Macrophage-targeted photodynamic therapy: scavenger receptor expression and activation state

Macrophage-targeted photodynamic therapy (PDT) may have applications in the selective killing of cells involved in atherosclerosis, inflammation and tumor. We have previously shown that a conjugate between the photosensitizer chlorin(e6) (ce6) and maleylated bovine serum albumin (BSA-mal) gives highly selective targeting to macrophages. In this report we examine the effect of macrophage activation and scavenger receptor class A (SRA) expression on this targeting in two murine macrophage tumor cell lines (RAW264.7 and P388D1) and a control murine mammary sarcoma cell line (EMT-6). Cells were pretreated with interferon gamma (IFNgamma) and/or lipopolysaccharide (LPS) followed byBSA-ce6-mal addition, and SRA expression, tumor necrosis factor alpha (TNFalpha) release, conjugate uptake and PDT killing were measured. Both macrophage cell lines expressed SRA and took up conjugate specifically in an SRA-dependent manner, but differences were observed in their response to activation. RAW264.7 expressed increasingly more SRA and took up increasingly more BSA-ce6-mal in response to IFNgamma, LPS, and IFNgamma+LPS, respectively. The PDT killing did not follow the same pattern as the uptake of the photosensitizer. The increase in uptake in the IFNgamma treated cells did not lead to an increase in PDT killing, while stimulation with LPS or IFNgamma + LPS resulted in a significant protection against PDT, despite a significant increase in photosensitizer uptake. P388D1 was responsive to neither IFNgamma, nor to LPS, or to IFNgamma +LPS with respect to SRA expression, conjugate uptake, and PDT killing. These data may have implications for the use of PDT to target physiologically undesirable macrophage subtypes implicated in disease, and on how manipulation of the activation status of the macrophage will influence the PDT effect.