Skewed Distribution of Natural Killer Cells in Psoriasis Skin Lesions

Background

Psoriasis is a chronic inflammatory disease of the skin with significant morbidity (1). Whether NK cells are involved in the immunopathogenesis of psoriasis remains controversial, although genetic and immunological analyses support a role (2-4). NK cells function without prior sensitization and could initiate plaque development (5, 6).

NK cells express a heterogeneous repertoire of receptors that regulate their effector function (7, 8). Although NK cell function (cytotoxicity and cytokine secretion) is related to their stage of development, both immature CD56^brightCD16^neg and mature CD56^dimCD16⁺ NK cell subpopulations secrete IFNγ, critical in psoriasis pathogenesis (6).

Expression of CD57 identifies terminally differentiated T cells (9). CD57 is also expressed on highly mature cells within the CD56^dimCD16⁺ NK cell compartment and might provide a marker of “memory” NK cells (10-12). NK cell differentiation is described as a progression from a less mature phenotype displaying high CD56 expression together with NKG2A but lacking CD16, to a more differentiated phenotype expressing CD16 and CD57 with a decreasing ability to replicate in vitro(10-12).

Questions Addressed

We investigated the distribution of CD57⁺ NK cells in lesional skin, non-lesional skin and peripheral blood of psoriasis patients compared to controls. We also investigated the expression of the inhibitory receptor NKG2A and the activating receptor NKG2C on NK cells in the skin and blood of patients and controls.

Experimental Design

Peripheral blood mononuclear cells (PBMCs) and skin biopsies were collected from 12 patients with psoriasis and PBMC alone were collected from 10 healthy controls.

The inclusion criteria and protocols are described in Supplemental Materials and Methods.

Results

Nine males and 3 females were included, median age 48 years (range 21 to 84). Five subjects had plaque and 7 subjects had guttate psoriasis. The majority of subjects had mild disease (n = 8), 3 had moderate disease, and 1 had severe disease. Matched skin and PBMC samples were available from 9 out of 12 psoriasis subjects. Median age of healthy controls was 45 years.

Both lesional and non-lesional skin of psoriasis patients had similar frequencies of CD56⁺CD16⁺ NK cells; however, in the blood patients had a trend toward an increased frequency of CD56⁺CD16⁺ NK cells compared to controls (Fig. 1a). The frequency of CD57⁺ NK cells in lesional skin (less than 10%) was significantly lower than in non-lesional skin (∼ 40%) (Fig. 1b).

An assessment of NKG2A and NKG2C expression on skin CD56⁺CD16⁺ NK cells revealed a significant expansion of NKG2A⁺ NK cells in skin of patients (Fig. 1c). The frequency of NKG2A⁺ NK cells in PBMC was similar between patients and controls, while patients had a significantly greater frequency of circulating NKG2C⁺ NK cells compared to controls (Fig. 1d).

Co-expression of CD57, NKG2A, and NKG2C on skin NK cells was different between patients and controls (Supplemental Fig. 1). NK cells from the skin of patients preferentially expressed NKG2A, particularly in lesional skin. In contrast, NK cells in the skin of healthy donors contained CD57⁺ NK cells that were NKG2A⁻ and NKG2C⁻. These data suggest that NK cells in the skin of this cohort of patients are less mature.

There was a trend toward an increased frequency of CD56⁺CD16⁻ NK cells in psoriasis lesional and unaffected skin compared to control skin (Fig. 2a). CD57 expression on CD56⁺CD16⁻ NK cells was significantly reduced in lesional and non-lesional skin of patients compared to controls (Fig. 2b). A trend of greater NKG2A expression in lesional skin compared to unaffected skin and control skin was observed (Fig. 2c), with lack of difference for NKG2C expression in the skin or blood of patients versus controls (Fig. 2d).

Comparison between plaque and guttate psoriasis for all measured parameters demonstrated no difference, although numbers for each group were small (Supplemental Figs. 2 and 3).

Conclusions

In this study, we found variations in NK cell distribution in blood and tissue of psoriasis patients compared to healthy controls.

We observed a substantially lower percentage of CD57⁺CD56⁺CD16⁺ NK cells in lesional skin compared to control skin. CD57⁻CD56⁺CD16⁺ correspond to activated cells with a higher turnover and degranulation ability (13). They are highly responsive to IL-2 stimulation, which could determine their preferential migration to the skin (14). CD57⁻CD56⁺CD16⁺ NK cells produce higher amounts of IFNγ following cytokine stimulation compared to CD57⁺CD56⁺CD16⁺ NK cells, and could contribute to the inflammatory environment (11, 13).

We observed a significant difference in the proportion of NKG2A⁺ cells in lesional skin. NKG2A is inducible by IL-12, which is likely to be highly expressed in lesions. The acquisition of functional capacities by NK cells, specifically IFNγ production, is correlated with NKG2A expression on CD56⁺CD16⁺ NK cells (15).

The difference observed in the proportion of circulating NKG2C⁺ NK cells could be due to differences in CMV serostatus, as NK cells expressing NKG2C are preferentially expanded in CMV seropositive individuals (16).

Together, these data suggest that NK cells from this group of psoriasis patients harbor a less differentiated phenotype. Future studies are needed to determine the functional significance of these less differentiated NK cells in psoriasis lesions, as this study included only a small number of patients with heterogenous characteristics.

Supplementary Material