Neonatal polymorphonuclear leukocytes (PMNs) are deficient in the expression of the adherence protein C3bi (Mo1), and are associated with reduced physiological inflammatory responses. We evaluated the priming and direct stimulating effect of recombinant human granulocyte-macrophage colony stimulating factor (rhGM-CSF) on newborn PMN expression of C3bi (Mo1), PMN adherence and PMN aggregation. Cord PMNs were incubated with rhGM-CSF (Amgen 4 x 10(7) U/mg) for 0-15 min, and C3bi surface receptor expression measured by immunofluorescence with CD11b, adherence with nylon wool, and aggregation using a Payton aggregometer. RhGM-CSF (15 min) significantly induced Mo1 expression: 250 pM/L 129.4 +/- 5.3% of C (p less than 0.001) 500 pM/L 141.5 +/- 4.1% (p less than 0.001), 1,000 pM/L 150.2 +/- 1.3% (p less than 0.0001). RhGM-CSF (1,000 pM/L x 5 min) followed by A23187 also primed newborn PMNs for increased Mo1 expression 122 +/- 4.5% of C (p less than 0.001). Additionally, rhGM-CSF (10 min) induced significant PMN adherence 50 pM/L 117.9 +/- 8.3% and 100 pM/L 131.5 +/- 5.7% of C (p less than 0.04). RhGM-CSF additionally primed newborn PMNs (100 pM/L) for increased adherence following A23187 (107.9 +/- 0.6% of C), p less than 0.02. Lastly, rhGM-CSF primed newborn PMNs for increased aggregation following FMLP: 100 pM/L, 15 min, 138.1 +/- 14.1%, p less than 0.0001. Co-incubating murine-antihuman GM-CSF AB 100 micrograms/ml neutralized 86.8 +/- 7.0% of newborn PMN Mo1 up-regulation. These studies demonstrate that rhGM-CSF primes and directly stimulates newborn PMNs for increased in vitro expression of Mo1, adherence, and aggregation.(ABSTRACT TRUNCATED AT 250 WORDS)