PROCESSING OF ENDOGENOUSLY SYNTHESIZED HEN EGG-WHITE LYSOZYME RETAINED IN THE ENDOPLASMIC-RETICULUM OR IN SECRETORY FORM GIVES RISE TO A SIMILAR BUT NOT IDENTICAL SET OF EPITOPES RECOGNIZED BY CLASS-II-RESTRICTED T-CELLS

To study the processing and presentation of endogenously synthesized A g to class II MHC-restricted T cells, hen egg lysozyme (HEL), either t agged with a peptide that confers retention in the endoplasmic reticul um (HEL.KDEL), or in the secretory form (HELs), was stably expressed i n LK-35.2 B hybridoma cells. Presentation of HEL peptides bound to cla ss II molecules was assessed by activation of specific T cell hybridom as recognizing seven different epitopes derived from exogenous HEL. Th e presentation of endogenously synthesized HEL was not caused by reupt ake of secreted or shed Ag. All the HEL epitopes examined were efficie ntly presented after processing of endogenous HEL by HELs-transfected LK-35.2 cells. Processing of HEL tagged with KDEL, however, gave rise to presentation of only six of the seven HEL epitopes. The epitope inc luded in the HEL sequence 112-124 was not presented by HEL.KDEL-transf ected B cells. In addition, two of the four T cell hybridomas recogniz ing HEL 116-129 together with I-A(k) molecules were not activated by H EL.KDEL, and three other epitopes were presented with lower efficiency as compared with HELs. Thus, endogenously synthesized HEL in secretor y form gives rise to a set of class II-binding epitopes indistinguisha ble from exogenous HEL, whereas endoplasmic reticulum-retained HEL gen erates a similar but not identical set of epitopes. The endosomal prot ease inhibitor leupeptin prevented presentation of the epitope 108-11 6, but not 46-61, both by HELs and HEL.KDEL transfected cells, indicat ing a requirement for endosomal processing in both cases. In addition, the presentation of peptides derived from endogenously synthesized, e ither secretory or endoplasmic reticulum-retained HEL, could be inhibi ted by lysosomotropic amines, further indicating that the intracellula r route of class II molecules presenting peptides derived from endogen ous Ag intersects the acidic endosomal compartment.