GCSF with sustained action increases the availability of monocytes in hematopoietic organs and monocyte recruitment into the degenerating muscle. Spleen size was decreased at late stage mutant SOD1 mice as assessed by weight (A, p <0.01, n = 6-11) and length (B, p <0.001, n = 6-11) as compared to wt control mice at the same age. Pegfilgrastim treatment increased the spleen size of mutant SOD1 mice (A, B, p <0.001, n = 6-11). The dotted line represents the spleen size of wt mice after long-term pegfilgrastim treatment (A, B). Long-term pegfilgrastim treatment also increased the number of splenocytes (C, p <0.001, n = 3) as analyzed from 20-week old mutant SOD1 mice. The number of leukocytes in BM was determined from the same mice (D): pegfilgrastim did not affect the total leukocyte number in BM but altered the composition of BM cell populations increasing the number of monocytes (E, p <0.01, n = 3). The similar effect on cell composition was detected is spleen cell populations (F, p <0.05, n = 3). The monocyte population which number was greatly increased, namely Ly6Cint had low or no production of cytokine after LPS stimulus (G, middle), in comparison to Ly6Chi monocytes which represented high cytokine production for inflammatory stimulus (G, below) as analyzed by flow cytometry. When mononuclear cell population was analyzed from the thigh muscle including the sciatic nerve in mutant SOD1 mice at symptomatic stage, the Ly6C monocytosis was increased while lymphocytosis was decreased (H, p <0.05, n = 3).