Surgical manipulation compromises leukocyte mobilization responses and inflammation after experimental cerebral ischemia in mice

Abstract

Acute brain injury results in peripheral inflammatory changes, although the impact of these processes on neuronal death and neuroinflammation is currently unclear. To facilitate the translation of experimental studies to clinical benefit, it is vital to characterize the mechanisms by which acute brain injury induces peripheral inflammatory changes, and how these are affected by surgical manipulation in experimental models. Here we show that in mice, even mild surgical manipulation of extracranial tissues induced marked granulocyte mobilization (300%) and systemic induction of cytokines. However, intracranial changes induced by craniotomy, or subsequent induction of focal cerebral ischemia were required to induce egress of CXCR2-positive granulocytes from the bone marrow. CXCR2 blockade resulted in reduced mobilization of granulocytes from the bone marrow, caused an unexpected increase in circulating granulocytes, but failed to affect brain injury induced by cerebral ischemia. We also demonstrate that isoflurane anaesthesia interferes with circulating leukocyte responses, which could contribute to the reported vascular and neuroprotective effects of isoflurane. In addition, no immunosuppression develops in the bone marrow after experimental stroke. Thus, experimental models of cerebral ischemia are compromised by surgery and anaesthesia in proportion to the severity of surgical intervention and overall tissue injury. Understanding the inherent confounding effects of surgical manipulation and development of new models of cerebral ischemia with minimal surgical intervention could facilitate better understanding of interactions between inflammation and brain injury.

Recurrent mild cerebral ischemia: enhanced brain injury following acute compared to subacute recurrence in the rat

“…The origin of the additional granulocytes can vary with the type of injury but includes mobilization from the spleen and bone marrow and potentially increased production and decreased apoptosis [ 34 , 37 ]. The mechanism mediating the increase likely includes a stress response [ 23 , 41 ] which is consistent with previous reports and our observation of elevated numbers of granulocytes and decreased numbers of lymphocytes, also in animals subjected to sham surgery [ 40 , 47 , 48 ]. With a mild transient ischemic insult, we observed an increase in granulocytes that was temporary with a normalization by 3 days post MCAO.…”

“…With a mild transient ischemic insult, we observed an increase in granulocytes that was temporary with a normalization by 3 days post MCAO. A similar return towards baseline was observed in mice with either sham surgery or MCAO using a transient ligature [ 48 ]. Important to note is that the stress induced neutrophilia would appear insufficient to enhance damage because a sham surgery 1 day prior to mild transient MCAO reduced ischemic damage indicating that a combination of both transient cerebral ischemia and neutrophilia would be involved in exacerbating damage.…”

Role of Neutrophils in Exacerbation of Brain Injury After Focal Cerebral Ischemia in Hyperlipidemic Mice

“…While the latter studies induced 45 and 60 min MCAO we used a mild ischemia model leading to much smaller infarcts and a lesser degree of neutrophil accumulation, suggesting that a certain threshold of cerebral neutrophil counts needs to be exceeded to uncover the cells’ pathogenic role. However, even if more severe injury models are used, several studies interfering with CXCR2 signaling and thereby reducing neutrophil infiltration failed to influence stroke outcome in normolipidemic wildtype mice 27–29 . But, most of these studies assessed stroke outcome at 24 hours post injury which might be too early to detect secondary neurodegeneration after the complex inflammatory cascade has fully established.…”

Brain-immune interactions in health and disease

“…Appropriate reprogramming of key immune mechanisms could be used to develop novel stroke therapies including possible prevention of injury through stroke in vulnerable individuals. The research paper by Denes et al demonstrates that brain injury, anesthesia, and surgical interventions have diverse systemic consequences, including altered leukocyte responses in several organs of the body and rapid mobilization of granulocytes (Denes et al, 2013 ). This could have important implications for animal models of cerebral ischemia as well as for patients with brain injury or for those undergoing surgeries or exposed to prolonged anesthesia.…”