Three major cell types, dendritic cells, macrophages, and B cells, present peptides bound to MHC class II molecules for recognition by CD4 T cells. In general, these peptides are derived from proteins...

Three major cell types, dendritic cells, macrophages, and B cells, present peptides bound to MHC class II molecules for recognition by CD4 T cells. In general, these peptides are derived from proteins or pathogens taken up by the cell by endocytosis, phagocytosis, or macropinocytosis. Based on these pathways of antigen uptake, some of the enzymes that degrade proteins to generate peptides for MHC class II presentation are:

1. Ubiquitin ligases that tag proteins for degradation by the proteasome


2. ATP transporter proteins that deliver endocytic proteins into the cytosol for degradation


3. Cysteine proteases like cathepsins that function at acidic pH


4. The lysosomal thiol reductase found in the endosomes


5. The lysosome-associated membrane trafficking protein, LAMP-2