Anne V. Powles

Normal keratinocytes express Toll‐like receptors (TLRs) 1, 2 and 5: modulation of TLR expression in chronic plaque psoriasis

Summary Backgroundu2003Toll‐like receptors (TLRs) are part of the innate immune system involved in the response to microbial pathogens. TLR2 recognizes various ligands expressed by Gram‐positive bacteria, while TLR3, TLR4 and TLR5 are specific for double‐stranded RNA, Gram‐negative lipopolysaccharides and bacterial flagellin, respectively.

Comparison of bacterial microbiota in skin biopsies from normal and psoriatic skin

Microorganisms have been implicated in the pathogenesis of psoriasis. Previous studies of psoriasis and normal skin have used swabs from the surface rather than skin biopsies. In this study, biopsies were taken from 10 patients with psoriasis and 12 control subjects from unmatched sites. Samples were analysed with massive parallel pyrosequencing on the 454 platform targeting the 16S rRNA gene and the variable regions V3–V4. The samples grouped into 19 phyla, 265 taxon and 652 operational units (OTUs) at 97% identity. A cut-off abundance level was set at 1%. The three most common phyla in both normal and psoriasis skin were Firmicutes (39% psoriasis, 43% normal skin), Proteobacteria (38% psoriasis, 27% normal skin) and Actinobacteria (5% psoriasis, 16% normal skin, pxa0=xa00.034). In trunk skin, Proteobacteria were present at significantly higher levels in psoriasis compared to controls (52 vs. 32%, pxa0=xa00.0113). The commonest genera were Streptococci in both psoriasis (32%) and normal skin (26%). Staphylococci were less common in psoriasis (5%) than in controls (16%), as were Propionibacteria (psoriasis 0.0001669%, controls 0.0254%). Both Staphylococci and Propionibacteria were significantly lower in psoriasis versus control limb skin (pxa0=xa00.051, 0.046, respectively). This study has shown some differences in microbiota between psoriasis and normal skin. Whether these are of primary aetiological significance, or secondary to the altered skin of psoriasis remains to be determined.

Psoriasis and streptococci: the natural selection of psoriasis revisited.

We have previously postulated that surviving invasive streptococcal infections may have been a factor in psoriasis becoming a common skin disease in some parts of the world. Many of the candidate genes linked to psoriasis are associated with the acquired or innate immune system, which are also important in host defence to invasive streptococcal infections. High rates of positive streptococcal throat swabs among patients with chronic plaque psoriasis suggest that they are efficient at internalizing/carrying β‐haemolytic streptococci. Internalization of streptococci in the throat is dependent upon the transforming growth factor (TGF)‐β/fibronectin/α5β1 integrin pathway. The immune cell Th17 and its related cytokine network are important in mucosal defence, being very effective against extracellular microbes but having little effect on intracellular organisms. The TGF‐β/fibronectin/α5β1 integrin pathway and the Th17 cell network also appear to be operative in psoriasis, animal models of both TGF‐β and α5β1 cutaneous overexpression being associated with characteristic psoriasis lesions. We postulate that some of the genotypic/phenotypic changes in different immunological pathways in psoriasis, including the acquired T‐cell response, the innate immune response, the TGF‐β/fibronectin/α5β1 integrin pathway and the Th17 cell system, confer protection against mortality during epidemics of invasive streptococcal infections, heightened efficiency in internalizing and allowing carriage of streptococci as well as predisposition to the development of psoriasis.

