Mast cells partially contribute to mucosal adjuvanticity of surfactin in mice

Surfactin (SF) is a cyclic lipopeptide that has potent mucosal adjuvant properties. However, immunological mechanisms of SF adjuvant action have not yet been elucidated. As some cyclic lipopeptides, such as polymyxin, can stimulate histamine release from mast cells, we hypothesized that mast cell activation is critical for SF adjuvanticity.

We observed that following intranasal immunization with ovalbumin (OVA) plus SF, the titers of the OVA-specific antibody (Ab) in the mucosal secretions and plasma of mast cell-deficient mice were significantly lower than those in congenic normal mice, although OVA-specific Ab did not entirely disappear from mast cell-deficient mice. SF induced degranulation of mast cells and release of histamine in vitro. To investigate whether SF stimulated mast cells in vivo, we measured body temperature of mice immunized intranasally with OVA plus SF because histamine level affects body temperature. Following immunizations, body temperature of immunized congenic normal mice transiently decreased, whereas body temperature of mast cell-deficient mice did not change. Plasma levels of OVA-specific IgE Ab were not significantly different in mast cell-deficient and congenic normal mice. These findings suggest that SF directly affected mast cells in an IgE Ab-independent fashion. Furthermore, we analyzed the effects of SF on MC/9 mast cells cultured in vitro. MC/9 cells stimulated by SF released not only histamine but also leukotriene B₄ and prostaglandin D₂ . Moreover, SF up-regulated mRNA expression levels of Tnf, Ccr5, and Il4 genes in mast cells. These cytokines may play a facilitating role in OVA-specific immune responses in mice.

Overall, our results showed that mast cell activation partially mediated SF adjuvanticity.