Imaging Immune Cell Infiltration and Function in Brain Injury

Lay Summary

How Brain Surgery May Alter Structure and Function Through an Inflammatory Response

Investigators will use two-photon microscopy in laboratory animals undergoing surgical removal of a part of the skull, to determine how the surgery activates an immune inflammatory response that may result in damage to synaptic connections between brain cells.

Brain surgery in laboratory animals provides a model for studying the effects of surgery in humans and is also a model of brain injury. The investigators will remove a part of the skull in laboratory animals, replace it with a glass plate, and image the response of immune cells. These include innate immune cells called “microglia” that reside in the brain, These cells activate glial cells, which support the synaptic connections between the brain’s neurons. This activation is thought to be an important response to brain injury, which produces inflammation. Immune overreaction to inflammation, however, may lead to damaging changes in the brain’s structure and function. The researchers hypothesize that this damage occurs not by the microglial cells, but by the circulating immune cells that they recruit into the brain to serve as reinforcements.

Using cellular imaging through the glass plate, investigators will determine how activation of microglia, and their subsequent recruitment of circulating immune cells, occurs and subsequently regulates changes in synapses. They also will determine whether the resultant loss and gain (“remodeling”) of synapses requires primarily the resident microglial cells or also the recruited immune cells. Since the months required for stabilizing synaptic changes in the laboratory animal is similar to the time required by patients undergoing surgery or injured by trauma or stroke, the research may lead to new insights into therapies for stabilizing brain connections that could be tested in humans.

Significance: The research may lead to development of new anti-inflammatory treatment strategies to accelerate synapse stabilization following stroke, brain injury, or brain surgery.

Abstract

Imaging Immune Cell Infiltration and Function in Brain Injury

Immune cell infiltration, glial activation, and neuronal damage are common pathological features in various modes of brain injury and disease. How immune cells such as microglia and T cells populate the brain after injury and whether they contribute to the remodeling of neuronal connections are not known. Recently, we have used intravital two-photon laser scanning imaging to directly observe dynamic changes of neuronal connections and immune cells in the adult mouse cerebral cortex through either a thinned-skull window or an open-skull window. We found that in superficial layers of the cerebral cortex, postsynaptic dendritic spines imaged through a thinned-skull window are remarkably stable with 1–2% turnover over 3 days and ~5% over 1 month. In contrast, in the same cortical regions, when the skull is removed and replaced with a glass window (open-skull), dendritic spines first undergo a substantial loss (~22%) within the first two weeks after surgery and subsequently become remarkably plastic, with ~30% turnover over 1 month.

Importantly, we observed massive activation and accumulation of microglia underneath the open-skull window but not thinned-skull window within the first 2 weeks of surgery. The open-skull window is also associated with extensive astrocytes activation for at least 4 weeks after surgery. These findings provide an outstanding experimental system for assessing the infiltration and function of immune cells in the adult brain at sites of tissue inflammation and damage.

In this application, we will test the hypothesis that inflammation induced by innate and adaptive immune system cells contributes to dramatic changes in synaptic connections following injury to the cerebral cortex. Specifically, we will determine the role of chemokine receptors CCR2 and CXCR6 in regulating the infiltration of immune cells into the injured brain and investigate the role of endogenous versus recruited immune cells in structural changes of synapses. The proposed studies will provide new insights into mechanisms by which autoimmunity and brain infections cause loss of neuronal structure and function.