Clinical and mechanistic aspects of photopheresis.

Photopheresis is an extracorporeal form of photochemotherapy with 8-methoxypsoralen (8-MOP) and ultraviolet A (UVA) radiation. Photopheresis is used for the management of T-cell-mediated diseases, and such treatment leads to the induction of antigen-specific immune suppression directed to the pathogenic clone of T cells. Photopheresis is used to treat a wide variety of diseases--such as cutaneous T-cell lymphoma, systemic sclerosis; rheumatoid arthritis, lupus erythematosus--and is also successfully applied in the suppression of graft rejection. In addition to the clinical achievements, attention will be paid to results from animal studies. An important outcome of these studies is that photopheresis can be used to treat airway hyperreactivity. Furthermore, it was shown that the therapeutic strategy can be changed drastically: the presence of plasma during irradiation should be avoided and the amount of blood that must be treated to obtain the desired antigen-specific immunosuppression can be greatly decreased. Also, results from cellular experiments are discussed. An example of this is the increase in the major histocompatibility complex expression on the surface of cells found after treatment. The mechanism that underlies photopheresis has not yet been elucidated, but progress has been made. The following related points will be reviewed: models for investigation; and mechanistic aspects, with the emphasis on cellular biomacromolecules and on photosensitizers (drugs) other than 8-MOP.

Singlet oxygen producing photosensitizers in photophoresis.

In this study, the immunosuppressive properties of two photosensitizers (benzoporphyrin derivative monoacid ring A (BPD) and Photofrin (HPD)), used for the photodynamic therapy of cancer, were investigated. The investigations were performed in our rat model for photophoresis. The validity of this model has been amply demonstrated. It enables a distinction to be made between antigen-specific and antigen non-specific immune suppression. With this model, the immune response which can be specifically suppressed is the contact hypersensitivity (CHS). CHS is induced by 2,4-dinitrofluorobenzene (DNFB). Both BPD and HPD are able to suppress CHS induced by DNFB. Furthermore, this generated suppression is transferable by the spleen cells of treated animals and is antigen non-specific.

Photopheresis, a possible therapy for airway hyperreactivity?

Airway hyperreactivity is an almost universal feature of asthma. The exact origin of this phenomenon is poorly understood. However, there is increasing evidence that T cells play an important role in the pathogenesis of this disorder. This fact makes it challenging to photopheresis to suppress the pulmonar hyperreactivity response. Photopheresis is a therapy for T cell mediated diseases aiming at specific suppression of the pathogenic clone of T cells involved. The use of photopheresis for the treatment of airway hyperreactivity was investigated in this study. We performed experiments in a murine model for airway hyperreactivity. In short, mice were sensitized by cutaneous application of 2,4,6-trinitrochlorobenzene. The immune system was challenged by an intratracheal injection of 2,4,6-trinitrobenzenesulfonic acid and a bronchoalveolar lavage was performed. In this lavage the total number of leukocytes was established and the number of macrophages was determined. It was found that photopheresis treatment was capable to suppress the airway hyperreactivity response for about 80%. In addition, the generated suppression proved to be transferable by splenocytes of treated animals. We conclude that photopheresis can be an interesting therapy for airway hyperreactivity (and perhaps also for asthma) especially when one takes into account that photopheresis induces specific immune suppression and has hardly any adverse effects.

Chlorpromazine, a candidate drug for photopheresis.

Photopheresis is a therapy for several T-cell-mediated disorders, aiming at a specific immune response against the pathogenic clone of T cells involved. With photopheresis, a mixture of patients' buffy coat and plasma, which contains 8-methoxypsoralen (8-MOP), is diluted with saline and exposed to ultraviolet A radiation (UVA). After the irradiation the treated fraction is reinfused. To improve this therapy and to broaden its scope, insight into the underlying mechanism is essential. Regarding the mechanism, photomodification of biomacromolecules is considered to be crucial in photopheresis. Up to the present, much emphasis has been put on the photobinding to DNA. However, photobinding to proteins can also play an important role in photopheresis, because much of the communication between the various parts of the immune system occurs via protein-protein interactions. In this study we compared the activity of 8-MOP and chlorpromazine (CPZ) in our animal model for photopheresis based on contact hypersensitivity. It proved that CPZ was able to induce specific immune suppression, just as 8-MOP. In addition, photobinding of CPZ and 8-MOP to lymphocytes was determined. It was shown that under conditions relevant for photopheresis the photobinding to DNA of 8-MOP exceeds that of CPZ by a factor of 22. This holds for both rat and human lymphocytes. With the photobinding to proteins it is just the other way round; the photobinding of CPZ exceeded that of 8-MOP by a factor of 23 for rat lymphocytes and 28 for human lymphocytes. This study proves that CPZ is an interesting candidate drug for photopheresis and shows that not only photobinding to DNA is crucial in photopheresis, but photobinding to proteins is important as well.

Photopheresis; the risk of photoallergy.

