Introduction:The pathophysiology and clinical outcomes of traumatic brain injury (TBI) in patients with sickle cell disease (SCD) is grossly understudied. TBI triggers a complex neuroinflammatory response involving a balance between pro- and anti-inflammatory mechanisms that is critical for optimal recovery. In the acute phase, the initial response is primarily pro-inflammatory, characterized by the activation of resident microglia, release of pro-inflammatory cytokines such as IL-6 and TNF-alpha, and recruitment of immune cells to the injury site. In the chronic phase, there is a shift towards an anti-inflammatory response, involving the production of anti-inflammatory cytokines such as IL-10 and the promotion of tissue repair and regeneration. Given that SCD is characterized by a chronic baseline pro-inflammatory state, we examine whether TBI in mice with SCD reflects a new homeostatic setpoint for neuroinflammatory response. To our knowledge, this is the first preclinical pilot study of the inflammatory response to TBI in SCD.

Methods:6 Berkeley (SCD) ɑ-/-, β-/-, Tg(HBɑ-HBBs)41Paz)/J hemizygote and 6 C57BL/6J (wild type) controls aged 12-18 weeks were split into 2 cohorts (n=3/group) for TBI and control (sham). Following a midline scalp incision, a 5mm craniotomy was performed via dental drill over the left parietal cortex-2.7mm lateral to midline and 2.5mm posterior to Bregma. Using a pneumatic impactor (Bimba, Monee, IL) with a 3 mm rounded-tipped impounder, a controlled cortical impact (CCI) model was performed over the craniotomy site (3.5 m/sec velocity; 1.0 mm depth) for the TBI groups. Sham controls were both anesthetized and surgically prepared without cortical impact. Plasma samples were collected at baseline, 24 hours, and 72 hours post-injury/sham. A multiplex assay (MesoScale Discovery, Rockville MD) quantified markers of inflammation in plasma samples. Two-way ANOVAs and post-hoc t-tests identified significant group mean differences in inflammatory marker concentrations. Tukey's honestly significant difference test was used to correct for multiple comparisons.

Results:During the study, between 24-28 hours post-injury, one Berkeley-CCI mice experienced mortality while another was prematurely sacrificed due to significant weight loss and frailty. Causes of death and weight loss were unknown. IL-10 plasma concentrations at baseline were significantly higher in SCD mice (mean [SEM]: 2.559 [0.150] pg/mL; p=0.0008) compared to wild type (WT) mice (1.337 [0.178] pg/mL). Conversely, IL-1ß levels 24 hours post-injury/sham were significantly lower in SCD mice (33.18 [8.923] pg/mL; p=0.0320) in comparison to WT mice (105.80 [30.48] pg/mL). Similarly, IFN-y concentrations in SCD mice 24 hours post-CCI/sham (0.163 [0.0293] pg/mL) were significantly lower when compared to WT mice (0.308 [0.0394] pg/mL; p=0.0264). IL-2 levels showed significant difference between SCD-sham (0.210 [0.0156] pg/mL) and SCD-CCI (0.382 [0.0142] pg/mL; p=0.0276) groups. Analyses of IL-4, IL-5, IL-6, KC/GRO and TNF-a did not yield statistical significance. The two SCD-CCI mice with mortality showed the highest levels of IL-10 (36.255 and 10.021 pg/mL) and lowest levels of IL-1ß (7.179 and 10.482 pg/mL) at 24 hours post-injury/sham across all animals.

Conclusion:This pilot study suggests an altered inflammatory response following TBI in SCD, with dampening of key pro-inflammatory markers such as IL-1ß and IFN-y. Baseline elevations in IL-10 suggests priming of anti-inflammatory signaling as a response to the basal inflammatory state present in SCD. However, this may inhibit critical neuroprotective effects of inflammation in the acute phase post-TBI, predisposing individuals with SCD to serious complications and mortality. Further exploration and larger studies are needed to fully understand this balance and characterize TBI responses and outcomes in SCD.

Disclosures

No relevant conflicts of interest to declare.

This content is only available as a PDF.
2024
Sign in via your Institution