Calpains move from cell death to immune checkpoint biology
Calpains are no longer confined to the biology of apoptosis. In this systematic review, calpain activity emerges as a wider immunologic regulator, shaping the balance between resting T-cell quiescence and activation through selective proteolysis of signaling proteins such as ZAP-70, NF-κB, Cyclin D1, and c-Myc. The central argument is straightforward: the same proteases long associated with structural remodeling and death can also set the activation threshold for lymphocytes and help preserve self-tolerance.
A key strength of the review lies in its scope. The literature search followed PRISMA 2020 guidance and screened PubMed, Web of Science, Scopus, Google Scholar, and the Cochrane Library for studies published from 1990 through September 2026. From 281 records identified, 125 studies met inclusion criteria after title, abstract, and full-text assessment. Those studies consistently pointed to calpains as selective regulators rather than indiscriminate degraders, with proteolysis often limited, context dependent, and capable of altering protein function without fully destroying the substrate.
Resting T cells are not inert. They sit in a metabolically restrained state while remaining poised for antigenic challenge, and that poised state depends on tightly controlled signaling chemistry. Calpain activity appears to contribute to that readiness by preserving signaling competence in quiescent T cells and by shaping how vigorously those cells respond once antigen arrives. In practical terms, excessive calpain activity can push toward immune suppression, while insufficient activity can remove a brake on activation and contribute to autoimmunity.
Limited proteolysis shapes T-cell activation thresholds
Calpains differ from many proteases because they do not rely on a simple consensus cleavage motif. Substrate recognition depends on structural context, solvent exposure, and the local protein assembly. This matters in lymphocytes, where a cleavage event need not eliminate a protein to change its behavior; a fragment can alter signaling location, stability, interaction partners, or downstream transcriptional effects.
Among the proteins discussed in the review, ZAP-70 stands out because it sits close to the T-cell receptor signaling hub. Calpain-mediated cleavage of ZAP-70 after antigen stimulation suggests a mechanism for damping proximal receptor signaling once activation has begun. Cyclin D1 and c-Myc point to a second layer of control. By influencing cell-cycle entry and metabolic growth programs, calpains can restrain the transition from a quiescent lymphocyte into a proliferating effector cell. The review also links calpain activity to NF-κB-associated inflammatory signaling, tying proteolysis to transcriptional programs that determine whether the immune response stays proportionate or becomes excessive.
One sentence captures the biology sharply: calpains can change the fate of a T cell without killing it.
The calpain-calpastatin axis and self-tolerance
The review places the traditional calpain-calpastatin system at the center of immune homeostasis. Calpain-1 and calpain-2, together with calpastatin, are broadly expressed and appear to dominate the regulation of signaling, cytoskeletal reorganization, and post-translational modification in immune cells. Basal calpain activity is present in resting human CD4+ and CD8+ T cells, and inhibition of that activity in resting cells reduces proliferation and cytokine secretion. That pattern supports a model in which physiologic calpain activity is required for functional competence, but only within narrow limits.
Self-tolerance emerges from this same logic. The review discusses proposed calpain regulation of checkpoint molecules such as PD-1 and CTLA-4, although direct evidence of cleavage remains limited. Even so, the concept is biologically plausible: if calpains influence receptor-proximal signaling, cell-cycle progression, and checkpoint tone, then they can act as a molecular governor on adaptive immunity. Too much proteolysis could destabilize inhibitory circuits and contribute to immune deficiency. Too little could leave autoreactive signaling insufficiently restrained and permit autoimmune disease.
A second sentence lands with equal force: the direction of the proteolytic imbalance determines whether the immune system drifts toward suppression or self-reactivity.
Autoimmunity, inflammation, and the translational gap
Preclinical work in the review points toward therapeutic inhibition of calpains as a way to reduce autoimmune disease severity and inflammation. Pharmacological suppression of calpain activity has repeatedly lowered inflammatory readouts in experimental settings, and calpastatin deficiency has been tied to enhanced IκB cleavage, increased NF-κB nuclear translocation, and greater IL-6 and TNF-α production in response to bacterial stimuli. Those findings matter because they connect a protease system to a recognizable inflammatory cascade that feeds tissue injury.
Still, the translational boundary remains wide. Human efficacy data for calpain inhibition in autoimmune disease have not been established. That leaves the field with a clear mechanistic signal but no clinical proof that suppressing calpains will help patients in a durable and safe way. Because calpains also participate in apoptosis, cytoskeletal remodeling, and broader cell signaling, indiscriminate inhibition could produce effects that are helpful in one immune compartment and harmful in another.
The review also points to the opposite direction of pathology. When proteolysis is insufficient, T-cell activation may escape normal restraint; when proteolysis is excessive, immune competence may collapse. This duality helps explain why calpain dysregulation has been linked not only to autoimmunity but also to immunosuppression, chronic infection, and cancer-related immune exhaustion. In that sense, calpains sit at a crossroads shared by immunity, tolerance, and failure of immune surveillance.
Apoptosis remains relevant, but it is not the whole story
The review does not discard apoptosis; it reframes it. Calpain-mediated cleavage can feed intrinsic apoptotic pathways, including Bax processing and cytochrome c release, and calpain-caspase feedback loops have been described in drug-induced T-cell death models. Calpain cleavage of calpastatin can also reinforce protease activity during stress, amplifying apoptosis when the cellular environment shifts toward injury. These interactions show why calpains should not be treated as a single-purpose death enzyme.
Instead, the biology suggests a continuum. At one end, basal calpain activity supports quiescent competence and normal activation thresholds. In the middle, controlled limited proteolysis fine-tunes signaling and checkpoint behavior. At the far end, dysregulated cleavage can tip cells into apoptosis or leave them improperly activated. That continuum is the most useful clinical insight in the review because it defines the therapeutic challenge: modulate proteolysis without breaking the immune system’s restraint mechanisms.
Why the review matters for autoimmune drug development
For clinicians and translational scientists, the review offers a clear message. Calpains deserve attention as regulators of immune quiescence and self-tolerance, not merely as executors of cell death. Their substrates place them directly at the interface of T-cell receptor signaling, cell-cycle entry, inflammatory transcription, and checkpoint control. Any future therapeutic program will need to distinguish between isoforms, define the relevant cell populations, and determine whether transient inhibition or partial modulation is safer than broad suppression.
The review also exposes what remains unknown. Direct evidence for cleavage of some checkpoint molecules is still thin. Human efficacy data are absent. The review is built on a large preclinical and mechanistic literature, but mechanistic abundance is not the same as clinical validation. For now, calpains stand as attractive but unproven drug targets in autoimmunity and perhaps in chronic infection or cancer, where immune activation thresholds are already distorted.
The decisive question is not whether calpains matter. It is how much protease activity a T cell can lose before tolerance gives way to dysfunction.