Direct versus indirect allorecognition: Visualization of dendritic cell distribution and interactions during rejection and tolerization

Interactions of donor and recipient dendritic cells (DCs) with CD4 ⁺ T cells determine the alloantigenic response in organ transplantation, where recipient T cells respond either directly to donor MHC, or indirectly to processed donor MHC allopeptides in the context of recipient MHC molecules. The present study evaluates donor and recipient alloantigen-presenting DC trafficking and their interactions with CD4 ⁺ T cells in the lymph nodes (LNs) and the spleen under tolerogenic treatment with anti-CD2 plus anti-CD3 mAb compared with untreated rejecting conditions. CX₃CR1^GFP BALB/c (I-A^d) donor hearts were transplanted into C57BL/6 (I-A^b) mice and quantification of donor DC direct (GFP⁺ or I-A^d+) and recipient DC indirect (YAe⁺) trafficking and interactions with host CD4 ⁺ T cells was performed by fluorescent microscopy. Our data indicate that although both direct and indirect interactions between CD4⁺ T cells and donor and recipient DCs occur shortly after engraftment, only indirect presentation persists in the LN, but not the spleen, of tolerized recipients. These data suggest that distinct anatomic lymphoid compartments play a critical role in peripheral tolerance induction and maintenance, and persistent indirect presentation to CD4⁺ T cells within the LNs is an important process during tolerization.