Deletion of Alloantigen-Activated Cells by Aminolevulinic Acid-Based Photodynamic Therapy

Abstract

Protoporphyrin IX (PpIX), an endogenously synthesized photosensitizer, can transiently accumulate in activated lymphocytes following administration of the heme precursor 5-aminolevulinic acid (ALA). One possible mechanism of this in lymphocyte accumulation is that actively dividing cells use intracellular iron stores for cytochrome and DNA synthesis and thus do not inactivate PpIX, the photoactive precursor of heme, by iron incorporation. This selective accumulation in activated cells should allow targeting by photodynamic therapy (PDT). To determine the effect of this accumulation, we studied PDT effects on the in vitro correlate of transplantation rejection: the one-way mixed lymphocyte reaction (MLR). Selective phototoxicity was determined by photoirradiating ALA-treated, MLR-activated cells and measuring subsequent stimulation either in a secondary MLR or with phytohemagglutinin (PHA). We found that proliferation of MLR-activated lymphocytes incubated with ALA and treated with light was only 12-20% of controls (ALA+, no light) after rechallenge with the stimulator cells (P < 0.05), although their response to nonspecific PHA stimulation was similar to controls. Thus alloantigen-specific depletion was shown. The data suggest a role for ALA-PDT in the treatment of diseases that require the selective elimination of activated lymphocytes and possibly as an immunomodulator.

Selective Action of the Photosensitizer QLT0074 on Activated Human T Lymphocytes¶

“…Mouse T cells activated with the anti‐CD3 antibody exhibited greater verteporfin uptake and underwent apoptosis at lower concentrations of this photosensitizer than did nonactivated T cells (7). Mitogen‐stimulated human T cells synthesized higher amounts of the endogenous photosensitizer protoporphyrin IX (PPIX) than did their resting equivalents after incubation with the PPIX precursor aminolevulinic acid (8,9). This feature was associated with a selective killing of the activated T cell population when combined with light exposure (8,9).…”

“…Mitogen‐stimulated human T cells synthesized higher amounts of the endogenous photosensitizer protoporphyrin IX (PPIX) than did their resting equivalents after incubation with the PPIX precursor aminolevulinic acid (8,9). This feature was associated with a selective killing of the activated T cell population when combined with light exposure (8,9). Thus, activated T cells treated with a variety of photosensitizing agents consistently exhibit an increased sensitivity to PDT relative to T cells in an unstimulated state.…”

Activation of the IL-10 Gene Promoter Following Photodynamic Therapy of Murine Keratinocytes¶

“…Several mechanisms have been proposed for the suppression of autoimmunity by PDT. Activated lymphocytes have been shown to be more sensitive to PDT than naive cells (7)(8)(9); thus PDT may act to eliminate the autoreactive effector cells. PDT has also been shown to alter the antigen presenting capabilities, and therefore T cell stimulatory function, of dendritic cells by reducing the levels of major histocompatibility antigens (MHC) and costimulatory molecules (CD80, CD86) expressed on the cell surface (10).…”

Action of the photosensitizer QLT0074 upon human T lymphocytes in vitro

“…Highly purified mouse T cells activated with anti-CD3 antibody exhibited greater verteporfin uptake and underwent apoptosis at lower photosensitizer concentrations than non-activated T cells (7). Mitogenactivated human T cells synthesized higher amounts of protoporphyrin IX (PPIX) than their resting equivalents following incubation with the PPIX precursor amino levulenic acid (ALA) (8,9). This was associated with a selective killing of the activated T cell population following light exposure (8,9).…”