The Janus Face of Neutrophil Specific Antigen CD177 in Bacterial Infection: Focal Disease Versus Systemic Disease

Abstract 1108

CD177, also known as NB1 (or HNA-2) is a GPI-linked glycoprotein, which is exclusively expressed on neutrophils. Approximately 3 to 5% of healthy individuals do not express this antigen on their neutrophils (NBnull).

Recent data demonstrated a strong upregulation of NB1 on neutrophils in patients with bacterial, but not viral infections. The mechanism underlying this phenomenon, however, is unknown. Our previous studies showed that NB1 functions as a partner of endothelial PECAM-1 and therefore plays a role on neutrophil diapedesis. Consequently, neutrophils carrying NB1 (NB1plus) migrate faster through endothelial cells than NBnull neutrophils. However, several studies have documented an abrogation of neutrophil migratory abilities in sepsis conditions. In this study, we aim to clarify the impact of neutrophil NB1 expression in bacterial sepsis.

To mimic this condition in vitro, we first compared the transendothelial migration ability of NB1 phenotyped neutrophils after stimulation with the bacterial peptide fMLP in a transwell system. Lower transmigration ability (75% vs. 40%) was observed in fMLP-treated NB1plus neutrophils (n = 5) in comparison to untreated neutrophils. In contrast, no significant difference in the migration ability was observed between fMLP-treated and untreated NB1null neutrophils (n = 3). Expression analysis by flow cytometry showed a dose-dependent upregulation of NB1 after stimulation with fMLP (10⁻⁶ to 10⁻⁸ μM) in NB1plus neutrophils. Interestingly, down regulation of PECAM-1 expression was observed in these treated cells. Contrary, no PECAM-1 downregulation was detected in NBnull fMLP-treated neutrophils. These results could be confirmed by immunoblotting analysis using specific antibodies directed against different epitopes on NB1 (mabs 7D8, MEM166) and against different PECAM-1 Ig domains (mabs PECAM1.1, 1.2 and 1.3). Analysis of the supernatants of fMLP-treated neutrophils demonstrated the shedding of PECAM-1 from NB1plus, but not from NB1null neutrophils. These results indicate that shedding of PECAM-1 from neutrophils during bacterial infections depends on NB1 expression.

Recent studies showed that a single nucleotide polymorphism (42C>G) located in NB1 promoter region is associated with the regulation of NB1 expression. Individuals homozygous for C allele express high NB1 surface density in comparison to individuals homozygous for G allele. Our association study showed higher frequency of C allele in patients with bacterial sepsis (n=98) compared with healthy cohort (n = 132) (8.16% vs. 12.88%; P<0.03). This observation indicates the role of C42 allele (or high NB1 expression) as genetic risk factor for bacterial sepsis.

All together, these data demonstrate a reverse role of NB1 expression in focal and systemic infection, indicating favourable effect of low NB1 in systemic bacterial infection. This phenomenon may be caused by reduction of neutrophil directionality and motility due to NB1-mediated PECAM-1 shedding.