A new member at the table: Granzyme K as a lymphocyte-born complement initiator

The complement system, classically defined by its three canonical activation pathways, that is, classical, lectin, and alternative has long been considered a liver-derived, plasma-resident immune surveillance system. These pathways converge at the cleavage of complement (C3) and C5, generating potent effectors, such as C3a, C3b, C5a, and C5b that orchestrate immune clearance via inflammation, opsonisation, and membrane attack complex (MAC) formation^{1, 2} (Figure 1a–c). However, emerging evidence over the past decade has radically reshaped this view. We now recognise a spectrum of non-canonical complement activation routes including intracellular complosome activity, proteolytic cleavage by thrombin, kallikrein, cathepsins, and redox-driven activation that integrate complement into broader immune, metabolic and stress response networks.^2-9

A recent study by Donado et al.¹⁰ introduced a novel addition to this expanding repertoire granzyme K (GZMK), a serine protease released by cytotoxic lymphocytes, as a direct initiator of complement activation. In contrast to thrombin, kallikrein, and cathepsin, which bypass early steps to directly cleave C3 and C5 intracellularly,^{3, 7, 9} GZMK uniquely cleaves C4 and C2 on the cell surface, generating the classical C3 convertase (C4b2a) and C5 convertase (C4b2a3b) independent of antibody–antigen complexes or MBL/Ficolin. This places GZMK mechanistically alongside C1r and C1s and MASPs but fundamentally distinct in origin and context cell-autonomous, lymphocyte-derived, and recognition-independent (Figure 2a–f). This discovery reframes complement as not just a fluid-phase sentinel but also a cellularly initiated effector system.

Granzymes, a conserved family of serine proteases (GZMA, GZMB, GZMK, GZMM, GZMH in humans), are best known for their role in perforin-mediated cytotoxicity by CD8⁺ T cells and natural killer (NK) cells.¹¹ Yet beyond cytolysis, granzymes increasingly appear as modulators of inflammation and tissue remodelling. GZMB, for instance, cleaves extracellular matrix proteins and promotes autoantigen formation in autoimmune diseases.^12-17 GZMA, GZMB, and GZMM are implicated in viral control and inflammation, often via extracellular routes.

Amongst them, GZMK stands out as a non-cytolytic effector with immunoregulatory and pro-inflammatory properties. It is expressed in γδ T cells, invariant natural killer T (NKT) cells, and CD56bright⁺ NK cells and is upregulated in aging and chronic immune activation replication.^18-20 Structurally homologous to trypsin and GZMA, GZMK contains a unique heparin-binding domain that enables its interaction with cell-surface heparan sulphate proteoglycans (HSPGs).^21-25 It cleaves substrates such as SET nuclear proto-oncogene (a nucleosome assembly protein (SET), heterogeneous nuclear ribonucleoprotein K (hnRNP K), and tubulins, affecting chromatin structure, mRNA metabolism, and cell integrity.^{26, 27} Notably, GZMK⁺ CD8⁺ T cells have been detected in a range of inflammatory diseases, such as Crohn's disease, lupus nephritis, rheumatoid arthritis, asthma, Alzheimer's disease, and cancer and their frequency increases with age, suggesting a role in inflammaging and chronic pathology.^28-32

Donado et al. further demonstrated that GZMK-deficient mice exhibit reduced complement activation and attenuated inflammation in models of dermatitis and arthritis. In human synovium, GZMK⁺ T cells co-localise with complement deposition, supporting their role as drivers of tissue injury through localised complement activation.¹⁰

Heparan sulphate (HS), a structurally diverse glycosaminoglycan present on virtually all cell surfaces and extracellular matrices, plays central roles in development, tissue repair, and immunity.^{33, 34} It also interfaces extensively with the complement system, binding numerous components (C1q, C4b, C5, Factor B, and H, MASPs, component C1q receptor, CR3, and CR) and modulating their function.^35-39 Notably, HS serves as a critical binding platform for both GZMK and complement, anchoring the former to the cell surface and facilitating localised cleavage of C4 and C2.¹⁰

This convergence suggests a broader immunological role for HS-rich microenvironments, such as inflamed tissues and mucosal surfaces as hubs for localised complement activation driven by lymphocyte effector proteins. HS may thus act not only as a structural scaffold but also as a biochemical amplifier of immune effector cascades under pathological conditions.

The identification of GZMK as a complement-activating protease opens new therapeutic opportunities but also raises critical challenges. GZMK is a double-edged sword, it promotes chronic inflammation via complement activation but also contributes to host defence against viruses and tumours.^{17, 40} Its substrates, such as hnRNP K are essential for RNA processing and cell survival,^{26, 27} underscoring the potential for unintended toxicity if targeted indiscriminately.

Similarly, systemic complement inhibition (e.g., C5 blockade) has transformed the treatment of diseases like paroxysmal nocturnal haemoglobinuria and atypical hemolytic uremic syndrome, yet it increases vulnerability to encapsulated bacterial infections requiring long-term antibiotic prophylaxis and vaccination.^41-49 These risks are amplified in patients with comorbid immunosuppression or chronic infections.

In disease contexts where GZMK⁺ lymphocytes and complement cooperatively drive pathology, for example, autoimmune arthritis, inflammatory bowel disease, lupus, rational combination strategies may be warranted. The dual inhibition of GZMK and C5aR1, combined with biomarker-guided patient selection and infection prophylaxis could offer a high-reward precision immunotherapy approach. For instance, patients with elevated GZMK⁺ CD8⁺ T cell infiltration and complement fragment deposition may benefit most from such interventions.

Future research should aim to define precise molecular contexts and biomarkers that delineate when and where GZMK-driven complement activation is pathogenic, protective, or both. Understanding this immunological dialectic will be a key to translating these insights into clinical benefit.

Manoj Kumar Pandey conceived the overarching structure and scientific narrative of the manuscript. He conducted comprehensive literature reviews, curated original figures, and led the drafting of the text. He was solely responsible for ensuring the scientific rigor, conceptual integrity, and clarity of the manuscript. He affirms fully applicable for all aspects of the work, including its accuracy and integrity.

The author declares no conflicts of interest.

This research received no external funding.

Not applicable.