Vaccination with photodynamic therapy-treated macrophages induces highly suppressive T-regulatory cells

The mode of cell death and uptake of dead cell by antigen-presenting cells influences a finely tuned balance between immune tolerance and a pathogenic auto-inflammatory response (1). Photodynamic therapy (PDT) induces apoptosis via permeabilization of the mitochondrial inner membrane by reactive oxygen species, which leads to mitochondrial swelling, cytochrome C release into the cytosol, and a caspase cascade (2–4). The uptake of apoptotic cells by immature dendritic cells has been shown to promote immune tolerance, whereas cell debris can elicit an inflammatory immune response (1). In the present study, we intend to test whether PDT-mediated apoptosis is tolerogenic and can suppress collateral tissue damage during infectious processes. We chose cutaneous leishmaniasis (CL) in susceptible BALB/c mice as our experimental model as in this model, an immune response is not only incapable of eliminating the etiologic agent but also results in collateral tissue damage (5, 6). Progressive involvement of the new tissues may lead to the complete destruction and self-amputation of the affected parts of the host (7). The understanding of the mechanisms underlying the counter-regulation of this inflammatory reaction may have therapeutic implications. There is some evidence that tolerance following presentation of apoptotic cells is mediated by CD4+CD25+Foxp3+ T regulatory (Treg) cells (8). Treg have been shown to control the severity of an inflammatory response, preserving surrounding tissues from unnecessary damage in many infectious diseases (9–11), including the early stage of CL (12, 13). We found that vaccination with the apoptotic, but not necrotic non-infected macrophages (MZ), was beneficial for the restriction of parasitic replication during an early stage of Leishmania infection and was mediated by Treg activation. The adoptive transfer of spleen-derived Treg from mice vaccinated with PDT-treated apoptotic, but not necrotic MZ restrained disease progression. These changes were mediated by the depletion of CD3+CD8+ and NKT cells and increased levels of IL-12p70 and interferon (IFN)-γ, IL-10, and TGF-β in the CL lesions.