Antigen processing by endosomal proteases determines which sites of sperm-whale myoglobin are eventually recognized by T cells.
This study reports an identification of the major processing products of an exogenous protein antigen, viz, sperm-whale myoglobin, as obtained after cell-free processing with partially purified macrophage endosomes. It is demonstrated that such a system yields fragments that are indistinguishable by high performance liquid chromatography analysis from those generated after uptake of myoglobin inside live macrophages. The concerted action of the endosomal proteases cathepsin D and cathepsin B can account for nearly all cleavages observed. Cathepsin D appears to be mainly responsible for the initial cleavage of myoglobin, while cathepsin B catalyzes the C-terminal trimming of initially released fragments. The fragments released by cathepsin D contain most, if not all, major epitopes for murine myoglobin-specific helper T cells. Interestingly, each known T cell epitope of myoglobin is located at the very N terminus of a different myoglobin fragment released upon processing. In order to explain this correspondence, noted also in several other protein antigens, a structural relationship is proposed between antigen processing by cathepsin D and antigen recognition by major histocompatibility complex (MHC) class II products. As is demonstrated here, this relationship may be used as a predictive tool for the identification of MHC-binding sequences as well as of T cell epitopes in their naturally occurring form.
Van Noort
J M
JM
TNO Medical Biological Laboratory, Rijswijk, The Netherlands.
Boon
J
J
Van der Drift
A C
AC
Wagenaar
J P
JP
Boots
A M
AM
Boog
C J
CJ
D000595:Amino Acid Sequence; D000818:Animals; D001666:Binding Sites, Antibody; D002401:Cathepsin B / Q000494:pharmacology; D002402:Cathepsin D / Q000494:pharmacology; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D010450:Endopeptidases / Q000502:physiology*; D000939:Epitopes; D000949:Histocompatibility Antigens Class II / Q000276:immunology; D066298:In Vitro Techniques; D008213:Lymphocyte Activation; D008264:Macrophages / Q000378:metabolism; D008969:Molecular Sequence Data; D009211:Myoglobin / Q000276:immunology*; D013601:T-Lymphocytes / Q000276:immunology*
D000939:Epitopes; D000949:Histocompatibility Antigens Class II; D009211:Myoglobin; D010450:Endopeptidases; D002401:Cathepsin B; D002402:Cathepsin D