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Inhibition by recombinant SLPI and half‐SLPI (Asn55‐Ala107) of elastase and cathepsin G activities: consequence for neutrophil‐platelet cooperation

1 The capacity of recombinant human secretory leukocyte proteinase inhibitor (SLPI) to inhibit human leukocyte elastase (HLE) and cathespin G (Cat G) was investigated and compared with a recombinant truncated form (carboxyl‐terminal domain, Asn55‐Ala107) called 1/2 SLPI. 2 Both compounds were efficient when tested against enzymatic activities of purified HLE and Cat G indicating that the HLE‐ and Cat G‐inhibitory sites were preserved in the truncated form. SLPI and 1/2 SLPI also affected platelet activation induced by 0.2 μm Cat G (IC50 = 112 ± 13 nm for SLPI and 280 ± 12 nm for 1/2 SLPI). 3 The effects of SLPI and 1/2 SLPI were then tested against polymorphonuclear neutrophil (PMN)‐mediated platelet activation, a cell‐to‐cell interaction mediated by HLE and Cat G released from PMN. In this experimental system, addition of SLPI or 1/2 SLPI before N‐formyl‐Met‐Leu‐Phe (fMLP) led to the inhibition of the resulting platelet activation. As was the case for Cat G enzymatic activity and Cat G‐induced platelet activation, SLPI was more efficient than 1/2 SLPI (IC50 = 676 ± 69 nm vs 1121 ± 150 nm). 4 The ratio of the IC50 against PMN‐mediated platelet activation compared to purified Cat G‐mediated platelet activation was 6.03 for SLPI and 4.32 for 1/2 SLPI. This difference may be due to the smaller size of the truncated form which could allow this molecule to diffuse more easily between PMN and platelets. 5 In conclusion, 1/2 SLPI could be a promising candidate in the treatment of pathological states linked to inflammation in which participation of HLE and Cat G has been evoked.