Minocycline reduces chronic microglial activation after brain trauma but increases neurodegeneration

Abstract

Survivors of a traumatic brain injury can deteriorate years later, developing brain atrophy and dementia. Traumatic brain injury triggers chronic microglial activation, but it is unclear whether this is harmful or beneficial. A successful chronic-phase treatment for traumatic brain injury might be to target microglia. In experimental models, the antibiotic minocycline inhibits microglial activation. We investigated the effect of minocycline on microglial activation and neurodegeneration using PET, MRI, and measurement of the axonal protein neurofilament light in plasma. Microglial activation was assessed using 11C-PBR28 PET. The relationships of microglial activation to measures of brain injury, and the effects of minocycline on disease progression, were assessed using structural and diffusion MRI, plasma neurofilament light, and cognitive assessment. Fifteen patients at least 6 months after a moderate-to-severe traumatic brain injury received either minocycline 100 mg orally twice daily or no drug, for 12 weeks. At baseline, 11C-PBR28 binding in patients was increased compared to controls in cerebral white matter and thalamus, and plasma neurofilament light levels were elevated. MRI measures of white matter damage were highest in areas of greater 11C-PBR28 binding. Minocycline reduced 11C-PBR28 binding (mean Δwhite matter binding = -23.30%, 95% confidence interval -40.9 to -5.64%, P = 0.018), but increased plasma neurofilament light levels. Faster rates of brain atrophy were found in patients with higher baseline neurofilament light levels. In this experimental medicine study, minocycline after traumatic brain injury reduced chronic microglial activation while increasing a marker of neurodegeneration. These findings suggest that microglial activation has a reparative effect in the chronic phase of traumatic brain injury.

Keywords: microglia; minocycline; neurodegeneration; positron emission tomography; traumatic brain injury.

Figure 1

Study design, TBI enrolment and control groups. Baseline data in TBI patients were compared to two control groups, and longitudinal MRI data in patients were compared to a third control group (Table 1). *Arterial blood sampling in two patients in the minocycline group failed (one baseline visit, one 12-week visit), so within-subject comparisons of PET data for these participants were not possible. ^†A third patient in the minocycline group had an anxiety episode at the start of the 12-week scan and immediately withdrew from the study, so all follow-up data from this participant were excluded.

Figure 2

Increased baseline ¹¹C-PBR28 binding in TBI in white matter and subcortical regions. (A) Individual standardized (z-score) images of baseline ¹¹C-PBR28 DVR are superimposed on axial T₁ MRIs. Voxels with increased DVR (z > 0) compared to the control mean, when controlling for age and TSPO genotype, are shown. Baseline images for 14 TBI patients and two representative controls are show. The age (years), gender and TSPO binding class (determined from the TSPO genotype) of participants is shown. In patients, the time since injury to baseline scanning is also shown. M = male; F = female; HAB = high affinity binder; MAB = medium affinity binder (Owen et al., 2012). (B) Red-yellow areas show significantly increased ¹¹C-PBR28 DVR in patients compared to controls. Results are thresholded using threshold free cluster enhancement (family-wise error correction P < 0.05).

Figure 3

Baseline white matter volume loss and reduced white matter tract structure are greater in areas of high ¹¹C-PBR28 binding in TBI patients. (A) Blue–light blue areas show significantly decreased white matter volume loss in patients compared to controls. Results are thresholded using threshold free cluster enhancement (family-wise error correction P < 0.05). (B) Significantly decreased grey matter volume in patients compared to controls. Thresholding and colour bar are as for A. (C) Blue–light blue areas show significantly decreased fractional anisotropy in patients compared to controls. The contrast is overlaid on the mean fractional anisotropy skeleton (green). Thresholding and colour bar are as for A. (D) Overlap map in patients of lesions visible on T₁ structural imaging. The colour of the map indicates the number of patients with a lesion in that area. Maps were computed by summation of the normalized binary lesion masks of individual patients. (E) In patients, white matter tissue probability, a measure of tissue volume expressed as a z-score with controls as a reference, is shown for areas of individually-defined high levels of white matter ¹¹C-PBR28 DVR (blue bar) and normal levels of DVR (grey bar). (F) As for E, but showing fractional anisotropy, expressed as a z-score with controls as a reference. Bars for E and F are mean ± standard error of the mean (SEM) **P < 0.01.

