SIRPA

Detailed Information

SIRPA (Signal Regulatory Protein Alpha) is located on the human chromosome 20p13 region and belongs to the immunoglobulin superfamily. The gene encodes the SIRPα protein, a transmembrane glycoprotein, which has structural features including three immunoglobulin-like domains (one V-set and two C1-set domains), a transmembrane region, and an intracellular domain. The intracellular domain contains four tyrosine phosphorylation sites that form the immunoreceptor tyrosine-based inhibition motif (ITIM), which is crucial for mediating inhibitory signals. The primary ligand for SIRPα is CD47, which is widely expressed on the surface of normal cells. This interaction plays an important role in the "self-recognition" mechanism. Bioinformatic analysis indicates that the SIRPA gene has multiple paralogous genes (such as SIRPB1 and SIRPG), which form tightly clustered gene families on the chromosome, gaining functional diversity through gene duplication and divergent evolution.

The expression regulation of SIRPα is cell-type specific. In myeloid cells (such as macrophages and dendritic cells), the transcription factors PU.1 and C/EBPα directly bind to the SIRPA promoter region to drive its high expression. In neurons, SOX9 and NFIA regulate SIRPα expression. Notably, the expression of SIRPA is regulated by its pseudogene, SIRPAP1, which affects SIRPA mRNA stability through competitive binding to microRNA. On the protein level, SIRPα undergoes tyrosine phosphorylation, and the phosphorylation state determines its ability to recruit SHP-1/SHP-2 phosphatases, which regulate downstream signaling. Additionally, the distribution of SIRPα on the cell membrane exhibits dynamic changes, and lipid raft microdomains can enhance its signaling efficiency. These complex regulatory mechanisms ensure that SIRPα plays a precise regulatory role in immune homeostasis.

Dual Role in Immune Regulation

SIRPα plays a dual role in the immune system, with its function depending on the cellular environment. In myeloid cells, the SIRPα-CD47 signaling axis serves as a classic "don't eat me" signal, inhibiting macrophage phagocytosis to maintain self-tolerance. When SIRPα binds to CD47, its intracellular ITIM domain gets phosphorylated, recruiting and activating SHP-1/SHP-2 phosphatases, leading to the dephosphorylation of myosin IIA, which inhibits phagocytosis and synapse formation. This mechanism prevents macrophages from phagocytosing normal blood cells, maintaining blood homeostasis. However, tumor cells often overexpress CD47, hijacking this pathway to escape immune surveillance. Studies have shown that blocking the CD47-SIRPα interaction can enhance macrophage phagocytosis of tumor cells, offering a potential strategy for tumor immunotherapy.

Figure 1. Biological significance of the CD47-SIRPA axis in macrophage homeostasis and the influ ences of CD47/SIRPA blockade on macrophage polarization. (Tang XX, et al., 2024)

In adaptive immunity, the role of SIRPα is more complex. On dendritic cells, SIRPα binding to CD47 suppresses the maturation process mediated by TLR signaling and reduces pro-inflammatory cytokine secretion. This negative regulation is critical for maintaining peripheral tolerance, but it also facilitates tumor immune evasion. In contrast, SIRPα on T cells exhibits co-stimulatory activity. T cells lacking SIRPα show a weakened proliferative response to antigen stimulation, with reduced IL-2 secretion, suggesting that SIRPα plays a positive regulatory role in T cell activation. This cell-type-dependent functional difference was confirmed in melanoma research: intrinsic SIRPα in tumor cells enhances the effectiveness of anti-PD-1 therapy, while SIRPα in myeloid cells exerts immunosuppressive effects. This functional duality presents a challenge for the precise targeting of SIRPα.

Key Role in the Nervous System

SIRPα is highly expressed in the nervous system, participating in synapse formation and functional regulation. During development, SIRPα promotes neuronal growth and branching by binding trans to CD47 on the neuron surface. In vitro experiments have shown that SIRPα-Fc fusion protein significantly enhances dendritic complexity in hippocampal neurons. At the synaptic level, SIRPα interacts with the postsynaptic density protein PSD-95, regulating NMDA receptor clustering and function, thus influencing synaptic plasticity. In Alzheimer's disease (AD), SIRPα and CD47 together form a synaptic protective signal, with decreased expression of this pathway closely linked to synapse loss.

