Response to Letter by Hermann and Kilic
We appreciate Kilic and Hermann’s comments on our publication “Meta-Analysis of the Efficacy of Granulocyte-Colony Stimulating Factor in Animal Models of Focal Cerebral Ischemia” and take this opportunity to address the critical points.
Kilic and Hermann criticize that the study from Taguchi and colleagues1 was excluded from the data analysis. This study using immunodeficient mice showed negative results regarding infarct size reduction and functional recovery improvement after G-CSF treatment. A potential concern, therefore, could be a bias of the overall results of our meta-analysis. In their letter Kilic and Hermann further emphasize that an exclusion of results from immunodeficient mice may not be appropriate because “G-CSF represents an anti-inflammatory strategy and stroke itself is followed by an immunodeficient state”.
In our article we clearly describe that the Taguchi study1 was excluded from the meta-analysis because of technical reasons. In this study infarct sizes were measured as infarct area or a brain atrophy index was calculated. Both methods of infarct size calculation are rather uncommon and may potentially distort true infarct sizes. Therefore, neither the infarct area nor the atrophy index are normally used for the calculation of infarct sizes as indicator for neuroprotective effects. This is also reflected by the studies included in our meta-analysis which all use a standardized infarct volume calculation. Nevertheless, we estimated the impact of the Taguchi study1 and another study2 which also measured infarct size as area: infarct volumes were approximated by assuming a spherical infarct shape. The results of this meta-analysis, however, were very similar to those reported in our article. Infarct volumes were reduced by 44% (95% CI, 29% to 55%) after inclusion of the 2 studies compared to an infarct size reduction of 42% (95% CI, 34% to 49%) as reported in our published meta-analysis. With respect to functional recovery assessment the Rotarod test, the neuroscore and the limb function were included in the meta-analysis as defined by predetermined criteria. Therefore, the open-field task used as neurobehavioral test in the Taguchi study could not be considered for our analysis.
We furthermore believe that there are pathophysiological reasons not to include this study with immunodeficient mice in this meta-analysis with wild-type animals. It seems to be inappropriate when effects of G-CSF on the immune system are reduced to its anti-inflammatory properties, because the factor is also known to improve anti-infectious defense.3 These immunomodulative properties of G-CSF may potentially be beneficial for stroke treatment. The contribution of anti-infectious properties of G-CSF on neuroprotection and functional recovery were, however, never convincingly proven so far. Moreover, one has to consider that stroke itself may lead to an immunocompromised status in humans and animals.4,5 An additional defect of the immune system due to genetic modification may cause unforeseeable effects on stroke pathophysiology. Interaction in such a pathopyhsiological situation with a factor that modulates the immune system in an unpredictable way makes the situation even more complicated. A comparison with all other studies included in the meta-analysis seems, therefore, to be inappropriate.
In conclusion, we still think that exclusion of the Taguchi study is appropriate for the above-mentioned reasons. However, we agree that studies with genetically modified animals are important, because they may provide an insight into the mode of action of candidate stroke drugs.
W.-R. Schäbitz is an inventor on a patent application regarding the neuroprotective effects of G-CSF.
Prass K, Meisel C, Hoflich C, Braun J, Halle E, Wolf T, Ruscher K, Victorov IV, Priller J, Dirnagl U, Volk HD, Meisel A. Stroke-induced immunodeficiency promotes spontaneous bacterial infections and is mediated by sympathetic activation reversal by poststroke T helper cell type 1-like immunostimulation. J Exp Med. 2003; 198: 725–736
Prass K, Braun JS, Dirnagl U, Meisel C, Meisel A. Stroke propagates bacterial aspiration to pneumonia in a model of cerebral ischemia. Stroke. 2006; 37: 2607–2612.