The role of ultraviolet radiation in lupus erythematosus and polymorphous light eruption

Abstract

Väitöskirjan aihepiirinä on ihon valoyliherkkyys toisaalta lupus erythematosus (LE)-nimisessä melko harvinaisessa, useista alatyypeistä koostuvassa tautiperheessä ja toisaalta hyvin yleisesti esiintyvässä monimuotoisessa valoihottumassa (MMVI). Näissä taudeissa ultraviolettisäteilyn (UV-säteily) aiheuttamat iho-oireet muistuttavat joskus toisiaan ja molemmat taudit saattavat esiintyä samalla potilaalla. Tutkimuksen yhtenä tarkoituksena olikin tarkemmin selvittää näiden tautien välistä yhteyttä haastattelu- ja henkilökohtaisella tutkimuksella. Toisena tarkoituksena oli kokeellisin menetelmin selvittää mm UV-säteilyn aiheuttamia ihon varhaisvaiheen solumuutoksia näissä taudeissa, sillä UV-säteilyn ihottumaan johtavat perusmekanismit ovat epäselviä.Yhtenä kliinisenä tutkimusmenetelmänä käytettiin pienen ihoalueen valottamista UV-säteilyllä tarkoituksena aikaansaada alueelle lievä ihottuma. Tutkituista 67 LE-potilaasta 67-100%:lle, alatyypistä riippuen, kehittyi ihottuma UV-säteilylle. Tämä ihottuma oli kuitenkin useammin MMVI:n kuin LE-ihottuman kaltaista sekä kliinisesti että histologisesti tarkastellen puoltaen aikaisempaa alustavaa havaintoa näiden tautien yhteisesiintyvyydestä. Laajan, 94 MMVI-potilaan seurantatutkimuksessa totesimme kuitenkin, että pitkänä seuranta-aikana LE-ihottumaa ei kehittynyt MMVI-potilaille useammin kuin normaaliväestölle. On kuitenkin mahdollista, että MMVI-potilaiden laajassa joukossa on LE:lle altistuneita henkilöitä, mahdollisesti perinnöllisesti määräytyneenä. Tämä olisikin mielenkiintoinen lisätutkimuskohde. Vaikka seurantatutkimuksessa ei todettukaan LE- taudin kehittymisalttiutta MMVI-potilailla, totesimme heille kehittyvän useita erilaisia autoimmuunioireita sekä kilpirauhashäiriöitä oletettua enemmän, joten näiden tautien huomioonottaminen MMVI-potilaiden seurannassa on tärkeää. Kokeellisissa tutkimuksissa totesimme UV-valotetulla iholla useita eroavaisuuksia LE-potilailla verrattuna MMVI-potilaisiin, mitkä erot viittaavat erilaisiin solutason perusmekanismeihin UV-säteilyn aiheuttamissa ihomuutoksissa näissä kahdessa eri tautimuodossa.

This thesis deals with two photoaggravated skin diseases, lupus erythematosus (LE) and polymorphous light eruption (PLE), and comprises two clinical and three experimental, original investigations. The thesis was carried out in collaboration with the Departments of Dermatology, Tampere and Oulu University Hospitals, Helsinki University Central Hospital, Finland and Karolinska Hospital, Sweden.

LE is a spectrum of diseases with only minor skin lesions in one extremity and a severe systemic disease with disabling symptoms from several organs in the other end. The three main subtypes of LE are chronic cutaneous LE (CCLE), subacute cutaneous LE (SCLE) and systemic lupus erythematosus (SLE). Each of them has distinct cutaneous symptoms, although some overlapping exists. CCLE usually presents with skin symptoms only, while in SCLE and SLE patients, other organ systems are often affected and serological abnormalities are observed as well. Photosensitivity is an important feature of all subsets of LE and in SLE, it is accepted as one of 11 criteria for the classification of SLE compiled by the American College of Rheumatology (ACR). However, in spite of its impressive role in diagnosing SLE, the definition of photosensitivity has remained vague. Also the type of photosensitivity has been inadequately examined. As PLE, the most common form of idiopathic photosensitivity diseases, may simulate clinically and histologically early skin lesions of CCLE, it has been speculated whether there is a relationship between these two photoaggravated skin disorders. Furthermore, in our earlier questionnaire study, we found out that 49% of the LE patients had experienced PLE-like symptom, in some cases more than ten years before the first LE symptoms. The possibility that PLE patients would be prone to develop LE in the long run has been inadequately examined.

