Role of memory effector T lymphocytes in the early phase of photosensitive psoriasis.

A minority of patients demonstrate severely photosensitive psoriasis (PP), but the underlying pathomechanism is poorly understood. We examined the early histological response to photoprovocation in PP (n=10) vs. non-photosensitive psoriasis (NP; n=10) and healthy volunteers (C; n=12). Low dose broadband UVA (20J/cm2 , 320-400 nm) was given to healthy ventral forearm skin on 3 consecutive days. Skin biopsies were taken 6h, 24h and 7d post-provocation, and from unchallenged healthy skin of the contralateral forearm. Samples were assessed immunohistochemically for CD4+, CD8+ and CD45RO+ T-lymphocyte subsets, and epidermal thickness quantifi ed by image analysis. Photoprovocation produced a clinical response in the majority of PP patients, while no visible change occurred in NP or controls. Histological changes of lymphocytic infi ltration and epidermal thickening occurred in 4/10 PP patients, but in no subjects in the other groups. This PP subgroup (PPH; n=4) was analysed separately from those with no histological change (PPN; n=6). Epidermal thickening occurred at 7d (mean ± SE μm, C: 53.4 ± 2.6; NP: 64.4 ± 3.6; PPN: 54.3 ± 5.5; PPH: 72.3 ± 9.0, p<0.05 for PPH vs. C). Whilst CD4+ cells were unchanged, CD8+ cells (cytotoxic T cells) were increased in the PPH sub-group compared to other groups and maximal 24h post-UVA provocation (mean ± SE cells/hpf, baseline, 12.1 ± 4.8; +6h, 9.0 ± 4; +24h, 18.2 ± 4.7; +7d, 13.2 ± 3.0; p<0.001 for PPH vs. C). CD45RO+ cell (memory effector T cell) expression mirrored CD8+ (baseline, 9.1 ± 2.4; +6h, 11.3 ± 4.1; +24h, 20.3 ± 11.8; +7d, 8.3 ±1.3; p<0.001 for PPH vs. C). Histological features consistent with psoriasis are rapidly photoprovoked in a subgroup of patients with severely photosensitive psoriasis; memory effector T cells may play a role in the early phase.