Microglia are the resident immune cells of the brain and play a central role in initiating and maintaining immune responses. After pathogenic stimulation, they can adopt two activation states, leading to either a pro-inflammatory (M1) or an anti-inflammatory (M2) phenotype (Al Mamun et al., 2020). Interferon regulatory factors (IRFs) mediate the activation of peripheral immune cells and macrophages in other inflammatory diseases. Interferon regulatory factors (IRFs) mediate the activation of peripheral immune cells and macrophages in other inflammatory diseases. A regulatory axis that balances the pro- and anti-inflammatory activation of microglia is formed by the IRF5-IRF4 signaling pathway (Al Mamun et al., 2020). After ischemic stroke, microglia exhibit both pro-inflammatory and anti-inflammatory responses (M1–M2 phenotypes) that coexist and antagonize each other. In microglial IRF5 CKO mice, the absence of the IRF5 signal leads to increased expression of IRF4, enhanced M2 activation, reduced pro-inflammatory response, and improved outcomes in ischemic stroke (Al Mamun et al., 2020). Because IRF4 signaling is absent in microglial IRF4 CKO animals, there is increased IRF5 expression, higher M1 activation, impaired pro-inflammatory responses, and decreased functional recovery (Al Mamun et al., 2020).

After ischemia, phagocytosis of cellular debris is essential for reducing inflammation and reestablishing tissue homeostasis. Triggering receptor expressed on myeloid cells 2 (TREM2), expressed on bone marrow cells, is a microglial surface receptor that transmits intracellular signals through the adaptor protein DAP12. In an anti-inflammatory environment, it can clear apoptotic material through phagocytosis (Sieber et al., 2013).

After ischemic stroke, TREM2-KO mice exhibit reduced microglial activity, with decreased transcription of pro-inflammatory cytokines TNFα, IL-1α, and IL-1β, as well as reduced transcription of chemokines CCL2 (MCP1), CCL3 (MIP1α), and chemokine receptor CX3CR1. Consequently, the infiltration of CD3-positive T cells is diminished. TREM2-KO mice exhibit a reduced subacute inflammatory response to ischemic stroke, whereas the infarct size remains unchanged (Sieber et al., 2013).

Vitamin D deficiency is associated with an increased risk of various diseases and adverse outcomes, including cardiovascular diseases, cancers, autoimmune diseases, and neurological disorders (Cui et al., 2023). There is a significant association between vitamin D deficiency and adverse cerebrovascular events, including an increased risk of ischemic stroke and poor stroke prognosis (Cui et al., 2023).

Following ischemic stroke, microglia/macrophage VDR-CKO mice exhibit an enhanced M1 phenotype in microglia/macrophages, with substantial secretion of TNF-α and IFN-γ. The blood–brain barrier is disrupted, peripheral T lymphocyte infiltration is enhanced, and CXCL10 is released from endothelial cells as a result of these inflammatory cytokines, aggravating ischemic brain injury (Cui et al., 2023).

The activation of microglia relies on the Na⁺/H⁺ exchange-mediated H⁺ homeostasis; preventing the accumulation of H⁺ and cytosolic acidification is crucial for maintaining the respiratory burst activity mediated by NADPH oxidase in microglia (Shi et al., 2011).

NHE-1 (sodium-hydrogen exchanger isoform 1) ^+/− mice show improved neurological and functional prognosis following ischemic stroke, as well as decreased pro-inflammatory responses, decreased NADPH oxidase activity, and lessened microglial activation (Shi et al., 2011).

Members of the plasma membrane cation-chloride cotransporter (CCC) family, such as the neuron-specific K⁺/Cl⁻ transporter KCC2 and the ubiquitously expressed Na⁺-K⁺-2Cl⁻ cotransporter NKCC1 (encoded by the Slc12a2 gene), are increasingly the focus of research in central nervous system (CNS) diseases, including neuropsychiatric disorders, epilepsy, stroke, and dementia (Tóth et al., 2022).

