T-cell alloimmunity and chronic allograft dysfunction Niloufar Safinia, Behdad Afzali, Kerem Atalar, Giovanna Lombardi, Robert I. Lechler  Kidney International  Volume 78, Pages S2-S12 (December 2010) DOI: 10.1038/ki.2010.416 Copyright © 2010 International Society of Nephrology Terms and Conditions

Figure 1 Direct, indirect, and semidirect pathways of allorecognition. (a) In the direct pathway, intact major histocompatibility complex (MHC) on donor antigen-presenting cells (APCs) is recognized directly by recipient CD4+ and CD8+ T cells. (b) The indirect pathway is characterized by recipient APC uptake of allogeneic donor MHC that has been shed through apoptosis or necrosis. This is then processed, resulting in presentation of donor antigens in the context of recipient MHC class II to recipient CD4+ T cells. (c) Semidirect allorecognition results from the transfer of cellular membrane components, including intact donor MHC, from donor APCs to recipient APCs. This process may occur through mechanisms such as cell–cell contact or through the transfer of donor exosomes that fuse with recipient APC cell membranes. Recipient APCs are then chimeric for MHC, and are able to stimulate both CD4+ and CD8+ donor T cells. Kidney International 2010 78, S2-S12DOI: (10.1038/ki.2010.416) Copyright © 2010 International Society of Nephrology Terms and Conditions