ligand interactions rather than by a few highly specific homing receptors. The precise function of Thy-I has been hard to define, although a role in cell-cell interactions has been suspected for a long time. Investigating the adhesive interactions between thymocytes and thymic epithelial cell lines, Hai Tan He presented evidence that Thy-1 functions as an adhesion molecule in this system via a heterophilic Ca + +-independent mechanism.
The binding of CD4 molecules to MHC class II molecules on other cells may be regarded as an adhesive interaction. However, as reported in the final talk of the session by Fabienne Mazerolles (Nogent), the CD4-MHC class II interaction does not seem to contribute to the adhesive strength of T-B cell conjugate formation. By contrast, ligand binding to CD4 appears to modulate T-cell adhesiveness, since conjugate formation between CD4+ and MHC class II+ cells (but not between cells lacking one or the other) results in reduced adhesiveness during a subsequent time period, an effect which can be mimicked by soluble CD4 ligands.
A N T I G E N PROCEE~_~ING A N D P R E S E N T A T I O N
A. Lanzavecchia Basel Institute for Immunology Grenzacherstrasse 487, 4005 Basel (Switzerland)
This session was mainly devoted to the analysis of the intracellular pathways of class 11and class II molecules and their assoc'iation with antigenic peptides. The mechanism for the preferential association1 of class II molecules with exogenous antigens has been reviewed by Peter Cresswell. Newly synthesized class II molecules are sorted to an endosomal compartment where they bind peptides derived from the proteo1.vtic degradation of exogenous antigens. Cresswell reported peptide-binding studies using different types of class II molecules purified from human EBV-B cells. Newly synthesized class II molecules which are associated with the invariant chain fail to bind peptides, while the class II molecules that spontaneously dissociate from the invariant chain bind peptides with an efficiency
close to 100 °70. Finally, these results demonstrate that one of the functions of the invariant chain is to protect the peptide-binding site of class II molecules until these molecules reach the endosomal compartment. The fate of membrane MHC molecules was studied by Colin Watts in human EBV-B cells. He showed that both class I and class II molecules are internalized and recycle through a small intracellular pool that can be increased by the addition of primaquine, a drug that slows receW_or recycling. He calculated tha' a single MHC molecule recycles 60 times during its lifespan. Jacques Davoust reported that mouse class II moJecules also recycle in a B-cell lymphoma, but not in fibroblasts. Although these reports were the first indication that MHC molecules are internalized
S U M M A R Y OF SESSION'S and recycle, it was not clear what could be the functional consequence of class I and class II recycling and whether this would necessarily lead to an exchange of immunogenic peptides. Vincenzo Cerundolo reviewed the class I pathway. Class I molecules bind peptides early in the endoplasmic reticulum and are then rapidly transported to the cell surface. Studies on class I have been greatly boosted by the availability of mutant cell lines of mouse (RMA-S) or human (0.174) origin with a defect in cell surface cxpression of class I molecules due to failure of peptide loading. These mutants have been used by several groups in a collaborative effort to understand the site and mechanism of peptide-class I association. Cerundolo reported that the defect of 0.174 cells can be mapped to the short arm of human chromosome 6 between DP and complement genes. It is not yet clear what this gene product .might be, but it is likely to be involved in peptide loading. Empty class I molecules are very unstable and can be stabilized by addition of peptides. It seems that this might occur on the cell surface and even in cell lysates. The availability of empty unstable class I molecules that can be stabilized by peptide
hinding
nff~rc
n n~ur
hlnrl~
. . . . . . . .
to measure the interaction between peptides and class i.
799
The identification of T-cell antigens by DNA transfection has enabled identification of the first tumour-specific antigens. Thierry Boon has reviewed his findings on the characterization of T u m - antigens which are generated by point mutations in normal cellular genes !eading to the production of an antigen~c peptide. Interestingly, not all tumourspecific antigens are produced by mutation, since at least one has been found to be a normal cellular constituent. Cees Melief has described the case of mouse tumours induced by adenovirus 5. In this case, the El viral product is required to maintain the transformed phenotype and can be recognized by specific T-ccll clones that eradicate the tumour in nude mi.ce. Relatively few reports dealt with structural aspects of T-cell recognition. Perarnau reported that allelic forms of mouse [3~microglobulin may influence antigen presentation to T cells. Romagnoli showed that Nickel, which is recognized by T c.ells from patients with atopic dermatitis, binds to class II molecules by interacting with histidine residues of peptides that are already bound in the groove. Finally, Terry Delovitch reported attempts to isolate and characterize naturally processed antigen fragments in v, v o a , nu a i t o w t : u t u u t t dendritic cells and thymic epithelial cells display different sets of antigen fragments.
PERIPH ERAL TOLERA NCE S. Cobbold
Cambridge University Department of Pathology, Level 3, Laboratories block, addenbrook's Hospital, Hill's Road, Cambridge CB2 2QQ
This session was probably the first devoted entirely to peripheral mechanisms of tolerance. The chairman
(H. Waldmann, Cambridge) used CD4 antibody-induced T-cell tolerance to human gamma globulin (HGG) to in-