Microglial NKCC1 KO mice show enhanced NLRP3 inflammasome activation and interleukin-1β (IL-1β) production after an ischemic stroke. A worsening of the neurological prognosis is brought on by cytotoxic swelling and edema production caused by impaired cell volume regulation. These conditions cause a marked increase in brain damage, inflammation, and cerebral edema (Tóth et al., 2022).

Signal transduction and transcriptional activator proteins, known as STATs, constitute a unique family of proteins capable of binding to DNA. The functional state of macrophages and microglia is regulated by multiple members of the STAT family. In particular, STAT6 signaling causes macrophages to exhibit an anti-inflammatory phenotype, whereas STAT1 activation encourages inflammatory activity. Moreover, IL-4, a STAT6 activator, can direct macrophages and microglia toward a favorable phenotype, aiding in the recovery process following a brain ischemic stroke (Cai et al., 2019).

After an ischemic stroke, microglia and macrophages’ STAT6 is activated. This activation modifies the microglia and macrophage phenotype via the STAT6/Arg1 pathway, accelerating the resolution of inflammation, enhancing phagocytosis, and ultimately improving the prognosis for stroke patients. The absence of STAT6 leads to impaired phagocytosis, exacerbated inflammation, and increased ischemic brain damage (Cai et al., 2019).

Recovery from a stroke and cerebral ischemia–reperfusion (I/R) injuries are both impacted by histone deacetylases (HDACs). After I/R, oxidative stress activates HDAC2, and blocking HDAC2 can improve cellular survival and neurogenesis after stroke. Ischemic preconditioning causes neurons’ HDAC3 to be degraded, which increases the neurons’ resistance to I/R. In neurons, inhibiting HDAC1 and HDAC6 can increase cellular viability while inhibiting HDAC4 and HDAC5 can decrease it (Liao et al., 2020).

After an ischemic stroke, HDAC3 regulates the nuclear localization and acetylation of microglial p65, which enhances the activation of the cGAS-STING pathway and transcriptionally promotes the production of cGAS. This process aids in the development of a pro-inflammatory milieu. The cGAS-STING pathway is less activated, inflammatory mediators are secreted less frequently, the inflammatory response is lessened, and the neurological and functional prognosis is improved in microglial HDAC3 CKO mice (Liao et al., 2020).

Integrin α9β1 upregulates neutrophil activation and works in concert with β2 integrins to stabilize neutrophil adhesion to activated endothelium. β1 is the sole partner for α9. Besides neutrophils, α9β1 is expressed in other cell types, including monocytes, smooth muscle cells, hepatocytes, endothelial cells, and epithelial cells (Patel et al., 2023).

The deficiency of neutrophil α9 limits the formation of ischemic thrombi associated with reduced fibrin deposition and platelet aggregation in the brain. When an ischemic stroke occurs, the lack of α9 in neutrophils limits the inflammatory response by lowering TNF-α, IL-1β, and IL-6 levels (Patel et al., 2023). Neutrophil α9^−/− mice after an ischemic stroke have a decreased thrombotic inflammatory response, which improves their neurological and functional results.

The glycolytic enzyme pyruvate kinase muscle 2 (PKM2) is an important regulator of aerobic glycolysis and also functions as an activator of transcription for pro-inflammatory cytokines such as interleukin-1 beta (IL-1β) and IL-6 (Dhanesha et al., 2022). Mammals have four different isoforms of pyruvate kinase (PK) (PKR, PKL, PKM1, PKM2), which are encoded by two separate genes, PKLR and PKM (Dhanesha et al., 2022).

PKM2 promotes STAT3 phosphorylation, which in turn controls peripheral neutrophil post-ischemic inflammation after an ischemic stroke. In bone marrow-specific PKM2^−/− mice, there is a reduction in fibrinogen, platelet deposition, neutrophil infiltration, and inflammatory cytokines in the brain, leading to improved neurological and functional outcomes (Dhanesha et al., 2022).

