In vitro eosinophil (EOS) adhesion to recombinant human (rh)-vascular cell adhesion molecule (VCAM)-1 stimulates superoxide anion (O2−) generation and enhances formyl-methionyl-leucyl phenylalanine (FMLP)-activated O2− generation. Therefore, EOS adhesion via VLA-4 to VCAM-1 expressed on endothelium may be instrumental in the selective recruitment and function of EOS in airway inflammation.
We hypothesized that EOS interaction with endothelial cells expressing VCAM-1 will undergo an enhancement in inflammatory function.
To determine this possibility, human umbilical vein endothelial cells (HUVEC) were stimulated with either a combination of interleukin (IL)-4 and tumour necrosis factor (TNF)-α (100 p M) or medium alone for 24 h; the expression of adhesion proteins on HUVEC and their effect on EOS O2− generation was subsequently determined.
As determined by both enzyme-linked immunosorbent assay and flow cytometry, IL-4 and TNFα acted synergistically to induce VCAM-1 expression on HUVEC. Treating HUVEC with IL-4/TNFα also increased EOS adhesion and primed subsequent FMLP (0.1 μM) activated EOS O2− generation. Although EOS adhesion was partially inhibited by both antiα4 and antiβ2 monoclonal antibodies (MoAbs), O2− generation was completely inhibited by either antiα4 integrin MoAb (HP1/2) or anti-VCAM MoAb (BBIG-V1). Furthermore, enhanced O2− generation, but not adhesion, associated with IL-4 + TNFα-treatment of HUVEC was inhibited when EOS were treated with the platelet activating factor (PAF)-antagonist WEB 2086 (20 μM), thus suggesting an involvement of PAF in priming EOS. However, paraformaldehyde fixation of IL-4/TFN-α treated HUVEC did not significantly alter EOS function.
These results suggest EOS adhesion to endothelial cells via an VLA-4/VCAM–1 interaction may be important in the development of the function of this cell. Furthermore, our results suggest that modulation of EOS function involves two priming factors: EOS adhesion to HUVEC expressing VCAM-1 and PAF.