SunanaaryEndocytosed protein antigens are believed to be fragmented in what appears to be a balance between proteolysis and MHC-mediated epitope protection, and the resulting peptide-MHC complexes are transported to the surface of the antigen-presenting cells (APC) and presented to T cells. The events that lead to antigenic peptide generation and the compartments where antigen processing takes place remains somewhat enigmatic. The importance of intracellular antigen processing has been well established; however, it is unclear whether additional processing occurs at the APC surface. To follow antigen processing, we have identified a pair ofT cell hybridomas that recognize a long vs. a short version of the same epitope. We have used prefixed APC and various protease inhibitors to demonstate that the APC surface has a considerable potential for antigen processing. Specific antibodies further identified the exopeptidase Aminopeptidase N (APN, CD13) as one of the enzymes involved in the observed cell-surface antigen processing. The NH2-terminal end of the longer peptide could, even while bound to major histocompatibility complex (MHC) class II molecules, be digested by APN with dramatic consequences for T cell antigen recognition. This could be demonstrated both in cellfree systems using purified reagents and in cellular systems. Thus, MHC class II and APN may act in concert to generate the final T cell epitopes.T h cells recognize protein antigens presented in the context of MHC class II molecules. The bulk of evidence suggests that protein antigens before presentation are internalized, transported to an acidic compartment, and partially degraded (reviewed in reference 1). Some of the resulting peptides are bound to class I1 molecules (2, 3), protected against further degradation (4, 5), and transported to the APC surface for Th cell scrutiny. Implicidy, antigen processing is viewed as an intracellular event, and tittle attention is currently paid to the cell surface as an auxiliary compartment for antigen processing. We have previously found that inhibitors ofproteolytic enzymes could alter the specificity and sensitivity of the of antigenic peptide presentation by prefixed APC (6). Since prefixed APC are unable to internalize antigen and thus to perform intracellular antigen processing, these results suggested that some antigen processing can occur at the cell surface. However, we were at that time unable to establish the exact identity and localization of the enzyme(s) involved. In this paper, we have used prefixed APCs and a pair of Ak-restricted T cell hybridomas specific for hen egg lysozyme (HEL)46-tt~ and HELs0-61~, respectively, to assess cell surface antigen processing and to identify one of the enzymes involved. The APC surface was able to perform considerable antigen processing. A careful analysis of the contribution of the various groups of proteases and a panel of mAbs allowed us to identify the ectopeptidase aminopeptidase N (APN, 1 also known as CD13), as one of the enzymes involved in cell-surface a...

The H-2Ab allele exerts a dominant down-regulatory effect on the anti-allo-HPPD (4-hydroxyphenylpyruvate dioxygenase) antibody response, through a hitherto unknown mechanism. In the present study, the allo-variable peptide bound to responder H-2Ak molecules with higher affinity than to H-2Ab ones, arguing against the operation of an affinity hierarchy. Quantitative polymerase chain reaction revealed differences in cytokine mRNA expression between suppressed and high-responder mice. Lymph node cells of responder but not suppressed mice contained high levels of interleukin (IL)-4 mRNA as early as 11 h post-immunization and continued to do so for at least 8 days; this early burst was paralleled by a small burst in transforming growth factor (TGF)-beta mRNA level. Differences in IL-12 mRNA were not detected, although an early IL-12 effect could not be excluded. Interferon (IFN)-gamma appeared to contribute to the suppression at later time points. Early treatment of responder mice with anti-IL-4 monoclonal antibody (11B11) down-regulated the antibody response. The proliferative T cell response from hyperimmunized mice was reduced but still detectable in the presence of an H-2Ab allele. Thus, in the presence of this allele, the Th1 response is enhanced and that of Th2 cells suppressed, apparently as a result of the bias of H-2Ab-restricted T cells in favor of the Th1 subset.