Figure 4

Longitudinal white matter atrophy over 6 months in TBI patients is greater in areas of high baseline ¹¹C-PBR28 binding. (A) Mean annualized Jacobian determinant (JD) images, indexing longitudinal change over 6 months in patients. Green colours represent little or no change over time, yellow-red colours reflect volumetric increases (expansion, positive Jacobian determinant), while blue–light blue colours reflect volumetric decreases (contraction, negative Jacobian determinant). Mean Jacobian determinant images are superimposed on the MNI T₁ template. (B) Blue–light blue areas show significantly decreased Jacobian determinant, indicating longitudinal atrophy in white matter (WM) over 6 months (white matter tissue-specific Jacobian determinant < 0), in patients. Results are thresholded using threshold free cluster enhancement (family-wise error correction < 0.05). (C) Regions of significantly decreased Jacobian determinant in grey matter (GM) over 6 months in patients, indicating longitudinal atrophy inn grey matter (grey matter tissue-specific Jacobian determinant<0). Thresholding and colour bar are as for B. (D) In patients, mean Jacobian determinant in white matter is shown for areas of individually-defined high levels of white matter ¹¹C-PBR28 binding (distribution volume ratio, DVR) (blue bar) and normal levels of DVR (grey bar). Bars are mean ± SEM **P < 0.01.

Figure 5

Plasma NFL and associations with ¹¹C-PBR28 white matter binding and time since injury. (A) Plasma NFL levels are shown for TBI patients (red dots) and controls (blue dots). Black bars show mean and 95% confidence intervals. Note y-axis is logarithmic. (B) Plasma NFL in TBI patients (y-axis) is plotted against time since injury in months (x-axis). Dotted horizontal line indicates upper limit of 95% confidence interval in controls. Note both axes are logarithmic. (C) ¹¹C-PBR28 DVR of the cerebral white matter (WM) region of interest in TBI patients (y-axis) plotted against plasma NFL level (x-axis, logarithmic).

Figure 6

Effect of minocycline treatment in TBI patients on ¹¹C-PBR28 binding, plasma NFL and longitudinal atrophy. (A) Mean change in ¹¹C-PBR28 volume of distribution (ΔV_T) between baseline and 12-week visits, expressed as a percentage of baseline V_T, in patients who received minocycline treatment (n = 7) (top row) and no drug (n = 5) (bottom row). See Fig. 1 for description of excluded data. Green colours represent little or no change over time, yellow-red colours reflect V_T increases over time, while blue-light blue colours reflect V_T decreases. Images are superimposed on the MNI T₁ template. (B) Blue-light blue areas show significantly decreased V_T between baseline and 12-week visits (ΔV_T < 0), in patients who received minocycline treatment (top row) and no drug (bottom row). Results are thresholded using threshold free cluster enhancement (family-wise error correction < 0.05). Neither group showed areas of significantly increased V_T (ΔV_T > 0). (C) Blue-light blue areas show significantly reduced ΔV_T in patients who received minocycline treatment compared to patients who received no drug. There were no areas of significantly increased ΔV_T. Thresholding and colour bar are as for B. (D) Mean ± SEM group change in V_T, expressed as a percentage of baseline V_T, in patients who received minocycline treatment (blue bars) and no drug (red bars), in white matter (WM), thalamus and cortical grey matter (GM) regions of interest. (E) Plasma NFL levels are shown for TBI patients treated with minocycline (red dots) and untreated patients (no drug, blue dots) for the three visits. Bars show visit mean (connected over time) and standard error. Note y-axis is logarithmic. (F) Percentage change in NFL between baseline and 12 weeks (y-axis) is plotted against percentage change in white matter ¹¹C-PBR28 VT (x-axis) measured over the same period. Colours are defined as in E. (G) Mean white matter Jacobian determinant (JD, y-axis) (measured between baseline and 6 months) is plotted against baseline NFL in patients.