title: Efficient flow cytometric assay for platelet-leukocyte aggregates in whole blood using fluorescence signal triggering.
abstract: BACKGROUND
  Platelet-leukocyte aggregates (PLAs) may be important in thrombotic and inflammatory disease states, but accurate assessment of PLA formation in vivo is hampered by the propensity for in vitro artefacts caused by sample manipulation. A whole blood flow cytometric assay for circulating PLAs, based on minimal sample manipulation, was thus developed.
  
  
  METHODS
  Citrated whole blood was labeled with a RPE-CD45 MAb (leukocyte marker) and an FITC-CD42a (GPIX) MAb (platelet marker). The latter was used to avoid possible influences of platelet glycoprotein proteolysis by neutrophil-derived proteases. The samples were mildly fixed with 0.5% formaldehyde saline. The cytometer was triggered by RPE-CD45 fluorescence. Leukocyte subpopulations were separated according to their typical light scattering and CD45 expression.
  
  
  RESULTS
  Minimal sample manipulation and mild sample fixation resulted in minor in vitro artefacts and good sample stability. Fluorescence triggering increased the efficiency of the flow cytometric analysis approximately 5-fold compared with triggering with light scatter, and allowed discrimination of leukocyte subpopulations. The majority of PLAs involved monocytes and neutrophils, rather than lymphocytes, both without and with in vitro stimulation by ADP or thrombin. A cocktail of blocking MAbs to CD62P, CD15, GPIIb/IIIa and the CD11b/CD18 complex had no effect on unstimulated samples, whilst totally inhibiting aggregation induced by 10(-5) M ADP, suggesting that the PLAs in unstimulated blood were preformed in vivo.
  
  
  CONCLUSIONS
  This whole blood flow cytometric assay for PLAs is simple and efficient, and appears to reflect closely platelet-leukocyte aggregates in circulating blood in vivo.
authors.affiliation: Department of Laboratory Medicine, Karolinska Hospital, Stockholm, Sweden.
authors.forename: N
authors.initials: N
authors.lastname: Li
authors.forename: A H
authors.initials: AH
authors.lastname: Goodall
authors.forename: P
authors.initials: P
authors.lastname: Hjemdahl
mesh_terms: D000328:Adult; D000911:Antibodies, Monoclonal / Q000032:analysis; D001792:Blood Platelets / Q000166:cytology*; D005260:Female; D005404:Fixatives / Q000032:analysis; D005434:Flow Cytometry / Q000379:methods*; D005456:Fluorescent Dyes / Q000032:analysis; D005557:Formaldehyde / Q000032:analysis; D006801:Humans; D007962:Leukocytes / Q000166:cytology*; D008297:Male; D008875:Middle Aged; D011108:Polymers / Q000032:analysis; D012016:Reference Values; D015203:Reproducibility of Results
chemical_list: D000911:Antibodies, Monoclonal; D005404:Fixatives; D005456:Fluorescent Dyes; D011108:Polymers; D005557:Formaldehyde; C003043:paraform