The spleen may be an important target of stem cell therapy for stroke

Abstract

Stroke is the most common cerebrovascular disease, the second leading cause of death behind heart disease and is a major cause of long-term disability worldwide. Currently, systemic immunomodulatory therapy based on intravenous cells is attracting attention. The immune response to acute stroke is a major factor in cerebral ischaemia (CI) pathobiology and outcomes. Over the past decade, the significant contribution of the spleen to ischaemic stroke has gained considerable attention in stroke research. The changes in the spleen after stroke are mainly reflected in morphology, immune cells and cytokines, and these changes are closely related to the stroke outcomes. Autonomic nervous system (ANS) activation, release of central nervous system (CNS) antigens and chemokine/chemokine receptor interactions have been documented to be essential for efficient brain-spleen cross-talk after stroke. In various experimental models, human umbilical cord blood cells (hUCBs), haematopoietic stem cells (HSCs), bone marrow stem cells (BMSCs), human amnion epithelial cells (hAECs), neural stem cells (NSCs) and multipotent adult progenitor cells (MAPCs) have been shown to reduce the neurological damage caused by stroke. The different effects of these cell types on the interleukin (IL)-10, interferon (IFN), and cholinergic anti-inflammatory pathways in the spleen after stroke may promote the development of new cell therapy targets and strategies. The spleen will become a potential target of various stem cell therapies for stroke represented by MAPC treatment.

Keywords: IL-10; Multipotent adult progenitor cells; Spleen; Stem cells; Stroke.

Fig. 1

Two cell therapeutic strategies for stroke. Replacement of necrotic cells and immunomodulation. Therapeutic stem cells have traditionally been known to differentiate into cells that make up nerve tissue to replace necrotic cells, thereby promoting nerve regeneration and angiogenesis. Recent studies have shown that the immune regulatory capacity of stem cells provides a favourable environment for nerve and vascular regeneration

Fig. 2

The main routes of administration of stem cell therapy for stroke. Although many preclinical studies and clinical applications have been carried out, the most adequate administration route for stroke is unclear. Each administration route has advantages and disadvantages for clinical translation to stroke patients. a Intranasal, b intracerebral, c intrathecal, d intra-arterial, e intraperitoneal and f intravenous

Fig. 3

Inflammation after stroke. DAMPs released from necrotic neurons activate macrophages through PRRs and the inflammasome. Activated macrophages enhance inflammation by releasing pro-inflammatory cytokines and recruiting T cells, which contribute to maintenance of inflammation through IL-17. DCs also activate and enhance antigen presentation to T cells. Gelatinase released by activated mast cells and MPP-9 produced by infiltrating neutrophils destroy the function of the BBB, resulting in brain oedema. Then, under the action of chemokines, leukocytes infiltrate into the damaged brain tissue, thereby expanding inflammation and injury. Several days after acute stroke, the cytokines produced by the innate immune system change to an anti-inflammatory phenotype, which contributes to inhibition of inflammation. The ratio and biodistribution of M1 and M2 microglia also changes, with anti-inflammatory M2 microglia becoming dominant again. Debris is cleaned up by microglia and macrophages. NSCs/NPCs are mobilised and migrate to the lesion. The environment becomes conducive to nerve regeneration, angiogenesis and BBB restructuring

Fig. 4

Changes in PIOs after stroke. Morphological and biochemical changes occur in the bone marrow, thymus, cervical lymph nodes and intestine after stroke and play respective roles in the stroke outcome

Fig. 5

Splenic sympathetic nerve-mediated anti-inflammatory pathway after stroke. After the acute stage of stroke, “brain-spleen cross-talk” not only inhibits the splenic inflammatory response by activating the SNS (reducing the production of inflammatory factors, such as TNF-α) [172] but also induces IL-10 production by lymphocytes in the spleen through activation of the NE-mediated PKA/cAMP pathway in other inflammatory-related disease models [174, 175]

Fig. 6

Therapeutic stem cells after stroke migrate to peripheral organs, such as the spleen. a After stroke, the patient was treated by intravenous injection of stem cells. b, c Stem cells in circulation are stimulated by IL-1β, IL-6 and TNF-α after stroke, and their chemokine ligand levels are elevated, thus enhancing the capability of stem cells to recruit inflammatory cells [194, 195]. d Through migration and adhesion, stem cells migrate rapidly to PIOs, such as the spleen, to play a regulatory role

Fig. 7

Effects of MSCs on various immune cells in circulation and in the spleen after stroke. a MSCs affect various immune cells in circulation and the spleen after stroke through soluble molecules and direct interactions [–200]. b T cell activation triggers the expansion of T cell clones and secretion of TNF-α and other inflammatory factors. Then, TNF-α activates the NF-κB signalling pathway in MSCs located in inflammatory environments through the TNF receptor, thereby inducing MSC-mediated immunosuppression [194, 201, 202]. c Similar to T lymphocytes, TNF-α produced by activation of the NF-κB signalling pathway in macrophages also induces MSC-mediated immunosuppression, which inhibits macrophage activation and converts macrophages from the pro-inflammatory M1 phenotype to the anti-inflammatory M2 phenotype by blockade of NF-κB. During this process, NF-κB signal activation in MSCs upregulates COX2 expression, which is turn increases the synthesis of prostaglandin E2 (PGE2). The secreted PGE2 binds to EP2 and EP4 receptors on macrophages, thereby increasing IL-10 secretion by macrophages to reduce inflammation [203, 204]

Fig. 8

Effects of Breg cells on other immune cells after stroke. Through IL-10, IL-35 and TGF-β production, Bregs can inhibit the differentiation of pro-inflammatory lymphocytes, such as TNF-α-secreting monocytes, IL-12-secreting dendritic cells, Th17 cells, Th1 cells, IL-17⁺ γδT cells and cytotoxic CD8⁺ T cells. Bregs can also induce the differentiation of immunosuppressive T cells, such as Foxp3⁺ Tregs and T regulatory 1 (Tr1) cells and contribute to the maintenance of iNKTs. Therefore, Bregs result in immune regulation at sites of inflammation, such as the CNS

Fig. 9

Stem cell therapy enhances recovery after stroke. In the untreated scenario, ischaemic stroke leads to activation of the peripheral immune system. During this process, the spleen atrophies, lymphocytes undergo apoptosis in the spleen, the inflammatory factor levels in the spleen increase, and inflammatory cells are released from the spleen into circulation. The antigen presentation of DCs is enhanced, and the levels of various chemokines are elevated. These pro-inflammatory mediators contribute to M1 microglia-mediated destruction of the BBB and CNS inflammation. Infiltration of leukocytes further aggravates inflammatory necrosis of neurons. Intravenous administration of stem cells reverses splenic atrophy and converts macrophages from the pro-inflammatory M1 phenotype to the anti-inflammatory M2 phenotype and Th cells from the pro-inflammatory Th1 phenotype to the anti-inflammatory Th2 phenotype. The inflammatory cytokine and cell levels in the spleen decrease, and anti-inflammatory cytokines and cells begin to be produced and released into circulation, which ultimately lead to less BBB restructuring and CNS inflammation and provide a favourable environment for nerve regeneration and angiogenesis