Get into the groove! Targeting antigens to MHC class II

The activation of MHC class II-restricted helper T cells is paramount to ad aptive immune responses. Vaccine development could therefore benefit from i mproved ways of targeting antigens into MHC class II molecules. In recent y ears, the natural pathways of MHC class II antigen presentation have been e xploited to achieve this goal. First, antigenic proteins and peptides have been modified to facilitate receptor-mediated uptake by professional antige n-presenting cells. Second, DNA constructs containing specific targeting se quences have been used to direct endogenously synthesized antigens to the M HC class II compartments. Both strategies proved to be highly effective. We review these data and describe how this knowledge is currently applied to the design of vaccines that activate helper T cells in vivo.