Photopheresis is a therapy for several T-cell-mediated disorders, aiming at a specific immune response against the pathogenic clone of T cells involved. With photopheresis, a mixture of patients' buffy coat and plasma, which contains 8-methoxypsoralen (8-MOP), is diluted with phosphate-buffered saline (PBS) and exposed to ultraviolet A radiation (UVA). After the irradiation the treated fraction is reinfused. As photomodification of biomacromolecules is considered to be crucial in photopheresis, the presence of plasma during irradiation can pose a problem. The fact is that photomodification of plasma proteins is supposed to be the causal step in the occurrence of photoallergy. Whether the presence of plasma during photopheresis is a risk for photoallergy was investigated with 8-MOP and chlorpromazine (CPZ; well-established photoallergen). It proved that both sensitizers can induce photoallergy, although the concentration of 8-MOP needed to induce photoallergy is 4.5 times higher than that of CPZ. It is concluded that the presence of plasma during irradiation should be avoided in order to prevent the risk of induction of photoallergy.

The lack of efficacy of 4,6,6'-trimethylangelicin to induce immune suppression in an animal model for photopheresis: a comparison with 8-MOP.

Photopheresis is an extracorporeal form of photochemotherapy with 8-methoxypsoralen (8-MOP) and UVA (PUVA). Patients ingest 8-MOP and then a psoralen-rich buffy coat is obtained by centrifugation and mixed with saline. This mixture is recirculated through a UVA radiation field and then reinfused. Photopheresis appears to be effective for several T cell-mediated disorders, because the treatment results in a specific immune response against the pathogenic clone of T cells involved. With PUVA therapy, the whole body of the patient is exposed to UVA, after ingestion of 8-MOP. Upon UVA exposure 8-MOP binds to, amongst others, DNA and induces DNA monoadducts and interstrand cross-links. As a result of these photoadducts photocarcinogenicity is a risk in PUVA. In PUVA for psoriasis, it proved that angular furocoumarins, although almost incapable of inducing DNA cross-links (less carcinogenic), are still effective. In order to determine if monoadducts induced by photopheresis could also be effective we used, specifically, 4,6,4'-trimethylangelicin (TMA). In this report, we compare the photodegradation of both TMA and 8-MOP under conditions relevant to the in vivo situation, as well as the effect both compounds have on the viability of rat lymphocytes as measured with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. We show that TMA did not induce immunosuppression in vivo, even after extensive irradiation. In addition a dose dependency of 8-MOP/UVA versus the induced immune suppression was carried out. It was shown that there is a log dose/response correlation of r=0.9205.

Non-specific systemic immune suppression induced by photodynamic treatment of lymph node cells with bacteriochlorin a.

Although photodynamic therapy is being used increasingly for the treatment of cancer, its effect on immune responses has received little attention. This aspect was studied in a rat model. Rats were given (intravenously) lymph node cells which were simultaneously exposed to a photosensitizer and light. As a result of this treatment the animals showed a decreased contact hypersensitivity response. Most important is that the generated immunosuppression proved to be non-specific. The consequence of this finding may be that photodynamic therapy of tumours can inhibit the immune response against malignant cells.

An animal model and new photosensitizers for photopheresis.

Recently, photopheresis was introduced as a therapy for several T cell-mediated disorders. The treatment results in a specific immune response against the pathogenic clone of T cells involved. However, although promising there is controversy concerning the use of photopheresis in some diseases, e.g. systemic sclerosis. One of the problems is that there is not yet sufficient insight into the mechanism underlying the therapy. This lack of knowledge is partly caused by the fact that there are no easy-to-handle animal models available for photopheresis. This report describes such a model--a Wistar-derived rat with contact hypersensitivity (CHS); a T cell-mediated immune response. White blood cells from CHS rats were simultaneously exposed to 8-methoxypsoralen (8-MOP) and ultraviolet A radiation (UVA) and subsequently intravenously injected into other syngeneic rats suffering from the same disorder. This treatment appears to be very efficacious in suppressing the immunological response against the applied contact allergen, 2,4-dinitrofluorobenzene (DNFB). In addition, the generated suppression of CHS is highly specific and transferable. Furthermore, drugs other than 8-MOP (chlordiazepoxide, nitrofurantoin and chlorpromazine) also appear to be active in our model.

An animal model for extracorporeal photochemotherapy based on contact hypersensitivity.

Recently, photopheresis was introduced as a specific immune suppressor in several T cell mediated disorders. In order to study photopheresis, animal models are indispensable. This report describes an easy to handle model for this purpose. It concerns the Wistar-derived rat with contact hypersensitivity (CHS), also a T cell mediated disorder that has already been studied extensively in several other fields of research. After subsequent exposure to 8-methoxypsoralen (8-MOP) and ultraviolet A radiation (UVA), white blood cells from CHS rats were intravenously injected into other syngeneic rats suffering from the same disorder. This treatment appears to be very efficacious in suppressing the immunological response against the applied contact allergen, 2,4-dinitrofluorobenzene (DNFB). Cells subsequently exposed to UVA and 8-MOP did not have any effect.