When it comes to ischemic brain injury, inflammatory cells have both beneficial and harmful functions. Myeloid cells’ autophagy, particularly that of macrophages, can reduce oxidative stress, stop the overproduction of pro-inflammatory cytokines, and stop the activation of inflammatory bodies (Kotoda et al., 2018).

Following ischemic stroke, autophagy can suppress the overall activity of inflammatory bodies by effectively removing endogenous inflammatory body activators, including reactive oxygen species and damaged mitochondrial DNA, thereby reducing secondary injury mediated by sustained and aberrant inflammation in the penumbra region (Patel et al., 2023). Mice with impaired myeloid cell autophagy (Atg5flox/flox LysMCre+) show increased levels of pro-inflammatory cytokines, which exacerbate ischemic brain damage by causing excessive inflammation in the subacute and chronic phases after ischemia brain injury (Kotoda et al., 2018; Wu et al., 2023).

Mast cells (MCs) are derived from hematopoiesis and are present in most tissues, with the lungs and intestines being the primary sites of their distribution. MCs serve as the initial line of defense against viruses and injuries, as well as environmental antigens and allergens. MCs release preformed and newly synthesized mediators, notably histamine (HA). HA is a neurotransmitter that promotes inflammation and is essential for the maturation of MC progenitor cells (Conesa et al., 2023).

Mast cells (MCs) react quickly to damage after an ischemic stroke and release histamine (HA). The main location of HA synthesis is the gastrointestinal system, and MCs are transported from the gut to the brain. Increased recruitment of MCs in the ischemic brain leads to neuroinflammation. In C-kit^−/− MC KO mice, the release of histamine can be prevented, which can alleviate neuroinflammation following ischemic stroke and improve neurological and functional outcomes (Conesa et al., 2023).

We examine how particular innate immune cells affect the response after an ischemic stroke in this section. Notably, animals with conditional deletion of the IRF5 and IRF4 genes in microglia following stroke showed clear behavioral changes. While mice with IRF4 CKO showed an increased M1-type pro-inflammatory response, mice with IRF5 CKO (conditional knockout) showed an improved M2-type anti-inflammatory response. These results imply that the inflammatory response and post-stroke recovery are strongly influenced by the activation state of microglia. Additionally, we observed that vitamin D signaling affects neuroinflammation and blood-brain barrier integrity in VDR CKO mice, and that TREM2-KO mice play a role in removing cellular debris and regulating anti-inflammatory responses. These findings highlight the possibility of treatment approaches that target particular innate immune cells, potentially enhancing stroke recovery.

The family of pattern recognition receptors (PRRs) known as toll-like receptors (TLRs) is able to recognize pathogen-associated molecular patterns (PAMPs) from microbes or damage-associated molecular patterns (DAMPs) generated from injured tissues (Tajalli-Nezhad et al., 2019). These receptors, by sensing different stimuli, recruit specific adapter proteins, activate a series of signal transduction cascades, and trigger specific immune responses, serving as a bridge between innate immunity and adaptive immune responses.

When TLR2/4 binds to the endogenous ligand HMGB1, it triggers a sequence of signal transduction cascades that result in the production of inflammatory cytokines such TNF-α, IL-1β, and IL-6, which exacerbates cerebral ischemia–reperfusion injury. Compared to WT mice, TLR2/4-deficient mice exhibit lower levels of these inflammatory cytokines, smaller infarct volumes, milder neuroinflammation, and higher neuronal survival rates in the ischemic hemisphere after cerebral ischemia–reperfusion injury (Tajalli-Nezhad et al., 2019; Qiu et al., 2019).

We specifically looked at the activity of Toll-like receptors (TLRs) when investigating the role of pattern recognition receptors in the innate immune response after stroke. TLRs are essential for controlling the inflammatory response following ischemic stroke, as evidenced by the reduced levels of inflammatory cytokines and smaller infarct volumes seen in mice with genetic deletion of TLR2/4. By modifying TLR activity, these findings offer a scientific basis for the creation of innovative treatment approaches that target TLRs and may lessen brain damage after stroke.

