Induction of inhibitory cells in vivo using recombinant GM-CSF alone. Administration of GM-CSF causes an increase in CD11b/Gr-1 double-positive splenocytes. BALB/c mice were injected either with saline alone (“Naive”), or saline containing β-gal-rVV (“β-gal immune”). After 3 wk, the two groups of mice were randomly separated into smaller groups of three animals each that were inoculated i.p. with HBSS alone (HBSS), or HBSS containing 5 μg of mouse GM-CSF (GM-CSF), twice daily for a total of 3 days. At the end of this cycle of inoculations, spleens were removed and cultured with β-gal peptide for 6 days. A, Cytofluorometric staining with FITC anti-Gr-1 and PE anti-CD11b mAbs of splenocytes from immune mice inoculated with GM-CSF (right panel) or with vehicle alone (left panel). Similar percentages of CD11b+/Gr-1+ cells were found in spleens of nonimmunized mice (data not shown). B and C, Functional consequence of CD11b+/Gr-1+ induction. The cytolytic response against a panel of β-gal-positive and -negative targets is shown in naive (B) and β-gal-immune (C) mice. E:T cell ratio was 100:1, then 3-fold dilutions (100:1, 33:1, 11:1, 4:1). Repeat experiments gave nearly identical results.