Protocols for circulating neutrophil depletion in neonatal C57Bl/6 mice

Murine neonatal neutrophil depletion strategies have problems achieving deep neutrophil clearance and accurate residual neutrophil fraction detection. An isotype switch method can achieve profound neutrophil clearance using a combination of anti-Ly6G and anti-rat κ Ig light chain antibodies in adult C57Bl/6 mice, proven by extra- and intracellular Ly6G detection by flow cytometry. We adapted this technique to neonatal mice, testing four neutrophil depletion strategies in the peripheral circulation, bone marrow, and spleen. Four protocols were tested: P3 Ly6G and P1-3 Ly6G (anti-Ly6G on postnatal days (P) 3 and 1-3 respectively), and P3 Dual and P1-3 Dual (anti-Ly6G and anti-rat κ Ig light chain on P3 and P1-3 respectively). Intracellular and extracellular Ly6G presence was detected using flow cytometry. Isotype control antibodies were used as controls. P1-3 Dual protocol achieved significantly better neutrophil depletion than the P1-3 Ly6G or P3 Ly6G protocols (97% vs. 74% and 97% vs. 50%, respectively) in the peripheral circulation. The P3 Dual protocol alone was enough to achieve significantly better neutrophil clearance (93%) than any of the Ly6G alone protocols. The Ly6G alone protocols led to near-total elimination of extracellular Ly6G. However, there was a significant presence of intracellular Ly6G in the CD45+ cell population, evading detection by extracellular Ly6G antibody-based detection methods. P3 protocols perform better than P1-3 protocols for bone marrow and splenic neutrophil clearance. Thus, the P3 Dual protocol might be the most effective and ethical protocol to induce profound neutrophil depletion in neonatal mice, an alternative to daily anti-Ly6G injections.