Mechanistic Basis of Complement C5 Inhibition in Generalized Myasthenia Gravis

In generalized myasthenia gravis (gMG), the therapeutic rationale for complement C5 inhibition is grounded in a well-defined immunopathological cascade. In patients who are AChR antibody-positive, pathogenic antibodies not only impair receptor function, but also activate the classical complement pathway, thereby causing additional damage to the postsynaptic membrane at the neuromuscular junction. Because C5 occupies a critical node in the terminal complement pathway, it serves as an effective target for interrupting the execution layer of tissue injury.

Keywords: generalized myasthenia gravis; AChR antibody; complement; C5; membrane attack complex; neuromuscular junction; terminal complement inhibition

1 Pathological Basis

1.1 Characteristics of Pathogenic Antibodies

(1) Basis of antibody subclasses

AChR antibody-positive myasthenia gravis is primarily associated with IgG1 and IgG3, both of which have strong complement-activating capacity and therefore are more likely to trigger the classical complement pathway locally at the motor endplate.

(2) Types of pathogenic effects

The pathogenic effects of AChR antibodies are mainly manifested in three aspects: first, direct interference with acetylcholine receptor function; second, promotion of receptor cross-linking, internalization, and degradation; and third, activation of complement leading to endplate membrane injury. Among these, complement-mediated terminal damage constitutes the direct mechanistic basis for C5 inhibition.

1.2 Vulnerability of the Neuromuscular Junction

(1) Structural features of the endplate

The postsynaptic membrane of the neuromuscular junction has a highly folded architecture, with densely clustered AChRs and a highly ordered local distribution of ion channels, representing a highly specialized membrane structure.

(2) Safety factor characteristics

Neuromuscular transmission depends on a certain safety factor for maintenance. Even mild endplate injury can reduce the efficiency of action potential triggering, making this structure highly sensitive to immune-mediated membrane damage.

(3) Characteristics of generalized involvement

In generalized disease, the abnormality is not restricted to the endplates of a single muscle group. Instead, neuromuscular junctions across multiple muscle groups are simultaneously maintained at a low safety factor, making the clinical consequences of cumulative complement injury more apparent.

2 Complement-Mediated Injury Cascade

2.1 Initiation of the Classical Pathway

(1) Formation of immune complexes

After AChR antibodies bind to endplate receptors, they form local immune complex conformations suitable for C1q recognition.

(2) Amplification through the complement cascade

Following C1q binding, the classical complement pathway is activated, leading sequentially to formation of the C3 convertase and C5 convertase, thereby amplifying a local antibody-binding event into a terminal complement reaction.

2.2 C5 Cleavage and Terminal Injury

(1) C5 cleavage products

After C5 is cleaved, C5a and C5b are generated. The former participates in local amplification of inflammation, while the latter initiates assembly of the terminal complement complex.

(2) Formation of the membrane attack complex

C5b binds sequentially to C6, C7, C8, and C9 to form C5b-9, namely the membrane attack complex.

(3) Structural consequences at the endplate

Once the membrane attack complex is deposited on the postsynaptic membrane, it can lead to simplification of junctional folds, structural disruption of the postsynaptic membrane, further loss of AChRs, and a sustained decline in the safety factor of neuromuscular transmission.

Table 1. Complement Injury Cascade in AChR Antibody-Positive Generalized Myasthenia Gravis

Pathological level	Key event	Main consequence
Antibody binding level	AChR antibodies bind endplate AChRs	Establishes the basis for complement initiation
Classical pathway level	C1q-mediated complement activation	Local complement amplification at the endplate
C5 node level	C5 is cleaved into C5a and C5b	Amplification of inflammation and initiation of the terminal pathway
Terminal execution level	Formation and deposition of C5b-9	Structural injury to the postsynaptic membrane
Functional outcome level	Reduced AChR and simplified endplate architecture	Failure of neuromuscular transmission

3 Mechanistic Significance of C5 as a Therapeutic Target

3.1 Terminal Node Property

(1) Hierarchical position of the target

C5 is not a distal peripheral regulatory factor, but rather the key node immediately upstream of the stage at which the terminal complement pathway enters the tissue injury phase.

