C1QB

C1QB encodes the B-chain of the C1q complement subcomponent and is expressed by microglia/macrophages, identified as the cellular source of brain C1qB expression following injury (PMID:1426121, PMID:7955342). C1qB is required for assembly and secretion of the complete C1q complex: a homozygous non-coding mutation abolishing C1qB mRNA and intracellular protein also reduced C1qA mRNA and produced total loss of C1q production (PMID:25454803). Its transcription is dynamically controlled — induced downstream of excitotoxic neuronal injury (PMID:7870303) and by neuronal COX-2 activity (PMID:11810182), positively regulated by direct TCF7L2 binding to the C1qB promoter (PMID:41534674), and negatively regulated by TGF-β1 in microglia (PMID:11074155). Functionally, C1qB promotes monocyte-to-macrophage differentiation, with knockdown reducing macrophage numbers and alleviating tissue damage in a diabetes model (PMID:36464147), and a TCF7L2→C1qB axis drives microglial activation and synaptic pruning-dependent neuronal injury in epilepsy (PMID:41534674). Loss-of-function C1qB mutations cause complete C1q deficiency with immunodeficiency/SLE-like disease (PMID:25454803).

1. 1992 Medium

   Established which brain cell type produces C1qB, a prerequisite for interpreting its role in CNS injury and inflammation.

   Evidence In situ hybridization with CR3 immunoreactivity in lesioned rat striatum

   PMID:1426121

   Open questions at the time

   • Does not address C1qB function or whether secreted C1q is assembled locally
   • Single injury model in one species
2. 1994 Medium

   Defined the developmental expression pattern of C1qB and argued against a role in programmed cell death, narrowing its candidate functions during brain development.

   Evidence Northern blot and in situ hybridization across rat brain development

   PMID:7955342

   Open questions at the time

   • Negative correlation does not exclude injury-specific roles
   • No loss-of-function test
3. 1994 Medium

   Placed C1qB induction causally downstream of excitotoxic neuronal injury, linking its expression to neurodegenerative signaling rather than constitutive baseline.

   Evidence Kainate lesion with barbiturate blockade in vivo plus glutamate-treated hippocampal cultures

   PMID:7870303

   Open questions at the time

   • Upstream inducing signal not molecularly identified
   • Whether C1qB induction is protective or injurious unresolved
4. 2000 Medium

   Identified TGF-β1 as a negative regulator of microglial C1qB expression, defining a cytokine input controlling C1qB transcription.

   Evidence Intraventricular TGF-β1 infusion in vivo and primary microglia cultures with ISH/immunocytochemistry

   PMID:11074155

   Open questions at the time

   • Direct vs indirect transcriptional mechanism not resolved
   • No promoter-level evidence
5. 2001 Medium

   Positioned COX-2 activity upstream of C1qB expression, connecting prostaglandin signaling to complement induction in neurons.

   Evidence Neuronal hCOX-2 transgenic mice with COX-2 inhibitor treatment and mRNA quantification

   PMID:11810182

   Open questions at the time

   • Mechanistic intermediary between COX-2 and C1qB transcription unknown
   • Neuronal C1qB expression here contrasts with microglial source
6. 2013 Low

   Demonstrated that C1qB loss-of-function mutations cause complete C1q deficiency and SLE-like disease, establishing clinical consequence of C1qB loss.

   Evidence Genetic sequencing and clinical complement assays in a patient with complete C1q deficiency

   PMID:24160257

   Open questions at the time

   • No in vitro functional reconstitution of the splicing defect
   • Single case report
7. 2014 Medium

   Showed that C1qB is mechanistically required for assembly and secretion of the entire C1q complex, not merely a passive subunit.

   Evidence Deep sequencing, western blot, ELISA, hemolytic assay and qPCR of C1q chains in a patient

   PMID:25454803

   Open questions at the time

   • Single patient
   • Structural basis of assembly dependence not defined
8. 2022 Medium

   Assigned C1qB a functional role in promoting monocyte-to-macrophage differentiation with disease relevance beyond the CNS.

   Evidence siRNA silencing in cultured monocytes and lentiviral knockdown in a rat T1DM model with histological assessment

   PMID:36464147

   Open questions at the time

   • Molecular pathway linking C1qB to differentiation unknown
   • Single lab and model
9. 2026 Medium

   Established a direct TCF7L2→C1qB transcriptional axis driving microglial activation and synaptic pruning-dependent neuronal injury in epilepsy.

   Evidence ChIP and luciferase reporter assays plus lentiviral Tcf7l2 knockdown in a kainic acid epilepsy mouse model

   PMID:41534674

   Open questions at the time

   • Whether C1q-mediated pruning is the sole effector mechanism not isolated
   • Other promoter regulators not surveyed
10. 2025 Low

    Provided single-cell resolution confirming microglia-exclusive C1QB expression and upregulation in Alzheimer's disease with sex differences.

    Evidence Single-nucleus RNA sequencing of human hippocampus (preprint)

    Open questions at the time

    • No functional validation of C1qB protein activity
    • Preprint, single study
    • Causal role in AD not tested

• How C1q produced by C1qB-expressing microglia mechanistically executes synaptic pruning and whether this is protective or pathogenic across disease contexts remains unresolved.
• No structural model of how C1qB drives C1q assembly
• Effector pathway from C1q deposition to neuronal injury not isolated
• Reconciliation of neuronal vs microglial C1qB expression unresolved