Cytoplasmic Processing Is a Prerequisite for Presentation of an Endogenous Antigen by Major Histocompatibility Complex Class II Proteins

Abstract

Biochemical and functional studies have demonstrated major histocompatibility complex (MHC) class II–restricted presentation of select epitopes derived from cytoplasmic antigens, with few insights into the processing reactions necessary for this alternate pathway. Efficient presentation of an immunodominant epitope derived from glutamate decarboxylase (GAD) was observed regardless of whether this antigen was delivered exogenously or via a cytoplasmic route into human histocompatibility leukocyte antigen class II–DR4+ antigen-presenting cells. Presentation of exogenous as well as cytoplasmic GAD required the intersection of GAD peptides and newly synthesized class II proteins. By contrast, proteolytic processing of this antigen was highly dependent upon the route of antigen delivery. Exogenous GAD followed the classical pathway for antigen processing, with an absolute requirement for endosomal/lysosomal acidification as well as cysteine and aspartyl proteases resident within these organelles. Presentation of endogenous GAD was dependent upon the action of cytoplasmic proteases, including the proteasome and calpain. Thus, translocation of processed antigen from the cytoplasm into membrane organelles is necessary for class II–restricted presentation via this alternate pathway. Further trimming of these peptides after translocation was mediated by acidic proteases within endosomes/lysosomes, possibly after or before class II antigen binding. These studies suggest that processing of exogenous and cytoplasmic proteins occurs through divergent but overlapping pathways. Furthermore, two cytoplasmic proteases, the proteasome and calpain, appear to play important roles in MHC class II–restricted antigen presentation.

Epigenetic modification augments the immunogenicity of human leukocyte antigen G serving as a tumor antigen for T cell-based immunotherapy

“…Interestingly, 5-AZA treatment did not upregulate the expression levels of HLA-G on the tumor cell surface, indicating that the production of the HLA-G 26-40 peptide epitope via antigen processing does not require protein cell surface expression. Because intracellular antigens can be processed as CD4 C epitopes in endosomal compartments 40 or by cytoplasmic proteases, 41 it is not that all surprising that tumor cells solely expressed intracellular HLA-G protein were capable of processing and presenting the HLA-Gderived CD4 C epitope on their surface MHC class II molecules (Figs. 4, 5).…”

Immune surveillance of intracellular pathogens via autophagy

“…46 A third pathway involves processing of cytosolic or nuclear proteins (e.g. glutamate decarboxylase 65 (GAD65) 33 ) by the proteasome and is TAP independent. 33,40 For this pathway, peptides seem to be imported directly into endosomal/lysosomal compartments, via a transporter that was recently suggested to be Lamp-2a, the transporter of chaperone-mediated autophagy.…”

“…glutamate decarboxylase 65 (GAD65) 33 ) by the proteasome and is TAP independent. 33,40 For this pathway, peptides seem to be imported directly into endosomal/lysosomal compartments, via a transporter that was recently suggested to be Lamp-2a, the transporter of chaperone-mediated autophagy. 47 In addition to these proteasome-dependent pathways, cytosolic and nuclear proteins can also be processed by a proteasome-and TAPindependent pathway: This fourth pathway involves the direct import of cytosolic/nuclear proteins (e.g.…”

“…For neomycin phosphotransferase II and EBNA1, autophagy was implicated in the delivery of antigens into lysosomes, while this has not been demonstrated for influenza matrix protein M1. However, when the half-life of M1 was modified with the N-end rule, only long-lived M1 (t 1/2 ¼ 5 h) was presented on MHC class II and able to stimulate CD4 þ T cells, while short-lived M1 (t 1/2 ¼ 10 min) 33 ) are degraded by the proteasome and then follow a TAP-independent pathway onto MHC class II. It is thought that peptides are directly imported into endosomal/lysosomal compartments via a peptide transporter, possibly Lamp-2a.…”

MHC class I and class II presentation of tumor antigen in retrovirally and adenovirally transduced dendritic cells

“…8 Certain endogenous antigens have previously been shown to be efficiently presented on MHC class II molecules, 14 and several groups have observed that immunizations with transduced DCs require CD4 + T-cell help for optimal antitumor effects; 15 however, presentation of endogenous tumor antigen to CD4 + T cells by transduced DCs is less well documented. Peptides bound to MHC class I molecules come predominantly from proteins degraded in the cytoplasm, whereas peptides bound to MHC class II molecules are mostly derived from exogenous or intravesicular sources.…”