(2) Interventional value

The significance of blocking C5 lies in simultaneously reducing C5a generation, preventing production of C5b, and further inhibiting continued assembly of C5b-9. Thus, C5 inhibition is not merely a means of lowering inflammatory intensity, but of directly interrupting the execution process of endplate injury.

3.2 Distinction from Other Therapeutic Levels

(1) Different from cholinesterase inhibition

Cholinesterase inhibitors improve the availability of acetylcholine in the synaptic cleft and therefore act at the level of transmission compensation.

(2) Different from general immunosuppression

Conventional immunosuppression mainly targets immune activation and antibody production and therefore acts at the upstream control layer.

(3) Different from antibody depletion strategies

FcRn-targeted intervention primarily reduces the circulating IgG burden and therefore acts at the antibody-pool intervention layer.

(4) Positioning of C5 inhibition

C5 inhibition acts directly at the execution layer of tissue injury, with the advantage of stronger pathological specificity.

Table 2. Mechanistic Positioning of C5 Inhibition in Myasthenia Gravis

Intervention level	Primary target	Main effect	Mechanistic positioning
Symptomatic level	Acetylcholine in the synaptic cleft	Improves transmission compensation	Functional compensation
Immune control level	Immune cells and antibody generation	Reduces immune drive	Upstream control
Antibody level	Circulating IgG pool	Reduces pathogenic antibody burden	Intermediate layer
Terminal complement level	C5 and C5b-9 formation	Blocks endplate membrane injury	Execution layer

4 Changes at the Neuromuscular Junction After C5 Inhibition

4.1 Structural Changes

(1) Blocking new MAC deposition

After C5 inhibition, new membrane attack complexes can no longer continue to deposit in the endplate region.

(2) Terminating ongoing destruction

The endplate is no longer continuously exposed to terminal complement attack, and the rate of further collapse of the postsynaptic membrane is markedly reduced.

4.2 Functional Changes

(1) Recovery of the safety factor

Although C5 inhibition cannot immediately restore all endplate structures, it can improve the transmission safety margin of the neuromuscular junction by blocking ongoing injury.

(2) More evident benefit in high-demand muscle groups

Bulbar swallowing muscles, speech muscles, neck muscles, and respiratory-related muscles are highly sensitive to changes in the endplate safety factor; therefore, clinical improvement often first becomes evident in these muscle groups.

5 Mechanistic Boundaries of the Applicable Population

5.1 Requirement for AChR Antibody Positivity

(1) Mechanistic matching

C5 inhibition is most consistent with the pathological mechanism of AChR antibody-positive generalized myasthenia gravis, because this subtype has a clearly complement-dependent basis of terminal endplate injury.

(2) Subtype differences

MuSK antibody-associated myasthenia gravis is characterized primarily by impairment of neuromuscular junction organization and maintenance rather than by typical terminal complement-mediated injury; therefore, the logic of C5 inhibition cannot be applied mechanically to that subtype.

5.2 Generalized Active Disease

(1) Stronger pathological progression

When patients continue to exhibit clear disease activity, this suggests that endplate injury is likely still progressing. Under such circumstances, C5 blockade is more likely to demonstrate direct mechanistic benefit.

(2) Widespread muscle involvement depends more on execution-layer blockade

Generalized patients often involve multiple high-demand muscle groups. Once terminal complement is effectively blocked, clinical improvement is more readily observed.

6 Problems Addressed and Not Addressed by C5 Inhibition

6.1 Aspects Already Addressed

(1) Terminal complement injury

C5 inhibition can markedly reduce formation of the membrane attack complex and ongoing destruction of the endplate membrane.

