Monocyte depletion attenuates the development of posttraumatic hydrocephalus and preserves white matter integrity after traumatic brain injury

Monocytes are amongst the first cells recruited into the brain after traumatic brain injury (TBI). We have shown monocyte depletion 24 hours prior to TBI reduces brain edema, decreases neutrophil infiltration and improves behavioral outcomes. Additionally, both lesion and ventricle size correlate with poor neurologic outcome after TBI. Therefore, we aimed to determine the association between monocyte infiltration, lesion size, and ventricle volume. We hypothesized that monocyte depletion would attenuate lesion size, decrease ventricle enlargement, and preserve white matter in mice after TBI. C57BL/6 mice underwent pan monocyte depletion via intravenous injection of liposome-encapsulated clodronate. Control mice were injected with liposome-encapsulated PBS. TBI was induced via an open-head, controlled cortical impact. Mice were imaged using magnetic resonance imaging (MRI) at 1, 7, and 14 days post-injury to evaluate progression of lesion and to detect morphological changes associated with injury (3D T1-weighted MRI) including regional alterations in white matter patterns (multi-direction diffusion MRI). Lesion size and ventricle volume were measured using semi-Automatic segmentation and active contour methods with the software program ITK-SNAP. Data was analyzed with the statistical software program PRISM. No significant effect of monocyte depletion on lesion size was detected using MRI following TBI (p = 0.4). However, progressive ventricle enlargement following TBI was observed to be attenuated in the monocyte-depleted cohort (5.3 ± 0.9mm³) as compared to the sham-depleted cohort (13.2 ± 3.1mm³; p = 0.02). Global white matter integrity and regional patterns were evaluated and quantified for each mouse after extracting fractional anisotropy maps from the multi-direction diffusion-MRI data using Siemens Syngo DTI analysis package. Fractional anisotropy (FA) values were preserved in the monocyte-depleted cohort (123.0 ± 4.4mm³) as compared to sham-depleted mice (94.9 ± 4.6mm³; p = 0.025) by 14 days post-TBI. All TBI mice exhibited FA values lower than those from a representative naïve control group with intact white matter tracts and FA~200 mm3). The MRI derived assessment of injury progression suggests that monocyte depletion at the time of injury may be a novel therapeutic strategy in the treatment of TBI. Furthermore, non-invasive longitudinal imaging allows for the evaluation of both TBI progression as well as therapeutic response over the course of injury.