A novel pathway of antigen presentation by dendritic and endothelial cells: Implications for allorecognition and infectious diseases.

Dendritic cells (DCs) are the major antigen presenting cells capable of stimulating T cell responses following either organ transplantation or a viral infection. In the context of allorecognition, T cells can be activated following presentation of alloantigens by donor DCs (direct), as well as by recipient DCs presenting processed donor major histocompatibility complex (MHC) as peptides (indirect). We have recently described another mechanism by which alloreactive T cells are activated. Recipient DCs can acquire donor MHC through cell-to-cell contact and this acquired MHC can stimulate a T cell response (the semidirect pathway). Similarly, during a viral infection, DCs are capable of stimulating T cells directly, as occurs when infected DCs present processed viral antigens, or indirectly by a process known as cross-presentation. Although cross-presentation of exogenous antigen is an important mechanism for controlling infectious diseases, it is possible that peptide:MHC acquisition (the semidirect pathway) may also play a part in immunity against pathogens. In this review, we discuss the possible contributions of the semidirect pathway/MHC transfer in infectious disease.