Focal and Systemic Immune Responses Following Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is a devastating type of acute brain injury with high mortality and disability. Acute brain injury swiftly alters the immune reactivity within and outside the brain; however, the mechanisms and influence on neurological outcome remains largely unknown. My dissertation investigated how ICH triggers focal and systemic immune responses and their impact hemorrhagic brain injury. At the focal level, a significant upregulation of interleukin (IL)-15 was identified in astrocytes of brain sections from ICH patients. A transgenic mouse line where the astrocytic IL-15 expression is controlled by a glial fibrillary acidic protein promoter (GFAP-IL-15tg) was generated to investigate its role in ICH. Astrocyte-targeted expression of IL-15 exacerbated brain edema and neurological deficits following ICH. Aggravated ICH injury was accompanied by an accumulation of pro-inflammatory microglia proximal to astrocytes in perihematomal tissues, microglial depletion attenuated the augmented ICH injury in GFAP-IL-15tg mice. These findings suggest that IL-15 mediates the crosstalk between astrocytes and microglia, which worsens ICH injury.Systemic immune response was investigated by leveraging the novel method of obtaining and analyzing bone marrow cells from the cranial bone flaps of ICH patients. A swift increase of hematopoietic stem cell (HSCs) population in the bone marrow was identified, along with a shift towards the myeloid cell lineage. Human findings were mirrored in an ICH mouse model. Fate mapping these HSCs revealed increased genesis of Ly6Clow monocytes in the bone marrow, which transmigrate into the hemorrhagic brain and give rise to alternative activation marker bearing macrophage. Blockade of the β3-adrenergic receptor or inhibition of Cdc42 abolished ICH-induced myeloid bias of HSCs. Importantly, mirabegron, a Food and Drug Administration-approved β3 adrenergic receptor agonist, and a Cdc42 activator, IL-3, enhanced bone marrow generation of Ly6Clow monocytes and improved recovery. These results suggest that brain injury modulates HSC lineage destination to curb distal brain inflammation, implicating the bone marrow as a unique niche for self-protective neuroimmune interactions. Together, these results demonstrate how acute brain injury exerts a profound yet distinct effect on immune responses within and outside the brain and sheds new light on neuroimmune interactions with potential clinical implications.