In the pathological process of AD, SIRPα expression shows age- and sex-dependent changes. Large-scale transcriptome analysis of 3849 human brain samples showed that complement system genes (C1QA/B/C, C3) and microglial regulators (ITGB2/ITGAM) are commonly upregulated, while the expression of SIRPA and CD47, the "brake signals," is downregulated. Notably, complement gene expression in male brains positively correlates with AD pathology, whereas females show higher baseline expression in the non-dementia stage, with this difference disappearing after AD onset. Additionally, SIRPα is involved in microglia-mediated synaptic pruning, and dysfunction of this process may lead to neurodegeneration. These findings highlight the protective role of SIRPα in neurodegenerative diseases.

Angiogenesis and Tissue Repair

The SIRPα-CD47 signaling pathway plays a critical role in endothelial cell function regulation. It is involved in maintaining vascular homeostasis and preventing pathological angiogenesis. In ischemic disease models, blocking CD47 enhances tissue angiogenesis and improves blood perfusion. Mechanistically, the CD47-SIRPα interaction inhibits endothelial cell migration and lumen formation, while blocking this signal activates the NO/cGMP pathway to promote angiogenesis. Additionally, SIRPα is involved in anti-angiogenic signaling mediated by thrombospondin-1 (THBS1), which activates NADPH oxidase and produces reactive oxygen species (ROS) to inhibit endothelial cell function.

During tissue repair, SIRPα regulates macrophage polarization towards the M2 (repair) phenotype. In injury models, SIRPα knockout mice exhibit delayed repair and increased fibrosis, which is related to impaired M2 macrophage differentiation. In myocardial infarction models, SIRPα-deficient mice show poor heart function recovery and reduced angiogenesis in the infarct area, indicating its involvement in cardiac repair. These studies reveal the multifaceted role of SIRPα in tissue homeostasis and repair, providing new therapeutic targets for ischemic and fibrotic diseases.

Antibody Drug Development Progress

Therapeutic antibodies targeting the SIRPα-CD47 axis have become a hotspot in tumor immunotherapy. Current development strategies are divided into three categories: anti-CD47 antibodies, anti-SIRPα antibodies, and bispecific molecules. Anti-CD47 antibodies (such as Magrolimab) enhance macrophage phagocytosis by blocking the CD47-SIRPα interaction. Phase I clinical trials show that Magrolimab combined with azacitidine in treating myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) yields objective response rates of 60-90%. However, some patients experience anemia as a side effect due to high CD47 expression on red blood cells. To reduce side effects, next-generation anti-CD47 antibodies have been developed with differentiated binding characteristics, achieving high affinity binding in the tumor microenvironment under acidic pH conditions, while showing reduced affinity in normal tissues under neutral pH conditions.

Anti-SIRPα antibodies have potential advantages, as their expression is more restricted (primarily in myeloid cells). SRF231, a fully human anti-SIRPα antibody, blocks all SIRPα variants from binding to CD47. Phase I clinical trials show that it is well tolerated in advanced solid tumor patients, with no severe hematological toxicities. Another strategy is to develop allosteric anti-SIRPα antibodies, such as GS0189, which does not compete with the CD47 binding interface but inhibits signaling through an allosteric effect, preserving other functions of SIRPα (such as neuroprotection). Additionally, bispecific antibodies, such as SIRPα×PD-L1 bispecifics, have entered clinical research stages, targeting both myeloid and tumor cells to enhance anti-tumor immunity.

Small Molecule Inhibitors and Combination Strategies

In addition to antibody drugs, small molecule inhibitors targeting downstream SIRPα signaling have also made progress. Small molecule inhibitors targeting SHP-1/2 phosphatases (such as SHP099) block the inhibitory signals mediated by SIRPα, enhancing macrophage function. Preclinical models show that SHP099 combined with radiotherapy significantly inhibits tumor growth. Another strategy is to develop SIRPα dimerization inhibitors that disrupt the clustering of SIRPα on the cell membrane, reducing its signaling. These small molecules have the advantage of oral bioavailability, making them suitable for long-term use.

Combination therapy is key to improving efficacy. Preclinical studies show that SIRPα blockade has synergistic effects with radiotherapy, chemotherapy, or targeted therapy. In colorectal cancer models, Sirpα knockout combined with anti-PD-L1 significantly inhibits tumor progression. Additionally, SIRPα blockade enhances the efficacy of CAR-M (chimeric antigen receptor macrophage) therapy. CAR-M cells expressing dominant-negative receptors for SIRPα resist CD47-mediated inhibitory signals, enhancing infiltration and cytotoxicity against solid tumors. These innovative combination strategies provide new insights for overcoming tumor immune suppression.