Functional distinction between CXC chemokines, interleukin-8 (IL-8), and growth related oncogene (GRO)α in neutrophil infiltration

Interleukin-8 (IL-8: CXCL8) and growth related oncogene alpha (GROalpha: CXCL1) are members of the CXC chemokines. In the present study, we explored the functional distinction between these CXC chemokines in the regulation of neutrophil infiltration. Injection of either rabbit IL-8 or GROalpha (10 mug each) into rabbit knee joints resulted in a massive neutrophil infiltration in the joints. At their peak time point (6 hours), the number of neutrophils induced by IL-8 was more than that induced by GROalpha. Each chemokine induced the other chemokine in the joints. TNFalpha activity was induced in the joints after administration of GROalpha, but not IL-8. Treatment with anti-GROalpha mAb and/or anti-TNFalpha mAb failed to inhibit IL-8-induced neutrophil infiltration. In contrast, either anti-IL-8 IgG or anti-TNFalpha mAb decreased GROalpha-induced response, and the inhibition was further enhanced by coadministration of these antibodies. Thus, it appears that IL-8 acts directly, whereas GROalpha acts indirectly, in part, on neutrophil infiltration. The distinct difference in TNFalpha production between IL-8 and GROalpha was further investigated. In vitro, GROa induced TNFalpha activity in cultured synovial cells, the cells producing TNFalpha in the joints after GROalpha-injection. However, IL-8 failed to produce TNFalpha activity from the cells, although equivalent levels of the mRNA expression were induced by IL-8 as compared with GROalpha. When recombinant rabbit TNFalpha was incubated with synovial fluids obtained at 2 hours after IL-8 injection, the resultant TNFalpha activity was significantly decreased, an event that was completely restored by a serine protease inhibitor, phenylmethylsulphonyl fluoride (PMSF). Furthermore, TNFalpha activity was unveiled in the joints when IL-8 was intra-articularly injected with PMSF. These data suggest that TNFalpha is degraded by serine protease(s) in the case of IL-8. Taken together, the data clearly demonstrate the functional distinction between IL-8 and GROalpha, which may influence the inflammatory responses.