Question:

S100A8 and S100A9 belong to a family of cytosolic calcium-binding proteins expressed in myeloid cells. Theyare secreted as heterodimers by neutrophils and activated monocytes and can be found at high levels at sites of inflammation. In this study we investigated the role of S100A8/S100A9 in leukocyte recruitment in vivo.

Methods:

We studied leukocyte rolling and adhesion in TNF-a-stimulated cremaster muscle venules of S100a9-deficient mice using intravital microscopy. In addition, ex vivo flow chamber assays were used to investigate rolling and adhesion of leukocytes of S100a9-deficient mice.

Results:

In cremaster muscle venules we observed a decrease in overall leukocyte adhesion and extravasation 2h after intrascrotal injection of TNF-a into S100a9-deficient mice compared to WT mice. Leukocyte rolling of S100a9-deficient leukocytes was slightly increased in comparison to WT leukocytes. In the ex vivo flow chamber assay, the number of adherent S100a9-deficient leukocytes was strongly reduced and the number of rolling leukocytes slightly increased as compared to their WT counterparts. Both, the in vivo and ex vivo findings suggest a defect of S100a9 deficient leukocytes in the transition from leukocyte rolling to arrest. Addition of soluble S100A8/A9 was able to almost completely reconstitute the decreased number of adherent leukocytes in S100a9-deficient cells to WT levels, pointing out a central role of soluble S100A8/A9 in the transition from leukocyte rolling to leukocyte arrest.

Conclusions:

We identified soluble S100A8/A9 as important pro-inflammatory modulators during leukocyte recruitment in vivo supporting leukocyte arrest on inflamed postcapillary venules and extravasation into inflamed tissue.