Study of the psoriasis proteome and analysis of MHC class I expression and function

Abstract

Psoriasis is a common inflammatory skin disease affecting approximately 2% of the population in Sweden. Psoriasis displays clinical heterogeneity leading to several different types of manifestations, among them chronic plaque and acute guttate psoriasis. Psoriasis is genetically determined with influence from multiple susceptibility loci identified in several linkage studies. The psoriasis susceptibility (PSORS) locus with the strongest association to psoriasis is PSORS1 on chromosome 6p21.3, which contains the human leukocyte antigen genes. This association is explained by the overrepresentation of psoriasis patients carrying HLA-Cw*0602. Other loci are known to contribute to psoriasis but their corresponding susceptibility genes and precise pathogenic mechanisms are as yet unknown. As a complex phenomenon, psoriasis is likely the result from interactions between environmental factors and single gene products or molecular networks.

The aims of this work were to compare the global protein expression patterns between skin from individuals with guttate and plaque psoriasis phenotypes, normal healthy skin, and acute contact eczema as an additional inflammatory skin disorder, and to investigate the specific protein expression of HLA-C. In addition, the relationship between HLA-C and HLA-E at the genetic level and functional consequences of such genotypes were investigated.

Protein expression patterns in skin from well-defined phenotypes of acute guttate and chronic plaque psoriasis, as well as from nickel induced contact eczema, were compared by two-dimensional gel electrophoresis. Four sets of differentially expressed protein spots allowed the classification of samples into clusters that correlated with the initial clinical categorization of the patients. In this cluster analysis lesional plaque psoriasis samples were clearly separated from the other phenotypes and lesional guttate psoriasis clustered closer to lesional contact eczema than the lesional plaque psoriasis samples, likely reflecting the acute character of the inflammatory reaction in these conditions. From all protein sets a total of 96 protein spots were identified by mass spectrometry, which belonged into many different pathways.

The expression pattern of HLA-C was investigated in skin lesions of patients with guttate or plaque psoriasis, and acute nickel contact eczema by immunohistochemistry and Western blot. The immunostaining showed that the site of expression in skin was different between the sample groups. The most prominent feature of the HLA-C staining pattern was that in lesional plaque psoriasis the expression level was low in the epidermis with polarization towards the basement membrane. Lesional eczema and normal skin showed staining throughout the epidermis. By Western blot analysis, the level of HLA-C expression was significantly higher in lesional eczema skin compared with the other sample groups, except for lesional plaque psoriasis that showed slightly elevated expression level.

Reduced expression of HLA-C in psoriasis epithelium might contribute to maintaining inflammation due to inefficient downregulation of activated lymphocytes. The distinct HLA-C expression patterns in psoriasis and eczema suggest a functional role in the specific psoriasis immune response and not only a general feature of inflammation.

Among Cw6 positive individuals, HLA-EG (high binding affinity) was five times less prevalent in patients. We also observed a similar expression pattern between HLA-E and HLA-C in psoriasis skin. In a subset of genotype-matched patients and controls, we also found, higher sHLA levels in patients, decreased HLA class I expression on lymphoblastoid cells after IFN-α stimulation, increased killing of LCL-Cw6 compared to K562 cells, lower cytotoxicity levels against K562 cells in Cw6/ER (low binding affinity) positive patients than in controls with the same genotype, streptococcal extract stimulated cytotoxic activity against K562 cells in both patients and controls. Taken together, the results of this study indicate that genotypes at the HLA-C and E loci influence immune factors and functions in psoriasis.