Antibodies to canine granulocyte colony-stimulating factor induce persistent neutropenia

A severe persistent neutropenia developed in a rabbit that was injected intradermally with 120, 60, 60, and 120 micrograms of recombinant canine granulocyte colony-stimulating factor (cG-CSF) on days 1, 22, 31, and 44, respectively. The neutropenia was present from day 44 to day 205. The nadir of the neutropenia (60 cells/microliters) occurred in conjunction with peak antibody titer (640,000) to cG-CSF on day 58. The immune antiserum from this rabbit reacted positively for cG-CSF on Western blot analysis. The immune antiserum also neutralized the activity of cG-CSF. On day 160, examination of the bone marrow showed marked granulocytic hypoplasia and mild erythroid hyperplasia. On day 205, the rabbit was still neutropenic (430 cells/microliters), even though the last injection of cG-CSF was given 161 previously. Necropsy on day 205 showed that there was still mild granulocytic hypoplasia with mild erythroid hyperplasia. Because of the lack of any inflammatory foci found at necropsy and the granulocytic hypoplasia, it was thought that the neutropenia was most likely due to decreased production and was not a consumptive process. It is hypothesized that the antibody that was produced to cG-CSF neutralized the effect of endogenous rabbit granulocyte colony-stimulating factor and prevented the normal proliferation and maturation of the rabbit neutrophils.