| PDT has been shown to enhance the host anti-tumor immune response, but the mechanism behind the enhancement is not well defined. We have hypothesized that PDT releases tumor antigens. The released antigens would then be engulfed by antigen presenting cells (APC's) which migrate to the tumor draining lymph nodes and initiate an anti-tumor immune response.;Surrogate antigens can be used to study antigen release by PDT. Line 1 murine lung carcinoma cells were transfected with two cytoplasmic surrogate antigens, green fluorescent protein (GFP) and ovalbumin (OVA). In vitro PDT treatment of stable GFP transfectants resulted in the release of GFP into the supernatant correlating with the degree of PDT induced cell kill. However, following PDT treatment, OVA was not detected by ELISA in either the supernatant or surviving cell fractions. OVA does not disappear when mechanically released by freeze/thaw or cell lysis buffer, but preliminary results suggest that ultraviolet radiation and hyperthermia lead to some degree of OVA disappearance. This suggests that OVA disappearance may be a broader phenomenon related to apoptosis and/or stress responses. This contrast between GFP and OVA may be due to GFP being resistant to proteolysis, whereas OVA is known to be readily degraded by a range of cellular proteolytic mechanisms including the proteasome. Protease, proteasome, and caspase inhibitors did not prevent the OVA disappearance phenomenon.;Antigen release is an important step towards the generation of a specific immune response, but the subsequent ability of a specific immune response to destroy a tumor also depends on the tumor's vulnerability to attack. One of the key characteristics that determines vulnerability is MHC class I status. We have obtained MHC class I inducible and non-inducible variants of the Line 1 murine lung carcinoma, and in both variants the baseline expression of MHC class I is very low. The inducible and non-inducible variants have similar in vitro growth rates, in vitro PDT dose responses, and in vivo tumor growth rates. However, they appear to differ in their in vivo PDT dose responses, with the non-inducible variant being more sensitive to PDT than the inducible variant. This differs from the hypothesis that higher MHC class I levels should provide a better target for cytotoxic T-cells (CTL's). However, MHC class I can be protective against natural killer (NK) cell lysis, and several recent publications have suggested a role for NK cells in PDT responses. Our anti-asialo-GM1 results support this explanation, showing an decrease in tumor-free survival when NK cells are depleted prior to PDT treatment. However, splenic NK cells did not appear to be activated following PDT, and flow cytometry results suggest that nL 1p tumors have a higher level of macrophage infiltration prior to PDT. PDT is known to stimulate the cytolytic activity macrophages, so those extra macrophages may also be responsible for the greater effect of PDT on nL 1p tumors. |
