Role Of The Spleen In Acute Ischemic Stroke

BACKGROUND AND PURPOSE:Current research has reveal the important role of the immune system in ischemic reperfusion injuries.In fact, it is now well established that a crosstalk between the immune system and the central nervous system not only contributes to secondary brain damage, but also it is also responsible for the systemic dampening of the immune system observed following stroke:stroke induced immuno depression(SIID).SIID is believed to be one of the main reasons behind the high susceptibility to infections following stroke. Therefore, understanding the relationship between the CNS and the immune system following acute stroke is of extreme importance in trying to find new therapeutic and diagnostics tools in stroke management. Soon after original ischemic insult, brain borne immune cells such as astrocytes and microglia become hyperactive. Activated microglia and astrocytes produce and release into surrounding brain tissues as well as the blood stream various pro-inflammatory cytokines including IL-1, IL-18 and TNF-a.The release of these pro-inflammatory cytokines can in turn activate and/or help recruit peripheral immune cells either via direct activation through the blood stream,or via the HPA axis and the sympathetic nervous system. Moreover, damaged brain cells from the ischemic site release damage associated molecular patterns(DAMPs)such as ATP,HMGB-1, HSP, in the surrounding area which can be sensed by surrounding immune cells(astrocytes mostly),which bind to these molecules through their corresponding membrane receptors(P2X7, TLR4, RAGE).This later phase results in the activation of downstream molecular pathways, including the activation of various transcription factors such as the NF-κB followed by the transcription of various genes coding for pro inflammatory cytokines and chemotractant molecules. This cascade of events is believed to be responsible for both post stroke immune response and SIID.Upon receiving the danger "signal" from the brain, peripheral immune cells such as lymphocyte, NK cells and leukocytes migrate from various immune organs such as the spleen,the thymus to the injured site, which in turn contributes to worsen the original brain damage. The toll like receptor 4(TLR4)-nuclear factor kappa B (NF-κB) inflammatory pathway Contributes to secondary inflammation in many diseases including stroke. Moreover, the neuroprotective effect of splenectomy in stroke is supported by a vast body of experimental evidence. Nevertheless, the underlying mechanism(s) by which splenectomy enhance neuroprotection in stroke is still poorly understood. Our study aimed to investigate whether post-ischemic splenectomy modulate the TLR4/NF-κB inflammatory pathway in stroke.METHODS:30 adults Sprague-Dawley (SD) rats were randomly divided into five groups as followed:sham group or SC (n=6,sham MCAO surgery followed by a sham splenectomy);MCAO control group (1d) or MC(1d)(n=6,MCAO without splenectomy with a reperfusion period of 1 day); MCAO control group (7d) or MC(1d)(n=6,MCAO without splenectomy with a reperfusion period of 7 days);MCAO+Splenectomy group (1d) or MS(1d) (n=6,MCAO surgery immediately followed by a splenectomy and a reperfusion period of 1 day); MCAO+ Splenectomy group (7d) or MS(7d) (n=6,MCAO surgery immediately followed by a splenectomy and a reperfusion period of 7 days).Immunohistochemistry was used to evaluate the levels of TLR4 and NF-κB expression in brain areas(parietal lobe, hippocampus and striatum).while, Apoptosis in these areas was assessed by TUNEL detection technique.RESULTS:Overall, The levels of TLR4and NF-κB expression were significantly reduced in splenectomized rats relative to the MC group(P<0.01) and both were increased compared to the sham group(P<0.01).In the parietal cortex,MS(1d) TLR4and NF-κB expression were reduced compare to the MC(1d) P<0.01.Similarly in the hippocampus and the striatum, MS(1d) TLR4and NF-κB expression were reduced compared to the Sham group P<0.01.However the difference of TLR4 and NF-κB expression between the two groups at the same timeframe did not reach any statistical significance P>0.05.Furthermore,MS(1d)TLR4and NF-κB expression in the hippocampus and striatum were reduced compared to the MC(1d) group P<0.01.In addition,MC(7d) TLR4and NF-κB expression in the parietal cortex, hippocampus as well as the striatum were reduced compared to the MC(7d) P<0.01. Additionally, the number of apoptotic cells in the ischemic hemisphere were significantly higher in both MCAO groups compare to the Sham group(P<0.05),between day 1 and day7.An exception was observed in the striatum where the difference in apoptotic rate between the MS and sham group was not statistically significant at the same time points. Moreover, the variation apoptotic rate in different cerebral zone correlated to variation in TLR4 and NF-κB expression.CONCLUSION:In summary, our study provides further evidence of neuroprotective effect of splenectomy in ischemic stroke. Our results suggest that such an effect might be due to the inhibition of theTLR4/NF-κB inflammatory pathway.