Neonatal Systemic Inflammation Induces Inflammatory Reactions and Brain Apoptosis in a Pathogen-Specific Manner

Abstract

Background: After neonatal asphyxia, therapeutic hypothermia (HT) is the only proven treatment option. Although established as a neuroprotective therapy, benefit from HT has been questioned when infection is a comorbidity to hypoxic-ischaemic (HI) brain injury. Gram-negative and gram-positive species activate the immune system through different pathogen recognition receptors and subsequent immunological systems. In rodent models, gram-negative (lipopolysaccharide [LPS]) and gram-positive (PAM₃CSK₄ [PAM]) inflammation similarly increase neuronal vulnerability to HI. Interestingly, while LPS pre-sensitisation negates the neuroprotective effect of HT, HT is highly beneficial after PAM-sensitised HI brain injury. Objective: We aimed to examine whether systemic gram-positive or gram-negative inflammatory sensitisation affects juvenile rat pups per se, without an HI insult. Methods: Neonatal 7-day-old rats (n = 215) received intraperitoneal injections of vehicle (0.9% NaCl), LPS (0.1 mg/kg), or PAM (1 mg/kg). Core temperature and weight gain were monitored. Brain cytokine expression (IL-6, IL-1β, TNF-α, and IL-10, via PCR), apoptosis (cleaved caspase 3, via Western blots), and microglial activation (Iba1, via immunohistochemistry) were examined. Results: LPS induced an immediate drop in core temperature followed by poor weight gain, none of which were seen after PAM. Furthermore, LPS induced brain apoptosis, while PAM did not. The magnitude and temporal profile of brain cytokine expression differed between LPS- and PAM-injected animals. Conclusion: These findings reveal sepsis-like conditions and neuroinflammation specific to the inflammatory stimulus (gram-positive vs. gram-negative) in the neonatal rat. They emphasise the importance of pre-clinical models being pathogen dependent, and should always be carefully tailored to their clinical scenario.

Hypothermia Is Neuroprotective after Severe Hypoxic-Ischaemic Brain Injury in Neonatal Rats Pre-Exposed to PAM₃CSK₄

“…This distinct difference between the 2 models, as well as the difference in susceptibility to hypothermic neuroprotection (Fig. 1) [15], is in line with other discrepancies between LPS- and PAM-triggered inflammation and how they affect the immature brain even without the HI insult [20]. LPS induced significant brain apoptosis, poor weight gain, and immediate loss of core temperature (median 31.2°C).…”

“…The effect of the studied intervention on thermoregulation is critical in preclinical models of neonatal HI and neuroprotection [38]. The profound mortality among LPS-exposed pups might partly be explained by the temperature drop that LPS induces prior to the HI insult [20]. This means that during the insult, we impose a much greater temperature elevation in LPS-exposed animals than we do in the PAM-exposed ones, and intrahypoxic temperature is relatively higher for the pups who received LPS.…”

“…This time point was based on the post-injection temporal development of intracerebral inflammatory cytokines from a previous study [20]. IL-6 and TNF-α peaked 2 h after an LPS injection, whereas after an injection of PAM, IL-6 and TNF-α were significantly upregulated after 6 h, indicating a more delayed intracerebral inflammatory activation.…”

Outcome and Feasibility after 7 Years of Therapeutic Hypothermia in Southern Brazil

“…18 The benefits of TH have been questioned when the infection/inflammation is associated with brain injury hypoxic-ischemic syndrome. 19 Studies evaluating the neuroprotective effect of TH where the rates of neonatal infection are higher did not show benefits of therapy, thus questioning whether comorbidities such as perinatal infection could reduce the neuroprotective effect of TH. 18,20 Unlike the protocol implemented by the Virginia group, 14 we did not exclude sepsis among our patients, although 64 (88.9%) used antibiotics for presumed early onset sepsis and only 2 had positive blood culture.…”

Involvement of CXCL1/CXCR2 During Microglia Activation Following Inflammation-Sensitized Hypoxic-Ischemic Brain Injury in Neonatal Rats

“…Hypoxia-ischemia triggers activation of microglia, the resident immune cells of the central nervous system (CNS) ( 7 , 8 ). In our recent study modeling inflammation-sensitized HI brain injury an upregulation of pro-inflammatory cytokines in microglia 24 h post injury was shown ( 9 ).…”