Exogenous peptides compete for the presentation of endogenous antigens to major histocompatibility complex class II-restricted T cells.

Abstract

SummaryAntigen-presenting cells (APC) transfected with a construct encoding the hen egg-white lysozyme (HEL) amino acid sequence 1-80 constitutively present HEL peptides complexed to major histocompatibility complex (MHC) class II molecules to specific T cell hybridomas, indicating that endogenous cellular antigens can be efficiently presented to class II-restricted T cells. Here we show that exogenous peptide competitors added to HEL-transfected APC can inhibit the presentation of endogenous HEL peptides to class II-restricted T cells. The inhibition is specific for the class II molecule binding the competitor peptide, and it affects to the same extent presentation of exogenous or endogenous HEL peptides. These results, demonstrating that an exogenous competitor can inhibit class II-restricted T cell activation induced by endogenous as well as exogenous antigen, suggest lack of strict compartmentalization between endogenous and exogenous pathways of antigen presentation. Since autoreactive T cells may recognize endogenous, as well as exogenous antigens, the results have implications for the treatment of autoimmune diseases by MHC blockade.T cells recognize antigen as peptides bound to MHCencoded molecules on the surface of APC (1). Two classes of MHC molecules are involved in antigen presentation to T cells: class I molecules, expressed on the surface of the majority of nucleated cells (2), and class II molecules, expressed mainly on B cells, macrophages, and dendritic cells (3). Although class I and class II MHC molecules appear to have similar antigen-binding sites (4), they are loaded with peptides at different intracellular locations (5-8) and interact with different T cell populations, CD8 + or CD4 +, respectively (9). Two separate pathways of antigen processing and presentation have been proposed, leading to selective association of peptides from endogenous cellular antigens to class I molecules and of peptides from exogenous protein antigens to class II MHC molecules (10). However, peptides derived from endogenous cellular antigens can bind to class II molecules and be presented to T cells (11)(12)(13)(14).Here we demonstrate inhibition by an exogenous MHCbinding competitor of class II-restricted T cell activation induced by endogenously derived antigenic peptides, suggesting lack of strict compartmentalization between the two pathways. These results imply that administration of exogenous class II blockers may also inhibit presentation of endogenous antigens, including those potentially able to activate class II-restricted autoreactive T cells leading to MHC-linked autoimmune diseases. Materials and MethodsAntigens. The synthesis, purification, and analysis of HEL peptides have been previously described (15).Cell Cultures. The establishment of class II-restricted, HEL specific T cell hybridomas has been described (15). Cultures containing 5 x 104 T hybridoma cells and the indicated number of APC were set up in microtiter plates with or without antigen in 0.2 ml of RPMI 1640 (Gibco Laboratories, Grand Isl...

Presentation of Endogenously Synthesized MHC Class II-Restricted Epitopes by MHC Class II Cancer Vaccines Is Independent of Transporter Associated with Ag Processing and the Proteasome

“…Four hybridomas that react to three different HEL peptides have been used: A2.A2 (HEL 46 -61 ), 2B6.3 (HEL [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] ), 3B11.1 (HEL 34 -45 ), and 3A9 (HEL 46 -61 ). Figs.…”

Thymic Dendritic Cells Express Inducible Nitric Oxide Synthase and Generate Nitric Oxide in Response to Self- and Alloantigens

“…To this purpose AML, which inhibits macropinocytosis in DCs (39,40), or ChlQ, which blocks presentation of Ags by neutralizing intracellular acidic compartments (41,42), or BfA, which interferes with the egress of newly synthesized MHC molecules (43)(44)(45)(46), was added before and maintained throughout the Ag pulsing. As shown in Fig.…”

“…APC floating after the incubation period were collected and DCs were purified and assayed for iNOS expression, by FACS and immunoblot, and NO production (see above). To assess the effect of Ag processing on the induction of NO synthesis in thymic DCs, we used AML (50 M), which inhibits Ag uptake by blocking macropinocytosis (39,40), ChlQ (15 M), which inhibits endocytic function and Ag processing (41,42), or BfA (1 g/ml), which blocks the endoplasmic reticulum-Golgi egress of nascent class II MHC molecules (43)(44)(45)(46).…”

CD4+T Cells Induced by a DNA Vaccine: Immunological Consequences of Epitope-Specific Lysosomal Targeting

“…Most CD4 ϩ T-cell responses are induced by proteins endocytosed from the extracellular milieu by specialized antigen-presenting cells (APCs) such as dendritic cells (DC); these proteins are then degraded in the acidic endosomal and lysosomal compartments, where they encounter major histocompatibility complex (MHC) class II molecules, leading to the eventual cell surface presentation of their encoded epitopes. However, although the underlying mechanisms are not fully understood, it has been clearly demonstrated that some proteins synthesized within an APC can be presented by MHC class II molecules and can induce CD4 ϩ T-cell responses (1,4,6,21,25,32). This observation suggested that a DNA vaccine could be designed which should direct endogenously synthesized proteins to the lysosomal compartment of APCs, thus enhancing the induction of CD4 ϩ T cells.…”