Background
IL-9 has been suggested to play an important role in chronic inflammation due to its effect on remodeling and cell recruitment. Since basophils can be found in inflamed tissue, we wanted to test if human basophils are capable of secreting IL-9 under the influence of growth or tissue factors like IL-3 or IL-33, respectively.

Methods
Basophils, purified from buffy coat blood by negative selection (purity > 95%), were cultured for 24 hours in RPMI culture medium (1x106 cells/ml) supplemented with 5% human serum, and IL-3 (10 ng/ml) and/or IL-33 (50 ng/ml). The cells were then analysed by FACS for ST2 expression where basophils were gated as FceRIa+CD3-CD14-CD19-CD56-. The supernatant was tested for IL-9 using a multiplex magnetic bead assay.

Results
Human basophils have recently been described to express the IL-33 receptor ST2. In agreement with previous findings, our results showed increased surface expression of ST2 on basophils stimulated with IL-3 for 24 h, whereas IL-33 stimulation did not influence surface expression of its receptor. Nevertheless, basophils stimulated with IL-33 released IL-9 at a level comparable to what was found when stimulating with IL-3. Moreover, there was a marked synergy between IL-3 and IL-33 on the IL-9 release.

Conclusion
To our knowledge, this is the first report that human basophils secrete IL-9. A strong synergy between IL-3 and IL-33 was found on the IL-9 induction, likely to be explained by the IL-3 upregulated ST2 expression.