(2) Structural deterioration process

By preventing new terminal complement deposition, C5 inhibition can slow the continued deterioration of the neuromuscular junction.

6.2 Aspects Not Directly Addressed

(1) Antibody source

C5 inhibition does not directly eliminate B cells or plasma cells, nor does it directly reduce antibody production.

(2) Thymus-related immune abnormalities

Its point of action is downstream of the immune source; therefore, it does not constitute immune reconstitution therapy.

(3) All MG subtypes

C5 inhibition is not a universally mechanistic solution for all myasthenia gravis subtypes.

7 Mechanistic Basis of Safety Management

7.1 Infection Risk

(1) Host defense role of terminal complement

The terminal complement pathway plays a critical role in defense against infections such as Neisseria meningitidis.

(2) Target-related risk

After C5 is inhibited, the host’s natural defense capacity against related pathogens declines. Therefore, infection risk is a mechanism-based risk rather than an incidental adverse reaction.

7.2 Consistency Between Risk and Target Engagement

This risk, in turn, confirms that the drug truly acts at the level of terminal complement. For myasthenia gravis, this means that the treatment is hitting the intended execution node of tissue injury.

8 Therapeutic Positioning from a Mechanistic Perspective

8.1 Precision stratification property

C5 inhibition is not a broad-spectrum immunosuppressive strategy in a general sense, but rather a precise intervention directed at the terminal complement execution layer in AChR antibody-positive generalized myasthenia gravis.

8.2 Preconditions for use

What truly determines whether C5 inhibition is appropriate is not simply whether a patient has been diagnosed with myasthenia gravis, but whether the current pathology is continuously driven by AChR antibodies and terminal complement activity.

9 Product Tables Related to Mechanistic Studies

Table 3. Core Target Product Table for C5 Inhibition

Product type	Catalog No.	Name	CAS No.	Grade and Purity	Suitable research direction/use
Anti-C5 monoclonal antibody	Ab170870	Eculizumab (anti-Complement C5)	219685-50-4	Carrier Free, Recombinant, ExactAb™, Low Endotoxin, Azide Free, Validated, Animal Free, ≥95%(SDS-PAGE&SEC-HPLC), See COA	Suitable for C5 blockade and mechanistic validation of terminal complement inhibition in generalized myasthenia gravis
Anti-C5 monoclonal antibody	Ab176615	Ravulizumab (anti-Complement C5)	1803171-55-2	Carrier Free, Recombinant, ExactAb™, Low Endotoxin, Azide Free, Validated, Animal Free, ≥95%(SDS-PAGE&SEC-HPLC), See COA	Suitable for long-acting C5 blockade studies
Anti-C5 monoclonal antibody	Ab182862	Crovalimab (anti-Complement C5)	1917321-26-6	Carrier Free, Recombinant, ExactAb™, Low Endotoxin, Azide Free, Validated, Animal Free, ≥95%(SDS-PAGE&SEC-HPLC), See COA	Suitable for studies of C5-targeted blockade
Anti-C5 monoclonal antibody	Ab182838	Pozelimab (anti-Complement C5)	2096328-94-6	Carrier Free, Recombinant, ExactAb™, Low Endotoxin, Azide Free, Validated, Animal Free, ≥95%(SDS-PAGE&SEC-HPLC), See COA	Suitable for mechanistic validation of C5 blockade
Anti-C5 monoclonal antibody	Ab191930	Lendalizumab (anti-Complement C5)	2210314-30-8	Carrier Free, Recombinant, ExactAb™, Low Endotoxin, Azide Free, Validated, Animal Free, ≥95%(SDS-PAGE&SEC-HPLC), See COA	Suitable for mechanistic studies targeting C5
Anti-C5 monoclonal antibody	Ab176617	Tesidolumab (anti-Complement C5)	1531594-08-7	Carrier Free, Recombinant, ExactAb™, Low Endotoxin, Azide Free, Validated, Animal Free, ≥95%(SDS-PAGE&SEC-HPLC), See COA	Suitable for terminal complement blockade studies
Anti-C5/albumin bispecific tool	Ab182984	Gefurulimab (anti-C5&Albumin)	2456407-94-4	Animal Free,Carrier Free,Recombinant,ExactAb™,Low Endotoxin,Azide Free,Validated,PBS Only,≥90%(SDS-PAGE&SEC-HPLC),See COA	Suitable for studies of C5 targeting and long-circulation strategies
C5 inhibitory peptide	rp175435	zilucoplan	1841136-73-9	Moligand™	Suitable for mechanistic studies of C5 inhibitory peptides
Small-molecule C5 inhibitor	C649199	Complement C5-IN-1	2365402-67-9	≥99%	Suitable for small-molecule inhibition studies of terminal complement
Small-molecule C5 inhibitor	C655749	Complement C5-IN-1	2365402-67-9	10mM in DMSO	Suitable for C5 inhibition experiments in cellular or in vitro systems
Target protein	C414658	C5 Protein	80295-53-0	BioReagent,PBS Only,1.0mg/ml,0.22 µm filtered	Suitable for C5 binding, blockade, and functional reconstitution experiments
Target protein	np001101	Complement C5 from Human Plasma	80295-53-0	BioReagent,Native,PBS Only,≥95%(SDS-PAGE),Protein concentration: See COA	Suitable for studies of native C5 function and blockade
Target protein	rp173718	complement C5	—	Moligand™	Suitable for C5-related mechanistic studies
Target detection antibody	G501431	Goat Anti-Human C5	—	＞40 mg/mL total protein concentration	Suitable for C5 detection and binding validation
Gene intervention tool	C1476488	C5 Human Pre-designed siRNA Set A	—		Suitable for C5 gene silencing studies

