Pglyrp3/4 Promote Treg and Inhibit Th17 Responses Figure 10. Induction of Treg cells by vitamin D reduces the inflammatory response to oxazolone and decreases the numbers of Th17 cells in the skin of Pglyrp32/2 mice. (A) Vitamin D applied to the skin of Pglyrp32/2 mice together with oxazolone reduces ear swelling compared to Pglyrp32/2 mice similarly treated with oxazolone alone. (B–D) Vitamin D applied to the skin of Pglyrp32/2 mice together with oxazolone increases the percentages of Treg cells in the ears and cervical lymph nodes and reduces the percentages of Th17 cells in the ears compared to the application of oxazolone alone, measured on day 20 by flow cytometry. Means 6 SEM of 8 mice/group (A–C; *, P,0.05; **, P,0.005; oxazolone versus oxazolone + vitamin D) or representative dot plots (D) are shown. doi:10.1371/journal.pone.0024961.g010 mice Pglyrp3 and Pglyrp4 promote recruitment and retention of Treg cells in the inflamed skin, which limits over-activation of Th17 cells and protects the skin from exaggerated inflammatory response to allergens. Our results do show prominent activation of Th2 cells in the oxazolone model of atopic dermatitis and prominent production of IgE, characteristic of Th2 bias in atopic diseases. In our model, however, the main difference at the peak of inflammation between WT and atopic dermatitis-sensitive Pglyrp-deficient mice, which determines the enhanced inflammatory responses and higher IgE production in Pglyrp-deficient mice, is the over- activation of Th17 cells and reduced numbers of Treg cells in the atopic dermatitis-sensitive mice. Our results indicate that initial Th2 response changes with time to Th17 response and that Th17 cells play an important role in enhancing inflammation and production of IgE in atopic dermatitis. Our results thus further extend recent findings of the enhancing role of Th17 cells in B cell maturation and differentiation [41–43]. Therefore, our results indicate that Pglyrp3 and Pglyrp4 are involved in controlling multiple functions of Treg and Th17 cells in the skin in atopic dermatitis. Contact dermatitis, which is a skin model of type IV hypersensitivity, is usually considered to be mediated by Th1 cells. However, this is an oversimplification, because recent findings show involvement of multiple cell types [2,4,5,44]. Our results show that Pglyrp1 (and other Pglyrps to a lesser extent) promotes Th1 responses, because Pglyrp12/2 mice (and also Pglyrp22/2 and Pglyrp42/2 mice) have reduced inflammation in oxazolone-induced contact dermatitis. This shift of balance to Th1 cells is likely beneficial for the desirable anti-microbial responses and may taper exaggerated inflammatory responses in the skin to allergens. Note that Pglyrp1 is mainly delivered to the sites of inflammation by PMNs, which are usually recruited to fight infections. Our results show that PGRPs, a family of innate immunity proteins, influence the functions of both innate and adaptive immune cells with an outcome of enhancing the activity of Treg cells and inhibiting the activity of Th17 cells. Pglyrp32/2 and PLoS ONE | www.plosone.org 12 September 2011 | Volume 6 | Issue 9 | e24961