Background: The skin of patients with atopic dermatitis (AD) exhibits a striking susceptibility to infection with gram-positive bacteria and herpes simplex virus (HSV), which are known to stimulate Toll-like receptor (TLR) 2.
Objective: We investigated whether TLR2-mediated proinflammatory cytokine production by monocytes is selectively impaired in patients with AD and, if so, whether high FcvarepsilonRI levels on the monocytes could be related to the impairment.
Methods: The 2 subpopulations of monocytes, CD14(dim) proinflammatory and CD14(bright) classical monocytes, from patients with AD and healthy control subjects were stimulated to produce IL-1beta and TNF-alpha with phorbol 12-myristate 13-acetate/ionomycin, LPS (TLR4 ligand), or Pam3Cys (TLR2 ligand) for 4 hours, and simultaneous flow cytometric assessment of surface phenotype and intracellular cytokine synthesis was performed. Surface expression of TLR2, TLR4, and FcvarepsilonRI on the monocyte subpopulations was also assessed by means of flow cytometry.
Results: TLR2-mediated IL-1beta and TNF-alpha production by either the CD14(dim) or CD14(bright) monocytes was found to be selectively impaired in patients with AD. The most remarkable reduction in TLR2-mediated proinflammatory cytokine production was observed in CD14(dim) monocytes expressing high FcvarepsilonRI levels from patients with AD. This reduction was restored by means of downregulation of their FcvarepsilonRI expression after preculture in the absence of IgE.
Conclusion: Monocytes, particularly the proinflammatory monocytes, from patients with AD are functionally defective in their capacity to produce proinflammatory cytokines on TLR2 stimuli in part because of the high levels of their FcvarepsilonRI expression.
Clinical implications: This selective impairment of monocytes would explain why patients with AD are specifically susceptible to cutaneous staphylococcal and streptococcal and HSV infections.