Table 4. Product Table for Terminal Complement Activation and Injury Readouts

Product type	Catalog No.	Name	Grade and Purity	Suitable research direction/use
C5 cleavage product	rp173611	C5a	Moligand™	Suitable for studies of the inflammatory branch downstream of C5 cleavage
C5 cleavage product	C414661	C5a Anaphylatoxin (Not Recombinant)	0.5 mg/mL,0.22 µm filtered	Suitable for functional studies of C5a
C5 cleavage product	C1433955	C5a Anaphylatoxin (human)		Suitable for functional studies of human C5a
C5 cleavage product	rp173613	C5a des-Arg	Moligand™	Suitable for studies of the C5a degradation product
C5a detection antibody	R501101	Rabbit Anti-Human rC5a	＞40 mg/mL total protein concentration	Suitable for C5a detection
C5b-6 complex	C414665	C5b,6 Complex	0.2 mg/mL,0.22 µm filtered	Suitable for studies of terminal complement assembly
SC5b-9 complex	S501175	SC5b-9 Complex	Moligand™,PBS Only,1.0 mg/mL,0.22 µm filtered	Suitable for functional studies of terminal complement complexes
SC5b-9 detection antibody	R501371	Rabbit Anti-Human SC5b-9 Neoantigen	High Performance,PBS Only,5 mg/mL purified IgG concentration,0.09 % sodium azide, filtered through 0.22 µm filter	Suitable for detection of SC5b-9 deposition
Human C5c ELISA	EJ1514891	Human Complement C5 Convertase (C5c) ELISA Kit	BioReagent	Suitable for assessment of upstream activation of C5 cleavage
Human C5 ELISA	EJ1514903	Human Complement Component 5 (C5) ELISA Kit	BioReagent	Suitable for quantitative detection of C5
Human C5a ELISA	EJ1514904	Human Complement Component 5a (C5a) ELISA Kit	BioReagent	Suitable for quantitative detection of C5a
Human C5b ELISA	EJ1514909	Human Complement Fragment 5b (C5b) ELISA Kit	BioReagent	Suitable for quantitative detection of C5b
Human SC5b-9 ELISA	EJ1514182	Human Terminal Complement Complex C5b-9 (Sc5b-9) ELISA Kit	BioReagent	Suitable for quantitative detection of terminal complement complexes
Rat C5a ELISA	EJ1512331	Rat Complement Fragment 5a (C5a) ELISA Kit	BioReagent	Suitable for rat C5a detection
Rat SC5b-9 ELISA	EJ1512084	Rat Terminal Complement Complexes (SC5b-9) ELISA Kit	BioReagent	Suitable for rat SC5b-9 detection
Mouse C5a ELISA	EJ1513240	Mouse Complement Fragment 5a (C5a) ELISA Kit	BioReagent	Suitable for mouse C5a detection
Mouse C5 ELISA	EJ1513245	Mouse Complement Component 5 (C5) ELISA Kit	BioReagent	Suitable for mouse C5 detection
Mouse C5b-9 ELISA	EJ1512823	Mouse Terminal Complement Complex C5b-9 (TCC C5b-9) ELISA Kit	BioReagent	Suitable for detection of mouse terminal complement complexes

