Old lysosomes, new tricks: MHC II dynamics in DCs

MHC II-restricted antigen presentation typically results in recognition of exogenous foreign peptides by T lymphocytes and the subsequent generation of a cellular immune response. It is well known that the intracellular events involved in the assembly of peptide–MHC II complexes occur in the endocytic pathway. However, a point not often considered by immunologists is that the nature of these events suggests contradictions between the fundamental characteristics of the endocytic pathway as defined in non-specialized cells and those features necessary to facilitate efficient MHC II antigen processing in antigen-presenting cells. We believe that one reason why dendritic cells are so efficient at T-cell stimulation is their ability to reconcile these contradictions by regulating the activities and dynamics of their otherwise ‘normal’ complement of endocytic organelles.