The complement system is a key component of the innate immune system that mediates the clearance of pathogens, apoptotic cells, and cellular debris. However, the complement system also has diverse roles in the central nervous system (CNS), where it regulates synaptic pruning, neural plasticity, and neuroinflammation. Dysregulation of the complement system has been implicated in various neurodegenerative disorders such as Alzheimer’s disease, multiple sclerosis, epilepsy, stroke, and traumatic brain injury. In these conditions, excessive or chronic activation of the complement system may lead to synaptic loss, neuronal damage, immune dysregulation, and inflammation, which leads to exacerbating the disease’s progression and severity. Moreover, the complement system may interact with infectious agents that invade the CNS, such as bacteria, viruses, fungi, and parasites, and modulate their pathogenicity and host response. Therefore, understanding the complex interplay between the complement system and the CNS is crucial for developing novel therapeutic strategies to prevent or treat neurodegenerative and neuroimmune disorders. Natural compounds, such as plant extracts, phytochemicals, and nutraceuticals, have emerged as promising candidates for modulating the complement system and its effects on the CNS. These compounds may exert anti-inflammatory, antioxidant, neuroprotective, and immunomodulatory effects by regulating the expression of various complement components and pathways. In this review, we summarized the current knowledge on the roles of the complement system in human neurodegenerative disorders and the benefits of natural compounds for complement-targeted therapy.

Complement is both evolutionary and scientifically old. It predates the adaptive immunity by some 600 million years and was first described in 1905 by Jules Bordet and Paul Ehrlich. For the most of its, the existence complement system has been ignored by most scientists and clinicians due to the perception of it being complicated and its relevance for the pathogenesis of human disease being unclear. With the recent US Food and Drug Administration (FDA) approvals of pegcetacoplan for both paroxysmal nocturnal haemoglobinuria (PNH) and geographic atrophy (GA), avacincaptad pegol for GA and iptacopan and danicopan for PNH, we are at a crucial juncture for complement-targeting therapies. A number of companies and academic institutions are developing next-generation complement therapies, which is resulting in an increasingly competitive landscape. If one looks at the serum complement cascade, all 3 pathways now have biotechnology or pharmaceutical industry players with 1 or multiple clinical-stage inhibitors that are expected to be FDA approved within the next few years. Furthermore, with the limited number of clinically validated targets in complement-mediated disease, the competition in this space is set to further intensify in the coming years. In this review, we will discuss the timeline of the academic discoveries that led to the development of the current crop of FDA-approved complement therapeutics. We follow with a discussion of an increasingly crowded complement therapy space and of the scientific advances that have emerged in recent two decades underpinning future innovation, including advances in our understanding of complement biology, such as local and intracellular complement, emerging complement targets, combinational approaches of complement and non-complement therapeutics to unlock new disease indications and new technologies such as gene therapy. We will also give a comprehensive overview of the gene therapy landscape and how it can be utilized to target complement dysregulation.

The complement component C1q plays a role as a pro-angiogenic factor in different contexts, acting in a complement-independent way. For example, this molecule is able to foster the remodeling of the spiral arteries for a physiological pregnancy and to promote the wound healing process. It is also involved in angiogenesis after post-stroke ischemia. Furthermore, it has a role in supporting the tumor vessel growth. Given its role in promoting angiogenesis both under physiological and pathological situations, other studies are needed to understand its potential therapeutic implications.