Table 5. Product Table for AChR-Classical Complement Initiation and Endplate Validation

Product type	Catalog No.	Name	Grade and Purity	Suitable research direction/use
Classical pathway initiating protein	C409490	C1q Protein from Normal human serum	ActiBioPure™, Bioactive, Native, High Performance, ≥98%(SDS-PAGE), See COA	Suitable for studies of AChR antibody-mediated classical complement initiation
C1q blocking antibody	Ab177901	ANX005 (anti-C1q)	Carrier Free, Recombinant, ExactAb™, Low Endotoxin, Azide Free, Validated, Animal Free, ≥95%(SDS-PAGE&SEC-HPLC), See COA	Suitable for blockade studies at the C1q initiation level
C1q detection antibody	G501295	Goat Anti-Human C1q	＞40 mg/mL total protein concentration	Suitable for human C1q detection
C1q detection antibody	G409463	Goat Anti-Mouse C1q	＞40 mg/ml total protein concentration	Suitable for mouse C1q detection
Human C1q ELISA	EJ1514888	Human Complement 1q (C1q) ELISA Kit	BioReagent	Suitable for quantitative detection of human C1q
Mouse C1q ELISA	EJ1513234	Mouse Complement 1q (C1q) ELISA Kit	BioReagent	Suitable for quantitative detection of mouse C1q
AChR antigen peptide	A1438154	AChRα(97-116)		Suitable for AChR-related antibody recognition and antigen peptide studies
AChR antigen peptide	A1438091	AChRα(97-116) TFA		Suitable for AChR antigen epitope studies
Human N-AChR ELISA	EJ1514280	Human Nicotinic Acetylcholine Receptor(N-AChR) ELISA Kit	BioReagent	Suitable for AChR-related quantitative detection
Mouse AChR ELISA	EJ1512620	Mouse Acetylcholine Receptor (AChR) ELISA Kit	BioReagent	Suitable for mouse AChR detection
Endplate labeling tool	B1437796	Biotin-α-Bungarotoxin		Suitable for labeling AChR clustering and endplate localization
Endplate labeling tool	B1438346	α-Bungarotoxin, FITC labeled		Suitable for fluorescent labeling of neuromuscular junction AChRs
Endplate labeling tool	rp175211	[³H]α-bungarotoxin	Moligand™	Suitable for AChR binding analysis
Endplate labeling tool	rp175028	[¹²⁵I]α-bungarotoxin	Moligand™	Suitable for radioligand binding detection of AChR

The mechanistic basis of complement C5 inhibition in generalized myasthenia gravis lies in its direct action on the terminal execution layer of neuromuscular junction injury. In AChR antibody-positive patients, antibody binding, classical complement activation, C5 cleavage, and membrane attack complex formation constitute a continuous pathological chain, and C5 is positioned at the most suitable point in that chain for pharmacological interruption.