Synthetic peptides containing ITIM‐like sequences of IREM‐1 inhibit BAFF‐mediated regulation of interleukin‐8 expression and phagocytosis through SHP‐1 and/or PI3K

Summary B‐cell activation factor of the tumour necrosis factor family (BAFF), an important regulator of B‐cell survival, has recently been found to be expressed on the surface of murine and human macrophages and engagement with its receptor was shown to trigger induction of pro‐inflammatory mediators and block phagocytic activity. In an effort to generate immunomodulatory agents that can regulate BAFF‐mediated signal, decapeptides representing the intracellular immunoreceptor tyrosine‐based inhibitory motifs (ITIMs) of immune receptor expressed on myeloid cells (IREM)‐1, an inhibitory transmembrane protein expressed on myeloid cells, were synthesized in conjugation with HIV‐transactivator of transcription (TAT) 48–57, which facilitates the internalization of peptides into cells. Interestingly, all five of these synthetic peptides, representing the five ITIM‐like sequences present in the cytoplasmic tail of IREM‐1, exhibited inhibitory action against BAFF‐mediated induction of matrix metalloproteinase‐9 and interleukin‐8 expression. Inhibitor assay and immunoprecipitation assay followed by Western blotting demonstrated that the inhibitory action was mediated by Src homology 2 (SH2)‐containing tyrosine phosphatase (SHP)‐1 and/or phosphoinositide 3‐kinase (PI3K). ELISA‐based nuclear factor‐κB DNA binding assay observed that the synthetic peptides blocked the activation of nuclear factor‐κB in an SHP‐1 and phosphoinositide 3‐kinase‐dependent manner. Three of these synthetic peptides exhibited varying degrees of inhibitory action against BAFF‐mediated blockage of phagocytosis in a SHP‐1 and PI3K‐dependent manner. These data indicate that the synthetic peptides are capable of blocking BAFF‐mediated regulation of macrophage activities through the activation of SHP‐1 and PI3K as well as inhibition of nuclear factor‐κB activation.