An Altered Response by Psoriatic Keratinocytes to Gamma Interferon

To determine whether psoriatic keratinocytes differ from normal keratinocytes in their response to gamma interferon, epidermal cell suspensions from normal and from lesional and uninvolved psoriatic skin were cultured in the presence of gamma interferon and the induction of HLA‐DR expression and inhibition of cell growth were measured The addition of 102 units of gamma interferon/ml during 7‐day culture period significantly increased mean HLA‐DR+ cell numbers in 21 epidermal suspensions of normal from 3.9 to 24.1% (P<0.0001), uninvolved psoriatic from 8.4 to 33.1% (P<0.01), and to a lesser extent lesional psoriatic biopsies from 12.6 to 18.3% (P<0.01). However, the increase in HLA‐DR+ cell numbers in these latter cultures was significantly less than that observed in either normal or uninvolved psoriatic epidermal cell cultures (P<0.0001). Furthermore, [3H]thymidine incorporation was substantially decreased by gamma interferon in 16 out of 22 (73%) cultures of normal epidermal cells; this decrease was statistically significant (P<0.0l). In contrast, only 4 out of 11 (36%) lesional and 9 out of 21 (43%) uninvolved psoriatic epidermal cultures showed comparable inhibition of proliferation. These findings suggest that psoriatic keratinocytes have an altered response to gamma interferon: this could explain the infrequency of keratinocyte HLA‐DR expression in psoriatic plaques in vivo and may also contribute to the increased epidermal proliferation that characterizes this disease.

Is chronic plaque psoriasis triggered by microbiota in the skin

There is a known association between psoriasis and Crohn disease (CD). Patients with CD are five times more likely to develop psoriasis, and, conversely, patients with psoriasis are more likely to develop CD. Many gastroenterologists now accept that CD results from a breakdown of immune tolerance to the microbiota of the intestine in genetically susceptible individuals. The microbiota of the skin have recently been investigated in psoriasis. Firmicutes was the most common phylum, and Streptococcus the most common genus identified. Beta‐haemolytic streptococci have been implicated in both guttate and chronic plaque psoriasis. Furthermore, the innate immune system has been shown to be activated in psoriasis, and many of the genes associated with the disease are concerned with the signalling pathways of the innate immune system, notably interleukin‐23 and nuclear factor κB. Patients with psoriasis also have an increased incidence of periodontitis, a disease thought to be due to an abnormal response to normal oral commensals. Based on the similarities between CD and psoriasis, we propose that psoriasis is due to a breakdown of immune tolerance to the microbiota of the skin. In support of this hypothesis we provide evidence for microbiota in the skin, activation of the innate immune system, and genetic abnormalities involving the innate immune system.

Association of psoriasis to PGLYRP and SPRR genes at PSORS4 locus on 1q shows heterogeneity between Finnish, Swedish and Irish families.

Abstract:u2002 A susceptibility locus for psoriasis, PSORS4, has been mapped to chromosome 1q21 in the region of the epidermal differentiation complex. The region has been refined to a 115u2003kb interval around the loricrin (LOR) gene. However, no evidence of association between polymorphisms in the LOR gene and psoriasis has been found. Therefore, we have analysed association to three candidate gene clusters of the region, the S100, small proline‐rich protein (SPRR) and PGLYRP (peptidoglycan recognition protein) genes, which all contain functionally interesting psoriasis candidate genes. In previous studies, the SPRR and S100 genes have shown altered expression in psoriasis. Also polymorphisms in the PGLYRP genes have shown to be associated with psoriasis. We genotyped altogether 29 single nucleotide polymorphisms (SNPs) in 255 Finnish psoriasis families and analysed association with psoriasis using transmission disequilibrium test. A five‐SNP haplotype of PGLYRP SNPs associated significantly with psoriasis. There was also suggestive evidence of association to SPRR gene locus in Finnish families. To confirm the putative associations, selected SNPs were genotyped also in a family collection of Swedish and Irish patients. The families supported association to the two gene regions, but there was also evidence of allelic heterogeneity.

Skin T cell proliferative response to M protein and other cell wall and membrane proteins of group A streptococci in chronic plaque psoriasis

