T cell-mediated autoimmunity to collagen V (col-V), a sequestered yet immunogenic self-protein, can induce chronic lung allograft rejection in rodent models. In this study we characterized the role of CD4+CD25 + regulatory T cells (Tregs) in regulating col-V autoimmunity in human lung transplant (LT) recipients. LT recipients revealed a high frequency of col-V-reactive, IL-10-producing CD4+ T cells (TIL-10 cells) with low IL-2-, IFN-γ-, IL-5-, and no IL-4-producing T cells. These TIL-10 cells were distinct from Tregs because they lacked constitutive expression of both CD25 and Foxp3. Expansion of TIL-10 cells during col-V stimulation in vitro involved CTLA-4 on Tregs, because both depleting and blocking Tregs with anti-CTLA4 F(ab′)2 mAbs resulted in loss of TIL-10 cells with a concomitant increase in IFN-γ producing Th1 cells (TIFN-γ cells). A Transwell culture of col-V-specific TIL-10 cells with Th1 cells (those generated in absence of Tregs) from the same patient resulted in marked inhibition of IFN-γ and proliferation of TIFN-γ cells, which was reversed by neutralizing IL-10. Furthermore, the TIL-10 cells were HLA class II restricted because blocking HLA class II on APCs resulted in the loss of IL-10 production. Chronic lung allograft rejection was associated with the loss of Tregs with a concomitant decrease in TIL-10 cells and an increase in TIFN-γ cells. We conclude that LT patients have col-V-specific T cells that can be detected in the peripheral blood. The predominant col-V-specific T cells produce IL-10 that suppresses autoreactive Th1 cells independently of direct cellular contact. Tregs are pivotal for the induction of these "suppressor" TIL-10 cells.