We read with great interest the recent paper by Koga et al.…

ARTICLE

Dear Editor,

We read with great interest the recent paper by Koga et al. [1] showing the presence of IFN-alpha immunostained cells in the psoriatic epidermis, which the authors take to be evidence for the presence of IFN-alpha producing epidermal T-cells in psoriasis. This is indisputable. However, it seems to us that the authors have overlooked another very interesting finding ; that is the expression of IFN-alpha by keratinocytes in the epidermis of psoriatic patients. In particular, in Fig. 2 and Fig. 3 of their paper [1], it seems that there is a significant amount of positive IFN-alpha immunostaining within the cytoplasm of keratinocytes. This surely cannot be background or non-specific staining ? We note that in another study [2] published in this journal, that micrographs of lesional psoriatic skin showing strong epidermal IFN-alpha -immunostaining, which was decreased by mometasone furoate treatment, were shown. Remarkably, the authors of that study also failed to remark on what would also appear to be expression of IFN-alpha by psoriatic keratinocytes.

We have noted that keratinocytes derived from non-lesional skin of psoriasis patients [3] and grown in serum free keratinocyte growth medium can produce significant amounts of secreted IFN-alpha which can be measured by ELISA in the culture supernatants. Keratinocyte cultures (80 % confluent) were harvested from 25 cm² flasks containing 10 mls of KSFM growth media [3]. Keratinocyte cultures from five different normal donors prepared by the same method [3] did not secrete any detectable IFN-alpha using an ELISA with a detection limit of 3 pg/ml. However, two of four cultures from psoriatic patients produced readily detectable IFN-alpha (mean = 32 ± 12 pg/ml). These cells had been cultured for 3-5 passages and cultures did not contain any T-cells.There is no record of human fibroblasts or melanocytes producing IFN-alpha , so keratinocytes would seem the most likely source of the cytokine.

The question remains what induces IFN-alpha production in keratinocytes ? Ourselves and others have shown that keratinocytes can produce interleukin (IL)-18 [4-7], and that in lesional psoriasitic skin there is strong epidermal immunostaining for IL-18 [7, 8]. We found staining through out the spinous and basal layers. There was a corresponding 2.7-fold increase in IL-18 mRNA over levels in normal skin, as measured by RT-PCR, in 14 patients [7]. IL-18 immunostaining is absent in normal skin, but is also present in the basal layer of non-lesional psoriatic skin. The basal layer of the epidermis is where the dividing cells which establish in vitro keratinocyte cultures originate from.

IL-18 participates in a number of inflammatory conditions including rheumatoid arthritis where it can contribute to disease pathology by inducing pro-inflammatory mediators such as IFN-alpha and TNF-b (reviewed in [9]). In addition, previously we have reported that psoriatic lesional epidermis expresses 5.2-fold higher levels of mRNA, for the IL-18 receptor measured by RT-PCR [7]. A possibility is that autocrine IL-18 release stimulates IFN-alpha expression in psoriatic keratinocytes. The chemokines IP-10, MCP-1 and RANTES ; the adhesion molecule ICAM-1 ; and HLA-DR are induced in keratinocytes by IFN-alpha and are expressed at high levels in psoriatic epidermis (reviewed in [10]). If keratinocyte damage induces IL-18, followed by IFN-alpha , this would provide an autocrine amplification loop for epidermal inflammation. What induces the IL-18 ? Keratinocyte damage by microbes ? Or by other environmental agents ? We know that some of these (streptococcal infection and physical trauma) are involved in triggering psoriasis.

There are some previous reports of IFN-alpha expression by keratinocytes. Basal keratinocytes in biopsies from lichen planus patients express IFN-alpha mRNA [11] as do kerati nocytes in skin from contact sensitisation reactions [12]. In the latter study, IFN-alpha protein was also detected by immunostaining and was co-localised with cytokeratin expression. IL-18 expression is induced in keratinocytes in vitro by contact sensitisers [13].

Moreover, the expression of IFN-alpha by keratinocytes may be important in determining the development of the T-helper cell development down the TH-1 pathway, which is seen in psoriasis. For many years a dogma has persisted that IFN-alpha is an exclusively "T-cell cytokine". We would like to propose that this is an outmoded concept and that under conditions of extreme inflammation, that the keratinocyte-psoriatic or otherwise- can also express IFN-alpha and direct the course of T-cell activation in the skin.

We would like to thank The Psoriasis Association (UK) and the Foundation for Skin Research (Edinburgh) for funding these studies and Felicity Boyd for performing RT-PCR.

References

1
Koga T, Duan H, Urabe K, Furue M. In situ localization of IFN-gamma-positive cells in psoriatic lesional epidermis Eur J Dermatol 2002; 12: 20-3.

2
Berti E, Cerri A, Marzano AV, Richelda R, Bianchi B, Caputo R. Mometasone furoate decreases adhesion molecule expression in psoriasis. Eur J Dermatol 1988; 8; 421-6.

3
Jackson M, Howie SEM, Weller R, Hunter JAA, McKenzie RC. Psoriatic keratinocytes show reduced IRF-1 and STAT-1alpha activation in response to gamma-IFN. FASEB J 1999; 13:495-502.

4
Companjen AR, van der Velden VHJ, Vooys A, Debets R, Benner R, Prens EP. Human keratinocytes are major producers of IL-18: predominant expression of the unprocessed form. Eur Cytokine Network 2000; 11: 383-90.

5
Mee JB, Alam Y, Groves RW. Human keratinocytes constitutively produce but do not process interleukin-18. Br J Dermatol 2000; 143: 330-6.

6
Koizumi H, Sato-Matsumura KC, Nakamura H, Shida K, Kikkawa S, Matsumoto M, Toyoshima K, Seya T. Distribution of IL-18 and IL-18 receptor in human skin: various forms of IL-18 are produced in keratinocytes. Arch Dermatol Res 2001; 293: 375-33.

7
McKenzie RC, Boyce F, Forsey R, Gracie A, Szepietowski JC, Weller R, Sabin E, Howie SEM. Psoriatic skin expresses high levels of interleukin-18 (IL-18) and IL-18 receptor (IL-18R). Br J Dermatol 2000; 142: 618.

8
Ohta Y, Hamada Y, Katsuoka K. Expression of IL-18 in psoriasis. Arch Dermatol Res 2001; 293: 334-42.

9
McInnes IB, Gracie JA, Leung BP, Liew FY. Interleukin 18: a participant in chronic inflammation. Immunology Today. 2000; 21: 312-5.

10
Krueger JG. The immunologic basis for the treatment of psoriasis with new biologic agents. J Acad Am J Dermatol 2002; 46: 1-23.

11
Fayyazi A, Schweyer S, Soruri A, Duong LQ, Radzun HJ, Peters J, Parwaresch R, Berger HT. Lymphocytes and altered keratinocytes express interferon-gamma and interleukin 6 in lichen planus. Arch Dermatol Res 1999; 291: 485-90.

12
Howie SEM, Aldridge RD, McVittie E, Forsey RJ, Sands C, Hunter JAA. Epidermal keratinocyte production of interferon-gamma immunoreactive protein and mRNA is an early event in allergic contact dermatitis. J Invest Dermatol 1996. 106: 1218-223.

13
Naik SM, Cannon G, Burbach JG, Singh SR, Swerlich RA, Wilcox JN, Ansel JG, Caughman SW. Human keratinocytes constitutively express interleukin-18 and secrete biologically active interleukin-18 after treatment with pro-inflammatory mediators and dichlorobenzene. J Invest Dermatol 1999; 113: 766-72.