To determine and compare the T cell response to M protein and other group A streptococcal (GAS) antigens, T cell lines (TCL) were cultured from the lesional skin of 33 psoriatic patients and 17 disease controls. GAS‐reactive skin TCL were tested in proliferation assays with recombinant M6 protein, and extracts of cell wall and membrane from type M6 GAS and its corresponding M gene deletion mutant. Initially, GAS‐reactive skin TCL were obtained from 16 of 25 (64%) psoriasis, and from seven of 17 (41%) control patients. Eleven psoriatic and four control GAS‐reactive TCL proliferated to M6 cell wall extract, whereas all the TCL from both groups responded to the extract of M6 membrane proteins. This difference in response to the two extracts was significant for both groups of patients (psoriasis, Pu2003=u20030·0335, controls, Pu2003=u20030·0156). GAS‐reactive TCL from a further eight psoriasis patients showed no difference in response to cell wall extract from M6 GAS (containing the M protein minus its C‐terminus) compared to that of its corresponding M gene deletion mutant. Furthermore, GAS‐reactive TCL did not proliferate to recombinant M6 protein. However, a small, but significant reduction in proliferation by the eight psoriatic GAS‐reactive TCL to the M‐negative (lacking the M protein C‐terminus) compared to M6‐positive membrane extract was observed (Pu2003=u20030·01). These findings suggest that GAS‐reactive T cells in skin lesions of chronic plaque psoriasis proliferate to streptococcal membrane and, to a lesser extent, cell wall proteins. However, psoriatic skin T cells do not recognize cell wall M protein.

Psoriasis and extra domain A fibronectin loops

We have previously postulated that as well as T‐helper (Th) 1 and Th17 cells, the transforming growth factor (TGF)‐β/fibronectin (FN)/α5β1 pathway is central to psoriasis pathogenesis. EDA+ FN refers to an alternatively spliced isoform of FN with an additional domain known as extra domain A. EDA+ FN has two important properties pertinent to psoriasis lesions: it stimulates keratinocyte hyperproliferation, and, through stimulation of Toll‐like receptor (TLR) 4, stimulates production of proinflammatory cytokines. EDA+ FN production induced by TGF‐β stimulation can be maintained in psoriasis lesions via two main feedback loops. Firstly, EDA+ FN stimulates proliferation of keratinocytes, which, in an autocrine fashion, will release more EDA+ FN. Secondly, EDA+ FN stimulates TLR4 expressed by antigen‐presenting cells resulting in the production of proinflammatory cytokines such as tumour necrosis factor‐α, interleukin (IL)‐1, IL‐6 and IL‐12. The resultant promotion of cutaneous inflammation results in the recruitment of Th1 cells, which also produce EDA+ FN. We propose that these ‘FN loops’ contribute to the maintenance and progression of psoriatic lesions. Finally, although the association between psoriasis and heart/thrombotic disease remains unclear one plausible link may be the promotion of atherosclerosis and thrombotic heart disease by EDA+ FN.

Evidence for the presence of bacteria in the blood of psoriasis patients

Evidence exists that microorganisms, particularly in the throat and skin, play a role in the pathogenesis of psoriasis. The aim of this study was to investigate whether evidence for the presence of bacteria, including Streptococcus pyogenes, can be demonstrated in the peripheral blood of patients with guttate and/or chronic plaque psoriasis. Peripheral blood samples from 20 patients with psoriasis, seven guttate, six chronic plaque and seven chronic plaque with associated guttate flare and from 16 control subjects were studied for the presence of bacteria by PCR using universal 16S ribosomal DNA primers and specific primers for S. pyogenes. Sequence analysis of amplified 16S rRNA sequences was used to determine taxonomic identity. Ribosomal bacterial DNA was detected in the blood of all 20 patients with psoriasis, but in none of the controls. Streptococci were detected in six of seven patients with guttate psoriasis, but none had staphylococci. In contrast, staphylococci were identified in 9 of 13 patients with chronic plaque psoriasis, whilst only 2 demonstrated streptococci. In three psoriasis patients, species other than streptococci and staphylococci were identified. These findings suggest that psoriasis is associated with bacteraemia, with distinct taxonomic groups present in guttate and chronic plaque psoriatic subtypes. The causes of the bacteraemia and its implications in psoriasis have yet to be determined.

HLA Cw*06 is not essential for streptococcal-induced psoriasis.

Backgroundu2002 Streptococcal throat infections and HLA Cw6 (Cw*06) have been implicated in the pathogenesis of psoriasis, particularly in the guttate form.