In order to get a more objective view of the photosensitivity, 67 LE patients were now photoprovoked with UVAR and UVBR. With these photoprovocations we could confirm the previous findings that both UVAR and UVBR induces a pathological UVR reaction in LE patients. Either UVR spectrum induced a pathological reaction in 64%, 70% and 100% of the DLE, SLE and SCLE patients, respectively. However, only 16% of the pathological reactions were strong and long-lasting, reminiscent of LE, while 48% were moderate or weak and transient, clinically reminding PLE. Also histologically, most (53%) of the reactions biopsied showed features of PLE or an unspecific inflammatory reaction.

In the second, questionnaire-based follow-up study, the possibility that LE develops in PLE patients in the long run was investigated. Of the cohort of 138 PLE patients, thoroughly evaluated 23 years earlier on the average, 94 patients returned the questionnaire on clinical characteristics of PLE disease and overall health status. Only two female patients had developed LE, one SLE and the other SCLE. However, the prevalence of any type of autoimmune disease, and especially of thyroid disease, was considerably higher, 15%. The prognosis of PLE symptoms was quite favourable: 24% of the 94 patients were free of PLE symptoms, 51% experienced milder symptoms, and 24% experienced equal or worse symptoms than during the previous follow-up 16 years earlier.

In conclusion of the clinical studies it can be stated that LE patients suffer from photoaggravated LE skin lesions but also from photosensitivity symptoms resembling clinically and histologically PLE. However, our long-term follow-up study of PLE patients revealed that PLE patients are not predisposed to LE specifically, although they are apt to develop some type of autoimmune disease or thyroid disorders in the long run.

The exact pathogenesis of UVR-induced LE and PLE skin lesions has not been resolved, although some hypotheses have been presented. Thus, antibody-dependent cellular cytotoxicity is the most favoured hypothesis for antibody-associated SLE and SCLE, whereas a DH mechanism has been suggested as the pathogenesis for non-antibody-associated CCLE lesions and PLE lesions. Three experimental studies concerning the photoimmunological events of the UVR-induced or spontaneous skin lesions in LE compared with PLE were conducted in the present thesis.

Firstly, epidermal urocanic acid (UCA) contents in 16 DLE and 12 PLE patients were examined. UCA is considered as one of the most important epidermal photoreceptors. A photoreceptor is a prerequisite for the initiation of any photobiological response. The cis-UCA content was significantly lower in light-protected uninvolved buttock skin in DLE patients than in PLE patients and in control persons. As cis-UCA is a strong immunosuppressant, this refers to the possibility of a less cis-UCA-mediated suppression of DH reaction towards UVR-induced neo-antigen in DLE skin lesions.

In the second experimental study, we examined the markers of naive (CD45RA and CD31 antigens) and memory (CD45RO antigen) T lymphocytes in epidermal and dermal inflammatory cell infiltrations. Typically, CD45RA+ cells were the prevailing inflammatory cell population in DLE lesions, whereas CD45RO+ cells prevailed in both diseases and in healthy controls. Thus, as CD45RO+ cells are the prominent T cell subtype in cell-mediated hypersensitivity reactions, the profound infiltration of these T cell subtypes now observed in both DLE and PLE lesions is in concert with the hypothesis of DH reaction. Furthermore, our results refer to the pathogenetical role of CD45RA+ cells in DLE lesions, but not in PLE lesions.

In addition to the absorption of UVR energy, the development of a UVR-induced neo-antigen is a prerequisite for either ADCC or DH reaction. Such a neo-antigen could be an intracellular autoantigen, such as Ro (SSA) or La (SSB) which are shown to relocalize on the surface of keratinocytes due to UVR-induced apoptotic cell death in in vitro experiments. As a third experimental study, UVA and UVB irradiation- induced keratinocyte apoptosis and localization of La (SSB), a possible autoantigen, were examined in DLE, SCLE, SLE and PLE patients. We observed that in some LE and PLE patients, apoptosis was more prolonged than in control persons. The nuclear La (SSB) signal was more intense and more frequent in healthy, intact skin than in skin lesions in any of the patient groups. In the patients, the La (SSB) signal was observed also in the cytoplasm, in LE patients even on the cell membrane. These results are concurrent with the previous findings performed in cultured keratinocytes from SLE patients.

In conclusion of the three experimental studies, the pathogenesis of UVR-induced LE skin lesions seems to differ from that of PLE lesions in terms of epidermal and dermal immune cell infiltration and antigen relocalization in UVR-irradiated keratinocytes. The dissimilar epidermal cis-UCA content in DLE and PLE patients may also regulate the immune response differently.