G protein-coupled receptors (GPCRs) are desensitized and internalized by adapter proteins called beta-arrestins, which stops G protein activation. They have been identified as scaffold proteins that regulate a wide range of biological processes by binding to different target molecules. The regulating function of beta-arrestin 2 (ARRB2) in inflammatory reactions is crucial (Chen et al., 2019).

Following cerebral ischemia–reperfusion (I/R) injury, ARRB2 negatively regulates the NF-κB inflammatory signaling triggered by NOD2 (nucleotide-binding oligomerization domain-containing protein 2) through its interaction with TRAF6 in microglia (Chen et al., 2019). When ischemia–reperfusion (I/R) injury occurs in β-arrestin 2 knockout (KO) mice, the lack of ARRB2 results in worsened outcomes of ischemia–reperfusion stroke generated by NOD2 stimulation.

A family of selenoenzymes known as glutathione peroxidases (Gpx) catalyzes the reduction of certain biological peroxides at the expense of reduced glutathione. Gpx1 is the most prevalent isoform, and it has been linked to neurodegenerative conditions such traumatic brain injury, Lewy body dementia, and Parkinson’s disease (Chen et al., 2011).

In order to shield the brain from oxidative stress and inflammation brought on by ischemia–reperfusion injury, Gpx1 performs a crucial regulatory role. The activation of pro-apoptotic p53 and Fas ligand (CD95/Apo1)-mediated pathways, a decrease in the Nrf2 antioxidant cascade, malfunctioning of the ubiquitin-proteasome system (UPS), and inhibition of the mitogen-activated protein kinase (MAPK) signaling pathway are observed in Gpx1 knockout (KO) mice after ischemic stroke. These findings exacerbate ischemic brain injury (Chen et al., 2011).

A member of the cysteine protease ubiquitin-specific protease family, ubiquitin-specific protease 10 (USP10) possesses the enzymatic ability to cleave ubiquitin from ubiquitin-conjugated protein substrates. Key signaling pathways, including those involving p53 and PTEN, that are involved in cell proliferation, apoptosis, inflammation, autophagy, and tumor metabolism are modulated by USP10 (Wang et al., 2019). The absence of USP10 leads to a significant increase in the activation of NF-κB (Wang et al., 2019).

By directly interacting with Transforming Growth Factor β-Activated Kinase 1 (TAK1), USP10 protects against cerebral I/R injury after ischemic stroke. USP10 KO mice show accelerated NFκB signaling pathway, increased inflammatory response in the ischemic cortex’s penumbra area, marked apoptotic induction, and worsened ischemia brain injury in comparison to WT mice (Wang et al., 2019).

Transforming Growth Factor β-Activated Kinase 1 (TAK1), also known as MAP3K7 (Mitogen-Activated Protein Kinase Kinase Kinase 7), serves a dual role as a regulator of cell death inducers and a variety of inflammatory signaling molecules. It includes key regulatory functions for IKK (I-κB Kinase), MAP2Ks, MKK3/6, MKK4/7, and AMPK (AMP-Activated Protein Kinase) (Wang et al., 2020).

Following cerebral ischemia–reperfusion, TAK1 plays a crucial role in the pro-inflammatory responses of macrophages (MΦ) and microglia (Mi). Pro-inflammatory genes and other myeloid cell activity-related functions in macrophages and microglia are downregulated or repressed in TAK1 Mi/MΦ-specific knockout mice. The specific conditional deletion of TAK1 in adult Mi/MΦ alleviates the pro-inflammatory response after ischemic stroke and improves long-term neurological function (Wang et al., 2020).

We examine how signaling pathways affect the innate immune response after an ischemic stroke in this section. In particular, we found that β-arrestin 2 knockout (KO) mice have worse stroke outcomes following ischemia-reperfusion injury because NOD2 stimulation exacerbates activation of the NF-κB inflammatory signaling pathway. These results demonstrate the critical function of β-arrestin 2 in controlling inflammatory reactions and could offer hints for the creation of novel treatment approaches.

