Semaphorin 7A on keratinocytes induces interleukin-8 production by monocytes

Semaphorin 7A (Sema7A) expressed on activated T cells stimulates cytokine production in monocytes through its receptor, α1β1 integrin.

To study the significance of Sema7A expressed on keratinocytes in skin inflammation where interaction between keratinocytes and β1-integrin expressing inflammatory cells, such as monocytes, takes place.

The regulation of Sema7A expression on keratinocytes by various cytokines was studied by flow cytometry and immunoblot. β1-integrin expressing human monocyte cell line, THP-1 cells, were co-cultured with paraformaldehyde-fixed normal human epidermal keratinocytes (NHK) and IL-8 production by THP-1 cells was studied. The significance of β1-integrin or Sema7A within this cell interaction was examined by the experiments using β1-integrin blocking antibody or Sema7A siRNA.

IFN-γ and TNF-α slightly increased Sema7A expression, while IL-4 decreased it. Among cytokines tested, TGF-β1 most strikingly increased the Sema7A expression on NHK. When NHK was stimulated by TGF-β1, paraformaldehyde-fixed, and co-cultured with THP-1 cells, IL-8 production by THP-1 cells was increased compared to THP-1 cells only. When THP-1 cells were pretreated with β1-integrin blocking antibody, this increase in IL-8 production by THP-1 cells was inhibited. Likewise, when NHK were pretreated with Sema7A siRNA before fixation and co-cultured with THP-1 cells, increase in IL-8 production by THP-1 cells was inhibited.

Our results suggest that Sema7A on keratinocytes and β1-integrin on monocytes contribute to monocyte activation by keratinocytes within skin inflammation, such as psoriasis or wound.