Peptidoglycan and peptidoglycan-specific Th1 cells in psoriatic skin lesions

We have previously demonstrated, in psoriatic skin lesions, the presence of a subset of dermal CD4⁺ T cells that produce interferon-γ (IFN-γ) in response to a mixture of cell wall proteins extracted from group A streptococci. However, the identity of the antigen(s) involved is unknown. To investigate the hypothesis that peptidoglycan (PG), the major constituent of the streptococcal cell wall, acts as a T cell activator in psoriasis, we performed in situ analysis to detect antigen-presenting cells containing PG in lesional versus non-lesional skin, and determined proliferation and IFN-γ responses of lesional skin T cells. Increased numbers of PG-containing cells were detected in the dermal papillae and cellular infiltrates of guttate and chronic plaque skin lesions compared with normal and non-lesional psoriatic skin. A varying proportion of these were CD68⁺ macrophages, but the remaining cells did not double stain for either Langerhans' or dendritic cell markers. Psoriatic dermal streptococcal-specific CD4⁺ T cell lines proliferated and produced IFN-γ in a self HLA-DR allele-restricted manner in response to streptococcal PG, excluding mitogenic or superantigenic stimulation, but were unresponsive to staphylococcal PG. Similarly, psoriatic staphylococcus-specific T cell lines recognized staphylococcal, but not streptococcal, PG by IFN-γ production. The presence of PG-containing macrophages in close association with PG-specific CD4⁺ T cells in lesional skin suggests that PG may be responsible, at least in part, for T cell activation in psoriasis.