Some binding of extracellular peptides to MHC molecules at the cell surface can occur, however, as the addition of peptides to living or even chemically fixed cells in vitro can generate peptide:MHC complexes that are recognized by T cells specific for those peptides. This has been readily demonstrated for many peptides that bind MHC class II and class I molecules. Whether this phenomenon is due to the presence of empty MHC proteins on the cells or to peptide exchange is not clear. Nevertheless it can happen and is a widely used technique to load synthetic peptides for analyzing the specificity of T cells.
6-9 Cessation of antigen processing occurs in dendritic cells after their activation through reduced expression of the MARCH-1
E3 ligase.
Dendritic cells that have not yet been activated by infection carry out active surveillance of the antigens in their location, for example, through macropinocytosis of soluble proteins. Peptides derived from proteins are continuously processed and loaded onto MHC class II molecules for expression on the cell surface. In addition, peptide:MHC complexes are also continuously being recycled from the surface and degraded in cells by ubiquitination and proteasomal degradation. MHC class II molecules contain a conserved lysine residue in the cytoplasmic tails of the β chain; this lysine residue is a target of an E3 ligase (see Section 3-7) called membrane associated ring finger (C3HC4) 1, or MARCH-1, expressed in B cells, dendritic cells, and macrophages. MARCH-1 is expressed constitutively in B cells and induced by the cytokine IL-10 in other cells. It resides in the membrane of a recycling endosomal compartment, where it ubiquitinates the cytoplasmic tail of MHC class II molecules, leading to their eventual degradation in lysosomes, thereby regulating their steady-state level of expression (Fig. 6.15).
The MARCH-1 pathway is shut down during infection to increase the stability of peptide:MHC complexes. Dendritic cells that capture antigens at sites of infection must first migrate to local lymph nodes in order to activate naive T cells; this may take many hours. Since continuous recycling limits the lifetime of peptide:MHC complexes on the cell surface, pathogen-derived peptide:MHC complexes could be lost during this migration, preventing T-cell activation. To prevent this situation, when dendritic cells are activated by pathogens, expression of MARCH-1 is shut off. This may be mediated directly by innate pathogen sensors, since TLR signaling in dendritic cells rapidly reduces the level of mRNA for MARCH-1. The MARCH-1 protein half-life is only around 30 minutes, so that activated dendritic cells soon accumulate peptide:MHC complexes on their cell surface produced at the time of encounter with pathogen.
In addition to regulating MHC class II expression in dendritic cells, MARCH-1 similarly regulates expression in dendritic cells of the co-stimulatory molecule (see Section 1-15) CD86 (or B7-2), which like MHC class II molecules, is also
MHC molecules accumulate on the cell surface, presenting peptides they acquired at the time of dendritic cell activation