One of the most important chemokines produced by neurons, astrocytes, and endothelial cells in response to hypoxia is monocyte chemoattractant protein-1 (MCP-1), commonly referred to as CCL2. It acts as the main chemoattractant, luring neutrophils, macrophages, and monocytes toward the center of the infarct (Strecker et al., 2013).

Following ischemic stroke, MCP-1−/− mice exhibit reduced expression of IL-1β, IL-6, and G-CSF, which helps prevent further damage to the blood–brain barrier in mice. Reductions in the infiltration of neutrophils, macrophages, and monocytes into the ischemic brain tissue result in better neurological and functional outcomes as well as a decrease in local inflammatory responses (Strecker et al., 2013).

A pro-inflammatory cytokine called interleukin-1 (IL-1) has been linked to a number of neurological illnesses, including ischemic stroke. The IL-1 type I receptor (IL-1R1), which is known to transduce intracellular signals, and the IL-1 type II receptor, which does not directly signal, are the two types of IL-1 receptors that have been found to date (Lazovic et al., 2005).

Due to the elimination of IL-1R1 signaling, IL-1R1 ^−/− mice can preserve energy status, interfere directly with the cytokine activation of immune cells, limit the damage caused by free radicals mediated by inducible nitric oxide synthase (iNOS), and disrupt the production of chemokines after an ischemic stroke. This results in better neurological and functional outcomes and less brain damage after an ischemic stroke (Lazovic et al., 2005).

The pathogenesis of stroke involves the complement system, which is a powerful inducer of inflammatory reactions. Brain damage progression is largely influenced by the lectin pathway (LP), one of the complement system’s activation routes. Mannose-binding lectin (MBL), one of the LP’s starting components, can be targeted in mice to reduce damage through pharmacological suppression or genetic deficit (Orsini et al., 2018).

Following ischemic stroke, circulating mannose-binding lectin MBL drives platelet activation. Activated platelets release interleukin-1 alpha (IL-1α), which upon binding to its receptor (IL-1R1) induces vascular inflammation and promotes vascular injury (Orsini et al., 2018). Endothelial injury results from the deposition of mannose-binding lectin (MBL) by damaged endothelial cells. This leads to complement activation and pro-inflammatory activation of the endothelium, including the upregulation of intercellular adhesion molecule-1 (ICAM-1) (Orsini et al., 2018). Following ischemic stroke, MBL^−/− mice exhibit reduced levels of IL-1α, an attenuation of the inflammatory phenotype, and improved neurological and functional outcomes.

Neuronal damage results from inflammation after an experimental stroke. Through influencing the upregulation of adhesion molecules, neutrophil chemotaxis, platelet activation, and the generation of reactive oxygen species, the complement cascade plays a crucial role in both starting and controlling the inflammatory process (Mocco et al., 2006). After cerebral ischemia–reperfusion injury in adult mice, the complement system, which is mostly dependent on C3, mediates inflammatory neuronal damage (Mocco et al., 2006).

C3-deficient (C3 ^−/−) mice show a substantial drop in infiltrative neutrophils and oxidative stress levels after an ischemic stroke, which diminishes brain damage and improves neurological and functional results (Mocco et al., 2006) (Table 1).

The function of innate immunity molecules in the innate immune response after stroke is examined in this section, with special focus on the outcomes of MCP-1 and IL-1R1 gene knockout mice. Following a stroke, MCP-1−/−mice show enhanced blood-brain barrier integrity and decreased production of inflammatory cytokines, whereas IL-1R1−/−mice show improved functional recovery and lower neuroinflammation. These findings raise the possibility of developing novel therapeutic approaches by indicating that modifying particular innate immune molecules can successfully control the inflammatory response after stroke. An alternate structure and a synopsis of the key points are shown in the following table.